Injection Molding

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Injection Molding

• 2.810 Fall 2002
• Professor Tim Gutowski

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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

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Outline

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

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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 studyWashing machine
  augers see on web page

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30 ton, 1.5 oz (45 cm3) Engel
Injection Molding Machine for wheel fabrication
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Process machine schematics

Source http//www.idsa-mp.org/proc/plastic/inje
ction/injection_process.htm
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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
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Typical pressure/temperature cycle


Cooling time generally dominates cycle time
Source http//islnotes.cps.msu.edu/trp/inj/inj_
time.html
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Calculate clamp force, shot size
FP X A 420 tons
3.8 lbs 2245 cm3 75 oz
Actual 2 cavity 800 ton
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Clamp force and machine cost
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Heat transfer Note aTool gt apolymer
1-dimensional heat conduction equation
qx
qx Dqx
Fouriers law
Boundary Conditions
The boundary condition of 1st kind applies to
injection molding since the tool is often
maintained at a constant temperature
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Heat transfer
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.
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Temperature in a slab
Centerline, q 0.1, Fo at/L2 1
Bi-1 k/hL
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Reynolds Number
Reynolds Number
For typical injection molding
For Die casting
Source http//www.idsa-mp.org/proc/plastic/inje
ction/injection_process.htm
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Viscous Shearing of Fluids
Newtonian Viscosity
Generalization
Typical shear rate for Polymer processes
(sec)-1 Extrusion 102103 Calendering 10102 Inje
ction molding 103104 Comp. Molding 110
Injection molding
Shear Thinning
1 sec-1 for PE
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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
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Non-Isothermal Flow
Flow rate 1/t V/Lx
v
Heat transfer rate 1/t a/(Lz/2)2
Small value gt Short shot
For injection molding
For Die casting of aluminum
Very small, therefore it requires thick runners
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Injection mold die cast mold
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Fountain Flow


Source http//islnotes.cps.msu.edu/trp/inj/flw_
froz.html Z. Tadmore and C. Gogos,
Principles of Polymer Processing
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Shrinkage distributions
Transverse direction
sample
V3.5cm/s
V8cm/s
Source G. Menges and W. Wubken, Influence of
processing conditions on Molecular Orientation in
Injection Molds
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Gate Location and Warping
Shrinkage Direction of flow 0.020
in/in Perpendicular to flow 0.012
2.0
60?
1.96
Sprue
60.32?
1.976
2.0
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
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Effects of mold temperature and pressure on
shrinkage
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Where would you gate this part?
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Weld line, Sink mark
Gate
Weld line
Mold Filling
Solidified part
Basic rules in designing ribs to minimize sink
marks
Source http//www.idsa-mp.org/proc/plastic/inje
ction/injection_design_7.htm
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Injection Molding
Source http//www.idsa-mp.org/proc/plastic/inje
ction/injection_design_2.htm
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Where is injection molding?
DLtotal DLmold DLshrinkage
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Effects of mold temperature and pressure on
shrinkage
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Tooling Basics
Core Plate
Cavity Plate
Moulding
Core
Cavity
Basic mould consisting of cavity and core plate
Cavity
Runner
Gate
Melt Delivery
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Tooling for a plastic cup
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Tooling for a plastic cup
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Tooling







Source http//www.idsa-mp.org/proc/plastic/inje
ction/ http//www.hzs.co.jp/english/products/e
_trainer/mold/basic/basic.htm (E-trainer by HZS
Co.,Ltd.)
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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

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New developments- Gas assisted injection molding
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New developments injection molding with cores
Injection Molded Housing shown in class
Cores used in Injection Molding
Cores and Part Molded in Clear Plastic
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Environmental issues

• Petroleum and refining
• Primary processing
• Out gassing energy during processing
• End of life

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Environmental loads by manufacturing sector
EPA 2001, DOE 2001
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The estimated environmental performance of
various mfg processes (not including auxiliary
requirements)
Energy per wt. normalized by the melt energy
total raw matl normalized by the part wt.
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The printer goes in the hopper
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And comes out.
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The problem with plastics is
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Or remanufacture.
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Summary

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