Injection Molding

2.810 Fall 2002Professor Tim Gutowski

Short history of plastics

1862 first synthetic plastic1866 Celluloid1891 Rayon1907 Bakelite1913 Cellophane1926 PVC1933 Polyethylene1938 Teflon1939 Nylon stockings1957 velcro1967 “The Graduate”

Outline

Basic operation

Cycle time and heat transfer

Flow and solidification

Part design

Tooling

New developments

Environment

Readings

Tadmore and Gogos

Molding and Casting pp584 -610

Boothroyd Dewhurst

Design for Injection Molding pp 319 - 359

Kalpakjian see Ch 18

Injection molding case study;Washing machine

augers; see on web page

30 ton, 1.5 oz (45 cm3) Engel

Injection Molding Machinefor wheel fabrication

Process & machine schematics

*

* Source: http://www.idsa-mp.org/proc/plastic/injection/injection_process.htm

*

Schematic of thermoplastic Injection molding machine

Process OperationTemperature: barrel zones, tool, die zonePressures: injection max, holdTimes: injection, hold, tool openingShot size: screw travel

Flash

Melt

Thermaldegradation

Short-shot

Temp.

Pressure

Processing window

Typical pressure/temperature cycle

polymersfor sec10

thicknesshalf

33

2

cm

tcool

Time(sec)

Cooling time generally dominates cycle time

Time(sec)

* Source: http://islnotes.cps.msu.edu/trp/inj/inj_time.html

**

Calculate clamp force, & shot size

F=P X A = 420 tons

3.8 lbs = 2245 cm3

=75 oz

Actual ; 2 cavity 800 ton

Clamp force and machine cost

Heat transfer Note; Tool > polymer

yxx

qyxTc

tx

p

)(

)()'( kind 3rd

constant)'( kind 2nd

constant)'( kind1st

TThxxx

Tk

xxx

Tk

xxTBoundary Conditions:

1-dimensional heat conduction equation :

The boundary condition of 1st kind applies to injection molding since the tool is often maintained at a constant temperature

x

Tkqx

qx qx + qx

2

2

2

2

or x

T

t

T

x

Tk

t

Tcp

Fourier’s law

Heat transfer

TW

Tii

t

x+L-L

Let Lch = H/2 (half thickness) = L ; tch = L2/;Tch = Ti – TW (initial temp. – wall temp.)

Non-dimensionalize: 2 ;1 ;

L

tF

L

x

TT

TTO

Wi

W

2

2

OFDimensionless equation:

Initial condition 1 0 OF

Boundary condition

0 2

0 0

Separation of variables ; matching B.C.; matching I.C. )()(),( gFfF OO

Centerline, = 0.1, Fo = t/L2 = 1Temperature in a slab

Bi-1 =k/hL

Reynolds Number

* Source: http://www.idsa-mp.org/proc/plastic/injection/injection_process.htm

VL

LVL

V

s viscou

inertia Re

2

2

Reynolds Number:

For typical injection molding

231

32433

10 ;1

10

timeFill

lengthPart

ess thickn10 ;101

msNs

V

mLsmNcmg Z

410Re

For Die casting

30010

1010103Re

3

313

Viscous Shearing of Fluidsv

F

h

h

v

A

F

F/A

v/h

1

Newtonian Viscosity

h

v

Generalization:

)(

Injection molding

rateshear :

Typical shear rate for Polymer processes (sec)-1

Extrusion102~103

Calendering 10~102

Injection molding103~104

Comp. Molding 1~10

“Shear Thinning”

~ 1 sec-1 for PE

Viscous HeatingRate of Heating = Rate of Viscous Work

2

h

v

h

v

A

F

Vol

vF

Vol

P

Rate of Temperature rise 22

or

h

v

cdt

dT

h

v

dt

dTc

pp

Rate of Conduction out22

2

~h

T

c

k

dx

Td

c

k

dt

dT

pp

Tk

v

2

Conduction

heating Viscous Brinkman number

For injection molding, order of magnitude ~ 0.1 to 10

Non-Isothermal Flow

vFlow rate: 1/t ~V/Lx

Heat transfer rate: 1/t ~a/(Lz/2)2

x

zz

x

z

L

LVL

L

LV

4

1

4~

rateHeat xfer

rate Flow 2

For injection molding

5.210

1.0

/10

1.0/10

4

1~

rateHeat xfer

rate Flow23

cm

cm

scm

cmscm

For Die casting of aluminum

22

1010

1.0

/3.0

1.0/10

4

1~

rateHeat xfer

rate Flow

cm

cm

scm

cmscm

* Very small, therefore it requires thick runners

Small value=> Short shot

Injection mold die cast mold

Fountain Flow

* Source: http://islnotes.cps.msu.edu/trp/inj/flw_froz.html ; ** Z. Tadmore and C. Gogos, “Principles of Polymer Processing”

*

**

Shrinkage distributions

* Source: G. Menges and W. Wubken, “Influence of processing conditions on Molecular Orientation in Injection Molds”

V=3.5cm/s

V=8cm/s

sample Transverse direction

Gate Location and Warping

Center gate: radial flow – severe distortion

Diagonal gate: radial flow – twisting End gates: linear flow – minimum warping

Gate

Air entrapment

Edge gate: warp free, air entrapment

Sprue

2.0

2.0 60

Before shrinkage

60.321.96

1.976

After shrinkage

ShrinkageDirection of flow – 0.020 in/inPerpendicular to flow – 0.012

Effects of mold temperature and pressure on shrinkage

0.030

0.000

0.010

0.005

0.015

0.020

0.025

100 120 140 160 180 200 220 240

Mold Temperature (F)

LDPE PP

Nylon 6/6

PMMA

Acetal

Sh

rin

kag

e

0.030

0.000

0.010

0.005

0.015

0.020

0.025

Sh

rin

kag

e6000

800010000

1200014000

16000

Pressure on injection plunger (psi)

AcetalLDPE

Nylon 6/6

PP with flow

18000

PP across

flow

PMMA

Where would you gate this part?

Weld line, Sink mark

* Source: http://www.idsa-mp.org/proc/plastic/injection/injection_design_7.htm

Weld line

Mold Filling

Gate

Solidified part

Sink markBasic rules in designing ribs to minimize sink marks

Injection Molding*

*

* Source: http://www.idsa-mp.org/proc/plastic/injection/injection_design_2.htm

Where is injection molding?

Ltotal = Lmold + Lshrinkage

Effects of mold temperature and pressure on shrinkage

0.030

0.000

0.010

0.005

0.015

0.020

0.025

Sh

rin

kag

e

6000

8000

10000

12000

14000

16000

Pressure on injection plunger (psi)

Acetal

LDPE

Nylon 6/6

PP with flow

18000

PP across flow

PMMA

Tooling Basics

Cavity Plate

Cavity

MouldingCore

Core Plate

Basic mould consisting of cavity and core plate

Runner

Cavity

Gate

NozzleSprue

Melt Delivery

Part

Cavity

Core

Stripper plate

Tooling for a plastic cup

Runner

Knob

Nozzle

Tooling for a plastic cup

Runner

Part

Cavity

Nozzle

Part

Cavity

Knob

Stripper plate

Runner

Part

Cavity

Nozzle

Tooling

* Source: http://www.idsa-mp.org/proc/plastic/injection/; ** http://www.hzs.co.jp/english/products/e_trainer/mold/basic/basic.htm (E-trainer by HZS Co.,Ltd.)

**

*

*

*

* **

Part design rules Simple shapes to reduce tooling cost

No undercuts, etc.

Draft angle to remove part In some cases, small angles (1/4) will do Problem for gears

Even wall thickness Minimum wall thickness ~ 0.025 in Avoid sharp corners Hide weld lines

Holes may be molded 2/3 of the way through the wall only, with final drilling to eliminate weld lines

New developments- Gas assisted injection molding

New developments ; injection molding with cores

Cores and Part Molded in Clear Plastic

Cores used in Injection Molding

Injection Molded Housing shown in class

Environmental issues

Petroleum and refining

Primary processing

Out gassing & energy during

processing

End of life

Environmental loads by manufacturing sector

Carbon Dioxide and Toxic Materials per Value of Shipments

00.5

11.5

22.5

33.5

44.5

5

Che

mic

als

Pet

role

uman

d C

oal

Pla

stic

s an

dR

ubbe

r

Prim

ary

Met

al

Fab

ricat

edM

etal

Mac

hine

ry

Ele

ctro

nic

Tra

nspo

rtat

ion

Manufacturing industries

Wei

ght/D

olla

rs

CO2 (metric ton/$10,000)

Toxic Mat'ls (lb/$1000)

EPA 2001, DOE 2001

The estimated environmental performance of various mfg processes (not including auxiliary

requirements)

*Energy per wt. normalizedby the melt energy

** total raw mat’l normalizedby the part wt.

The printer goes in the hopper…

And comes out….

The problem with plastics is…

Or remanufacture….

Summary

Basic operation

Cycle time and heat transfer

Flow and solidification

Part design

Tooling

New developments

Environment