1. 2.810 Fall 2002 Professor Tim Gutowski
Injection Molding
2.810 Fall 2002
Professor Tim Gutowski

2. Short history of plastics
1862 first synthetic plastic
1866 Celluloid
1891 Rayon
1907 Bakelite
1913 Cellophane
1926 PVC
1933 Polyethylene
1938 Teflon
1939 Nylon stockings
1957 velcro
1967 “The Graduate”

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

4. Readings Tadmore and Gogos Boothroyd Dewhurst Kalpakjian see Ch 18
Molding and Casting pp
Boothroyd Dewhurst
Design for Injection Molding pp
Kalpakjian see Ch 18
Injection molding case study;Washing machine augers; see on web page

5. Injection Molding Machine
30 ton, 1.5 oz (45 cm3) Engel
Injection Molding Machine
for wheel fabrication

6. Process & machine schematics* *
Schematic of thermoplastic Injection molding machine
* Source:

7. 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
Short-shot
Melt
Pressure

8. Typical pressure/temperature cycle
Time(sec) * *
Time(sec)
Cooling time generally dominates cycle time
* Source:

9. Calculate clamp force, & shot size
F=P X A = 420 tons
3.8 lbs = 2245 cm3
=75 oz
Actual ; 2 cavity 800 ton

10. Clamp force and machine cost

11. Heat transfer Note; aTool > apolymer
1-dimensional heat conduction equation : qx
qx + Dqx
Fourier’s law
Boundary Conditions:
The boundary condition of 1st kind applies to injection molding since the tool is often maintained at a constant temperature

12. Heat transfer Let Lch = H/2 (half thickness) = L ; tch = L2/a ;TW
Tii t x +L -L
Let Lch = H/2 (half thickness) = L ; tch = L2/a ;
DTch = Ti – TW (initial temp. – wall temp.)
Non-dimensionalize:
Dimensionless equation:
Initial condition
Boundary condition
Separation of variables ;
matching B.C.; matching I.C.

13. Centerline, q = 0.1, Fo = at/L2 = 1
Temperature in a slab
Centerline, q = 0.1, Fo = at/L2 = 1
Bi-1 =k/hL

14. Reynolds Number Reynolds Number: For typical injection molding
For Die casting
* Source:

15. Viscous Shearing of Fluids
F/A
v/h 1 m
Newtonian Viscosity
Generalization:
Typical shear rate for
Polymer processes (sec)-1
Extrusion 102~103
Calendering 10~102
Injection molding 103~104
Comp. Molding 1~10
Injection molding
“Shear Thinning”
~ 1 sec-1 for PE

16. Viscous Heating Rate of Heating = Rate of Viscous Work
Rate of Temperature rise
Rate of Conduction out
Brinkman number
For injection molding, order of magnitude ~ 0.1 to 10

17. Non-Isothermal Flow Flow rate: 1/t ~V/Lx
Heat transfer rate: 1/t ~a/(Lz/2)2
Small value
=> Short shot
For injection molding
For Die casting of aluminum
* Very small, therefore it requires thick runners

18. Injection mold die cast mold

19. Fountain Flow * **
* Source: ; ** Z. Tadmore and C. Gogos, “Principles of Polymer Processing”

20. Shrinkage distributions
Transverse direction
sample
V=3.5cm/s
V=8cm/s
* Source: G. Menges and W. Wubken, “Influence of processing conditions on Molecular Orientation in Injection Molds”

21. Gate Location and Warping
Shrinkage
Direction of flow – in/in
Perpendicular to flow – 0.012
2.0
60
1.96
Sprue
60.32
2.0
1.976
Before shrinkage
After shrinkage
Air entrapment
Gate
Center gate: radial flow – severe distortion
Edge gate: warp free, air entrapment
Diagonal gate: radial flow – twisting
End gates: linear flow – minimum warping

22. 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
Shrinkage
0.030
0.000
0.010
0.005
0.015
0.020
0.025
Shrinkage
6000
8000
10000
12000
14000
16000
Pressure on injection plunger (psi)
Acetal
LDPE
Nylon
6/6
PP with flow
18000
PP across flow
PMMA

23. Where would you gate this part?

24. Weld line, Sink mark Mold Filling Solidified part
Gate
Weld line
Mold Filling
Solidified part
Sink mark
Basic rules in designing ribs to minimize sink marks
* Source:

25. Injection Molding * *
* Source:

26. DLtotal = DLmold + DLshrinkage
Where is injection
molding?
Consider the assembly of airplane parts where dimensions are on the order of 10 – 100 ft, and tolerances are on the order of 10-3 in
This gives ratios like 10-5 and In this case, assembly usually requires a lot of hand work, fitting, shimming, hand drilling, etc.
Some very small components may also have such tight tolerance. In those cases special processes are required such as honing , polishing, lapping
Etc.
DLtotal = DLmold + DLshrinkage

27. Effects of mold temperature and pressure on shrinkage
0.030
0.000
0.010
0.005
0.015
0.020
0.025
Shrinkage
6000
8000
10000
12000
14000
16000
Pressure on injection plunger (psi)
Acetal
LDPE
Nylon
6/6
PP with flow
18000
PP across flow
PMMA

28. Tooling Basics Basic mould consisting of cavity and core plate
Nozzle
Sprue
Cavity Plate
Core Plate
Moulding
Core
Cavity
Basic mould consisting of cavity and core plate
Cavity
Runner
Gate
Melt Delivery

29. Tooling for a plastic cup
Knob
Nozzle
Runner
Cavity
Core
Part
Stripper plate

30. Tooling for a plastic cup
Runner
Part
Cavity
Nozzle
Part
Cavity
Knob
Stripper plate
Runner
Part
Cavity
Nozzle

31. Tooling * * * * * ** *
* Source: ** (E-trainer by HZS Co.,Ltd.)

32. 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 ~ 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

33. New developments- Gas assisted injection molding

34. New developments ; injection molding with cores
Injection Molded Housing shown in class
Cores used in Injection Molding
Cores and Part Molded in Clear Plastic

35. Environmental issues Petroleum and refining Primary processing
Out gassing & energy during processing
End of life

36. Environmental loads by manufacturing sector
EPA 2001, DOE 2001

37. The estimated environmental performance of various mfg processes (not including auxiliary requirements)
*Energy per wt. normalized
by the melt energy
** total raw mat’l normalized
by the part wt.

38. The printer goes in the hopper…

39. And comes out….

40. The problem with plastics is…

41. Or remanufacture….

42. Summary Basic operation Cycle time and heat transfer
Flow and solidification
Part design
Tooling
New developments
Environment