Injection Molding
2.810 Fall 2004 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
Readings
Tadmore and Gogos
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Molding and Casting pp584 -610
Boothroyd Dewhurst
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Design for Injection Molding pp 319 - 359
Kalpakjian see Ch 18 Injection molding case study;Washing machine augers; see on web page
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
* Source: http://islnotes.cps.msu.edu/trp/inj/inj_time.html
thickness ) tcool = 10 3 cm3 sec for polymers
2
(half =
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
qx qx + qx
Note; Tool > polymer
1-dimensional heat conduction equation :
Fouriers law
q ( c p T ) xy = x xy t x T q x = k x T 2T T 2T cp = k 2 or = 2 t x t x
1st kind 2nd kind 3rd kind T ( x = x' ) = constant T ( x = x' ) = constant x T k ( x = x' ) = h (T T ) x k
Boundary Conditions:
The boundary condition of 1st kind applies to injection molding since the tool is often maintained at a constant temperature
Heat transfer
T ii t TW x
Let Lch = H/2 (half thickness) = L ; tch = L2/ ; Tch = Ti TW (initial temp. wall temp.) Non-dimensionalize: =
-L
+L
T TW x t ; = + 1; FO = 2 Ti TW L L
Dimensionless equation: Initial condition Boundary condition
2 = 2 FO
FO = 0
=0 =2
=1
=0 =0
Separation of variables ; matching B.C.; matching I.C.
( , FO ) = f ( FO ) g ( )
Temperature in a slab Centerline, = 0.1, Fo = t/L2 = 1
Bi-1 =k/hL
Reynolds Number
Reynolds Number:
V2 inertia VL L Re = = V 2 viscous L
For typical injection molding
= 1 g cm3 = 10 3 N m 4 s 2 ; LZ = 103 m V Part length 10 = ; Fill time 1s
1
thickness
= 103 N s m 2
Re = 10 4
For Die casting
3 10 3 10 1 10 3 Re = 300 3 10
* Source: http://www.idsa-mp.org/proc/plastic/injection/injection_process.htm
Viscous Shearing of Fluids
F h 1 v
F/A
F v A h
Generalization:
= (& ) &
v = h
v/h
Newtonian Viscosity
= &
& : shear rate
Injection molding
Typical shear rate for Polymer processes (sec)-1 Extrusion Calendering Injection molding Comp. Molding 102~103 10~102 103~104 1~10
Shear Thinning ~ 1 sec-1 for PE
&
Viscous Heating
Rate of Heating = Rate of Viscous Work Rate of Temperature rise
P F v F v v = = = Vol Vol A h h
cp
dT v = dt h
or
dT v = dt c p h
Rate of Conduction out
dT k d 2T k T = ~ dt c p dx2 c p h2
Viscous heating v 2 = Conduction kT
Brinkman number
For injection molding, order of magnitude ~ 0.1 to 10
Non-Isothermal Flow
v
Flow rate: 1/t ~V/Lx Heat transfer rate: 1/t ~a/(Lz/2)2
Flow rate V L2 1 VL z Lz z ~ = Heat xfer rate 4 Lx 4 Lx
For injection molding
Small value => Short shot
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 0.3cm2 / s 10cm
* Very small, therefore it requires thick 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 1.96 60.32
Shrinkage Direction of flow 0.020 in/in Perpendicular to flow 0.012
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 0.025 0.030 0.025 0.020 0.015 0.010 0.005
LDPE
PP Acetal Shrinkage Nylon 6/6
LDPE Acetal PP with flow PP across flow
Shrinkage
0.020
0.015 0.010 0.005
Nylon 6/6
PMMA
0.000 100 120 140 160 180 200 220 240 0.000 6000
PMMA
8000 10000 12000 14000 16000 18000
Mold Temperature (F)
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?
Ltotal = Lmold + Lshrinkage
Effects of mold temperature and pressure on shrinkage
0.030
LDPE Acetal PP with flow
0.025
0.020
Shrinkage
0.015
PP across flow
Nylon 6/6
0.010
0.005
PMMA
6000 8000 10000 12000 14000 16000 18000
0.000
Pressure on injection plunger (psi)
Tooling Basics
Sprue Nozzle Cavity Plate Core Plate
Moulding Cavity
Core
Cavity
Basic mould consisting of cavity and core plate
Gate
Runner
Melt Delivery
Tooling for a plastic cup
Nozzle
Knob Runner Cavity
Part Stripper plate Core
Tooling for a plastic cup
Nozzle Nozzle Runner Cavity Runner Part Cavity Cavity Knob
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.)
Part design rules
Simple shapes to reduce tooling cost
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No undercuts, etc. In some cases, small angles (1/4) will do Problem for gears
Draft angle to remove part
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Even wall thickness Minimum wall thickness ~ 0.025 in Avoid sharp corners Hide weld lines
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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
Environmental loads by manufacturing sector
Carbon Dioxide and Toxic Materials per Value of Shipments
5 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0 Plastics and Rubber Petroleum and Coal Chemicals
Weight/Dollars
CO2 (metric ton/$10,000)
Toxic Mat'ls (lb/$1000)
Primary Metal
Manufacturing industries
Transportation
Fabricated Metal
Machinery
Electronic
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 matl 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
