Injection molding

Basic operation Cycle time and heat transfer Flow and solidification Part design Tooling New developments Environment... L V~ rate Heat xfer rate For injection molding 5... Gate Locatio

Injection Molding

2.810 Fall 2002

Professor Tim Gutowski

Short history of plastics

1862 first synthetic plastic

Basic operation

Cycle time and heat transfer

Flow and solidification

Part design

Tooling

New developments

Environment

Tadmore and Gogos

 Molding and Casting pp584 -610

30 ton, 1.5 oz (45 cm3) Engel

Injection Molding Machine for wheel fabrication

Process & machine schematics

Process Operation

Temperature: barrel zones, tool, die zone

Pressures: injection max, hold

Times: injection, hold, tool opening

Shot size: screw travel

Flash

Melt

Thermal degradation

shot Temp.

Short-Pressure

Processing window

Typical pressure/temperature cycle

polymers for

sec 10

thickness half

3 3

Calculate clamp force, & shot size

Clamp force and machine cost

Heat transfer Note; αTool > αpolymer

y

x x

q y

x T

c t

)'(

kind

3rd

constant)

'(

kind

2nd

constant)

'(

kind1st

x x

T k

x

x x

T k

x x T

Boundary 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

T k

T x

T k t

Heat transfer

TW

Tiit

x

+L -L

Let Lch = H/2 (half thickness) = L ; tch = L2/α ;

∆Tch = Ti – TW (initial temp – wall temp.)Non-dimensionalize: ; 1 ; 2

L

t F

L

x T

T

T

T

O W

2

0

θ ξ

Separation of variables ;

matching B.C.; matching I.C θ ( ξ , FO) = ∑ f ( FO) g ( ξ )

Centerline, θ = 0.1, Fo = α t/L2 = 1

Temperature in a slab

Bi -1 =k/hL

Reynolds Number

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

µ

ρ µ

inertia

Re

1

3 2

4 3

3

10

; 1

10 time

Fill

length Part

ess thickn 10

; 10

1

m s

N s

V

m L

s m N cm

10 10

10

3

3 1

Viscous Shearing of Fluids

v F

h

h

v A

v A

F Vol

v

F Vol

dt

dT h

v dt

dT c

p

p

ρ

µ µ

ρ

Rate of Conduction out

2 2

2

~

h

T c

k dx

T

d c

k dt

dT

p p

T k

For injection molding, order of magnitude ~ 0.1 to 10

L V

~ rate Heat xfer

rate

For injection molding

5

2 10

1

0 /

10

1 0 /

10 4

1

~ rate Heat xfer

cm

cm s

0 /

3 0

1 0 /

10 4

1

~ rate Heat xfer

cm

cm s

cm

* 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”

*

**

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

60.32 °

1.96

1.976 After shrinkage

Shrinkage Direction of flow – 0.020 in/in Perpendicular to flow – 0.012

Effects of mold temperature and pressure on shrinkage

0.015 0.020 0.025

Nylon 6/6

PP with flow

18000

PP across flow

PMMA

Where would you gate this part?

Weld line, Sink mark

Basic 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

Nylon 6/6

PP with flow

18000

PP across flow

PMMA

Basic mould consisting of cavity and core plate

Runner

Cavity

Gate

Nozzle Sprue

Melt Delivery

Tooling for a plastic cup

Runner

Part Cavity Nozzle

Part Cavity

Knob

Stripper plate

Runner Part Cavity

Nozzle

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

The estimated environmental performance of various

mfg processes (not including auxiliary requirements)

*Energy per wt normalized

by 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….

Basic operation

Cycle time and heat transfer

Flow and solidification

Part design

Tooling

New developments

Environment