Gas Assisted Injection Molding

Gas Assisted Injection Molding

Gas Assisted Injection Molding

Bhupendra Singh Plastics and PPV Department University of Mumbai Institute of Chemical Technology Matunga, Mumbai- 400 019

Introduction Gas Assisted Injection Molding (GAIM) is the fastest growing injection molding technology. This process is gaining popularity and is being used in place of conventional injection molding and structural foam processing.(1)GAIM is basically designed for producing hollow plastic parts and for parts having separate internal void spaces or channels.(2) There is a great demand for thin wall high quality moldings made in higher specification materials. The GAIM process is capable of producing hollow, light weight, rigid parts that are free of sink marks and less warpage. GAIM can be operated on virtually any injection molding machine by connecting gas injections phased pressure control equipment. The purpose of this paper is to present an overview of the fundamental design, process and developments in the gas assisted injection molding process.

GAIM process GAIM process consists of injection of gas, normally nitrogen, under controlled pressure into molten plastic; which is partially filled in the mold. The mold is partially filled with plastic (short- shot). The plastic flow is stopped and a controlled volume of inert gas (usually nitrogen) is injected into the centre of the flow at a point which is predefined depending on the shape and size of the final product. The gas pressure is held constant as the plastic cools in the mold, and then is relived (vented) just prior to mold opening.(3) Schematic diagrams for GAIM is represented as shown below in the following figures.

By- Bhupendra Singh

Gas Assisted Injection Molding

Step 1:-The resin is injected into the empty mold cavity through an edge gate. A shortshot of polymer is injected into the mold filling it partially with predetermined weight.

Step 2:- Gas injection starts using either a nozzle or gas pin, which penetrates material and further advances melt front by displacement of molten plastic. The gas is injected into the center of the hot resin. The gas than directs the resin to flow in the least resistance path.

Step 3:- The resin shot completes its injection into the mold. Bubble forms inside the article which stretches the skin to the end of the mold.

By- Bhupendra Singh

Gas Assisted Injection Molding

Step 4:- The gas bubble continues to be pressurized creating an internal cushion to compensate for volumetric shrinkage of the plastic as it cools.

Step 5:- the article is cooled. The gas is vented prior to the mold opening to avoid explosion. The molten plastic from the thicker section which is present in molten condition is displaced by the gas into the extremities of the tool, thus packing out the molded part. The gas channel acts as internal flow runners; replacing external hot and cold runners. This gas channel provides constant gas pressure through the part, thus creating a uniform pressure on all surfaces. As the plastic closest to the mould is cooler; the air stays in the middle of the plastic and press it into every cavity. The hollow gas channels compensate for the tendency of plastic to form the sink marks at thicker cross sections. The constant gas pressure eliminates part warpage and reduces “molded in stress.” Depending on the injection method being used, the gas is injected into the melt at different moments and positions.

By- Bhupendra Singh

Gas Assisted Injection Molding

Molding cycles A relationship of injection pressure Vs time curve for conventional injection molding is given below in fig. (1)

200 Pressure MPa

0

t1

t2

Time t3

Fig. (1): Pressure Vs Time curve for conventional injection molding Conventional injection molding P (t1) = 50 MPa P (t2) =150 MPa P (t3) =0 In the case of conventional injection molding the majority of the mold volume (90-95 %) is filled in the time period t1. the maximum pressure required to completely fill the mold at t2 determines the machine clamp tonnage required to mold the part. The pressure decreases as the plastic cools during t2-t3. At time t3 the mold opens. In GAIM the short-shot is injected during time period t 1. The plastic flow is stopped by using positive shut off valve. The gas is then injected during the period from t2 to t3. The “dwell time” prior to gas injection (t1 to t2) must be controlled accurately. The “dwell time” can be set between zero to several seconds. The gas pressure is held constant between the time period t3 to t4. The gas is vented at time t4 through a nozzle. The mold cycle completes at t5. The relationship of injection pressure Vs time curve for GAIM is given below in fig. (2)

By- Bhupendra Singh

Gas Assisted Injection Molding

100 Pressure MPa

0 t1 t2 t3

Time

t4 t5

Fig. (2): Pressure Vs Time curve for GAIM GAIM molding P (t1) = 22 MPa P (t2) = 20 MPa P (t3) = 14 MPa P (t4) = 14 MPa P (t5) = 0 The hold time is generally 10-80 % of the initial pressure and depends on the geometry of the molded parts. Melt temperatures remain similar to that of conventional injection molding.

Gas Gas may be injected into the plastic either directly into the mold cavity or into the molding machine nozzle. Gas travels the path of least resistance, and hence travels into thicker sections where the plastic is still in molten condition. Preferred direction for gas travel Once inside the mold, the gas will travel in a direction having least resistance to gas flow. The resistance for the gas flow depends on the process variables such as  Resin flow length  Cross-section area of cavity  Melt temperature and  Existence of short-shot. Consider a polymer melt injected inside a pipe mold. The gas is injected at a point X. the melt length at the left hand side is smaller then the melt length at the right hand side. In this case the resistance for the flow to the gas is less in left hand side direction. Hence the left hand side direction is the preferred direction for gas flow. Higher gas pressure and shorter delay time results in shorter primary gas penetration length with thinner polymer skin, and vice-versa. Volumetric fill time decreases with increasing gas pressure, higher melt temperature and lower melt viscosity. Gas should be

By- Bhupendra Singh

Gas Assisted Injection Molding

injected at the proper gas pressure to avoid excessive deceleration or acceleration of polymer melt, which can cause short-shots, hesitation marks, material degradation, or discoloration. The GAIM requires careful attention to controlling the channel wall thickness i.e. the thickness of the material surrounding the hollow core. Wall thickness influences the extent of gas penetration and ultimately the part performance. The solid layer which forms on the mold walls as hot polymer contacts the colder mold surface can be thinned out by decreasing gas delay time, increasing melt temperature, increasing mold temperature or by changing materials to one with a lower thermal conductivity and specific heat. The molten layer thickness is determined by the velocity of the gas bubble through the molten core and the rheological properties of the resin. As the polymer viscosity increases, the gas channel wall thickness will increase. A combination of parameters such as increased gas temperature, increased melt temperature and decreased shot size is used to reduce the thickness of the molten layer. As the wall thickness increases the gas will penetrate farther than desired. Also increasing shot size will often inhibit adequate gas penetration. Complex parts consisting of both thin and thick sections should have gas channels designed to pass through the heavy sections to eliminate sink marks and reduce cycle time. Conversion equipment A gas injection nozzle can be adapted to a conventional machine with slight modification. The gas injection system consists of gas system control panel and add-on conversion equipment for precise gas pressure, timing and volume control. The injection molding machine should be capable of delivering a controlled short shot and retracting the barrel and breaking the plastic sprue to vent the gas. Gas recycling Recycling of nitrogen for use in subsequent cycles is done in certain cases but it is not common. Recovery rates ranges from 70-90% but gas recovery is not possible in certain resins. Many processors are planning to install on site systems which for separating nitrogen from air. These types of systems have the advantage of eliminating the inconvenience and danger of continually changing the gas bottles. Such systems also have the potential of cutting gas cost.(4) A system which combines gas recovery system with pressure- swing absorption (PSA) generator can recover gas up to 95 %. In this system nitrogen is recovered through the in mold gas injection nozzles after each cycle, filtered and fed to a fixed volume receiver. Isopropyl alcohol liquid; which transforms to a gas upon injection in to the mold, can be used instead of nitrogen gas. This system can be retrofitted to conventional injection molding machine, enabling material savings of 30-40%.

Material consideration GAIM has been found to work with any thermoplastic materials (with or without filler), however as the process control demands that thick sections remains fluid while the plastic at the mould surface solidifies, the process can not be used for thermoset materials. Difficulty has been faced with some high shrinkage and temperature sensitive materials.

By- Bhupendra Singh

Gas Assisted Injection Molding

The plastic should posses a certain level of hot melt strength in order to achieve controlled gas channel formation and to prevent gas blow through. GAIM works well with high MW resins, low melt flow materials, highly filled resins and alloys and blends. High melt viscosity helps control the formation of the hollow gas channels. The large flow channel reduces fiber breakup during injection of fiber-filled materials, which further improve the part performance. The following list gives some of the commonly used resins on GAIM machines.  HDPE, because of its low cost, ease of processing, chemical resistance and low temperature impact strength is used in a wide variety of applications.  PP is a low cost resin with excellent processing characteristics and good chemical resistance.  HIPS is used in furniture and cosmetic applications. It can be easily painted. It is also used for large panels requiring stiffness and low cost.  PS is used for applications such as television and other electronics appliances.  PC exhibits both higher stiffness and high impact strength. Proper processing is critical in PC to obtain full potential of the resin as moisture or heat degradation causes a loss of impact strength.(3,5)

Advantages of GAIM GAIM offers a number of advantages like: • High strength to weight ratio • Reduced cycle time • Low injection pressure • Lower clamp tonnage • Reduced stress and warpage • Tool design freedom • Cost effective production of large parts having good surface finish • Reduction in part weight • Improved material properties and part qualities • Part consolidation with both thick and thin sections • Placement of delicate pins in the cavity. Some of the advantages offered by GAIM can be explained as follows. Reduced molding stress and warpage: In GAIM the gas pressure is equal everywhere on the surfaces are markedly reduced. As the gas pressure is constant during cooling cycle; it reduces the tendency towards part distortion. Elimination of sink marks: Sink marks are caused at ribs or bases on the back side of a part, due to volume contraction of plastic during cooling. In GAIM the sink marks are minimized to a greater extent and also can be eliminated by directing a hollow gas channel between the surface of the part and the back side detail as shown below in the fig.(3)

By- Bhupendra Singh

Gas Assisted Injection Molding

Fig: (3) The hollow gas channel at the base of the rib eliminates sink marks on the visible surface. The base of the rib is made thicker to help direct the gas channel. Reduced clamp tonnage: the lower pressure used in GAIM results in a 25 to 90 % reduction in clamp tonnage as compared to conventional injection molding machines. The clamp tonnage in GAIM is only slightly higher than the clamping force used with structural foam molding. The low pressure used in GAIM reduces problems associated with flash, tool wear and “lifter read through.” Elimination of external runner: As flow runners can be designed into the part, all external runners (hot and cold) can be easily eliminated and the plastic is injected through a single gate. This result into reduced tool cost, reduced regrinding operation, and improved temperature control of the melt. Use of internal flow runners also improves the flow pattern and controls knit line locations resulting from multiple gates. Possibility of different wall thickness: With GAIM different wall thickness are possible if a gas flow channel is designed into the part at the transition point as shown in the fig. (4)

Fig. (4)

Gas channel

Fig. (5) Gas channel

By- Bhupendra Singh

Gas Assisted Injection Molding

The gas channel permits uniform material flow and avoids the high stresses resulting from the design geometry. The gas channel can be used to form tubular sections particularly at the edge of the part as shown in the fig. (5) GAIM design flexibility allows the elimination of core pullers in the mold.

Disadvantages of GAIM GAIM exhibits minor limitations as compared with the significant benefits. Vent holes: The gas channel is vented prior to opening of the mold. This leaves a hole on the appearance or function or secondary operations such as chrome plating. Hence it may be necessary to seal or finish the hole. Surface blush: The gas channel may cause a surface blush, which arises from differences in surface gloss level. This can be partially masked by a surface grain, or by placing the channel behind a surface feature such as groove or character line. The tendency to blush is a function of processing conditions and type of plastic. Multicavity molds Precise short shot control is extremely difficult to achieve as the gas can not push material from one cavity to another. Mold temperature control. Consistent wall thickness requires good temperature control. Mold temperature is significant, and the plumbing must be designed to avoid irregular wall formation arising from mold temperature gradient. Blisters Failure to vent the high pressure gas results in a blister. Discontinuous bubbles in the part leads to blisters, also in loss of process control due to the molds not filling out as required. Design problems Conversion of a conventional machine into GAIM is simpler, but conversion of a mold is time consuming and the modified injection molding tool may not yield an optimum process or design. Large hollow sections GAIM is not a substitution for extrusion or injection blow molding. GAIM is not well suited for thin walled hollow parts, such as bottle or tank and attempts to make such parts have generally resulted into irregular wall thickness and for gas “blow through.”

Amount of gas to total cavity volume for different parts A reduction of about 5 % in weight is achieved for applications such as shell structure, as shown in fig. (6).

Fig. (6): shell structure

By- Bhupendra Singh

Gas Assisted Injection Molding

In applications such as armrests a reduction of around 40-50 % in weight may be achieved. These parts are easier to process as gas propagates through a clearly defined path, and the parts has no thin walled areas which must remain gas free. These parts exhibits gain in strength to weight ratio of 40% or more as compared to injection molded parts, as shown in fig. (7).

Fig (7) Arm rest Basin wash tub: Using the GAIN system, the shot size reduction by 14% can be achieved as shown in fig. (8).

Fig (8) Wash basin

Applications of GAIM GAIM can be used for wide variety of applications such as: 1. Hollow parts GAIM, achieves a 30 to 40 % weight reduction in hollow areas, hence is a good choice for applications in which some injection molded type details are must. GAIM has ability to combine a hollow part with a flat piece. 2. Thin wall structural parts In such applications the GAIM technique can be used to make parts having the cost of thin wall and the strength of thick wall. The hollow tube within the molded article gives extremely high strength to weight ratios. 3. Thick wall structural parts In this class of parts the foam is replaced with an interconnected web of hollow sections. The gas webs provide an internal cushion the way a foam does. As gas assist parts have the ability to obtain stiffness from oversized ribs, increasing wall thickness takes away the lower weight benefit inherent in thin wall gas assist.(5) The following list puts emphasis on some of the applications of GAIM.

By- Bhupendra Singh

Gas Assisted Injection Molding

• •

•

GAIM is used for making tub and rod like parts which saves the material and reduces the cycle time. Examples include cloth hangers, grab handles, chair armrests, shower heads and water faucet spouts. GAIM process is used in applications such as automotive panels, business machine housings, satellite dishes and outdoor furniture. In such type of applications the process of GAIM is used for reducing part warpage and clamp tonnage. It also enhances the rigidity and surface quality of the product. In applications of complex parts having both thick and thin sections, the GAIM process reduces the cost associated with the manufacturing cost by consolidating several assembled parts into one single design. Examples for such applications include television cabinates, computer printer housing and automotive parts.

New variants of GAIN Partial frame process technique In these techniques, the design is such that it avoids the formation of long void channels as with conventional gas process. A tiny shot of compressed air is injected into semi molten plastic in one or more parts of the mold, where stress is likely to build up and create sink marks. This shot of air expands in the direction of slowest cooling. As it expands it absorbs most of the stresses. The typical final diameter of a void nucleus is 1 to 2 mm. Voids form in a part when a certain amount of stress builds up. By injection of a void nucleus, voids grow sooner, when only a fraction of the stress is developed. By minimizing tensile build up of stress, there is a less chance large voids will occur and create sink marks. And there is less internal stress. PFP technology also allows a weight reduction of several percent. It also permits molding at reduced pressure.(6) A schematic diagram for void formation in partial frame process is as shown in the fig. (9)

By- Bhupendra Singh

Gas Assisted Injection Molding

Molten resin Solid resin Void

Pin injects air, or void, in molten part

void

Void expands reducing stress, preventing sinks

Fig: (9) Void formation in partial frame process Finish of the parts molded with PFP techniques is as good as one molded with a gas assist process.

CO2 process CO2 is injected into the mold as well as the melt. End result is superior surface replication. Possible applications for molding technology for CO2 include thin wall portable information terminals and light guide panels, lenses and optical disks. The Amotec process developed by Asahi molding technology entails injection of gas into the injection cylinder and the mold. 2 to 3 wt. % of CO 2 is injected into the cylinder. It acts as a plasticizer, enhancing flowability of the melt. The net effect is a layer of resin with CO2 dissolved throughout which has a Tg significantly lower than the base resin or compounded. Test with Amotec using PS, exhibited melt flow rate improvement by a factor of 5 to 10. This allowed thin walling or the option of using processing temperatures 30 to 50ºC lower than normal.

By- Bhupendra Singh

Gas Assisted Injection Molding

80 % of the CO2 disappears from a 2 mm thick PS part within one day at room temperature, while after five days no gas is detectable. The resulting part is transparent and void free. With acrylics heating is required to purge all the gas from the part, but the process can be used in optical applications.(7)

Vibrated Gas Assisted Injection Molding In this process the gas is inserted in the mold prior to melt injection and vibrated from the subsonic (1 to 30 Hz) to the ultrasonic (15 to 20 KHz) range. The gas induces resonance and manipulates the melt as it enters and fills the mold. The vibrations are generated by electrical or mechanical transducers. A chemically inert gas, such as nitrogen is generally used in the process. This technology can improve one or more mechanical properties by altering the morphology of the plastics. It also improves surface finish, reduces internal stresses and warpage and cut down on sink marks and voids. Pressurized vibrated gas when used inside the mold can help fill and pack the mold, core out hollow parts, and help balance flow in multicavity molds.(8)

External gas molding (EGM) EGM involves the application of gas pressure to one surface of the plastic in order to improve the opposite surface quality. Improved designs and reduced processor control costs are contributing to reduced prices for the moulder. This process is used for applications such as mobile phone casing, handheld and laptop computers, cameras and sound recorders.

Conclusions GAIM is gaining upper hand over the conventional injection molding as a result of higher quality products at less costs. The equipment suppliers are coming up with new designs, techniques and innovative process control systems. GAIM is offering several benefits which includes cost benefits, reduced cycle time, enhanced mechanical properties, quality improvement and several design options. It is getting popular and is being used in place of conventional injection molding. The applications for GAIM will accelerate with new designs and techniques.

References 1. 2. 3. 4. 5. 6. 7. 8.

S. shah, J. of Injection Molding Technology, Sept.2001 L. S. Turng, J. of Injection Molding Technology, Dec. 2000 Rusch K., Plastic Engineering, 1989, 45 (7) 35-38 Moore S., Modern Plastics International, Aug. 1994, 24,50 Ham S. , Plastic Engineering, 2001, 57 (6), 52 Moore S., Modern Plastics International, Nov. 1992, 22,29 Moore S., Modern Plastics International, 2001, 31(3),62 Ibar J. P., Plastic Engineering, 1999, 55 (9) 51

By- Bhupendra Singh