Injection Molding

Injection Molding
2.810
T. Gutowski

D. Roylance

Short history of plastics
1862
1866
1891
1907
1913
1926
1933
1938
1939
1957
1967

first synthetic plastic
Celluloid
Rayon
Bakelite
Cellophane
PVC
Polyethylene
Teflon
Nylon stockings
velcro
“The Graduate”

Ref Kalpakjian and Schmid

McCrum, Buckley, Buckknall

Outline
Basic operation
Cycle time and heat transfer
Flow and solidification
Part design
Tooling
New developments
Environment

30 ton, 1.5 oz (45 cm3) Engel

Injection Molding Machine
for wheel fabrication

Process & machine schematics
*

*

Schematic of thermoplastic Injection molding machine
* Source: http://www.idsa-mp.org/proc/plastic/injection/injection_process.htm

Process Operation
Temperature: barrel zones, tool, die zone
Pressures: injection max, hold
Times: injection, hold, tool opening
Shot size: screw travel
Processing window
Temp.

Thermal
degradation
Flash
Shortshot

Melt

Pressure

Typical pressure/temperature cycle
*

*

Time(sec)

Time(sec)

Cooling time generally dominates cycle time

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

2

half thickness 




  10 3 cm 2 sec for polymers

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

1-dimensional heat conduction equation :
qx

qx + qx

Fourier’s law

Boundary Conditions:


q
(  c T )xy   x xy
t
x
T
q x  k
x
T
 2T
T
 2T
c
k 2
or
 2
t
x
t
x
1st kind
2nd kind
3rd kind

T ( x  x' )  constant
T
k
( x  x' )  constant
x
T
k
( x  x' )  h (T  T )
x

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

Heat transfer
Tii
t
TW

-L

x

+L

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

T  TW
x
 t
;    1; FO  2
Non-dimensionalize:  
Ti  TW
L
L

Dimensionless equation:
Initial condition
Boundary condition

Separation of variables ;
matching B.C.; matching I.C.


 2
 2
FO 
FO  0

 1

 0
 2

 0
 0

 ( , FO )   f ( FO ) g ( )

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

See Heat Transfer Text
By Lienhard on line

Bi-1 =k/hL

Reynolds Number
Reynolds Number:

V2

inertia
VL
L
Re 

V

 2 viscous
L

For typical injection molding

  1g cm 3  10 3 N m 4 s 2 ; LZ  10 3 m thickness
1

Part length 10
V

;
Fill time
1s
For Die casting

  10 3 N  s m 2

3 103 101 103
Re 
 300
3
10

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

Re  104

Viscous Shearing of Fluids
F

v

F/A



h

v
 
h

1

F v

A h

v/h

Newtonian Viscosity

Generalization:

  


  ( )


 : shear rate
Injection molding

“Shear Thinning”
~ 1 sec-1 for PE



Typical shear rate for
Polymer processes (sec)-1

Extrusion
Calendering
Injection molding
Comp. Molding

102~103
10~102
103~104
1~10

Viscous Heating
P
F v F v
 v

  :  
 h
Vol Vol A h

Rate of Heating
= Rate of Viscous Work
Rate of Temperature rise

Rate of Conduction out

  cp

dT
v
  
dt
h

2

or

2

dT
 v

 
dt   c p  h 

2

dT
k d 2T
k T

~
dt   c p dx 2   c p h 2

Viscous heating v 2

Conduction
kT

Brinkman number

For injection molding, order of magnitude ~ 0.1 to 10

Non-Isothermal Flow
Flow rate: 1/t ~V/Lx

v

Péclet N o.

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

Flow rate
V  L2z
1 VLz Lz
~


Heat xfer rate 4  Lx 4  Lx

Small value
=> Short shot

For injection molding

Flow rate
1 10cm / s  0.1cm 0.1cm
~

 2.5
3
2
Heat xfer rate 4 10 cm / s
10cm
For Die casting of aluminum

Flow rate
1 10cm / s  0.1cm 0.1cm
2
~


10
Heat xfer rate 4 0.3cm 2 / s
10cm
* Very small, therefore it requires thick runners

Non-Isothermal Flow
Flow rate: 1/t ~V/Lx

v

Péclet N o.

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

Flow rate
V  L2z
1 VLz Lz
~


Heat xfer rate 4  Lx 4  Lx

Small value
=> Short shot

For injection molding

Flow rate
1 10cm / s  0.1cm 0.1cm
~

 2.5
3
2
Heat xfer rate 4 10 cm / s
10cm
For Die casting of aluminum

Flow rate
1 10cm / s  0.1cm 0.1cm
2
~


10
Heat xfer rate 4 0.3cm 2 / s
10cm
Very small value for aluminum requires thicker runners

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
sample

Transverse direction

V=3.5cm/s

V=8cm/s

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

Gate Location and Warping
Sprue

2.0

60

2.0

Before shrinkage

1.96

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

60.32

1.976

After shrinkage

Air entrapment
Gate

Center gate: radial flow – severe distortion

Diagonal gate: radial flow – twisting

Edge gate: warp free, air entrapment

End gates: linear flow – minimum warping

Effects of mold temperature and
pressure on shrinkage

LDPE

LDPE

0.030

PP

0.025

Acetal

0.025

0.020

Nylon 6/6

0.020

Shrinkage

Shrinkage

0.030

0.015

0.010

0.015

Acetal
PP with
flow
PP across
flow

0.010
0.005

0.005

Nylon
6/6

PMMA

PMMA
0.000

0.000
100

120

140

160

180

200

Mold Temperature (F)

220

240

6000

10000

8000

14000

12000

18000

16000

Pressure on injection plunger (psi)

Where would you gate this part?

Weld line, Sink mark
Gate

Weld line

Mold Filling

Solidified part

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

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?
Controlled by shrinkage
and warping. Hence,
polymer, fillers, mold
geometry and processing
conditions can all
influence the final
tolerance.
Shrinkage is of order
10-100/1000 for unfilled
and
1-10/1000 for filled across
the thickness

Effects of mold pressure on
shrinkage
LDPE

0.030

Acetal
PP with
flow

0.025

Shrinkage

0.020

0.015

PP across
flow

Nylon
6/6

0.010

0.005

PMMA

0.000
6000

10000
8000

14000
12000

18000
16000

Pressure on injection plunger (psi)

Tooling Basics
Sprue

Nozzle
Core Plate

Cavity Plate

Moulding

Core

Cavity

Cavity

Gate

Runner

Melt Delivery

Basic mould consisting of cavity and core plate

Tooling for a plastic cup
Nozzle

Knob
Runner

Cavity

Part
Stripper plate
Core

Tooling for a plastic cup
Nozzle
Nozzle

Knob
Runner

Cavity

Runner
Part

Cavity

Cavity

Part

Part
Stripper
plate

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.)

Tooling Alternatives

Kalpakjian & Schmid

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
Injection Molded Housing

Cores used in Injection Molding

Cores and
Part Molded in Clear Plastic

Micro injection molding

Micro embossing
Replacing serial processes with parallel processes
at small scales

B. Kim UMass

Environmental issues
System boundaries
Polymer production
Compounding

Machine types
Out gassing & energy during processing

CRADLE

Additives

Naphtha, Oil.
Natural Gas

Ancilliary Raw
Materials

Compounder

Thermoplastic Production
(Boustead)

Polymer
Delivery

Internal Transport

Drying

Extrusion

Pelletizing

Emissions
to air,
water &
land

Building (lights,heating, ect..)
Emissions to
air, water, &
land

Polymer

Delivery

Injection Molder
Energy Production Industry

Internal Transport

Drying

Injection Molding
Emissions to air, water, & land
Scrap
Anciliary Raw
Materials

Emissions
to air,
water &
land

Building (lights,heating, ect..)
Packaging

Note to Reader:
= Focus of this Analysis

FACTORY GATE

1 kg of Injection Molded Polymer

= Also included in the Paper

Service Period
Waste Management

Polymer Production
Largest Player in the Injection Molding LCI
What is a polymer:

How much energy does it take to make 1 kg of polymer = a lot !!!
Sources
Boustead
Ashby
Patel
Kindler/Nickles
[Patel 1999]
Worrell et al.
[Patel 1999]
E3 Handbook
[OIT 1997]
Energieweb

HDPE
76.56
111.50
-------

LLDPE
77.79
-------------

LDPE
73.55
92.00
64.60

PP
72.49
111.50
-------

PVC
58.41
79.50
53.20

PS
86.46
118.00
70.80

PC
115.45
------80.30

PET
77.14
------59.40

-------

-------

71.00

-------

53.00

81.00

107.00

96.00

-------

-------

67.80

-------

52.40

82.70

78.20

131.65

121.18

136.07

126.07

33.24

-------

-------

-------

80.00

-------

68.00

64.00

57.00

84.00

-------

81.00

Values are in MJ per kg of polymer produced. Thiriez ‘06

Compounding - extrusion
An extruder is used to mix additives with a polymer base, to
bestow the polymer with the required characteristics.
Similar to an injection molding machine, but without a mold
and continuous production.
Thus it has a similar energy consumption profile.

Environmentally Unfriendly Additives:
•Fluorinated blowing agents (GHG’s)
•Phalates (some toxic to human
liver, kidney and testicles)

•Organotin stabilizers (toxic and
damage marine wildlife)

Injection Molding Process
Source:
http://cache.husky.ca/pdf/br
ochures/br-hylectric03a.pdf

Machine types: Hydraulic, electric, hydro-electric

All-electrics have very low fixed energy costs (small
idling power). SEC is constant as throughput
increases.

SEC  pv

9
8
7

SEC (MJ/kg)

All-Electric - 85 tons
6
Hydraulic - 85 tons
5
Material: PP

4
3
2
1
0
0

5

10
Throughput (kg/hr)

15

20

Source: [Thiriez]

For Hydraulics and Hybrids as throughput
increases, SEC  k.
Variable Pump Hydraulic Injection Molding Machines.
HP 52
HP 05
HP 06
HP 57
HP 001
Low Enthalpy - Raise Resin to Inj. Temp - PVC
High Enthalpy - Raise Resin to Inj. Temp - HDPE

8
7

SEC (MJ/kg)

6
5
4
3
2
1
0
0

50

100
Throughput (kg/hr)

150

Does not account for the electric grid.

200

Source: [Thiriez]

Enthalpy value to melt plastics is just 0.1 to 0.7 MJ/kg !!!

All-electric vs. hybrid
Ton
Buildup

120

Power Required (kW)

Cool

100

Clamp open-close

Plasticize

Inject high

80

ton

60

Inject low

40
20
0
0

1

2

3

4

5

6
7
8
9
10
11
Time (seconds)
MM 550 Hybrid
NT 440 All-Electric

12

13

14

Source: [Thiriez]

The hydraulic plot would be even higher than the hybrid curve

Driers

Used to dry internal moisture in hygroscopic polymers and external
moisture in non-hygroscopic ones.
It is done before extruding and injection molding.

Specific Power Consumption
(MJ/kg)

1.8
Power Trendline

W300

1.6

2

R = 0.8225

W400

1.4

W200

1.2
1
0.8

W150

0.6
W3200

W1600

W600

0.4

W5000

W800

0.2

W1000

W2400

0
0

500

1000

1500

2000

2500

3000

3500

Throughput (kg/hr)

Source: [Thiriez]

Same as

P0
P E
  SEC 
k
m m
m

LCI Summarized Results
ENERGY CONSUMPTION BY STAGE in MJ/kg of shot

Thermoplastic Production

avg
low
high

HDPE LLDPE LDPE
89.8
79.7
73.1
77.9
79.7
64.6
111.5 79.7
92.0

PP
83.0
64.0
111.5

PVC
59.2
52.4
79.5

PS
87.2
70.8
118.0

Polymer Delivery

Extras
Generic by Amount
PET
Consumed Inj. Molded PC
81.2
74.6
95.7
78.8
69.7
62.8
78.2
59.4
102.7
97.6
117.4
96.0
avg
low
high

0.19
0.12
0.24

Compounder

avg
low
high
Subtotal

Internal
Transport
0.09
--------avg
low
high

Drying
0.70
0.30
1.62

Extrusion
3.57
1.82
5.00

Building (lights,
heating, ect..)
0.99
---------

Pelletizing
0.16
0.06
0.31

5.51
3.25
8.01
Polymer Delivery

avg
low
high

0.19
0.12
0.24

Injection Molder

avg
low
high

Internal
Transport
0.04
---------

avg
low
high
Subtotal

TOTAL w/
Generic Inj.
Molded
Polymer
TOTAL w/o
Polymer Prod
Notes

Drying

Injection Molding
(look below)

Scrap (Granulating)

0.70
0.30
1.62

0.05
0.03
0.12

Building (lights,
heating, ect..)
0.99
---------

Injection Molding - Choose One
Hydraulic
Hybrid
All-Electric
11.29
5.56
4.89
3.99
3.11
1.80
69.79
8.45
15.29

avg
low
high

13.08
5.35
72.57

7.35
4.47
11.22

6.68
3.17
18.06

avg
low
high

Hydraulic
93.60
71.65
178.68

Hybrid
87.87
70.77
117.34

All-Electric
87.20
69.46
124.18

avg
low
high

18.97
8.84
81.04

13.24
7.96
19.70

12.57
6.66
26.54

Drying - the values presented assume no knowledge of the materials' hygroscopia. In order words, they are
averages between hygroscopic and non-hygroscopic values. For hygroscopic materials such as PC and PET
additional drying energy is needed (0.65 MJ/kg in the case of PC and 0.52 MJ/kg in the case of PET)
Pelletizing - in the case of pelletizing an extra 0.3 MJ/kg is needed for PP
Granulating - a scarp rate of 10 % is assumed

Source: [Thiriez]

Energy Production Industry
United States Electricity Composition by Source
Hydro
7.1%

Nuclear
19.6%

Other
0.0%

Coal
50.7%

Oil
3.1%

Gas
16.7%

The Grid is about 30% efficient
For every MJ of electricity we also get:
171.94 g of CO2

0.76 g of SO2
 0.31 g of NOx
 6.24 g of CH4
 0.0032 mg of Hg

Waste/
Renewable
2.2%

Scale
HDPE,
LDPE,
LLDPE,
PP, PS,
PVC

Compounder and Injection
Molder
6 Main Thermoplastics
All Plastics

U.S.
GJ/year
9.34E+07
2.06E+08

Global
GJ/year
4.01E+08
6.68E+08

The Injection Molding Industry in the U.S. consumes 6.19 x
107 GJ of electricity (or 2.06 x 108 GJ in total energy).
This is larger than the entire electric production of some
small countries.

In such a scale imagine what a 0.1 % energy savings mean !!!

Do Polymers get recycled?

Ref Ashby 2009

The printer goes in the hopper…

And comes out….

Readings
Tadmore and Gogos


Molding and Casting pp 584 -610

Boothroyd Dewhurst


Design for Injection Molding pp 319 - 359

Kalpakjian Ch 7 & 19
Thiriez et al, "An Environmental Analysis of Injection
Molding“
"Injection Molding Case Study“ (Gas Assist)