Injection Molding - Introduction

Overview • • • • • • •

Injection Molding ver 1

ENG 4793: Composite Materials and Processes

Equipment Process Flow in barrel Flow in cavity Clamp force Ejection force Design rules

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Equipment Clamp

Mold

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ENG 4793: Composite Materials and Processes

Equipment

Barrel Hopper

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ENG 4793: Composite Materials and Processes

Equipment

Cross-section pellets clamp

nozzle

hopper

barrel

throat

mold cavity

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screw

heaters

ENG 4793: Composite Materials and Processes

motor / drive

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1

Mold

Mold

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ENG 4793: Composite Materials and Processes

Mold

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Process

runner

sprue

cavity

gate

ENG 4793: Composite Materials and Processes

sprue

nozzle

front

ejection pin

side

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Process

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Process

• Pellets placed in hopper • Pellets fall into barrel through throat • Pellets packed to form solid bed

• Melted plastic forms shot in front of screw – screw moves back as plastic moves forward (reciprocating screw)

– air forced out through hopper

• Pellets melted by mechanical shear between barrel and screw

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ENG 4793: Composite Materials and Processes

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Process

Process

• Screw moves forward to inject plastic into mold cavity

• Mold opens • Ejection pins move forward to eject part

• Part cools and solidifies

• Mold closes • Process starts again

– next shot is made

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ENG 4793: Composite Materials and Processes

Solid bed

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Melting zone

• Pellets crushed together • Air ejected

• Solid bed melted by mechanical shear barrel

secondary mixing flow ENG 4793: Composite Materials and Processes

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Homogenization zone

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Injection pressure

• Melted polymer is homogenized thermally and color-wise.

ENG 4793: Composite Materials and Processes

screw flight solid bed

• Typically 15,000 psi • Ranges from 3,000 to 40,000 psi • Hydraulic pressure is about 10x less

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3

Process time

Cycle time

Close mold Injection

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Pack and hold Part cooling

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Cooling 50

25

Pack

Gate freeze off

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Injection

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Fast closing

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Part ejection

Mold closed time Cavity pressure (MPa)

Open and eject part

Pressure history

0 0

5

15

33 35

Time (s) 19

ENG 4793: Composite Materials and Processes

ENG 4793: Composite Materials and Processes

Temperature history

20

Gates and freeze-off

Cooling Part ejection

Pack

gate

Gate freeze off

100

Injection

part Fast closing

Temperature (oC)

200

0 0

5

15

33 35

Time (s) ENG 4793: Composite Materials and Processes

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ENG 4793: Composite Materials and Processes

Minimum cooling time (tc) h2 4 æT −T tc = ln ç M W απ 2 π çè TE − TW

Flow in screw

ö ÷ ÷ ø

• Understood through simple fluid analysis • Unroll barrel from screw

Example: – – – – – –

α = thermal diffusivity ~ 10-7 m2/s h = plate thickness ~ 3 x 10-3 m TW = mold wall temperature ~ 50oC TM = melt temperature ~ 250oC TE = ejection temperature ~ 100oC Minimum cooling time for the center line to reach TE is tc~ 23 sec. ENG 4793: Composite Materials and Processes

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– rectangular trough and lid v=πDN

θ vz

vx B 23

w ENG 4793: Composite Materials and Processes

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Flow analysis

Flow rate

• Barrel slides across channel at the helix angle • vz = pumping • vx = stirring v=πDN

flow rate = f(exit pressure, vbarrel, µ, d, w, l)

θ vz

vx B

• vz shows viscous traction work against exit pressure

w

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ENG 4793: Composite Materials and Processes

Flow analysis

Drag flow (QD) v = vo * y/B QD = vo/2 * wB

• Simplify by using Newtonian fluid • Separate into drag and pressure flows

vo

• Add solutions (superposition)

ENG 4793: Composite Materials and Processes

y

B

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ENG 4793: Composite Materials and Processes

Pressure flow

Pressure flow

τ

( p − ( p + dp ))× 2 y − 2τdz = 0

y 2y

p + dp

p

z

τ = −y

τ dz

dp dz

Newtonian fluid

Equilibrium (p-(p+dp)) * 2y - 2τdz = 0 ENG 4793: Composite Materials and Processes

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τ =µ

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dv dy ENG 4793: Composite Materials and Processes

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Pressure flow

Total pressure flow (Qp)

• Eliminating τ

B

− 1 dp ydy dv = µ dz • Integrating and noting

wB 3 dp Q p = w ò vdy = 12 µ dz −B 2

2

@ y = +/- B/2, v = 0

v=

1 dp é B 2 y 2 ù − ú µ dz êë 8 2û 31

ENG 4793: Composite Materials and Processes

ENG 4793: Composite Materials and Processes

Total flow (Q) é vz B

Q = QD − Q p = wê ë

2

f(screw speed)

−

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Flow rate

B 3 dp ù ú 12µ dz û

2ω

flow rate

ω

f(pressure drop) output pressure

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ENG 4793: Composite Materials and Processes

Flow in mold runner (round)

z

dr dz

p + dp

π [(r + dr )2 − r 2 ] dp = 2π [(r + dr )(τ + dτ ) − rτ ]dz Neglecting HOT

p

2πdrdp = 2π (τdr + rdτ )dz

τ

dp τdr + rdτ = dz rdr

Equilibrium

π [(r + dr )2 − r 2 ] dp = 2π [(r + dr )(τ + dτ ) − rτ ]dz ENG 4793: Composite Materials and Processes

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Flow in round runner

τ + dτ r

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Flow in round runner

Flow in round runner

τdr + rdτ = d (τr )

• At center, τ = 0 • At edge of tube (R), τ = max

dτ ∆p τ = − dr 2 L r

τ=

τ max =

∆p r 2L

Newtonian fluid

τ =µ 37

ENG 4793: Composite Materials and Processes

dv dy

ENG 4793: Composite Materials and Processes

Flow in round runner

γ& =

∆pR 2L

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Flow in round runner

du ∆pr = dr 2 Lµ

u=

∆p 2 r − R2 4 µL

(

)

finally

∆p 2 r − R2 u= 4 µL

(

π∆pR 4 Q = ò 2πrudr = 8µL 0 R

) 39

ENG 4793: Composite Materials and Processes

log η

• Typically 50 tons/oz of injected material • Can be approximated by

log ηo n-1

1

– injection pressure x projected area of part at parting line.

log γ&

m, n are consistency and power law index

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Clamp force

Power law viscosity

η (γ& ) = mγ& n−1

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Ejection force

Thin-walled cylinder with closed ends

• Ejection pins force the part out of the mold after the part has cooled and solidified enough. • The part will shrink onto any cores, leading to an interference fit. • Model as a thin walled cylinder with closed ends (plastic part) on a rigid core (metal mold). ENG 4793: Composite Materials and Processes

σt = σa =

pd = σ1 2t pd =σ2 4t

σr = 0 =σ3

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ENG 4793: Composite Materials and Processes

Biaxial strain ε1 =

σ 1 νσ 2 pd pd − = − ν E E 2tE 4tE

ε1 =

pæ d d ö − ν÷ ç E è 2t 4t ø

Ejection force p=

Eα∆T d ö æd − ν÷ ç 2 t 4 t ø è

Fejection =

ε 1 = α∆ T

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Fejection = µpA

µEAα∆T d ö æd − ν÷ ç 2 t 4 t ø è ENG 4793: Composite Materials and Processes

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Gate types

Nomenclature • A = area • d = core diameter • E = Young’s modulus • p = pressure • t = part thickness

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• α = thermal expansion coefficient • ∆T = temperature differential • ν = Poisson’s ratio • µ = friction coefficient

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Fountaining

Effects of gate number and location

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Fiber orientation and effects

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Fiber orientation

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random

60%

30%

90%

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Flash

Weldline development

• Over-filling of the mold forces mold open – inadequate clamp force

• This leads to flash around the edges of the part at the parting line

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Injection Molding Evaluation

Design rules •

• • • •

Walls should be uniform in thickness Walls should be thin Appropriate draft angles should be used Adequate ejection pin area should be used • Simplify molds • Design parting line to get part out • Put weld lines where they don’t matter

Investment

•

– High Capital Equipment – High Tooling

•

Materials – – – –

•

Mostly TP, some TS Mostly short fibers • Mostly < 30 vol % fibers Fibers < 1/2 inch, mostly 1/8 inch

Processing – Very fast (usually < 1 minute) – Low labor – Hazards remote – High pressure ( 2 - 20 ksi injection)

Quality – – – –

Modest properties Defects low Consistent parts Smooth surfaces (both sides)

Products – – – –

High volume Small projected areas Complex contours Low cost

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ENG 4793: Composite Materials and Processes

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ENG 4793: Composite Materials and Processes

Summary • • • • • • •

Equipment Process Flow in barrel Flow in cavity Clamp force Ejection force Design rules

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