Mold Advance Course Book

PITAC

Mold Design (Advance)

Page 1

1- Temperature Control Basics Temperature control for a mold refers to a control of receiving and releasing heat on the mold. In this connection knowledge of heat conductivity is important for consideration of heat reception and heat dissipation. Thus the basics of thermal conductivity will be reviewed as follows. 1-1 Heat transfer When there is a certain temperature difference in an object or between objects, heat will transfer to keep thermal equilibrium in a system. Heat will be transferred from high side to low side and the transfer modes are classified as follows: Heat transfer

Heat conduction Convection heat transfer (Heat delivery) Radiation heat transfer

Above three occur in a complex manner, but one normally dominates others. 1-1-1 Heat conduction Characteristic of heat conduction is that the conductor does not move. Thus heat transfer in a solid object is considered to be the result of genuine heat conduction. To a certain extent heat conduction occurs in gas and liquid but the conductivity there is prohibitively small in comparison with that of solid body. Transfer of heat is made from high temperature area to low temperature area and the transfer rate is proportional to the temperature gradient and the cross section area of heat passage. This is called Fourier’s Heat Conduction Law and the formula is shown below (Fig. 1-1-1.1). Q = λ•

∆T1 • A1 ................................................................. Formula (1.1.1.1) S

Where Q: Heat transfer rate: Heat flow (kcal/h) ΔT1 : Temperature difference between 2 points (℃) S: Distance between 2 points (m) A1: Cross-section area perpendicular to heat flow (m2) λ : Heat conductivity (kcal/m•h•℃) In the case of heat transfer from resin to mold in the molding process, both heat conduction and heat convection occur simultaneously during the injection process but heat conduction dominates during cooling process under holding pressure after the injection process. Heat transfer from cavity surface to wall surface of cooling water pipe is made under genuine heat conduction because it is a heat transfer in a solid body. Incidentally, heat conductivity of S50C steel, which is often used as mold material, is about 46 kcal/m•h•℃, while heat conductivity of HDPE, which has rather high heat conductivity among resins, is 0.4 kcal/m•h•℃ and that of GPPS, lower heat conductivity among resins, is about 0.1 kcal/m•h•℃. The ratio to S50C is 1:115 and 1:460 respectively (Fig. 1-1-1.2).

Mold Design (Advance)

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PITAC Section Area

Section A-A

S = Distance

Heat Flow

"Solid"

Temperature Gradient

Temperature Difference

Fig. 1-1-1.1 Image of Heat Transfer

Air Water * Notice log scaling.

Pure Copper Heat Conductivity: λ (kcal/m•h•℃)

Fig. 1-1-1.2 Heat Conductivity of Various Materials (Ambient Temp. 20℃) Mold Design (Advance)

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PITAC 1-1-2 Convection heat transfer (Heat delivery)

Looking into heat transfer between liquid and solid, effect of heat transfer along with movement of liquid is much greater than heat conduction. It is called convection heat transfer or heat delivery. The heat transfer rate is proportional to temperature difference between solid and liquid and transfer area between the same. The formula is shown below: Q = α•ΔT2•A2 (kcal/h) ....................................................... Formula (1.1.2.1) Where A2: Heat transfer area between solid and liquid (m2) ΔT2 : Temperature difference between solid and liquid (℃) α : Heat transfer coefficient (kcal/m2•h•℃) Difference between heat delivery and heat conduction is that in the heat delivery heat transfers along with moving liquid media and heat transfer coefficient α is not a specific constant for material like λ (formula 1.3.8.1) and varies depending upon flow condition. It is considered that there exists a stable film of liquid (or gas), named boundary film, between solid and flow media. This film is not subject to convection heat transfer but conduction heat transfer only. As heat conductivity of flow medium is small in comparison to solid, this boundary film can be treated as a kind of insulation layer made of flow medium (Fig. 1-1-2.1). Accordingly if a flow makes the film thinner, the heat transfer coefficient α becomes greater and the heat transfer rate becomes faster. Generally in the case of slow flow velocity, the flow forms so called laminate flow in which liquid is not mixed. In this case the boundary film is thicker. On the other hand the film is thinner if the flow is under turbulent flow with high velocity where liquid is well mixed. As explained, heat transfer coefficient α is the one having a boundary film in between and influenced substantially by the film thickness. Thus it may be called as boundary film heat transfer coefficient. It is important how to determine α in the convection heat transfer. One way is to determine α on the basis of Nusselt Number (Nu) which represents magnitude of heat transfer between solid and flow medium. α = Nu λ F／D (kcal/m2•h•℃) ............................................ Formula (1.1.2.2)

Where λ F: Heat conductivity of fluid (kcal/m•h•℃) D: Internal diameter of pipe (m) Convection heat transfer can be classified by two. One is natural convection heat transfer and another is enforced convection heat transfer. Heat discharge from mold to atmosphere is mainly influenced by natural convection heat transfer together with radiation heat transfer to be explained later. While, heat transfer from mold (internal wall of cooling water tube) to cooling medium (water or oil) is mainly affected by enforced convection heat transfer.

Mold Design (Advance)

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PITAC

Liquid

Solid

Temperature Difference

Boundary Film A2: Conduction Area Heat Flow: Q = α•∆T2•A2 (kcal/h)

Fig. 1-1-2.1 Heat Transfer from Solid to Liquid

1-1-3 Radiation heat transfer

Thermal energy from the sun is brought to the earth through a space without any transfer media. This is because heat transfers as electro magnetic wave as same as light and electric wave. This sort of heat transfer is called radiation heat transfer or simply radiation. Any material radiates heat unless its temperature is 0°K (-273℃) in absolute temperature. The radiation is mutually absorbed, reflected or passed trough. The heat transfer rate in radiation is proportional to difference of the 4th power of absolute temperature (Kelvin’s temperature). It is shown below (Fig. 1-1-3.1). Q = K (TA4 ― TB4) (kcal/h) ................................................. Formula (1.1.3.1) Where TA: Absolute temperature of object A (°K) TB: Absolute temperature of object B (°K) K: Proportion constant Proportion constant k includes various elements. This k is not given based on physical property like heat conductivity (λ) but calculation like heat transfer coefficient (α). In the case of radiation from a mold, a formula is given below considering object a (mold) is surrounded by object b (air) and radiation area ratio (AA/AB) is negligibly small. Mold Design (Advance)

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PITAC ⎧⎪⎛ T 4 ⎞ ⎛ T 4 ⎞⎪⎫ Q = ⎨⎜⎜ A ⎟⎟ − ⎜⎜ B ⎟⎟⎬ (kcal/h) ........................................... Formula (1.1.3.2) ⎪⎩⎝ 100 ⎠ ⎝ 100 ⎠⎪⎭

Where AA:Surface area of object a (m2) σ : Black Radiation constant = 4.88 kcal/m2•h•k4 ε : Radiation rate of object A In the formula (1.1.3.2), let us see the influence to heat flow due to radiation of mold temperature by varying the temperature TA like 40℃, 80℃, and 120℃. Room temperature TB is assumed to be 25℃. Result shows when TA changes to 2 times and 3 times, resultant Q changes 4.5 times and 9.3 times. This tells you that radiation transfer cannot be ignored if temperature difference between room temperature and heated object temperature is big when the object is exposed to atmosphere.

Absorption

Reflection

Fig. 1-1-3.1 Image of Heat Radiation Transfer

Mold Design (Advance)

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PITAC 1-2 Received heat of a mold In terms of received heat (QI) of a mold, the biggest source must be from resin (QA). Other sources may be of heat from nozzle (QB) of nozzle touch area of injection machine or received heat from hot runner manifold and hot tip area (QC) in the case of hot runner mold (Fig. 1-2.1). QI = QA + QB + QC (kcal/h)....................................................... Formula (1.2.1) Let’s take up received heat from resin (QA). When W (kg/h) is resin weight injected per hour, received heat (QA) can be calculated by applying following formula. QA = W • {CP (TP - TR) + L • C} (kcal/kg•℃) .......................... Formula (1.2.2) Where CP: Specific heat of resin (kcal/kg•℃) TP: Resin Temperature (℃) TR: Temperature at mold separation (℃) L: Latent heat of crystalline resin (kcal/kg) C: Crystallinity of crystalline resin (0.1~0.8) In the formula (1.2.2), temperature at mold separation (TR) can be replaced by thermal deformation temperature to assure the temperature in the center of the thickest portion of the product to be lower than the heat distortion temperature. In this case, try to set the temperature 10~30℃lower than the heat distortion temperature to entertain safety consideration. A part of the formula {CP(TP-TR) +L•C} can be roughly estimated by resin material, when the value is represented by total heat amount (Q), formula (1.2.2) can be shown as below. QA = W•Q (kcal/kg) .................................................................. Formula (1.2.3) Table 1-2.1 shows estimated values of Q by resin material in the safe direction (estimating q in the bigger side).

3) Released heat from a mold If there is no temperature control device on a mold (natural radiation only), released heat from a mold (QO) should consist of transferred heat to platen of injection machine (QD) and radiated heat to atmosphere (QE) (Fig. 1-2.2). QO = QD + QE (kcal/h)............................................................... Formula (1.2.4) QD is calculated as heat passing through composite wall surfaces, but estimation of heat resistance between mold clamping plate and platen of injection machine is very difficult. QE is considered as a mixture of convection and radiation heat transfer. It is influenced by molding conditions such as mold temperature, airflow, mold open time, etc. Mold Design (Advance)

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PITAC In case of cold runner : QI = Qa + Qb In case of hot runner : QI = Qa + Qc

Heat from nozzle touch area

Heat from melted resin

Heat from hot runner area

Fig. 1-2.1 Various Heat Received by Mold (QI) Specific Heat Cp Latent heat L

Crystalline Rate C

Total Heat q

Non-crystalline

Crystalline

Resin Material

Table 1-2.1 Heat Specifics of Resin Material Platen of Injection Machine

Mold

a. Released Heat at Mold Binding

b. Released Heat at Mold Opening

Fig. 1-2.2 Released Heat from Mold (Q0) Mold Design (Advance)

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PITAC 1-3 Heat to be removed from a mold

A mold reaches to thermally balanced condition through heat receiving and heat releasing process. Thus theoretically speaking, molding can be made without temperature control device as long as the balanced temperature is suitable for the plastic molding. However it is advised not to precede molding without temperature control device because a long time will be needed before reaching to a balanced condition and moreover mold temperature cannot be stable being influenced by environmental disturbances. If receiving heat is more than releasing heat and thermal balanced point is higher than required temperature range, cooling device is needed. On the other hand, if receiving heat is less than releasing heat and balanced temperature point is lower than required temperature range, heating device should be arranged (Fig. 1-3.1). Here in this section, condition QO< QI, in other words, condition required to cool off a mold, will be taken and heat to be removed from a mold will be discussed. Removed heat QR can be expressed in a molding cycle where receiving and releasing heat are to be balanced. QI = QO + QR (kcal/h)................................................................ Formula (1.3.1) Then QR = QI – QO (kcal/h) ................................................................ Formula (1.3.2) If you understand basics behind the formula (1.3.3), you may simplify the calculation as follows. In the cold runner mold, QB can be traded off by (QD + QE) because (QD + QE) is usually bigger than QB. By trading them off, cooling calculation will come to safe side (increased requirement for cooling). In this way, you may treat heat to be removed (QR) is equivalent to received heat (QA) from resin. QR ≒QA ≒ W • Q (kcal/h) ....................................................... Formula (1.3.4) In the heat transfer calculation, you may apply formula (1.3.4) for approximate result because q in formula (1.2.3) and table (1-2.1) are given in the safe side. However if you intend to apply formula (1.2.2), it is advised to incorporate 1.5 times safety factor taking account of possible requirement of cycle shortening and expected deterioration in the heat exchanger performance. QR = 1.5 QA = 1.5 W • {CP (TP – TR) + L • C} (kcal/h) ........................... Formula (1.3.5) Mold Design (Advance)

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PITAC Assuming QR can be applicable to all kinds of cooling medium, following formula can be derived. QR = WL • CPL (TW －TL) (kcal/h) ............................................ Formula (1.3.6) Where WL: Required weight of cooling agent (kg/h) CPL: Specific heat of cooling agent (kcal/kg•℃) TW: Internal wall temperature of cooling tube (℃) TL: Average temperature of cooling agent (℃) TL in formula (1.3.6) is average temperature of the cooling medium other than that in the boundary film. In the case of water as cooling medium, (TW–TL) can be regarded as about 2~3℃. WL and VL, required volume of cooling agent can be expressed as follows: WL = VL • ρL (kg/h)................................................................... Formula (1.3.7) VL = V • πD2/4 (m3/h) ............................................................... Formula (1.3.8) Where PL: Density of cooling medium (kg/m3) V: Flow velocity of cooling medium (m/h) D: Internal diameter of cooling tube (m) Flow velocity V can be derived from formulae (1.3.4) or (1.3.5), (1.3.6), (1.3.7) and (1.3.8), as follows: V=

4W • Q (m / h ) .................................... Formula (1.3.9) π • D 2 • ρ L • C PL (TW − TL )

More accurate formula must be:

V=

4 × 1.5W • {C P (TP − TR ) + L • C} (m / h ) ........................... Formula (1.3.10) π • D 2 • ρ L • C PL (TW − TL )

Internal diameter of cooling tube (D) in above formulae should be picked up from Table 13.1 temporally, and confirm them if it falls in the range of 10,000~30,000 of Reynolds number (RE) and then finalize the diameter. Be aware that unit of d is in m. RE = D • ρL • V／µ ................................................................... Formula (1.3.11) Where µ : Viscosity of cooling medium (kg/m•h) Mold Design (Advance)

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PITAC Next, Nusselt number (NU), important parameter in the convection heat transfer calculation, will be calculated. Prantle number (PR) in Nusslet number is defined as follows. PR = ν／A = µ • CPL／λF.......................................................... Formula (1.3.12) Where ν : Dinamic viscosity of cooling medium = µ / ρL (m2/h) A : Heat dissipation rate = λF/ρL • CPL (m2/h) Nusselt number is given as follows. Be aware NU formula varies slightly depending upon where to get the formula from. Nu = 0.023 • (RE) 0.8 • (PR) 1/3 ................................................. Formula (1.3.13) Formula (1.3.13) is effective only for turbulent flow. In the case of laminated flow or transition flow, in which Raynold’s number is less than 10,000, re-evaluation of mold temperature and cooling tube diameter must be carried out. Once Nusselt number (NU) is decided, heat transfer co-efficient α can be calculated by formula (1.1.2.1). And cooling tube surface area (AL) can be calculated by a converted formula from (1.2.2). AL = QR／α • (TW – TL) (m2) .................................................... Formula (1.3.14) As cooling tube diameter (D) is known, cooling tube length can be calculated as follows: LR = AL／πD (m) ...................................................................... Formula (1.3.15) As described, total cooling circuit length of cavity and core can be calculated. So far, all formulae assumed that heat from injected resin W (kg/h) is transferred perfectly to a mold. Here we should evaluate if such assumption is reasonable or not. Bahlman’s formula should be effective for the evaluation. It is to evaluate if W (kg/h) is possible by estimating molding cycle from theoretical cooling time calculation. TC =

H2 π

2

•A

⎡ 4 TP − TW ⎤ • ⎥ .............................................. Formula (1.3.16) ⎣ π TR − TW ⎦

• LN ⎢

Where Tc: Theoretical cooling time (h) H: Product thickness (m) A: Resin dissipation rate = λ/ρ•CP (m2/h) Mold Design (Advance)

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PITAC TP: Resin temperature (℃) TR: Mold separation temperature (℃) TW: Mold temperature (℃) This formula can be used as a guideline because all kinds of condition have to be assumed for calculation. Molding cycle should be evaluated by estimating injection time, mold opening time duration, mold take-out time duration. If you design temperature control system by applying above explained basics on the heat transfer thermal dynamics, you should be able to provide temperature control system with improved heat exchanging efficiency comparing with traditional system, which was made based on past examples. In the end of basics on thermal transfer theory, a calculation example is shown below for your better understanding. Try to solve the example before you read the answer to follow. Mind units are to be carefully treated.

Mold Design (Advance)

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PITAC 【A calculation example】

Calculate total circuit length of a cooling water system required for a mold for following conditions. Resin: ABS Number of product per mold: 2 Product dimension: 100mm×100mm×2mm Molding cycle target: 20 seconds (Calculation) ① To estimate molding conditions along with physical properties of ABS resin.

• Density: ρ = 1030kg/m3 • Specific heat: CP = 0.35 kcal/kg•℃ • Heat conductivity: λ = 0.2 kcal/m•h•℃ • Resin temperature: TP = 220℃ • Mold separation temperature: TR = 70℃ • Internal wall temperature (water tube): TW = 52℃ • Water temperature: TL = 50℃ ② To decide mold specification required for heat transfer calculation.

1) Physical property of water • Density: PL = 988 kg/m3 • Specific heat: CPL = 1.0 kcal/kg•℃ • Viscosity: µ = 5.58×10-4 PA•S = 2.009 kg/m•h • Heat conductivity: λF = 0.552 kcal/m•h•℃ 2) To calculate weight of Sprue runner • Dimension: (φ5×130) mm = (φ0.005×0.13)m • Volume: (0.0052×π/4) ×0.13 = 2.55×10-6 m3 • Weight: (2.55×10-6) ×1030 = 2.63×10-3 kg 3) To calculate weight of product • Volume: • Weight:

(0.1×0.1×0.002) ×2 = 4.0×10-5 m3 (4.0×10-5) ×1030 = 0.0412 kg

4) To assume cooling tube diameter. • From Table 1-3.1, the diameter φ8 is assumed. D = 8 mm → 0.008 m ③ To check appropriateness of molding cycle

1) To calculate heat dissipation rate (A) A = 0.2／1030×0.35 = 5.55×10-4 m2/h

Mold Design (Advance)

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PITAC 2) To calculate theoretical cooling time 0.002 2 ⎡ 4 220 − 52 ⎤ • LN ⎢⎣ π × 70 − 52 ⎥⎦ π 2 × 5.55 × 10 − 4 =1.807×10-3h = 6.5 sec TC =

Injection machine (100~150 ton capacity) to be used for this size of product usually required additional 10 seconds maximum for other functions than cooling time. Thus target cycle time of 20 seconds is reasonable. It could be less than 15 seconds. ④ To calculate injection weight per hour

• Weight per one shot: 2.63×10-3 +0.0412 = 0.0438kg • W = 0.0438×3600 / 20 = 7.884 kg/h ⑤ To calculate heat (QR) to be removed

QR = 1.5 QA = 1.5×7.884 {0.35(220－70)} = 620.9 kcal/h ⑥ To calculate flow velocity of cooling water

V=

4 × 620.9 = 6251 m / h π × 0.008 2 × 988 × 1 (52 − 50)

⑦ To check Reynolds’s number

RE = 0.008×988×6251 / 2.009 = 24593 RE falls in the range of 10,000~30,000. Thus water tube diameter φ8 is ok. ⑧ To calculate Prantle number (PR).

PR = 2.009×1 / 0.552 = 3.64 ⑨ To calculate Nusselt number

NU = 0.023× (24593) 0.8×3.641/3 = 115.2 ⑩ To calculate heat transfer coefficient (α)

α = 115.2 ×0.552 / 0.008 = 7949 kcal/m2•h•℃ ⑪ To calculate cooling water tube area (AL).

AL = 620.9 / 7949× (52–50) = 0.039m2 ⑫ To calculate total cooling water tube length LR = 0.039 / π×0.008 = 1.55m

Mold Design (Advance)

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Cooling

Mold Temperature

Balanced Mold Temperature without temperature control device.

Heating

PITAC

Nominal Mold Temperature

Balanced Mold Temperature with temperature control device

Time

Time

a. Without Temperature Control Device (Deviated by outside influences)

b. With Cooling Device

Time

c. With Heating Device (natural cooling)

Fig. 1-3.1 Temperature Balance vs. Mold Temperature Control Device

Table 1-3.1 Mold Size vs. Channel Diameters Mold Size (Binding Fore)

Mold Design (Advance)

Channel Diameter

Page 15

PITAC

Table 1-3.2 Comparison in Mold Temperature Control Method Items Objective Mold Temperature Cooling Method Heating Method Temperature Control

Characteristics

Strong Points

Weak Points

Coolant Circulation

Heater

Cooling Lower than naturally balanced temperature (-20～130℃) To circulate heat transfer medium that is cooled by cold water. To circulate heat transfer medium heated by heater. To detect medium temperature and control the temperature. • Good flexibility in temperature control design. • Independent to mold heat capacity due to forced cooling. • Relatively high cost. • Relatively difficult in design and manufacturing.

Heating Higher than naturally balanced temperature (higher than 100℃) Heat transfer to platen or natural radiation. To heat the mold by heater. To detect and control mold temperature directly. • Relatively low cost. • Relatively easy in design and manufacturing.

• Less flexibility in temperature control design. • Dependent to mold heat capacity due to natural cooling.

Table 1-3.3 Selection Criteria in Temperature Control Method by Resin Resin

Mold Temperature °C

PE PP +++++++P 0S0 PVC ABS AS PMMA MPPE PA PBT POM PC PPS

Mold Design (Advance)

30~70 20~80 40~60 40~70 40~70 40~80 50~80 50~100 50~110 60~110 70~110 80~120 120~160

Water Circulation Method Supplied Pressure Added Pressure ○ △ ○ △ ○ △ ○ ○ ○ ○ △ △ △ △ △ ×

△ △ △ △ ○ ○ ○ ○ ○ △

Heater Method × × ×

× × × × △ △ △ △ ○ ○ ○ Proper △ So-so ×Improper

Page 16

PITAC 1-4

Design of Temperature Control Device

Coolant circulation method Important point for the design of coolant circulation system is to optimize the design in terms of coolant circulation channel, size and location in consideration of product quality and productivity. For the design of pressurized water circulation system, basic design concepts are the same but special attention should be paid for sealing and safety as well. It is suggested to refer technical information supplied by the system suppliers. Reduced pressure suction type of temperature control device will not be treated in this section because of its specialty in the circuit design. Hereafter, we will discuss about design concept for standard temperature control device utilizing water, an excellent coolant for heat exchanging. 1-4-1 Coolant channel diameter and flow velocity

Cooling efficiency is higher if coolant in the channel is under turbulent flow where boundary film is thin. Thus it is important to decide proper diameter of coolant channel so as to make the flow stable turbulent with Reynolds’s number RE 10.000~30.000. As you may refer to formula (1.3.11), RE seems proportional to diameter d; in other words, a big diameter seems to give a big RE. This may be true if other factors stay the same. But if flow volume is given constant, flow velocity is reversibly proportional to channel cross section area, which is proportional to the 2nd power of channel diameter in case of round channel. Therefore if flow volume is given constant, flow velocity and RE become small with big diameter D referring to formula (1.3.8) and formula (1.3.11). Accordingly if channel diameter D is too big, heat-exchanging performance drops due to smaller flow velocity, Reynolds’s number, Nusselt number and heat transfer coefficient. If the diameter is too small, the heat exchanging performance will also drop due to less flow volume and increased pressure loss in the flow channel. Thus the channel diameter should be appropriately designed referring to Formulae (1.1.2.2), (1.3.8) Table (1.1.2.1). When passage is not of round shape, equivalent diameter should be applicable as explained in the “Gate Runner System”. The equivalent diameter (DE) was as follows: DE = 4AS/LW ....................................................................... Formula (1.4.1) Where AS: Cross section area of coolant channel LW: Circumference length of AS Fig. 1-4.1 shows equivalent diameter for various cross sections such as half a round or square shape.

Mold Design (Advance)

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PITAC

Square

Rectangle

Half Circle

Channel Section (b = 0.5a)

Equivalent Diameter

Fig. 1-4.1Channel Section vs. Equivalent Diameter

Mold Design (Advance)

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PITAC 1-5

Heater capacity

Heater capacity can be calculated as follows: P = WM • CPM (TT－TI) ／TU • 860 • η (kW) ............................ Formula (1.5.1) Where P: Capacity (kw) WM: Mold weight (kg) CPM: Specific heat of mold (kcal/kg•℃) TI: Atmospheric temperature (℃) TT: Target temperature (℃) TU: Heating time (h) η : Efficiency (0.2~0.5) η Is a value due to heat transfer loss due to radiation loss or loss due to heater mounting, etc, and normally it is set as 0.5. If you utilize a heater with higher capacity and with adjustable power arrangement, you may ensure stable mold temperature by adjusting heating and radiation conditions in addition to shortened preparation time.

Mold Design (Advance)

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PITAC

Heat Insulation Heat Insulation Plate Platen of Injection Plate Machine (Movable Side) Platen of Injection Machine (Fixed Side)

Heat Insulation Plate (Movable Mold Plate, All around Receiving Plate)

Heat Insulation Plate (All around Fixed Side Mold Plate)

Fig. 1-5.1stallation Points of Heat Insulation Plate for High Temperature Application

Heater

Section A-A

Heater Mounting Bolt

Thermocouple

Fig. 1-5.2 Mold with Heated Temperature Control Device

Mold Design (Advance)

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PITAC 1-6 Clamping force

In order to calculate required clamping force of a mold used for a product, we need to know a force on the mold toward opening direction received from injected resin. This mold opening force (FO) can be expressed as the product of total projected area of a product and a runner and average molding pressure (cavity inside pressure) as follows FO = AA • PM • 10-3 (tf) .............................................................. Formula (1.6.1) Where AA: Total projection area (cm2) PM: Average molding pressure (kgf/ cm2) Projection area is the area of a product projected in the mold clamping direction (usually perpendicular to PL). Total area is shown below (Fig. 1-6.1) AA = N • AP + AR (cm2)............................................................. Formula (1.6.2) Where N: No. Of cavity AP: Product projection area per piece (cm2) AR: Runner projection area (cm2) In the case of 3-plate mold, notice that projection of cavity and runner overlaps. If separately calculated, overlapped area will be calculated twice. Assuming mold is of transparent, project parallel light from nozzle side, and then figure projected area on the movable platen surface (Fig. 1-6.2).

Mold Design (Advance)

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PITAC

Projection

Projected Area

a) Container Type Product

b) Disc Type Product

*Projected area has no influence from height dimension.

Fig. 1-6.1 Projected Area used for Binding Force Calculation

Product

Sprue, Runner

Projection

Total Projected Area (Aa)

Fig. 1-6.2 Total Projected Area of 3 Plate Mold

Mold Design (Advance)

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PITAC Formula (1.6.1) shows that mold-opening force is proportional to projected area if average clamping force is constant. Let us look into molding pressure. Injected mold by injection machine fills a mold space against pressure loss caused by nozzle, sprue, runner, gate and cavity. Thus there exists a big pressure difference between sprue area and a part of cavity where resin is filled in the end. Even after resin filling, molding pressure varies from place to place (Fig. 1-6.3). However, mind that you need to know average pressure, not all in different parts. Although average clamping force varies depending upon product shape, molding condition, mold structure, etc., Table 1-6.1 can be practically used for your guideline. If your calculation reveals clamping force exceeds average mold opening force, the machine should be well justified. In practice, the clamping force should be evaluated as 80% of maximum clamping force against mold opening force compensating estimated average of the opening force. Required clamping force (FC) then is shown as follows. 0.8 FC ≧ FO (tf)......................................................................... Formula (1.6.3) FC ≧ FO／0.8 (tf)...................................................................... Formula (1.6.4) FC ≧ 1.25 FO (tf)....................................................................... Formula (1.6.5) If your selected injection machine has a clamping force more than above described force, you will be able to mold products without burrs on PL surface. But be minded that too big a clamping force may cause you a trouble such as partial concentration of the force in the center or ineffective clamping force due to excessive size of locating hole, etc. (Fig. 1-6.4). Rule of thumb is not to go beyond 20% of above formula. FO／0.2 ≧ FC ≧ FO／0.8 (tf)................................................... Formula (1.6.3) Or 5 FO ≧ FC ≧ 1.25 FO (tf) ......................................................... Formula (1.6.3)

Mold Design (Advance)

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PITAC Average Resin Pressure

Binding Force

Fig. 1-6.3 Resin Pressure Works toward Mold Opening Incorporated to Projection Area Table 1-6.1 Various Resin Materials and Average Resin Pressure in Cavity Resin Material

Average Resin Pressure in Cavity (Kgf/cm2)

Locating Ring Hole

Too big a locating ring hole against mold size.

Deformation of platen due to partial binding force on the middle (particularly for toggle type)

Binding force cannot be applied properly to fixed side mold due to too big locating ring hole.

Fig. 1-6.4 Problems due to too Big Injection Machine Mold Design (Advance)

Page 24

PITAC 【Calculation example 1】

Calculate required clamping force for a product shown in Fig. 1-6.5. Material is ABS and numbers of cavity are two. [Calculation] ① Estimate runner layout (Fig. 1-6.6). ② Calculate runner projection area (AR).

AR = 0.5×5 = 2.5 ≒ 3 cm2 (Round up the first decimal.) ③ Calculate product projection area (AP).

AP = (5×8)－(π×12) = 37 cm2 ④ Calculate total projection area (AA).

AA = (2×37) + 3 = 77 cm2 ⑤ Calculate mold-opening force (FO).

From Table 1-6.1, PM = 300 kgf/cm2, FO = 77×300×10-3 = 23.1 tf ⑥ Calculate required clamping force (FC).

FC ≧ 1.25×23.1 = 28.9 tf FC ≤ 5×23.1 = 115.5 tf Thus, applicable injection machine should be of the clamping force more than 30ton and less than 110 ton (or 100 ton may be).

Mold Design (Advance)

Page 25

PITAC

(All Circumference)

Fig. 1-6.5

Product Projection Area (Ap) Sprue

Expected Runner Layout

Runner Projection Area (Ar)

Fig. 1-6.6 Mold Design (Advance)

Page 26

PITAC 1.7 Required injection capacity

A general injection machine provides 3 screw size options per clamping unit. Accordingly you should know which=0.h screw size is selected for evaluation of required injection capacity. As screw size is given bigger, maximum injection capacity becomes bigger. But injection pressure goes opposite direction. In other words, as screw size is given bigger, maximum injection pressure becomes smaller (Fig. 1-7.1) as long as diameter of hydraulic cylinder is the same. Thus under the same clamping force, injection machine with small screw is suited for precision thin wall products and injection machine with large screw is suited for large products with thick walls. Injection capacity is a product of internal cross section area of injection cylinder (screw cross section area) and injection stroke. VI =AI • SI (cm3)........................................................................ Formula (1.7.1) AI = πD2／4 (cm2)..................................................................... Formula (1.7.2) Where VI: Injection volume (cm3) AI: Internal cross section area of injection cylinder (cm2) SI: Injection stroke (cm) D: Screw diameter (cm) Maximum injection capacity shown in the specification of injection machine is calculated by above formula. This means a capacity of an object (air for example) injected under normal temperature and pressure by screw size. Practically molding is operated by injecting plastic with viscosity and elasticity under high temperature and pressure. Therefore maximum injection capacity for injecting PS-GP (General purpose polystyrene) is usually given together with theoretical value. Actually, internal pressure of injection cylinder is high, but density of the plastic there is smaller than that under normal temperature and pressure condition because plastic in the cylinder is expanded due to high temperature (Fig. 1-7.2). Calculation of required injection capacity has two folds. The first step is to calculate expanded capacity of plastic per one shot for product, sprue and runner under high pressure Mold Design (Advance)

Page 27

PITAC and temperature condition, then the second step to compare it with maximum injection capacity assigned for an injection machine. Specifically, total injection capacity can be given as follows: VA = N • VP + VR (cm3)............................................................. Formula (1.7.3) Where VA:Total injection capacity (cm3) N: Number of cavity VP: Mold volume per one piece (cm3) VR: Volume of sprue and runner (cm3) To figure volume expansion accurately, we need PVT data per plastic material, but in our purpose it is not necessary to go to those details. Although there is some variations in pressure and temperature conditions, we may approximately estimate 90% of density (1.11 times volume expansion) for amorphous resin, of which specific volume is not much influenced by temperature, and 80% of density (1.25 times volume expansion) for crystalline resin, of which specific volume is much affected by temperature. Furthermore taking account of injection efficiency due to back flow and cushion amount, additional safety factor 80% shall be introduced. To make a formula applicable to all resin, regardless of crystalline and amorphous, density in the injection cylinder is now assumed to be 85% of the value under normal pressure and temperature condition. Then required injection capacity (VS) will be as follows: VS≧VA／ (0.8×0.85) = VA／0.68 (cm3) ................................ Formula (1.7.4) Or VS = 1.47 VA (cm3).................................................................... Formula (1.7.5) Injection machine with larger injection capacity than above can be utilized, but it should not be too large. Expected problem is that resin starts decomposition in the cylinder if it stays too long in the cylinder. As a minor problem, measuring accuracy drops due to small measuring stroke. Thus similarly to the case of clamping force, calculated injection capacity (VS) should not be less than 20% of theoretical maximum injection capacity of the injection machine. Accordingly formulae (1.7.4) and (1.7.5) can be expressed as follows. VA／0.16 (VA／0.8×0.2) ≧VS≧VA／0.68 (cm3) ................ Formula (1.7.6) Or 6.25 VA≧VS≧1.47 VA (cm3) ................................................... Formula (1.7.7) Approximately 6 VA≧VS≧1.5 VA (cm3) .......................................................... Formula (1.7.8) Mold Design (Advance)

Page 28

PITAC 【Calculation example】

Calculate required injection capacity of the product. [Calculation] ① Calculate volume of sprue and runner.

Sprue: (π×0.52) ／4×7 = 1.4 (Taper portion is assumed to be cylindrical.) Runner: (π×0.52) ／4×5 = 1.0 Thus VR = 1.4 + 1.0 = 2.4 cm3 ② Calculate volume of the product.

VP = (2×5×8)－ (1.8×4.6×7.6) － (π×12×0.2) = 16.5 cm3 ③ Calculate total injection amount.

VA = (2×16.5) + 2.4 = 35.4cm3 ④ Calculate required injection capacity.

VS ≧1.5×35.4 = 53.1≒54 cm3 VS ≦6×35.4 = 212.4≒212 cm3 Thus you can apply an injection machine of which theoretical maximum injection capacity is more than 54 cm3 and less than 212 cm3.

Mold Design (Advance)

Page 29

PITAC Hydraulic Pressure (Po)

Injection Pressure (Pi) Principle

Pi•D2 = Po•Do2 Screw Diameter (D)

Big

Small

Screw Section Area (Ai)

π•D2/4

Big

Small

Injection capacity (Vi)

Ai•S

Big

Small

Injection Pressure (Pi)

Po•Do2/D2

Small

Big

Big Product. Thick Wall Product

Precision Product Thin Wall Product

Application

Fig. 1-7.1 Internal Injection Cylinder (Screw Diameter) vs. Injection Capacity / Injection Pressure

Material: PP

Specific Volume

Specific Volume

Material: PMMA

Temperature (℃)

Temperature (℃)

Fig. 1-7.2 PVT Diagram of Resin

Mold Design (Advance)

Page 30

PITAC 1-8 Mold Strength Mold strength should be evaluated to see if the deformation of the mold due to molding pressure (injection pressure or holding pressure) stays within allowable tolerance limit. Two considerations must be highlighted. One is how to estimate molding pressure. Another is how to decide allowable tolerance limit. Molding pressure comes from resin. With some exceptions, you need not take up a pressure during injection but a pressure after injection. This is nothing more than the molding pressure inside cavity pressure that was explained and calculated in the previous section for calculating required clamping force. But in this section we choose 500 kgfcm2 with some margin. Tolerable deformation varies depending upon product accuracy, mold structure, locations, etc. One practical reference is if the deformed amount results in generation of burr or not. Clearance to generate burr can be considered as the depth of air ventilation. If burr cannot be a reference, the allowable tolerance should be looked into from the aspect of allowable repeated stress on the mold or product accuracy. Generally allowable deformation amount is 0.1~0.2mm unless the mold is extremely small in size. 1) Side walls of rectangular cavity There are two types. One is of split type consisting of sidewalls and bottom plate. Another is made from one block, in other words one-piece cavity. Split type can be machined easily with high accuracy but weaker in strength. Let us see the difference in strength between split type and one-piece rectangular cavity. 1-8-1)

Split type

In the split type, calculation disregards restraint of bottom plate. Actually sidewalls are bolted together with bottom plate or mounting plate. Thus calculation results in the value with safety factor by disregarding binding and friction influence from bolts (Fig. 1-8-1.1). We apply a model of a beam both side fixed and with equal weight distribution to cavity walls for strength analysis (Fig. 1-8-1.2). Be minded molding pressure uses cm unit, while strength calculation uses mm unit. The maximum deformation (δMAX) on the both side fixed beam appears in the middle as follows:

δ MAX =

PAL4 384EI

Where

P: A: L: E: I:

( mm) ........................................................................ Formula (1.8.1.1)

Molding pressure (kgf/mm2) Product height (mm) Product length (mm) Vertical elasticity coefficient (kgf/mm2) Cross section moment of inertia

Cross section of the beam is of rectangular, thus moment of inertia is as follows:

I=

BH3 12

Where

( mm) .................................................................................. Formula (1.8.1.2) B: Cavity thickness (mm) H: Cavity wall thickness (mm)

Wall thickness (H) is derived from formulae (1.8.1.1) and (1.8.1.2) as follows: Mold Design (Advance)

Page 31

PITAC H=

3

12PAL4 384Ebδ MAX

( mm) .................................................................. Formula (1.8.1.3)

Pm: Molding Pressure (kgf/cm2)

δMax: Maximum Deformation (mm) P: Evenly Distributed Weight (Molding Pressure) (kfg/mm2) E: Vertical Elasticity Coefficient (kfg/mm2)

Fig. 1-8-1.1 Molding Pressure on Cavity with Split Structure

δmax = pal4/384EI

I = bh3/12 I : Cross Section Moment of Inertia

Fig. 1-8-1.2 Model of a Beam, Both Side Fixed, for Cavity Wall

Mold Design (Advance)

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PITAC 【Calculation example】

Calculate wall thickness (H) of split type cavity shown in Fig. 1-8-1.3. Resin material is ABS and vertical elasticity coefficient of the mold is assumed to be E = 2.1×104 kgf/mm2. [Calculation] Molding pressure (P), allowable deformation (δMAX) is assumed as follows. Then formula (1.8.3) is applied to calculate wall thickness (H). P = 500 kgf／cm2 = 5 kgf／mm2 δMAX = 0.025mm (in the case of ABS) H=3

12 × 5 × 20 × 100 4 = 24.6 (mm) 384 × 2.1 × 10 4 × 40 × 0.025

Thus, you may decide the wall thickness 25mm if there is no space available, but if possible, decide 30mm for safety consideration.

Fig. 1-8-1.3

Mold Design (Advance)

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PITAC 1-8-2) One-piece type

One-piece type cannot be simplified as split type. Table 1-8-2.1 shows coefficient (C) corresponding to ratio (L/A) of product length to height. Then allowable deformation (δMAX) is calculated as below (Fig. 1-8-2.1). δ MAX =

CPA 4 (mm) ................................................................. Formula (1.8.2.1) EH 3

Where P: A: E: H:

Molding pressure (kgf/mm2) product height (mm) Vertical elasticity coefficient (kgf/mm2) Cavity wall thickness (mm)

Cavity wall thickness can be derived from formula (1.8.2.1) as follows. H=3

CPA 4 (mm) .................................................................. Formula (1.8.2.2) Eδ MAX

【Calculation example】

Assuming last example is of one-piece type; calculate wall thickness (H). Other conditions are the same. [Calculation] ① Calculate volume of sprue and runner.

Sprue: (π×0.52)／4×7 = 1.4 (Taper portion is assumed to be cylindrical.) Runner: (π×0.52) ／4×5 = 1.0 Thus VR = 1.4 + 1.0 = 2.4 cm3 ① L/A = 100/20 = 5 kgf/mm2

From Table 1-8-2.1, C = 0.142. ② Calculate cavity wall thickness (H) from formula (1.8.2.2).

P = 500 kfg/cm2 = 5 kfg/mm2 δ = 0.025mm (in case of ABS) E = 2.1×104 kfg/mm2 A = 20mm Thus, H = 3 Mold Design (Advance)

0.142 × 5 × 200000 = 6 (mm) 2.1 × 10 4 × 0.025 Page 34

PITAC Wall thickness has come up with 6mm, but it is advised to select 10mm for safety consideration.

Pm: Molding Pressure (kgf/cm2)

Fig. 1-8-2.1 Molding Pressure Operated on Cavity with One-piece Structure

Table 1-8.2.1 Various Resin Materials and Average Resin Pressure in Cavity Resin Material

Mold Design (Advance)

Average Resin Pressure in Cavity (Kgf/cm2)

Page 35

PITAC 1-8-3

Wall thickness of cylinder type cavity

Thick wall cylinder is applicable in material strength analysis. Similarly to rectangular type cavity calculation, we will try to make the clearance resulted from deformation smaller than the clearance to cause burr for different resin materials. It will be complicated if we try to calculate wall thickness of cylinder tube. Therefore we evaluate if the deformation is within allowable tolerance under a given wall thickness. Deformation (δ) in Fig. 1-8-3.1 is given as follows. δ=

rP ⎡ R 2 + r 2 ⎤ + λ ⎥ (mm) ....................................................... Formula (1.8.3.1) ⎢ 2 2 E ⎣R − r ⎦

Where P: R: R: E: λ

Molding pressure (kfg／mm2) Cavity outside diameter (mm) Cavity insider diameter (mm) Vertical elasticity coefficient (kfg／mm2) : Poisson’s ratio (λ = 0.3 for steel)

【Calculation example】

Evaluate if the deformation of cylinder type cavity in Fig. 1-8-3.2 can be within tolerance limit or not. Resin material is ABS. [Calculation]

Calculate δ of formula (1.8.3.2) and compare it with the clearance 0.03mm to cause burr for ABS resin. P = 500 kfg/cm2 = 5 kfg/mm2 E = 2.1×104 kfg/mm2 r = 20mm R = 30mm λ = 0.3 Thus, δ =

20 × 5 ⎡ 302 + 202 ⎤ + 0.3⎥ = 0.014 (mm) ⎢ 2.1 ⎣ 302 − 202 ⎦

Calculation result reveals that the deformation is less than minimum clearance (0.03mm) to cause burr of ABS resin. Thus the wall thickness is appropriate. In the case of cylinder type cavity, inserts are used in the plate. As the clearance between inserts and holes on the plate is around 0.01~0.02mm, the plate can share some stress when deformation exceeds the clearance above. Mold Design (Advance)

Page 36

PITAC

Fig. 1-8-3.1 Molding Pressure Operated on Cavity with Cylinder Shape

Fig. 1-8-3.2

Mold Design (Advance)

Page 37

PITAC 1-8-4

Mold weight and center of gravity

Calculation for mold weight and center of gravity is required to determine size and position of hook bolts for hoisting a mold. Normally the shape of a mold is of rectangular sections. Thus the volume can be easily figured, so as weight by multiplying specific weight of mold material. In case of steel, apply 7.87. Regarding center of gravity, it can be determined by estimating the position in the direction of mold thickness. Mold is normally symmetrical with a centerline in the injecting direction. The center of gravity should locate on the centerline. Calculation proceeds firstly to calculate weight on the center of gravity of each plate and secondly to find a point where each moment can be balanced. Referring to Fig. 1-8-4.1, calculate each moment as a product of weight on the center of gravity of each plate and distance based on fixed side clamping plate. The total moment should balance with a moment as a product of total mold weight and distance from the reference point to the center of gravity. w • x = ∑WN • LN ..................................................................... Formula (1.8.4.1)

Where w: Mold weight x : Distance from reference point to center of gravity. WN: Weight on each plate. LN: Distance from reference point to center of each plate. Distance from reference point to center of gravity can be derived as follows: x=

∑W L N

w

Mold Design (Advance)

N

........................................................................... Formula (1.8.4.2)

Page 38

Fixed Side Mounting Face

PITAC

Fig. 1-8-4.1 Calculation of Mold Center of Gravity

Mold Design (Advance)

Page 39

PITAC 【Calculation example】

Calculate center of gravity of the mold shown in Fig. 1-8-4.2. Mold material is S50C. [Calculation] ① Specific weight 7.87 is applied for S50C. ② Calculate weight of each plate.

W1 = 7.87×25×300×300×10-6 = 17.7 kg W2 = 7.87×50×250×300×10-6 = 29.5 kg W3 = 7.87×40×250×300×10-6 = 23.6 kg W4 = 7.87×40×250×300×10-6 = 23.6 kg W5 = 7.87×70×38×2×300×10-6 = 12.6 kg W6 = 7.87×35×170×300×10-6 = 14.1 kg W7 = 7.87×25×300×300×10-6 = 17.7 kg ③ Calculate total mold weight.

W = 17.7 + 29.5 + 23.6 + 23.6 + 12.6 + 14.1 + 17.7 = 138.8 kg ④ Calculate distance from fixed side mounting face to the center of plate. (Fig. 1-8-4.3) ⑤ Calculate moment of each plate.

(No need to convert mm to m. Weight of each plate is assumed to be on the center.) Fixed side clamping plate: Fixed side mold plate: Movable side mold plate: Movable side support plate: Spacer: Ejector plate (upper ＆ lower): Movable side clamping plate:

W1•L1 = 17.7×12.5 = 221 kg•mm W2•L 2 = 29.5×50 = 1475 kg•mm W3•L 3 = 23.6×95 = 2242 kg•mm W4•L 4 = 23.6×135 = 3186 kg•mm W5•L 5 = 12.6×190 = 2394 kg•mm W6•L 6 = 14.1×207.5 = 2926 kg•mm W7•L 7 = 17.7×237.5 = 4204 kg•mm

⑥ Calculate gravity position (X) from formula (1.3.9.23).

x=

221+1475+2242+3186+2394+2926+4204 ≒ 120 (mm) 138.8

Mold Design (Advance)

Page 40

PITAC

Fixed Side Mounting Face

Fig. 1-8-4.2

Fig. 1-8-4.3

Mold Design (Advance)

Page 41

PITAC In the case of calculation example 1.3.9.6, center of gravity locates at 120 mm from fixed side mounting face. If you place a hook bolt at this location, you can hoist the mold without tilting the mold. But notice this location is near parting surface of movable mold plate and support plate. Thus a screw cannot be tapped there. Two solutions can be proposed here. One is to use a lifting bar and another is to move the screw position a bit toward fixed side clamping plate (Fig. 1-8-4.4). The moved amount should be selected for the mold not to tilt more than 10°. If it is moved in the opposite direction, workability to position a locating ring to the nozzle hole of injection machine will be affected (Fig. 1-8-4.5). In our case here, if the screw position is moved toward fixed side mounting face, it comes to much closer to the parting surface. Thus it is recommended to apply a lifting bar.

Mold Design (Advance)

Page 42

PITAC Difficult to position screw hole for hoisting if gravity center is too close to parting surface of mold plate.

Lifting Bar Hook bolt

Receiving Plate

Movable Side Mold Plate

a. By Lifting Bar

Calculated Gravity Center

Receiving Plate

Movable Side Mold Plate

b. By shifting screw position

Fig. 1-8-4.4 Design Consideration when Gravity Center is Close to Plate Parting Surface

Platen of Injection Machine

Platen of Injection Machine

Gravity Center

a: Proper (Good workability)

b: Improper (Poor workability)

Fig. 1-8-4.5 Mold Inclination and Workability

Mold Design (Advance)

Page 43

PITAC 1-8-5

Return force of ejector plate

Ejected ejector plate is normally returned to its original position by spring force of a spring installed on the periphery of return pin. If the spring force is too weak, the ejector plate cannot fully return to its original position. If it is too strong, operational balance will be affected and galling may be caused on the return pin. Thus we may define the return force of ejector plate should stand weight of ejector plates (upper and lower) and their friction force. Friction force is related to friction coefficient of ejector plates (upper and lower). As we know the maximum friction coefficient is 1, it must be enough to estimate the friction force 2 times of the plate weight. Generally, number of springs to be installed on the periphery of return pin is 4. Thus you should calculate the shared friction load per spring is 1/2 of ejector plate weight (2×1/4). The spring is better to have a smaller spring constant value to assure smoother load transfer to the spring while stroking (Fig. 1-8-5.1). In addition initial deflection of the spring is better not to exceed thread length of stripper bolt (normally 10~15), otherwise you will have a difficulty in installing stripper bolt to female thread hole because of the long spring (Fig. 18-5.2). Following checkpoints may be useful for selecting correct spring from available ones in the market. ① Internal diameter of the spring should be at least 1 mm larger than return pin outside

diameter. ② Maximum deflection in usage should be within allowable limit. ③ The spring should have enough returning force at the initial deflection. ④ When spot facing is made for the spring, clearance around return pin can be secured (two times pin diameter) and there should be no interference with coolant channel.

Mold Design (Advance)

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PITAC

Load – Deflection Diagram for Spring A Load – Deflection Diagram for Spring B Load

Initial Deflection of Spring A Initial Deflection of Spring B Ejection Plate Stroke Initial Load Maximum Deflection of Spring A Maximum Deflection of Spring B Deflection

Fig. 1-8-5.1 Spring Constant vs. Load – Deflection Diagram

Ejector Plate Fastening Bolt

Screw does not reach to tap hole if initial deflection of return pin spring is too big.

Return Pin Spring Return Pin

Fig. 1-8-5.2 Initial Deflection of Return Pin Spring and Mounting Workability of Ejector Plate

Mold Design (Advance)

Page 45

PITAC 【Calculation example】

Conditions of an ejector plate are as below. Select appropriate length of springs shown in Fig. 1-8-5.3. [Ejector plate conditions] • Upper ejector plate dimension: 250×110×13mm • Lower ejector plate dimension: 250×110×15mm • Return pin diameter: φ12mm (4pcs) • Maximum ejector stroke: 17mm • Initial deflection: 8mm [Calculation] ① Calculate weight of ejector plates.

25×11×(1.3×1.5)×7.87×10-3≒6.1kg ② Calculate ejector plate return force per spring.

6.1×2/4 = 3.05 kg (or more) ③ Calculate spring constant.

3.05/8 = 0.38 kgf/mm ④ Calculate maximum deflection in use.

17+8 = 25mm ⑤ Select springs in the list to meet required spring constant and deflection.

From Fig. 1-8-5.3, springs having length (L) more than 45mm and less than 100mm are appropriate. In practice, select one of appropriate springs evaluating influence of spot facing and interference with coolant channel.

Mold Design (Advance)

Page 46

PITAC

\ Price N (kgf) Load

1~19

Fig. 1-8-5.3 Partial List of Coil Spring in the Market (fromミスミフェイス)

Mold Design (Advance)

Page 47

PITAC

Calculate the appropriate diameter of support pin.

Calculation for the appropriate diameter of support pin and Deflection of Support pin Mold Design (Advance)

Page 48

PITAC

Bending Moment MMAX = W l Deflection Wl3 δMAX = 3EI I =Cross Sectional Secondary Moment = E= Young’s modulus (Vertical elastic modulus) = Ε =

∈= Deflection ∂ = Rectangular stress

δ

MAX

δ

MAX

d=

4

=

π d2 64

∂ = 2.1 x 104 kgf/mm2 ∈

Wl3 3EI

Wl3 = π d4 3E 4 Wl3 . 64 3E . π . δ MAX

Example:Support pin length = 250 No. of Support pins = 4 Weight of Cavity Plate = 400 kg

E= Young’s modulus = 2.1 x 103 Allowable Deflection = δMAX = 0.01 length of SP Weight of Cavity Plate Young's Modulus Allowable Deflection Diameter of SP Diameter of single SP

400 220 21000 0.01 102.6792791 25.66981977

mm kg kgf/mm2 mm mm mm

Ejector Pin, Ejector Sleeve Strength Calculation Mold Design (Advance)

Page 49

PITAC

Mold Design (Advance)

Page 50

PITAC

` Mold Design (Advance)

Page 51

PITAC

Chapter - 2

Mold Material & Heat treatment

Mold Design (Advance)

Page 52

PITAC 2- Mold Material

Limiting our scope to a mold for plastic injection molding, steel is the most popular material. Particularly JIS S50C and S55C are mostly applied because these are standard materials of the mold bases in the market. It is important to select right material to satisfy purpose of the mold and its application on the part of end users. If necessary, heat-treating or surface finish must be carried out to satisfy requirements. Here we will take up materials to be used for main parts of the mold, cavity and core. 2-1

Mold Material

2-1-1 Basics Normally users’ specification specifies if the material is of heat-treated (quenched) or not heat-treated (raw) for cavity and core material. Note that pre-hardened steel, which is heattreated when supplied but will not be heat treated after machining, is classified as raw steel.

Materials for Cavity and Core

Not heat-treated

As-rolled steel

→ Raw type

Pre-hardened steel

→ Raw type

Heat treated → Heat treatment

→ Quenched type

Table 2.1.1.1 shows various steel for plastic mold with bland names. Molds made of asrolled steel and pre-hardened steel belong to raw type. Pre-hardened steel is heat-treated having 30~40 HRC hardness and yet having a good machineability. Molds made of prehardened steel are used without heat treatment. Thus the mold processing is the same as that of as-rolled steel. Cost of a mold is also similar in both cases.

On the other hand, there are two types in quenched type. One is to harden and temper the mold after machining and to finish the mold just by simple polishing. Another is to finish a heat-treated mold with a certain deformation clearance by a grinder or EDM (Electric discharge machine). The former is used for a mold, which does not require high precision but only erosion resistance. Thus the cost is on the same level as raw type. But the latter involves time consuming finishing on the hardened steel surface. Thus the cost is much higher than raw type. Qualified material for mold should satisfy following points. ① ② ③ ④ ⑤ ⑥ ⑦ ⑧ ⑨ ⑩

Good machineability. High abrasion resistance. High corrosion resistance. High toughness. High strength. Homogeneous property without segregation and pin holes. Good heat-treating with less deformation. Good heat conductivity. Reasonable price. Easy procurement.

No single material satisfies all items above. Particularly the extent of abrasion resistance to determine mold life is deeply related to machineability that affects cost of a mold (Fig. 2-11.1). Major factors for determining mold material in the user’s specification are number of injection shot, application and molding material. Mold Design (Advance)

Page 53

PITAC QT : Quenching and Tempering PH : Prehardened I : Improved Steel Ideal Mold Material

SKD 11

13Cr Class QT SKD 61

I

Wear Resistance

I AISI P21

Free-Cutting Class PH Free-Cutting Winkle Class PH Class PH

Winkle Class PH

13Cr Class QT I

I

SCM Class

I SC Class

Machineability

Fig. 2-1-1.1 Wear Resistance and Machineability of Plastic Mold Material

2-1-2 Number of shots and mold material Total number of shots is a product of monthly production volume and mold life. Total number of shots = monthly production volume×mold life (month) Naturally, a mold having less number of shots will be made of cost conscious material because depreciation cost per shot needs to be lowered as much as possible, while a mold having high number of shots will be made of life conscious material. Note that in this case cost means machining cost rather than material cost. Characteristics and application of popular steel materials for mold for plastic molding is shown below in the order of low durability (Table 2.1.1.1). 2-1-2-1 SC steel S50C and S55C are used for material of mold bases available in the market. They are widely used for cavity and core material of which total shots are less than 100,000. Particularly they are applicable for large molds. Table 2.1.1.1 shows SC steel under both as- rolled steel and pre-hardened steel. It is recommended to use pre-hardened one for cavity and core due to better abrasion resistance. 2-1-2-2 SCM steel Generally machineability of SCM steel is not so good comparing with SC steel. Prehardened steel adjusted for better machineability with 28~33HRC hardness is often used for cavity and core material, mold base material, mold plates and holders that require hardness to certain extent. Mold Design (Advance)

Page 54

PITAC 2-1-2-3 AISI-P21 steel

This is a kind of pre-hardened steel, precipitation hardened with 40 HRC hardness, originated to AISI-P21 of US specification. This should stand for 500,000 shots for usual resins. There are two types. One is a material with improved machinebility, close to S50C and S55C, by adding lead (Pb) and sulfur (S). Another is a material with improved machinebility for electric discharge machining, texturing ability and polishing. Apply one way or another depending upon mold characteristics. So far steel materials that are not quenched after machining have been introduced. These materials have benefits of easier machining, costs and delivery comparing with quenched type. Currently 40HRC hardness of pre-hardened steel is the hardest, but it is expected to be 50 HRC hardness in view of recent development in high speed and high precision machining capability for hard metals. 2-1-2-4 SKD-61

SKD-61 steel is normally used for die-cast mold as tool steel for hot processing. But it also is applicable for plastic mold for relatively large production volume. Table 2.1.1.1 shows this material under pre-hardened steel with 40 HRC hardness. But normally raw steel is machined and quenched to 50 HRC hardness after machining. A life of quenched mold can stand for at least one million shots for usual resins. If conditions are met, 2~3 times longer life can be expected.

SKD-61 can be nitride to the extent of 0.05mm in depth with 900HV hardness or more. It means that nitride layer still exists after finishing as much as 0.01~0.02mm. Therefore nitride SKD-61 is quite effective for a mold that is subject to galling or seizing. 2-1-2-5 SKD-11 SKD-11 steel is normally used for press mold as tool steel for cold processing. But it is also applicable for mold for plastic with reinforced fiberglass or for mass production.

SKD-11 has high resistance to abrasion. When it is quenched at 58~60 HRC hardness, SKD11 can stand for around 5 million shots without special coating on the surface. Weakness may be poor machineability and toughness. Steel suppliers are developing improved SKD11 to cover such weakness. As Table 2.1.1.1 shows, grains are laid out in dense and homogeneity. Thus powder forging is made available. SKD-11 is applicable for molds that requires mirror polishing and abrasion resistance. Mold Design (Advance)

Page 55

PITAC 2-1-2-6

Powder metal

This is applicable for a mold for super mass production, super engineering plastics with reinforced fibers, IC, etc.. Similarly to SKD-11, this is made from powder metallurgy process. Powder metal is superior to high-speed tool steel (SKH-51) in terms of hardness and toughness, but the cost is much higher. Therefore this is often used partially in the form of inserts wherever high abrasion resistance is required. So far we have discussed about typical mold materials in relation with number of shots required for a mold. It is advised to analyze available materials, applications, number of shots, etc. for mold design. a sample of which is shown in Table 2.1.2.6.1

Mold Design (Advance)

Page 56

PITAC Table 2.1.2.6.1 Total Shots vs. Mold Material Total Shots (×1000) 5,000≤ Resin, Application

Resin for General Application

Engineering Plastic (Not Reinforced)

Engineering Plastics (Reinforced)

Fire Retarded Grade

1,000≤ <5,000

500≤ <1,000

100≤ <500

AL alloy (HB 150) No No

SKD-61 (I) Q/T (HRC 48-50) No

P/H Steel (M, T) Q/T No (HRC 40) No

S50C, S55C No No

Powder Metal Q/T (HRC 64-66) PVD

Powder Metal Q/T (HRC 62-64) No/PVD

SKD-11 (I) Q/T (HRC 58-60) No/Nitride

SKD-61 (I) Q/T (HRC 48-50) No

Cemented Carbide Steel Insert No (HRA85-90) No/PVD

SKD-11 (I)

SKD-61 (I)

P/H Steel (M, T)

Q/T (HRC 58-60) No/PVD

Q/T (HRC 48-50) No/Nitride

Q/T No (HRC 40) No

Powder SUS Class Q/T (HRC 56-58) No/PVD

13 CrSUS Class Q/T (HRC 50-52) No/PVD

13 CrSUS Class Q/T (HRC 50-52) No

13 CrSUS Class Q/T (HRC 33) No

13 CrSUS Class Q/T (HRC 33) No

Powder SUS Class

13 CrSUS Class

13 CrSUS Class

P/H Steel (T)

S50C, S55C

Q/T (HRC 56-58) No/PVD

Q/T (HRC 50-52) No/PVD

Q/T (HRC 50-52) No

Q/T (HRC 40) No

No No

Powder Metal Q/T (62-64 HRC) No/PVD

SKD-11 (I) Q/T (HRC 58-60) No

SKD-61 (I) Q/T (HRC 48-50) No

P/H Steel (M, T) No (HRC 40) No

Mold Material Heat Treatment Surface Treatment

SKD-11 (I) Q/T (HRC 58-60) No/PVD

SKD-61 (I) Q/T (HRC 48-50) No/Nitrided

P/H Steel (M, T) No (HRC 40) No/Nitrided

SCM (I) No (HRC 33) No

Powder Metal Q/T (HRC 62-64) No/PVD

SKD-11 (I) Q/T (HRC 58-60) No/PVD

Mold Material Heat Treatment Surface Treatment

Mold Material Heat Treatment Surface Treatment

Mold Material Heat Treatment Surface Treatment

Mold Material Transparent Heat Treatment Product, Optical Surface Treatment Product

Cemented Carbide Steel Insert No (HRA85-90) No/PVD

Note: Q / T: Quenched / Tempered P / H (M): Prehardened (Free-Cutting class)

Mold Design (Advance)

<20

S50C, S55C No No

Mold Material Heat Treatment Surface Treatment

Mold Material Heat Treatment Surface Treatment

20≤ <100

PVD: Physical Vapor Deposition (Ion Plating) P / H (T): Prehardened (Wrinkle class)

I: Improved

Page 57

PITAC 2-1-3 Plastic materials and mold materials

Depending upon plastic material that may include, reinforced fibers or additives, requirements for abrasion or corrosion resistance vary on the part of cavity and core. 2-1-3-1 Reinforced plastic

Reinforced plastic with filled material such as fiberglass causes high abrasion on the mold. The extent of abrasion is higher if the amount of filled material is greater and the material is harder. For example when glass fiber content is more than 30%, the mold life will become 10~ 20% of the life otherwise. Mold materials for anti-abrasion were discussed in the previous section. Be minded that hard steel material may cause chipping due to inferior toughness. It may be necessary to lower the hardness and compensate it by surface treatment such as PVD. 2-1-3-2 Flame retardant plastic

Flame retardant plastic that includes halogen (bromine) or fluororesin produces corrosive gas under heat and pressure in the molding. This will shorten the life of a mold. In the case of PVC, chlorine gas is generated. Thus you need to select mold material with high corrosion resistance. 2-1-3-2-1 13Cr stainless steel

This is a stainless steel material to include 13% of chromium. This may be called 13Cr steel or SUS420 in JIS. 13Cr SUS is not quite high in corrosion resistance, but being prehardened steel of Martensite structure it can be used as it is due to its reasonable hardness 33HRC or can be quenched to 50HRC if needed. Thus 13Cr SUS can be used for a mold to be mirror polished or to be used for fire retard resin or fluororesin. 2-1-3-2-2

SUS 630

This is a precipitation-hardened stainless steel having high corrosion resistance. This is supplied as prehardened steel with 35HRC hardness. SUS 630 stainless steel is applicable for a mold for highly corrosive resin such as PVC. 2-1-3-2-3

Transparent resin

Cavity and core need to be mirror polished when transparent resins such as GPPS, AS, Mold Design (Advance)

Page 58

PITAC PMMA, PC, etc. are molded. Particularly for photoproducts such as optical discs or lenses, high grade of transparency is required. Although JIS provides no specific standard for mold steel for plastic injection molding, special steel suppliers made such standard available for our application. Referring to such references select appropriate steel material to be mirror polished for transparent resin molding. Improved materials are often processed by special smelting processing such as vacuum process which brings about homogeneous and dense grain structure with minimum segregation and pin holes so as to assure a mold to satisfy with precise transcription capability. 13Cr stainless steel for such purpose is normally made by vacuum process to suit precision mirror polishing. 2-1-3-2-4 Thin products

Steel material for thin core or fine core is required to be with high rigidity and high toughness, particularly when injection is made from one side only. For such application, Maraging steel to include 18% Ni is recommended. Maraging steel is supplied as solution treated condition and is to be hardened to 53HRC through age hardening. This material is often used for thin wall core, mirror polished core, and ejector pin with thin wall or small diameter. 2-1-4 Other mold materials

As explained so far, steel material is most balanced in properties as mold material. Thus it is widely used. Other materials than steel are being introduced for particular applications. 2-1-4-1 Aluminum alloy

Mold for extremely small production volume is not necessary is of steel material. Mold made of aluminum alloy can stand for 20,000~30,000 shots. You may extend the life even more by hardening the mold surface with alumite processing. But be aware that aluminum alloy is always subject to damage on its surface because of soft material by nature. Benefits gained from this material must be low cost, short delivery and improved cycle time due to high thermal conductivity. 2-1-4-2 Copper Alloys

Beryllium copper (BeCu) is a typical copper alloy used for copper alloy mold for plastic molding. This material can be improved in abrasion resistance through age hardening. Advantage of copper alloy is its high thermal conductivity, while disadvantage must be its high cost. Therefore copper alloy is used for inserts to remove heat from hot spots. Mold Design (Advance)

Page 59

PITAC Application to a whole cavity is limited to pressure casting and precision casting, which will be explained afterward (Fig. 2-1-4-2.1). In processing BeCu by EDM, be equipped with partial ventilation facility due to generation of toxic gas. As polar consumption is high, processing BeCu by EDM is better be avoided. BeCu has limitation for corrosion resistance, but it can be improved by electro less nickelplating on the surface as much as 0.01mm. In this way it will be also improved in abrasion resistance. 2-1-4-3 Tungsten carbide alloy

Tungsten carbide alloy consists of tungsten carbide (WC), cobalt (Co) and nickel (Ni). Tungsten carbide alloy with more of cobalt, which has high transverse strength, is used for mold applied to disc mold (CD), mold for highly reinforced resin, IC mold, etc.. Strength of carbide alloy mold is its high abrasion resistance, while weakness is its high cost. Thus this material should be used just as inserts to the more extent than BeCu. In order to cover its small transverse strength, which is a half of steel, it is recommended to apply shrink fitting wherever applicable. Also be aware that its thermal expansion coefficient is different from that of steel. Therefore pay attention to fitting accuracy when it is used as an insert for high temperature application.

Main Core

Main Core

BeCu Insert (for 4 corners)

Fig. 2-1-4-2.1 Application of BeCu Insert

Mold Design (Advance)

Page 60

PITAC 2-2

Heat Treatment and Coating

Physical properties of steel such as tensile strength, hardness, elongation, etc. vary in accordance with amount of carbon contents. To a greater extent, heat treatment will influence to physical properties. We can say that good steel characteristics can be realized depending upon how the steel is heat-treated. In these days, not only heat treatment but also surface hardening process such as PVD is applied on mold to satisfy expected longer life of molds or requirements from engineering plastic molding. Importance of heat treatment is sometimes overlooked because we normally subcontract heat treatment to outside vendors without being involved. But it is important to understand basics of heat treatment and surface coating to be able to specify appropriate processing to satisfy objectives of the mold. 2-2-1 Heat treatment

Let us review basics of heat treatment. 2-2-1-1 Basics of heat treatment

Steel changes in atomic sequence due to allotropic transformation and structure due to solid solution and separation of carbide in steel under thermal influence. Heat treatment is to utilize such changes in characteristics of steel material. 2-2-1-1-1 Allotropic transformation

Pure iron has a form of α iron of body-centered cubic structure up to 911℃, and transforms to γ iron of face-centered cubic structure from 911℃ to 1392℃. When α iron transforms to γ iron, the volume shrinks. The reverse transformation causes expansion (Fig. 2-2-1-1-1.1). In the case of steel, transformation temperature and structure vary substantially depending upon carbon contents. This relationship is given in a graph known as ‘Iron-carbon equilibrium chart’. You may refer it to textbook or handbook supplied by steel manufacturer. A sample is shown in Fig. 2-2-1-1-1.2.

Mold Design (Advance)

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PITAC

Length

α Iron

γ Iron

δ Iron

Temperature (℃)

α Iron δ Iron (Body-Centered Lattice)

γ Iron (Face-Centered Lattice)

Fig. 2-2-1-1-1.1 Iron Transformation and Atomic Structure A1 Transformation Point: P-S-K Line A3 Transformation Point: G-S Line

Temperature (℃)

δ Iron+Molten Iron

Molten Iron

Austenite + Molten Iron Austenite Carbon Compound + Molten Iron Ferrite + Austenite

Ferrite

Austenite + Carbon Compound

Ferrite + Carbon Compound

Fig. 2-2-1-1-1.2 Equilibrium Diagram of Iron – Carbon Compound (from日立金属Hand Book) Mold Design (Advance)

Page 62

PITAC 2-2-1-2 Heat-treating Method

Important point for heat treatment lies in how to heat and how to cool. When heating, temperature is the important factor, while in cooling the cooling speed is the important factor. ① Heating

Heating rate: Heating should be done slowly except for surface quenching. The rate of 30 minuets per one inch for rising ambient to designated temperature is well accepted standard. Simultaneous temperature rising from surface to the center is ideal. Heating temperature: Tempering and annealing are carried out at lower than A1, transformation temperature (727℃). Complete annealing and quenching are carried out at A3 transformation temperature (or A1) + 50℃. In the case of alloy tool steel, often used for mold, the temperature is 800 ~ 880℃ for SKS steel and 950 ~ 1050℃ for SKD steel taking account of influence of alloy elements. Temperature is determined referring to technical data from steel suppliers and JIS as well. ② Cooling

Cooling rate: Basic is to anneal slowly and quench fast. But low carbon steel requires fast annealing and certain steel can be quenched under slow cooling rate. Particularly influence of cooling rate varies substantially for alloy tool steel. Thus you should refer specific transformation curve (TTT curve or S curve) given in hand book or catalog supplied by steel suppliers for appropriate cooling rate. Cooling range: Referring to Fig. 2-2-1-2.1, steel with poor quenching characteristic shows nose of S curve in a short time, while steel with good quenching characteristic shows the nose in the late stage. In quenching, cooling rate should be controlled in a way that temperature up to Ms point (Martensite point) should stay out of the nose in question. In short, cooling rate should be controlled to cool fast from heated point to Ms point and to cool slowly after Ms point to assure homogeneous Martensite.

Mold Design (Advance)

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PITAC

Nose of S-Curve

Temperature (℃)

“Nose “of S-Curve appears at temperature 430℃ and time 15 sec. This means that the material can be quenched if it is cooled from 850℃ to 430℃ within 15 seconds. Thus oil cooling is needed.

Time (S) 850℃ Heated Isothermal Transformation Curve of SGT (SKS-3) Nose of S-Curve

Temperature (℃)

“Nose “of S-Curve appears at temperature 700℃ and time 300 sec. This means that the material can be quenched if it is cooled from 1000℃ to 700℃ within 300 seconds (5min.) thus this material is easy in quenching just by air cooling.

Time (S) 1000℃ Heated Isothermal Transformation Curve of SLD (SKD-11)

Fig. 2-2-1-2.1 Steel Quenching and Isothermal Transformation Curve (from日立金属Hand Book)

Mold Design (Advance)

Page 64

PITAC 2-2-1-3 Quenching and tempering

The purpose of annealing and normalizing is to soften the steel, to relieve internal strain or to improve internal structure. While, in the case of mold, quenching and tempering are conducted to improve hardness, strength and abrasion resistance. Quenching is a process in which heated steel in Austenite temperature is changed to Martensite grain structure by being cooled quickly. As explained, once steel is heated to quenching temperature, α iron and carbide changes to γ iron solid solution (austenite) with shrinkage, and in cooling process, γ solid solution changes to α solid solution with expansion. High carbon steel or high alloy steel, which is often used for mold, tends to leave austenite structure in martensite structure. Important point to assure dimensional stability of mold is how to minimize retained austenite content through appropriate heat-treating. Tempering is done immediately after quenching at lower temperature than A1, transformation point (727℃). In the tempering, low tempering is done at 150℃ ~ 200℃ and high tempering is done at 400 ~ 650℃. A few important considerations in quenching and tempering mold will be explained below. ① To regard quenching and tempering as one process

Tempering must be done immediately after quenching. You should never skip tempering nor temper after elapsed time. Even if quenching temperature is happened to be bit low for a material which is not hardened by tempering, you should carry out tempering with low temperature around 100℃. In this way toughness will be improved without losing hardness. Sometimes tempering is conducted at low temperature at 180℃ intending to improve toughness knowing some sacrifice in losing hardness. Normally it is advised to quench at austenite temperature and then to temper at 400 ~ 600℃ to assure intended hardness. ② To temper at high temperature

It is advised to apply high temperature (400 ~ 650℃) for tempering mold for not only high temperature molding for thermosetting resin or super engineering plastics but also for usual thermo plastic resins. High temperature tempering can minimize remaining austenite structure to cause dimensional deflection as time elapses, and can minimize deflection due to heat treatment when surface hardening such as PVD is conducted. When high temperature tempering is conducted on many kinds of alloy tool steel, hardness can be improved at 500℃ due to improved conversion to martensite grain structure (Fig. 2-2-1-3.1). Refer to catalogs and handbooks supplied by tool steel manufactures for further details.

Mold Design (Advance)

Page 65

Hardness (HRC)

PITAC

Tempering Temperature (℃)

Remaining Austenite (%)

AC: Air Cooled

As Quenched Tempering Temperature (℃)

Fig. 2-2-1-3.1 Quenching / Tempering of HPM31 (Improved SKD 11) and Remaining Austenite (from日立金属)

Mold Design (Advance)

Page 66

PITAC ③ Subzero treatment for long life precision mold

Problem incurred from remaining austenite can be solved by high temperature tempering for usual molds. If a mold is for high precision to be used at high temperature (100℃ or higher) for long period of time, subzero treatment is recommended. Subzero treatment is conducted immediately after quenching when austenite is not stabilized. It is to hold the mold for certain time length at minus 100℃, and to temper at high temperature afterward. Subzero treatment minimizes retained austenite structure, thus assures to minimize deformation after elapsed time, and in addition improves hardness (Fig. 2-2-1-3.2). ④ To apply steel with high quenching characteristic

Temperature (℃)

Mass effect is a phenomenon in that cooling rate at the center of material cannot be as fast as the surface so that the quenched hardness cannot be attained in the center of thick material. Mass effect is associated with thickness of the material to be quenched and quenching characteristic of the material. You need not too much concern about mass effect on the mold for plastic molding because quenched hardness is not required in the center the mold wall usually. However be minded in this respect if cavity and core are odd shaped so that quenching of a material with poor quenching characteristic may invite cracks or deformation due to mass effect.

Time (Hr) Quenching

Sub-Zero

Tempering

Fig. Fig. 2-2-1-3.2 Heat Treatment Process of SKD-61 Steel

Mold Design (Advance)

Page 67

PITAC 2-2-1-4 Vacuum heat treatment

Vacuum heat treatment is conducted in a vacuum environment of a certain vacuum rate. For quenching, usually 10-2 ~ 10-5 tore (mmHg) of vacuum rate is applied. The vacuum furnace is made to vacuum condition before heating. This is to take oxygen out to protect the mold from oxidization. And then the furnace is heated by adding nitrogen 0.5 torr to minimize evaporation of steel element. Characteristics of vacuum heat treatment are as follows. Shiny surface can be attained without oxidization influence. Deformation can be minimized through proper installation of a product in the furnace. Automated heat treatment reduces manpower overhead. Working environment is clean and comfortable because the furnace is insulated. ⑤ There is no environmental issue like salt bath furnace. ⑥ Cost of facility is high. ① ② ③ ④

Fan Motor Insulation Material

Cooling Fan Cooling Coil

Cooling Door (Opened)

Heater

View Window

Housing Door

Basket Furnace Bed Insulation Material (Ceramic Fiber) Cooling Door (Opened)

Fig. 2-2-1-4.1 Vacuum Heat Treatment Oven of Gas Quenching Type (from石川島播磨重工catalog)

Mold Design (Advance)

Page 68

Temperature (℃)

PITAC

Vacuum (Tore)

Time (min)

(*Numbers are incorporated to explanation in the text book.)

Fig. Fig. 2-2-1-4.2 Vacuum Quenching Chart of SKD-61

2-2-2 Surface treatment

Limiting our scope to surface hardening, various processes are classified as shown in Table 2.2.2.1. Below explained is some of the surface hardening processing often used for the mold of plastic molding. In order to improve surface hardening quality, some combined processes are applied.

Mold Design (Advance)

Page 69

PITAC Table Fig. 2.2.2.1 Various Surface Treatments No. Classification 1

Surface Quenching

Treatment Method High Frequency Flaming Electro Beam Laser

2

Diffusion Penetration

Cementation

Nitriding

Soft Nitriding Boron 3

Wet Plating

Hard Chrome Electro less Nickel

4

Dry Plating

Welding Thermal Spraying PVD (Physical Vapor Deposition)

CVD (Chemical Vapor Deposition)

Mold Design (Advance)

Remarks Suitable for bars and axises. This is to apply high frequency induction heating. Partially quenched by acetylene gas burner. It requires some skills. Electro beam is applied in vacuum chamber. Facility cost is high. Laser is applied. Quenching is done in atmospheric environment. Little heat strain. Carbon is diffused on low carbon steel with 0.2% and the surface is hardened as high carbon steel. Solid cementation, liquid cementation, gas cementation, vacuum cementation and ion cementation are made available. To harden the surface by nitrogen diffusion. Gas, salt bath and ion Nitriding methods are available. Good application to mold. Nitrogen and carbon are applied. Gas, salt bath and ion soft Nitriding methods are available. Boron is applied on steel surface. Gas, salt bath and powder methods are available. Chrome is plated by electric plating. Wide application to mold. Nickel is plated chemically. Plate thickness is more even than hard chrome plating. To add harder steel alloy on the steel by welding. To add harder steel alloy on the steel by plasma or flame thermal spraying. Coating material is vaporized in vacuum chamber and deposited on the steel surface. Vacuum, iron plating and spattering methods are available. Suitable to mold application. Coating material gas is chemically react with heated steel surface and hard coating is formed. Due to high temperature, application to mold is limited. Low temperature CVD can be widely applied to mold.

Page 70

PITAC 2-2-2-1

Gas Nitriding

An object is heated in the atmosphere of nitride gas such as ammonia gas (NH3) to be diffusionpermeated by carbon and nitrogen. Characteristics of this process can be summarized as follows: ① Any small surface of any kind such as internal surface of small hole can be hardened. ② Assuming proper tempering at higher temperature than nitride temperature is conducted,

deformation is small due to processing in rather low temperature (500℃ for alloy tool steel). ③ Does not influence surface roughness. ④ All steel except stainless steel can be treated, particularly effective for SCM steel, SKD-61 steel

and prehardened steel of precipitation hardened type. ⑤ There is no environmental problem as salt bath Nitriding. ⑥ White layer (or ε layer), hard and brittle composite, is formed on the surface. But this can be

minimized by controlling temperature and nitrogen concentration precisely. In view of various characteristics above, a mold can be gas-nitride after finishing or before final finishing. In the case of precision mold, gas Nitriding is conducted before final finish by leaving finishing margin 0.01 ~ 0.02 mm. Nitride depths is at most 0.05 mm even for SKD-61. Therefore amount of margin for finishing should be limited. Gas Nitriding is effective against galling and seizing. Therefore you can apply this surface hardening not only for cavity and core but also for sliding surface in the mold components. Gas Nitriding is difficult to apply on stainless steel, as explained, because its surface is made of stable oxidized steel. But ionized Nitriding to apply glow discharge under low-pressure gas can make stainless steel nitride. 2-2-2-2

Ion Plating

Ion plating is a kind of physical vapor deposition (PVD). This is a surface treatment method to ionize vaporized coating elements such as carbide and nitride and to deposit them on the surface of an object with negative voltage (Fig. 2-2-2-2.1). PVD includes vacuum vapor deposition and spattering. But they are usually not applicable for surface hardening. Characteristics of ion plating are as follows: ① Deformation is extremely small because temperature under treatment is only 300 ~ 500℃. Be

minded tempering should be conducted at the higher temperature. ② Film thickness 1 ~ 4 µm is given evenly. ③ Super hard coating HV 2000 ~ 3000 can be attained by applying Tin and TiCN. ④ No influence to surface roughness.

Mold Design (Advance)

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PITAC ⑤ Applicable to all kinds of steel. ⑥ Work environment is favorable because all activities are carried out in the vacuum chamber. ⑦ Generally adhesive strength is not as high as CVD. The adhesive strength is much affected by

surface condition and treatment temperature. ⑧ Coating on the surface such as internal surface of small hole, on which deposit is unlikely made,

is rather difficult. In view of above characteristics, this method is applicable to cavity with flat shape and smooth surface, core and core insert with simple shape. But you need to check carefully items ①, ⑦ and ⑧. Referring to ⑤, ion plating is applicable to aluminum alloy and copper ally as well. But adhesive strength is not so high because hardness of such material is not hard enough. Thus it is recommended to limit this application to steel having hardness more than 50 HRC. Ion plating can be combined with gas Nitriding for better surface hardening. In this case white layer should be removed by shot pining in order to assure adhesive strength in ion plating. The surface condition before ion plating processing should be metallurgically active. This can be said to the surface when electro discharge machining is conducted. Ion bombardment processing is a popular method in this respect by bombarding ionized argon gas on the object surface before ion plating is processed.

Electric Gun Vaporization

Electric Beam Ar Gas Product

Vacuum Chamber Reactor Gas Product

Vacuum chamber is of cylindrical shape in general. Coating material is placed in the middle and electric gun is installed to melt it. Products are laid out around. Products rotate around crucible to assure even coating on the surface.

Exhaust

Crucible

Coating Material

Fig. 2-2-2-2-1 Principle of Iron Plating Device

Mold Design (Advance)

Page 72

PITAC 2-2-2-3 Other surface treatments

Traditionally hard chrome plating and electro less nickel plating have been widely applied. Treatment temperature for hard chrome plating is as low as 45 ~ 65℃. Hard chrome plating is of low cost but with good abrasion resistance, mold separation and corrosion resistance. It has been widely applied to mold, which is not complex in shape, or IC mold. Comparing with PVD, plated film is thick, 0.01 ~ 0.03 mm, and in addition there will be a build up at edges or corners. Thus you need to evaluate the usage carefully before application. Table 2.2.2.3.1 illustrates characteristics of hard chrome plating and electro less nickel-plating for your reference. Surface treatment by spattering, which is a dry type instead of a wet type in hard chrome plating, is highlighted in these days. This has better adhesion and even film thickness, but cannot be applied to small area such as internal surface of fine hole. With regard to coated film, CVD, the same dry type as spattering, is far better in adhesive strength, but its weakness lies in deformation to precision parts due to high temperature treatment. In this respect it should be worth attention that CVD in low temperature treatment or plasma CVD is under development. Such new technology should provide you with a new insight for a superior surface treatment. Table 2.2.2.3.1 Wet Plating Methods and Characteristics Item

Hard Chrome Plating

Electro less Nickel Plating

Electric plating

Chemical plating

Temperature

Liquid 45 ~ 65℃

Liquid 90 ~ 95℃

Plating Layer

Cr

Ni 92 % ~ 98%

Thickness

5 ~ 50μm (usually 10µm)

10 ~ 30µm

Hardness (Without Heat Treatment) (With Heat Treatment)

800 ~ 900 HV 750 ~ 850 HV (300℃)

450 ~ 550 HV 650 ~ 900 HV (250 ~ 350℃)

Strong Points

• High wear resistance. • Good separation. • High acid resistance. • Easy reworking (re-plating). • Low cost.

• Even plating thickness. • High wear resistance. • High corrosion resistance. • High adhesion. • No pinhole. No crack.

Weak Points

• Weak adhesion • Uneven plating thickness. • Corroded by halogen gas.

• High cost (Plating Liquid) • If plating liquid flow is not smooth, even coating thickness cannot be attained.

Characteristics

Methods

Mold Design (Advance)

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PITAC

Chapter - 3

Under-Cut Handling

Mold Design (Advance)

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PITAC 3-

Undercut Handling

Undercut has nature in which ejection of a product in the direction of mold open/close cannot be possible. As it affects mold cost and product cost as well, undercut should be avoided at the stage of product evaluation. On the contrary, tendency is toward increased undercut to satisfy needs to reduce number of parts and to simplify joints by utilizing resin elasticity. Undercut mechanism as well as ejection mechanism is one of a few mechanical operations in the injection mold. It is expected to make the mechanism reliable. Here we will discuss various methods in handling undercut, and then typical methods, slide core and inclined core, will be taken for details. 3-1

Classification

There should be various ways to classify undercut. Generally accepted way is to classify undercut to two, namely whether the undercut locates outside of the product or inside, in other words whether the undercut is to be handled from outside of the product or from inside. Outside undercut Undercut Inside undercut Through hole on the outside of a product may be classified to either way. It should be appropriate to be classified to outside undercut because a handling from outside must be easier. 1) Outside undercut (Fig. 3-1.1) Outside undercut is easier than inside undercut because enough space may be made available. In the most cases, slide core mechanism, which will be explained later, is used for the outside undercut handling. The weakness must be that parting line of slide core and cavity comes on the outside of a product. This may influence appearance quality of the product. 2) Inside undercut (Fig. 3-1.1) Parting line is hidden in the inside of a product because undercut is located in the inside (core side). But the design is difficult due to limited space. Various undercut handling methods are proposed, in which inclined core (loose core) mechanism is widely accepted. 3-2

Undercut Handling Methods

There are many handling methods are proposed depending upon shape and size of undercut. Typical methods are as follows.

Undercut handling methods

Slide core Inclined core (loose core) Dogleg cam Elastic core Removable core Enforcing

Power source of undercut handling is from mold open/close, ejection, outside force, manual, etc. as showing in (Table 3.2.1). Mold Design (Advance)

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PITAC

(a) Outside Undercut

(b) Inside Undercut

Fig. 3-1.1 Undercut Classification

Table 3.2.1 Undercut handling Method Classification Undercut Parting Force Handling Method

Slide Core

Mold Open / Close

Ejection Force

◎

Application Characteristic

External Force

Manual

Out Side

Inside

○

○

◎

○

Generally accepted handling method.

Slant Core (Loose Core)

◎

○

◎

Used for inside undercut handling.

Dogleg Cam

◎

○

◎

Used for small undercut. Small lot production.

Elastic Core

◎

○

◎

Used for small undercut. Medium and large production.

◎

○

○

Extremely small production. Low cost.

○

○

○

Depends on part shape and material.

Fixed Core Enforced

○

◎

○

◎: Suitable ○: Possible

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PITAC 3-3

Characteristics and Application of Various Undercut Handling Methods

1) Slide core Slide core is the most widely used undercut handling mechanism. Normally it is applied for outside undercut, but can be used for inside undercut depending upon the product requirement. This mechanism is to release undercut by means of transfer of a core, which forms undercut, in the parallel direction to the parting line. The name comes from sliding transfer of the core. Driving force for the sliding comes from either mold opening force, which is converted to horizontal force through angular pin, and outsourced force such as hydraulic cylinder (Fig. 3-3.1). Mold opening (Angular pin, cam) Slide core driving force Outsourced (hydraulic cylinder, air cylinder) Generally, angular pin or angular cam is mostly applied for a mechanism to utilize a mold opening force. In this case slide core is located on the movable side and the slide stroke is relatively small. If the slide stroke is big, mold design will be difficult due to restraints from angular pin, length of the pin, mold thickness, etc.. In addition loss in molding cycle due to big opening stroke will be invited. In this case you should consider application of outsourced device to allow big sliding stroke. You may apply hydraulic cylinder, pneumatic cylinder, actuator to convert motor rotation to linear force through rack and pinion gear, etc.. If a sliding core is installed on the fixed side, outsourced application makes the mechanism simple and gives more flexibility for various product shapes with higher reliability. When mold opens, product will separate from the fixed side mold (cavity) and transfer with the movable side mold (core). Thus it is required to release undercut from the cavity before mold opens. When the driving force is sourced outside, you should design an electrical inter locking device on the injection machine not to start mold opening or ejection process until slide core comes to the designated position.

Mold Design (Advance)

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PITAC Angular Pin Slide Core

Slide Core

Coupling Hydraulic / Pneumatic Cylinder

(a) To apply mold opening force

(b) To apply outside force

Fig. 3-3.1 Driving Mechanism of Slide Core

Undercut Release

(a) Injection Filling Process

(b) Cooling Completion Process (c) Mold Opening Process

Fig. 3-3.1 Sequential Movement of Fix Side Slide

Mold Design (Advance)

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PITAC 2) Inclined core (loose core) Inclined core, which may be called loose core, is typical inside undercut handling mechanism. This may be called ‘inclined ejector pin’ because this has a function of ejector pin as the core to form undercut is installed on the ejector plate with designated angle. The undercut handling is done in such a way that the inclined core transfers to the direction to release undercut incorporating to ejection stroke. Ejector plate installed on the inclined core should slide smoothly on the ejector plate not to cause galling. Traditional inclined core has a structure in which ejection point and guide area are located too far a part. Then the ejection force works as a bending moment on the inclined core. Thus traditional structure was considered as troublesome undercut handling mechanism (Fig. 33.2). But in these days, improved design is proposed, in which the bending moment on the inclined core is eliminated by installing a slide guide rod diagonally on the receiving plate and the movable side mounting plate. This improved type is available in the market (Fig. 33.3). If you adopt above reliable mechanism, you can extend the inclined core application to wide range. You will have a benefit to minimize cavity pitch for multi cavity application with outside undercut because it does not require a wide operation space that is needed if slide core method is adopted. 3) Dogleg Cam Dogleg cam is applied for a mold for multiple cavity in which the space is limited. In this case undercut should be relatively small (Fig. 3-3.4). If undercut area locates in the movable side, you may apply it on the both outside and inside. But in practice inside undercut is more in cases. Core to form undercut is in the shape of a dogleg. Similarly to the inclined core, the core will slide on the ejector plate in the releasing direction from undercut area incorporating the movement of ejection stroke. Accordingly, galling may be incurred if the core does not slide smoothly on the ejector plate similar to the inclined core. Another weakness may be that the core hole edge is subject to abrasion because the dogleg cam hits on the hole edge every time when ejector plate returns to the original position.

Mold Design (Advance)

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inclined core and ejection force point are far apart.

PITAC

Fig. 3-3.2 Problem in Traditional Inclined Core Structure Inclined Core (Loose Core)

Guide Plate Slide Base

Guide Rod (This if to cancel a bending moment on the inclined core.)

Holder Bushing (a) Inside undercut handling mechanism (b) Outside undercut handling mechanism (a) Inside undercut handling mechanism

Fig. 3-3.3 New Inclined Core Mechanisms (from catalog of タカオ設計事務所) Dogleg Cam Edge of this part tends to be worn away.

Operational trouble will be incurred unless this part moves smoothly.

Fig. 3-3.4 Dogleg Cam Application

Mold Design (Advance)

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PITAC 4) Elastic core (collapsible core) The undercut core is made of an elastic steel which is inclined if there is no load. The core, which is reformed by other parts, tries to be back to original position at the ejection process. Undercut is handled by this motion. There are two types. One is cylinder type to take care of inside undercut of round parts. Another is bar type to take care of small undercut. They are available in the market under the name of ‘Collapsible Core’ and ‘Spring Core’ respectively (Fig. 3-3.5). Advantage is that molding cycle can be made fast and reliable as there is no operation mechanism like sliding. But you cannot apply it to a product with a big undercut because undercut is handled only by the inclined core. Collapsible core is often applied to a product, which has female threads inside like a bottle cap. Spring core is used similarly to dogleg cam and inclined core (Fig. 3-3.6). As it is more reliable than dogleg cam or the inclined core, consider the spring core first if situation is allowed.

Mold Design (Advance)

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PITAC

Collapsible Core

Spring Core

Screw

Center Pin

(a) Spring Core (from日本金型産業catalog)

Sleeve

(b) Collapsible Core (from日本ディエムイーcatalog)

Fig. 3-3.5 Elastic Core Application

(a) Dogleg Cam

(b) Inclined Core

(c) Elastic Core

Fig. 3-3.6 Various Undercut Handling Applications Mold Design (Advance)

Page 82

PITAC 3-3-5)

Removable core

This is used for a small lot production. This is not operated through mold mechanism but to remove the core, placed in undercut area, manually after the product is ejected with the core in it (Fig. 3-3-5.1). Accordingly the core cost can be low, but productivity should be low, too. This kind of core is seldom used in Japan where labor expense is high. It is used only for a prototype mold before production mold is produced. Removable core is set at the undercut area in the mold like insert. Normally two removable cores are prepared per one undercut so that one can be removed while another is under molding processes. Basically similar consideration to insert molding should be paid for the design of removable core. ① To evaluate the method of the core removal at the time of mold design. ② To design the removal core that can be easily inserted in the mold with a foolproof shaping. ③ To design the removable core to be positioned exactly in the mold. It should not be dislocated by the mold clamping motion. ④ To layout an ejector pin for the removable core. ⑤ To use light metal with high thermal conductivity. Aluminum alloy is recommended.

3-3-6)

Enforcing

This is to take the undercut out just by enforcing the product manually or by ejector depending on the elasticity of the material. The quality of the product will be very much influenced by the shape and the kind of material. Thus a thorough evaluation is essential at the design stage (Fig. 3-3-6.1). Following considerations are needed in the design when enforcing method is chosen. ① Undercut should be within a size so that distorted product could be elastically regained to the designed shape. ② Edge of the undercut area should be designed to have smooth R corners. ③ If undercut is enforced by an ejector pin or a stripper plate, the structure should be made so that the product can be elastically deformed. ④ Material should have enough elasticity to be distorted and to be regained to the designed shape. Gene rally, crystalline non-reinforced resin such as PE, PP, PA, etc. is relatively applicable for the enforcing. Mold Design (Advance)

Page 83

PITAC

Removable

After product is taken out, apply a spanner to this part for removal.

Ejector Pin

Fig. 3-3-5.1 Screw Forming by Removable Core

Pull cavity insert out so that product can be elastically deformed.

Cavity Insert

Undercut Area

(a) Molding Completed

(b) Before Enforcing

(c) After Enforcing

Fig. 3-3-6.1 Enforced Pullout of Tape Guide Roller Mold Design (Advance)

Page 84

PITAC 3-4

Design of Slide Core Mechanism 3-4-1 Driving mechanisms

As explained, driving mechanism of slide core has two ways depending upon how to source the driving force. One is to utilize mold open/close force and another is to source the force from outside. Here we will discuss the design criteria of the driving mechanism of an angular pin, which is most commonly applied. ① One angular pin per one slide core

The function of an angular pin is only to activate sliding core. It has neither function for bearing injection pressure, nor for positioning. Thus positioning between angular pin and sliding core is not severe, and in the same principle, high rigidity is not required. Rough positioning between angular pins and sliding core is associated with difficulty to keep relative position accurately in machining. This tells you that if 2 angular pins are installed for a sliding core, one is in contact but another is not. This will give a moment to sliding core, then result in galling on the sliding area. Thus, one angular pin should be prepared for one slide core in the normal practice, but if you need to install 2 angular pins for some reason, you need to machine relative positions accurately (Fig. 3-4-1.1). ② To keep angle less than 25°

Angle of an angular pin is better be 10°~25°. In practice, 15° or 20° are often used. If the angle is set beyond 25° in the case of large stroke, initial resistance due to mold separation becomes too big to risk damage of the mold.(Fig. 3-4-1.2). If you need a large stroke, you should consider outsourced actuator or installation of an angular cam, which can change the angle in the ejection process. The angle of a locking block should be 2° plus pin angle in principle. ③ To assist mold opening by a spring.

It is advised to apply a spring to assist mold opening not only for a mold to have a sliding core on the top but also on any position. In principle the main function of the spring is to push the sliding core against the stopper in order to position the sliding core accurately, and the secondary function is to assist separation of the product from the mold. The spring should not be too strong not to induce unstable movement of the sliding core. There are 2 ways for spring installation. One is to install it between sliding core and main core. Another is to install it on the end of the sliding core by way of a stripper bolt. The formar is popular because of its compact layout. In this case it is advised to prepare a spring cover so that any foreign materials cannot be pinched by the spring (Fig. 3-41.2). Mold Design (Advance)

Page 85

PITAC Quenched Plate

Slide Core

Slide Core

Angular Pin Hole (2places per slide core)

Section A-A

(a) Improper • l1 < l2 • 2 Angular Pins per slide core.

(b) Proper

• l1 < l2 • One Angular Pin per slide core.

Fig. 3-4-1.1 Proper / Improper Slide Core Design

Slide Core Locking Block Extrusion-cut Surface Stopper Block Spring Cover Wear Plate

(Surface A plays a role of stopper, not extrusion surface.)

Ball Plunger

Fig. 3-4-1.2 Points of Slide Core Design

Mold Design (Advance)

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PITAC 3-4-2 Guide mechanisms

Generally 2 guide rails are to guide a side face of the sliding core. These design criteria are explained as follows. ① To design guide longer than a slide core width

The longer the guide is, the better the guiding stability will be. It must be ideal to have the length 1.5 times of the slide core width. If it is not possible, maintain the length at least more than the slide core width. If the length is less than the width, you will have a back lashing movement similar to old drawers. Naturally this will cause galling. The tendency toward a galling is evident if you try to handle multiple undercut areas by a wide sliding core (Fig. 3-4-1.2). In this case prepare a narrow guide, which is a guide of parallel key shape, on the bottom face of the sliding core and let the guide rails on the side work in the direction toward brim thickness only. ② To apply hardened metal for the guide

In wear consideration, hardened metal or other kind of metal such as brass is used on the sliding surface. This general principle should be applied to guide area of the sliding core. Or one of sliding core and sliding guide should be made of hardened metal. Normally sliding core of small and medium size mold is made of hardened metal, but mold bases are not. They may be of pre-hardened steel as it is. In this case you are advised to attach partially hardened wear plate on the sliding part of the mold base to improve wear resistance. If partial load is not expected, wear plates made of brass or oil-less metal can also be useful (Fig. 3-4.1.2). 3-4-3 Positioning mechanisms

Sliding core should be positioned precisely at the time of mold clamping and mold opening as well. Otherwise mold can be damaged. Positioning criteria of the sliding core will be discussed as follows: ① To use a stopper block in the mold opening direction

Occasionally a sliding core, installed horizontally from operation side to non-operation side, relies on the positioning of the last stroke by means of ball plunger only without spring and stopper block. Even for only horizontal direction, this type of positioning is very unstable and unreliable. As a spring of the ball plunger is not made strong, there is always a risk of disMold Design (Advance)

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PITAC positioning through vibration and moment inertia of the sliding core. This risk is particularly evident when toggle-clamping mechanism with a quick motion is applied to the injection machine. The final positioning of the sliding core stroke in the direction of mold opening should be set by a contact of the sliding core against a stopper block with a help of a spring force (Fig. 3-4-1.1). Thus a ball plunger should be used as a supplemental means for possible damage of the spring. ② To select the surface for positioning except touching surface in the mold clamping

direction Positioning toward mold opening can be done by a stopper block. Do not position the end of mold clamping against touching surface. Particularly if you use a small touching surface as a stopper for positioning, the surface is likely to have marks or to be concaved due to concentrated force on the surface. Positioning toward mold clamping direction should be made against a large surface such as core side, which does not affect product quality. The properly selected area should function as a stopper to withstand touching of the sliding core together with a locking block, and the touching face should be protected (Fig. 3-4-1.1). 3-4-4 Layouts

Slide core should be laid out horizontally in the direction of operation side to non-operation side, and avoid a vertical layout (Fig. 3-4-4.1). If you need to select a vertical layout for a product to require 4 directions sliding or for other reasons, safety consideration for the sliding core not to fall by its weight should be made carefully. A safety consideration may be to install a spring having 1.5~2.0 times strength of the sliding core weight and in addition to install a ball plunger under the sliding core in case of the spring failure. Similar safety consideration had better be paid for any kind of layout of the sliding core.

Mold Design (Advance)

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PITAC

Platen of Injection Machine

Top

Mold Not preferable sliding direction. Preferable sliding direction.

Bottom

Fig. 3-4-4.1 Layout of Slide Core

Mold Design (Advance)

Page 89

PITAC 3-5

Design of An Inclined Core

It was explained that traditional inclined core has a weakness in mold failure and parting failure because of a bending moment on the inclined core derived from too far a distance between ejection point and guiding area. It should be ideal to develop a new inclined core to eliminate such bending moment, but here we will discuss some idea for modification or design points based on traditional design concept of the inclined core. ① To prepare a guide on the support plate or on the core plate

A bending moment on the inclined core can be reduced if the guiding area of the inclined core comes closer to the ejection point. Specifically, a quenched guiding plate can be installed on the support plate or the lower core plate surface (the other side of PL surface). In this way two areas, core and support plate (or core plate), will take care of guiding. This will contribute to improved operational stability of the inclined core (Fig. 3-5.1). ② To minimize friction on the inclined core slide

Inclined core will slide on the ejector plate under an ejecting force applied to the mounting part on the ejector plate. If the sliding is very smooth, the ejection force can be converted effectively to a pushing force along the inclined direction not to a bending moment on the inclined core. You may apply non-lubricant type sliding plate available in the market. But modification by having a quenched plate on the ejector plate and a cam follower or a needle bearing to provide small friction resistance are more likely effective (Fig. 3-5.1). ③ To minimize product movement along with inclined core

Undercut handling of the sliding core is to release the undercut part by pushing it parallel to the plate being derived by the ejection force on the inclined core. Accordingly the product should not move angularly with the inclined core. Following points may be useful to cope with this problem. * If possible, to prepare a draft angle on the undercut area to reduce releasing resistance. * To have an ejector pin to cut into the product as much as 0.1~0.3 mm. * To make the height of the inclined core lower than the main core as much as 0.1~0.2mm. ④ To minimize the angle of the inclined core

In order to minimize a bending moment, minimize the angle. Imagine an ejection stroke without undercut consideration and then set an minimum angle to operate the undercut with the stroke. It is advised to limit the angle to maximum 15°. If it exceeds 15°, you are advised to consider a design with no bending moment. This new type of core is subject to patent issue. Thus you cannot manufacture it in house, but can purchase it in the market. Mold Design (Advance)

Page 90

PITAC

Draft Angle

Detail A Receiving plate Guide Plate Ejection Pin

Needle Bearing

Cam Follower

Wear plate

Fig. 3-5.1 Design Points for Traditional Inclined Core

Mold Design (Advance)

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PITAC

Chapter - 4

3-Plate Mold

Mold Design (Advance)

Page 92

PITAC 4- 3 plate structure

When a gate is designed on the upper surface of the product like pin point gate, a runner stripper plate is required for runner separation in addition to 2 plate mold. This is called 3 plate mold. 3 plate mold has one more plate on the 2 plate mold, but the structure and the operation is quite different from the 2 plate mold. The big difference is in open/close of the plate. 3 plate mold requires a big open/close stroke to open runner parting (PL2) for taking runner out in addition to open/close of main parting line (PL1) for taking product out. Therefore the mold cycle is slower than 2 plate molding. Another disadvantage in comparison with 2 plate mold is the higher cost due to increased parts required for its complex structure. However 3 plate mold is popular as it has advantage over 2 plate mold in molding comparatively large products maintaining a good balance in the shape. Open/close of PL surface of 3 plate mold is worked by transferring a force of movable side mold, initiated by open/close of the injection machine, to fixed side through mold mechanism. Concern must be on the sequence of PL surface opening. Factors to determine sequence of the opening are separation resistance of product and runner, and operation resistance from runner stripper plate. In the normal mold, either main PL surface or runner PL surface opens first and runner stripper plate opens next. Fig. 4.1 shows the relationship. There is a mechanism to lock PL surface (PL1) and let the runner PL surface (PL2) open first. This is applicable for a thin product of which releasing resistance is small on the core side. This enables the product to stay on the core surface. (Fig. 4.2) Basic structure of the movable side is the same. Thus various types of the mold explained for 2 plate mold are applicable to the case of 3 plate mold. In this way 3 plate mold can cope with the requirement of various shapes of the products.

Mold Design (Advance)

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PITAC

Runner Stripper Plate Cavity Plate Core Plate

F1: Core side separation resistance. F2: Cavity side separation resistance. F3: Runner separation resistance. F4: Gate cut-off resistance. F5: Cavity plate operational resistance. F6: Runner plate operational resistance. F7: Runner lock pin separation resistance. F8: Sprue separation resistance. [Required Conditions] (1) PL1 opens and then product stays on the core side. (F2 + F4) < F1 (2) Before PL3 opens, PL2 opens. (F3 + F4) < (F6 + F7 + F8) [Sufficient Conditions] (1) Before PL2 opens, PL1 opens. F2 < (F3 + F5) (2) Before PL1 opens, PL2 opens. F2 > (F3 + F5)

Fig. 4.1 Opening Sequence and Various Resistances for Three Plate Mold

Parting Lock

(1)Parting lock locks PL1 and PL2 opens first. (2)As PL2 precedes opening, then PL1 opens. (3) Lastly, PL3 opens.

Fig. 4.2 Mold Opening Control by Parting Lock

Mold Design (Advance)

Page 94

PITAC Arm for Product Automatic Take-out

Arm for Runner Automatic Take-out

Decide ‘Required’ and ‘Sufficient’ S1, S2 & S3 in mold design

Fig. 4.3 Mold Open Stroke of Three-Plate Mold 4-1 Support pin

Support pin is a unique part in the 3-plate mold. It is to guide fixed side mold plate and runner stripper plate in the process of the mold opening. Therefore both support pin and guide pin/bushing are used for standard mold base for 3-plate mold. Both share a function of guiding fixed and movable plate. But this structure limits a space for puller bolts and other components in the mold. Thus there is other type of standard mold base to enable support pin to perform double purpose by adding guiding function of movable mold plate (Fig. 41.1). This double purpose structure provides more space for other components but be aware that support pin collar cannot be used here. The support pin collar is a safety device to stop the fixed mold plate from coming off from the support pin when a head of the stop bolt happens to be broken. Therefore if the double purpose structure is adopted, you should either consider other method for safety consideration or use stop bolts and puller bolts of higher strength to ensure high reliability. Be advised that the double purpose structure is inferior to the functionally shared structure in terms of the positioning performance of the mold plate because there are possibility of pin deflection and clearance problem with guide bushing as the support pin fixed on the fixed side clamping plate has to guide the movable side mold plate. Considering all above, Mold Design (Advance)

Page 95

PITAC following design criteria can be presented for designing guide and support pin structure of 3 plate mold. ① In the case of a mold with movable side guide pin structure, like stripper plate ejector,

the basic structure should be that support pin and guide pin/bushing are laid out separately. ② In the case of a mold with a thick fixed side mold plate or with a large opening stroke in

PL2, select a basic structure to assure better performance for guiding mold plate because support pin deflects and positioning performance should drop. ③ In the case of double purpose mold that requires accurate positioning of the movable and

fixed side mold plate, increase positioning accuracy of the cavity and the core by applying a taper pin on the PL surface.

Guide Structure

Basic Structure

Flexible Structure

R plate

R plate

Drawing

Characteristic

• High positioning accuracy between cavity and core • Low flexibility in parts design such as puller bolt.

• High flexibility in parts design such as puller bolt. • Poor positioning accuracy between cavity and core, thus taper pin positioning should be considered for procision mold.

Fig. 4-1.1 Mold Guide Structure with Guide Pin and Support Pin (from catalog of フタバ)

Mold Design (Advance)

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PITAC 4-2 Control parts for the extent of opening.

4-2-1 Stop bolt A stop bolt is a part to limit the stroke of runner stripper plate. As Fig. 4-2-1.1 shows, various types of stop bolts are made available. Type a in Fig. 4-2-1.1 is the most popular kind of stop bolt and it can be located in the same location as puller bolt. Thus it is good in space utilization and in addition it saves machining on various plates. Other types are more or less the same shape as puller bolt, but a puller bolt is too long for the stop bolt. It is rather recommended to find a stopper bolt among stripper bolts which have better possibility to be used for a stop bolt and are available as standard press die parts. 4-2-2 Puller bolt A puller bolt is commonly used as a control part to control opening extent between runner stripper plate and cavity plate, in other wards opening extent of runner parting line (PL2). Also it is used for controlling opening extent of main parting line (PL1). This mechanism is preferable when space is widely available for a mold where a support pin has a dual function to guide movable side mold plate. One thing you should not forget is to pay enough consideration on the workability and interference with automated take-out device in the scope of product take-out. Puller bolt has two types. One is male screw and another is female screw type. Female screw type is widely used because it can be used as a pair with stop bolt. Even it is used independently, female screw type has better reliability for repeated load application because it can be fastened from runner stripper plate side (Fig. 4-2-2.1).

Mold Design (Advance)

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PITAC

Fixed Side Mounting Plate

Runner Stripper Plate

Puller Bolt Characteristics • High Space Efficiency • Poor in molding workability and maintainability.

• Good molding workability. • Poor reliability in strength of screw area. • Good maintainability.

• Good molding • Good molding workability. workability. • High reliability in • High reliability in strength of screw area. strength of screw area. • Good maintainability. • Poor maintainability. • High cost.

Fig. 4-2-1.1 Various Stop Bolts and Characteristics

Female Type

Male Type

Female Type

Fig. 4-2-2.1 Kinds of Puller Bolt and Application

Mold Design (Advance)

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PITAC 4-2-3 Tension link

Tension link is installed on the plate side of the mold to control the limit of mold opening. It is used for controlling opening of the main parting line (PL1) (Fig. 4-2-3.1). As explained, puller bolt is used for controlling opening of runner parting line (PL2), but tension link can be used for PL2 if layout space is not given enough to apply puller bolt. Like above example, tension link gives more flexibility in layouting parts for mold opening control, but close attention should be paid not to interfere with couplers mounted on the plate side face for temperature control. Also attention should be paid on the workability of product take-out, particularly of manual take-out, because tension link is installed on the side face of the mold plate. Thus the layout of the tension link should be carefully made in consideration of take-out method of product and runner as well. 4-2-4 Chain

Tension link and puller bolt are normally installed in the range of mold thickness, but once in a while there is a case in that the range of mold thickness is not wide enough for their installation when the extent of the opening is large relative to the mold thickness. In this case chain can be effectively applied (Fig. 4-2-4.1). As it is flexible, the chain can be used for most of the cases. When the mold is closed, the chain can be folded and stored within the mold plate thickness. However a chain may hang down when the mold is closed and may interfare with couplers for temperature control device. Thus a chain should be only used for special cases as in the case explained above.

Mold Design (Advance)

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PITAC Type

Short Stroke Type

Long Stroke Type

Drawing

Characteristic

• One end is fixed by a bolt. Thus operation is assured. • Possible to make mold open / close fast. • Less noise. • Not suited for a big opening.

• Can be used for a big opening. • Rather high noise. • Cannot make mold open / close fast

Fig 4-2-3.1 Extension Links and Characteristics (drawings are fromミスミcatalog) Chain

Fig. 4-2-4.1 Restriction of Mold Opening by Chain Mold Design (Advance)

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PITAC 4-3 Parts for opening sequence control

4-3-1

Parting lock

Parting lock is to control mold-opening sequence providing operational resistance in the opening mechanism through various methods. Generally it is used to lock main parting line (PL1) and let the opening of runner parting line (PL2) operate in advance. For example, in the case of shallow product, which has not, much difference in the releasing resistance of fixed side and movable side, the product tends to remain on the fixed side due to gate cutting resistance. In such case it is effective if PL1 is locked by a parting lock by eliminating a chance of the product to remain on the fixed side. In a ward, the parting lock stabilizes releasing balance of the fixed and movable side. Types Creation of operational resistance can be made by mechanical system, friction, magnetic force, etc.. Parting locks associated with various methods are classified as follows. Mechanical lock Spring lock Parting locks Plastic lock Magnet lock Features and application ① Mechanical Lock

Mechanical lock is installed on the plate side face of the mold and controls the sequence of the mold opening. There are various types of mechanical locks available in the market. There is a common mechanism. A portion is kept closed by means of combination of latches and springs until a certain stroke is achieved by another parting device (Fig. 4-31.1) example a locking device on the PL1 can be set not to release PL1 to open until PL2 opens to a certain position. As you see, this kind of mechanical locking device can eliminate mold parts, such as a tension link to control mold opening of PL1. The most important thing is the proper timing of the locking release. Parting line opens in the sequence of PL2→PL3→PL1. Release bar of the mechanical locking device should be adjusted to release the latch somehow earlier than the opening extent set by stop bolt or puller bolt. If the release if too early, it will result in insufficient opening of PL3 (PL between fixed side clamping plate and runner stripper plate). If the release is too late, it will result in the damage of stop bolt or puller bolt. Mechanical locking device can be said the most reliable parting locking device though it is costly. Mold Design (Advance)

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PITAC

Fig. 4-3-1.1 Parting Mechanism by Mechanical Locking Device (fromミスミcatalog)

Mold Design (Advance)

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PITAC ② Spring Locking Device

Spring locking device is to control sequence of the mold opening being installed on the plate side similar to mechanical locking device. It has a structure in which a plate spring or a roller falls into the undercut portion to resist mold opening, driven by the plate or dish spring force (Fig. 4-3-1.2). Traditionally the plate spring has been commonly used but adjustment of the spring force could not be made. In these days a roller to enable both locking and adjustment by a dish spring with a screw is made available in the market. This type is available in the market and the reliability has much improved. Spring locking device cannot control a stroke of the mold opening but is easier to be handled comparing to mechanical locking device. In the durability aspect, it is much better than plastic locking device to follow. ③ Plastic locking device

Plastic locking device has a structure to lock the parting line by the friction force of a extruded part of a plastic cylinder pressed in a hole on the mold plate by the mold clamping force. This plastic cylinder is installed on the PL surface of the fixed or movable side mold plate. The center of the extruded part of the plastic cylinder is made by a tapered bolt. Thus the outside diameter of the extruded part can be changed by screwing in the tapered bolt. Thus the friction force can be adjusted. (Fig. 4-3-1.3) This parting lock device is less costly and easy for installation but due to the stress relaxation of plastic material tapered bolt it tends to be loosened comparing to other options. Thus evaluate how to stop loosening of the bolt and be ready for spares for the replacement whenever needed. ④ Magnet locking device

Magnet locking device has a structure in which a permanent magnet is installed on the side face of the mold plate and is to pull steel block by magnet force (Fig. 4-3-1.4). This device has no better reliability comparing to mechanical locking device and no adjustment mechanism comparing to spring locking device and plastic locking device. But it can contribute to an efficient molding cycle because no mold clamping force is wasted, and you need not slow down the speed of mold closing. In addition the device can be made to standard parts because it can be easily installed and removed to and from the mold plate. Mold Design (Advance)

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PITAC Bolt for adjustment

Lock Roller (SUJ2 high frequency quenched.)

Lock Holder (S50C)

Screw for fixing of bolt for adjustment

Bolt for adjustment

Lock Holder Plate Spring

Internal Roller

Fixed Side

Movable Side

High Frequency Quenched

Lock Bushing

Lock Roller

MPLKB Lock Bushing (SK3)

Fig. 4-3-1.2 Spring Locking Device to Enable Adjustment of Holding Force (fromミスミcatalog)

Plastic Lock Bushing Dowell Pin

Tapered Screw

Fig. 4-3-1.3 Plastic Locking Device and Application (from HASCO

Mold Design (Advance)

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PITAC

Magnet Lock

Material SS400 Magnet MLK 40-100 Alnico Magnets (Max. temp. 80℃) MLK 80-200 Rare Earth (Max. temp. 200℃)

Steel Block

Magnet Lock

Square Magnet

Steel Block Application

Material SS400

Fig. 4-3-1.4 Magnet Locking Device (fromミスミcatalog)

Mold Design (Advance)

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PITAC 4-3-2 Runner lock pin Basic functions of the runner lock pin are as follows.

Functions ① To cut off a gate Main function of the runner lock pin for 3 plate mold is to create a cutting force to cut off a gate along with the mold opening. Undercut part at the tip of the runner lock pin is to hold the runner on the runner stripper plate against an integral force of parting resistance of the runner and the 2nd sprue and cutting resistance of the gate. ndDue to this holding force, the gate is cutted off and at the same time the runner and the 2 sprue can be taken out from the fixed side mold plate. Accordingly the runner can be taken out. ② Mold opening sequence control.

The secondary function of the runner lock pin is to control sequence of the mold opening. Opening of PL3 is for pushing off the runner. Thus PL3 should open after PL2 is opened. As undercut part of the runner lock pin locks PL3, the desired sequence can be maintained. Design considerations Design considerations to assure above functions are explained as follows. ① Shape and amount of undercut

Shape and amount of undercut at the tip of the runner lock pin should be appropriate to suit resin and runner size. If the amount is too small, the gate cannot be fully cut off and the runner may stay on the fixed side mold plate. If the amount is too big or corner edge portion is still remained, undercut portion will stay in the ring form and the gate cutting force cannot be created for the next injection (Fig. 4-3-2.1). ② Layout

Runner lock pin cannot be located at any place on the runner. It should be located at the same position of the gate or near the gate in order to effectively cut off the gate. If it is located far from the gate, the gate may not be cut off due to bending deflection of the runner when mold opens. (Fig. 4-3-2.2) ③ Structure and layout in terms of resin flow resistance

When a runner lock pin is laid out on the gate following the principle ② above, undercut portion of the runner lock pin may be subject to flow resistance. In such a case, a resin pool on the runner stripper plate can be prepared. Do not make it too deep. If too deep, injection cycle becomes long (Fig. 4-3-2.2). Or you may shift the position of the runner lock pin 2~3mm against resin flow direction. ④ Galling

When there are many runner lock pins, you may face galling trouble. In order to waive galling, the best solution is to match the taper of the runner lock pin with that of the runner stripper plate. But taper fitting is a difficult work. For easier solution it is advised to use heat-treated bushings on the runner stripper plate (Fig. 4-3-2.3). To protect from galling, another consideration is to give a certain flexibility for fixing method of the runner lock pin. For example, in the case that the runner lock pin is fixed by a screw bolt from the brim side, it is suggested to apply a collar, which is a bit thicker than the brim, and fasten the collar so that the brim can be designed to have a certain dimensional freedom on the both radial and axial directions (Fig. 4-3-2.3). Mold Design (Advance)

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PITAC Tip Details

Tip Details

Resin forms a ring due to poor design of runner lock pin.

* Tip should be with smooth R in shape

Fig. 4-3-2.1 Runner Lock Pin and its Tip Shape (fromミスミcatalog)

Runner Lock Pin Big

Big

Gate a. Poor Resin Flow (improper)

b. Poor Gate Cut-off (improper)

c., d. Good Resin Flow and Good Gate Cut-off (proper)

Fig. 4-3-2.2 Proper / Improper Design for Resin Flow and Gate Cut-off

Mold Design (Advance)

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PITAC Screwed Plug Collar

Bushing

(Improper)

(Proper)

(Proper)

(Proper)

(Best)

Fig. 4-3-2.3 Proper / Improper Structure for Galling of Runner Lock Pin

4-3-3

Push pin

Push pin is a supplementary part for PL2 and PL3 opening. As long as runner lock pin operates satisfactorily, it will perform supplementary function for gate cut-off and releasing of sprue and runner. Particularly for a mold to apply parting lock device, PL2 should open before PL1. Push pin enables sure opening sequence and gate cut-off as well. The similar function can be attained by installing a spring between the runner stripper plate and the fixed side mold plate being guided by a puller bolt. Generally this mechanism is used more frequently but this does not have a function to support opening of PL3. (Fig. 4-33.1) 4-3-4 Runner ejector

Runner ejector is an ejector installed on the runner stripper plate. This is to assure a runner to drop by a spring force after the runner separates from the sprue bushing. Therefore a runner ejector is effective when the product is taken out by the gravity force. If automated device is used for product take-out, this device is better not be installed. The runner ejector is sold in the market as a set including runner ejector pin, spring and housing. This can be easily installed if the specification meets the requirement (Fig. 4-34.1). When it is applied, the runner ejector should not use a pin smaller than runner width. A part of the pin should be in contact with PL2 surface of the fixed side mold plate, otherwise the pin will be inside of the groove of the runner and may be in touch with the surface of the draft angle or the bottom of the runner. In either case, function of ejector cannot be worked. (Fig. 4-3-4.2) Mold Design (Advance)

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PITAC

Push Pin

Mold Binding → Supplemental Device to open PL2 → Supplemental Device to open PL3

(b) Supplemental Device by spring (for PL2 opening)

(a) Push Pin

Fig. 4-3-3.1 Supplemental Device for PL Opening by Push Pin and Spring ①Housing

②Pin

①Housing Material Hardness

①Housing

②Pin Material Hardness

②Pin

③Spring Type/ OD-Free Length

Spring Constant

Fig. 4-3-4.1 Example of Runner Ejector Set (fromミスミフェイス) Pin Housing Spring

Runner A case of malfunction because pin is in contact with runner. (Pin is to be returned by resin pressure)

(a) Proper

Pin does not perform as an ejector because it goes into resin.

(b) Improper

Fig. 4-3-4.2 Proper / Improper Application of Runner Ejector Mold Design (Advance)

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PITAC 4-4 Pinpoint gate

Pinpoint gate may be called pin gate. This is of 3 plate mold structure and the gate is cut off automatically. Because of 3 plate structure, the manufacturing cost is higher than submarine gate, the same automated cut-off gate, but made of 2 plate. Advantage is that runner layout is flexible, thus this gate can be applied to small products and to big products as well. Disadvantage is that this gate does not suit to a resin of poor flowability such as acryl resin because the gate is small and nor to a resin to include fiberglass, which erodes the gate quickly. Automated cut-off in the pinpoint gate works like a pulling action generated by the mold opening force. Therefore the cut length varies. To cope with this problem gate area sometimes is designed to be 0.3~0.5mm in concave shape. To compensate decreased thickness of the product, it is advised to make the core side the same amount in convex shape if possible (Fig. 4-4.1).

Gate Shape

Characteristic

• Easy mold machining. • High mold strength • Gate cut – off length L should be allowed.

• Good balance in machinebility, strength and cutting performance.

• Priority is placed on cutting performance rather than machinebility and strength. • Applicable when high gate is not allowed.

Fig. 4-4.1 Various Pin Point Gate and Characteristic

Mold Design (Advance)

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PITAC

Mold Design (Advance)

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PITAC 5 Corrective Actions for Defective Molding Defective molding may be derived from improper setting of the injection parameters or material conditions. Corrective action or improved procedure should be in practice by finding out the cause of the defective molding. Typical phenomena of the defective molding, possible causes and actions for improvement shall be explained as follows: 5-1

Short filling (short shot)

A short shot is a condition in which resin cannot be fully filled into the mold. This will cause poor appearance and poor function of the product. Possible causes of the short shot are as follows: • Resin fluidity is poor. • Amount of resin measured by the machine is too less. • Product wall thickness is too small. • Air pocket due to flow pattern of the resin. • Air ventilation of the mold is poor. • Resin temperature is too low. • Temperature of the cavity surface is too low. • Injection speed is too slow. • Injection pressure is too less. 5-2

Flashes or burrs

Flashes are thin filmy resin adhered to edges of the product. Flashes may cause difficulty in assembly of the molded parts and may be hurting when handled. Flashes can be resulted from following conditions: • Resin viscosity is too low. • Clamping force of the mold is not enough. • Dimensional problem of the cavity. • Clearance on cavity parts is too much. • Rigidity of the mold plate is not enough. Mold Design (Advance)

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PITAC • Resin temperature is too high. • Cavity surface temperature is too high. • Injection speed is too fast. • Injection pressure is too high. 5-3

Sink mark

Sink mark is a concavity on the product surface. It causes poor appearance and poor function of the molded product. Sink marks can be resulted from following conditions: • Contraction factor of the resin is too high. • Wall thickness of the product is too thick. • The gate is too small. • Runner is too small. • The gate location is too far. • Cavity temperature is partially too high. • Cooling capacity of the mold is too less. • Holding pressure is too low. • Time for holding pressure is too short. • Cooling time is too short. 5-4

Void

Void is a bubble in the product. There can be classified to two causes. One is caused by water or air mixed into the material and another is caused by vacuum cavity created when material is contracted. Void results in weak strength of the product or poor appearance if the products need to be transparent. Void can be resulted from following reasons. (In the case of mixed air) • Preheating of the pellets is not enough. • Rotation speed of the screw is to fast. • Staying time of the resin is too long. • Resin temperature is too high. Mold Design (Advance)

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PITAC • Injection speed is too fast. • Air ventilation in the mold is not enough. (In the case of vacuum void) • Holding pressure is too low. • Time for holding pressure is too short. • Gate is too small. • Runner is too small. • Amount of cushion is too less. • The wall of the product is too thick. 5-5

Flow mark

Flow mark is a wavelike mark of the resin flow left on the surface of the product. Appearance and surface quality should be affected. Flow marks can be caused by following reasons: • Resin flow is poor. • Resin temperature is too low. • Gate is too small. • Injection pressure is too low. • Holding pressure is too low. • Surface temperature of the cavity is too low. • Injection speed is too low. 5-6

Weld line

Weld line is a line mark on the surface of the product appeared around merging area of the resin flow. Appearance and strength of the product should be affected. Weld line can be caused by following reasons: • Resin flow is poor. • Resin includes additives such as fiber. • Surface temperature of the cavity is too low. • Staying time of the resin is too long. • Location of the gate is not proper. Mold Design (Advance)

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PITAC • Air ventilation is not enough. • Injection pressure is too low. • Injection speed is too slow. • Holding pressure is too low. • Time for holding pressure is too short. • Preheating of the pellets is not enough. 5-7

Burn

Burn is a phenomenon in that resin burns when air is heated upon compression around final filling area and air pocket area. Burn should affect appearance and quality of the product. Burn can be caused by the following reasons: • Air pocket can be created. • Air ventilation is not enough. • A certain gas has plugged the vent line. • Injection speed is too fast. • Resin temperature is too high. • Gate is too small. 5-8

Jetting

Jetting is a snaking mark appeared on the product surface, this tends to appear when the gate is too small. Jetting can be caused by the following reasons: • Gate is too small. • Gate is located around the thick wall of the product. • Injection speed is too fast. • Injection temperature is too low. • Surface temperature of the cavity is too low. 5-9

Silver streak

Silver streak is a fine silvery line on the surface of the product. Silver streak can be caused by the following reasons: Mold Design (Advance)

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PITAC • Preheating of the pellets in not enough. • Air ventilation is not enough. • Gate is too small. • Resin temperature is too high. • Rotation speed of the screw is too fast. • Injection speed is too fast. 5-10 Poor luster

Poor luster is a phenomenon in that luster on the product surface is not enough or inconsistent. It affects appearance of the product. Poor luster can be caused by the following reasons: • Resin flow is poor. • Too much variation in the wall thickness of the product. • Surface finish of the cavity is too rough. • Preheating of the pellets is not enough. • Injection speed is too slow. • Injection pressure is too low. • Holding pressure is too low. • Time for holding pressure is too short. • Surface temperature of the cavity is too low. 5-11 Inclusion of foreign matters

Foreign matters such as dusts, dirt, bugs, etc. may be included in the material. Appearance and material strength may be affected. Causes may be as follows: • Preheating management is poor. • Foreign matters tend to be in at the crashing process. • Cover of the hopper dryer is left open. • There may be a fluorescent light above the machine. • 5 S is not in practice in the factory. • Management in keeping material bags is not proper. • Oil and separation agent are spread too much on the mold. • Air ventilation tube of the mold is plugged. Mold Design (Advance)

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PITAC • Cleaning of the mold is not sufficient. • Color change is not proper. • Staying time of the resin in the cylinder is too long. • Foreign matters are on the screw. 5-12 Poor releasing of the mold

Poor releasing of the mold is a phenomenon in that the product sticks to the cavity. There are three patterns for this problem. One is that the product sticks to the cavity when the mold is opened. The second is that the product sticks to the core when product is to be ejected. The third is that the product sticks to the slide core. When poor releasing happens, naturally continuous production is interrupted and product surface may be damaged. Poor releasing can be caused by the following reasons: • Releasing resistance of the resin is too big. • Draft angle is too small. • Cavity surface is too rough. • Direction of the cavity surface finishing is perpendicular to draft direction. • Cavity is machined in the shape of under cutting. • Measured resin is too much. • Amount of cushion is too much. • Injection pressure is too high. • Holding pressure is too high. • Time for holding pressure is too long. • Cooling time is too short. • Cooling capacity of the mold is not enough. 5-13 Bending and deformation

Bending and deformation is a phenomenon of the bent and deformed product. Bending and deformation can be cause by the following reasons: • Contraction rate of the resin is too big. • Wall thickness of the product varies too much. • Gate location is not proper. • Filling pattern is not even. • Surface temperature of the mold varies from one place to another. Mold Design (Advance)

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PITAC • Holding pressure is too low. • Time for holding pressure is too short. • Surface temperature of the cavity is too high. • Cooling time is not enough. • Layout of the ejector pins is imbalanced. • Draft angle is too small. 5-14 Dimension defect

Dimension defect is a phenomenon is that the product dimensions fall outside of specified tolerances. Dimension defects can be caused by the following reasons: • There is a fluctuation in the quality of the material by the lot. • Machining dimensions of the cavity may be in error. • Contraction rate of the material is out of scope. • Flow pattern of the resin is not stable. • Molding condition is not stable. • Surface temperature of the mold is not stable. • In the case of multi-molding, balance of the runner and gate is not appropriate. • Holding pressure is released before gate sealing. 1) “Reading of Plastic Molding”, written by Y. Sakurauchi, published by (株) 工業調査会 2) “Easy Injection Molding”, written by K. Takano, published by (株) 工業調査会 3) Molding Standard Parts Face 1999-2001, published by (株)ミスミ

Mold Design (Advance)

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PITAC Table 5.1 Variety of Defective Mold and Causes

Mold Design (Advance)

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PITAC

Fig. 5.1 Molding Cycle Diagram

Fig. 5.2 How to Experimentally Establish Appropriate Holding Pressure and Time for Holding Pressure Mold Design (Advance)

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PITAC

Fig. 5.3 Relationship between Injection Pressure and Pressure in the Cavity

Mold Design (Advance)

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