# 2.810 Fall 2002 Professor Tim Gutowski

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

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”

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

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

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

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

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

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

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; 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

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.

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

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

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

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

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

Injection mold die cast mold

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

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”

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

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

Where would you gate this part?

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:

Injection Molding * * * Source:

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

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

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

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

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

Tooling * * * * * ** * * Source: ** (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 ~ 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 shown in class Cores used in Injection Molding Cores and Part Molded in Clear Plastic

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

EPA 2001, DOE 2001

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.

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