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Injection Molding
2.810 Fall 2002Professor Tim Gutowski
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Short history of plastics
1862 first synthetic plastic1866 Celluloid1891 Rayon1907 Bakelite1913 Cellophane1926 PVC1933 Polyethylene1938 Teflon1939 Nylon stockings1957 velcro1967 “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 study;Washing machine
augers; see on web page
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30 ton, 1.5 oz (45 cm3) Engel
Injection Molding Machinefor wheel fabrication
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Process & machine schematics
*
* Source: http://www.idsa-mp.org/proc/plastic/injection/injection_process.htm
*
Schematic of thermoplastic Injection molding machine
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Process OperationTemperature: barrel zones, tool, die zonePressures: injection max, holdTimes: injection, hold, tool openingShot size: screw travel
Flash
Melt
Thermaldegradation
Short-shot
Temp.
Pressure
Processing window
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Typical pressure/temperature cycle
polymersfor sec10
thicknesshalf
33
2
cm
tcool
Time(sec)
Cooling time generally dominates cycle time
Time(sec)
* Source: http://islnotes.cps.msu.edu/trp/inj/inj_time.html
**
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Calculate clamp force, & shot size
F=P 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; Tool > polymer
yxx
qyxTc
tx
p
)(
)()'( kind 3rd
constant)'( kind 2nd
constant)'( kind1st
TThxxx
Tk
xxx
Tk
xxTBoundary Conditions:
1-dimensional heat conduction equation :
The boundary condition of 1st kind applies to injection molding since the tool is often maintained at a constant temperature
x
Tkqx
qx qx + qx
2
2
2
2
or x
T
t
T
x
Tk
t
Tcp
Fourier’s law
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Heat transfer
TW
Tii
t
x+L-L
Let Lch = H/2 (half thickness) = L ; tch = L2/;Tch = Ti – TW (initial temp. – wall temp.)
Non-dimensionalize: 2 ;1 ;
L
tF
L
x
TT
TTO
Wi
W
2
2
OFDimensionless equation:
Initial condition 1 0 OF
Boundary condition
0 2
0 0
Separation of variables ; matching B.C.; matching I.C. )()(),( gFfF OO
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Centerline, = 0.1, Fo = t/L2 = 1Temperature in a slab
Bi-1 =k/hL
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Reynolds Number
* Source: http://www.idsa-mp.org/proc/plastic/injection/injection_process.htm
VL
LVL
V
s viscou
inertia Re
2
2
Reynolds Number:
For typical injection molding
231
32433
10 ;1
10
timeFill
lengthPart
ess thickn10 ;101
msNs
V
mLsmNcmg Z
410Re
For Die casting
30010
1010103Re
3
313
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Viscous Shearing of Fluidsv
F
h
h
v
A
F
F/A
v/h
1
Newtonian Viscosity
h
v
Generalization:
)(
Injection molding
rateshear :
Typical shear rate for Polymer processes (sec)-1
Extrusion102~103
Calendering 10~102
Injection molding103~104
Comp. Molding 1~10
“Shear Thinning”
~ 1 sec-1 for PE
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Viscous HeatingRate of Heating = Rate of Viscous Work
2
h
v
h
v
A
F
Vol
vF
Vol
P
Rate of Temperature rise 22
or
h
v
cdt
dT
h
v
dt
dTc
pp
Rate of Conduction out22
2
~h
T
c
k
dx
Td
c
k
dt
dT
pp
Tk
v
2
Conduction
heating Viscous Brinkman number
For injection molding, order of magnitude ~ 0.1 to 10
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Non-Isothermal Flow
vFlow rate: 1/t ~V/Lx
Heat transfer rate: 1/t ~a/(Lz/2)2
x
zz
x
z
L
LVL
L
LV
4
1
4~
rateHeat xfer
rate Flow 2
For injection molding
5.210
1.0
/10
1.0/10
4
1~
rateHeat xfer
rate Flow23
cm
cm
scm
cmscm
For Die casting of aluminum
22
1010
1.0
/3.0
1.0/10
4
1~
rateHeat xfer
rate Flow
cm
cm
scm
cmscm
* Very small, therefore it requires thick runners
Small value=> Short shot
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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
* Source: G. Menges and W. Wubken, “Influence of processing conditions on Molecular Orientation in Injection Molds”
V=3.5cm/s
V=8cm/s
sample Transverse direction
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Gate Location and Warping
Center gate: radial flow – severe distortion
Diagonal gate: radial flow – twisting End gates: linear flow – minimum warping
Gate
Air entrapment
Edge gate: warp free, air entrapment
Sprue
2.0
2.0 60
Before shrinkage
60.321.96
1.976
After shrinkage
ShrinkageDirection of flow – 0.020 in/inPerpendicular to flow – 0.012
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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
Sh
rin
kag
e
0.030
0.000
0.010
0.005
0.015
0.020
0.025
Sh
rin
kag
e6000
800010000
1200014000
16000
Pressure on injection plunger (psi)
AcetalLDPE
Nylon 6/6
PP with flow
18000
PP across
flow
PMMA
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Where would you gate this part?
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Weld line, Sink mark
* Source: http://www.idsa-mp.org/proc/plastic/injection/injection_design_7.htm
Weld line
Mold Filling
Gate
Solidified part
Sink markBasic rules in designing ribs to minimize sink marks
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Injection Molding*
*
* Source: http://www.idsa-mp.org/proc/plastic/injection/injection_design_2.htm
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Where is injection molding?
Ltotal = Lmold + Lshrinkage
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Effects of mold temperature and pressure on shrinkage
0.030
0.000
0.010
0.005
0.015
0.020
0.025
Sh
rin
kag
e
6000
8000
10000
12000
14000
16000
Pressure on injection plunger (psi)
Acetal
LDPE
Nylon 6/6
PP with flow
18000
PP across flow
PMMA
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Tooling Basics
Cavity Plate
Cavity
MouldingCore
Core Plate
Basic mould consisting of cavity and core plate
Runner
Cavity
Gate
NozzleSprue
Melt Delivery
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Part
Cavity
Core
Stripper plate
Tooling for a plastic cup
Runner
Knob
Nozzle
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Tooling for a plastic cup
Runner
Part
Cavity
Nozzle
Part
Cavity
Knob
Stripper plate
Runner
Part
Cavity
Nozzle
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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.)
**
*
*
*
* **
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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
Cores and Part Molded in Clear Plastic
Cores used in Injection Molding
Injection Molded Housing shown in class
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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
Carbon Dioxide and Toxic Materials per Value of Shipments
00.5
11.5
22.5
33.5
44.5
5
Che
mic
als
Pet
role
uman
d C
oal
Pla
stic
s an
dR
ubbe
r
Prim
ary
Met
al
Fab
ricat
edM
etal
Mac
hine
ry
Ele
ctro
nic
Tra
nspo
rtat
ion
Manufacturing industries
Wei
ght/D
olla
rs
CO2 (metric ton/$10,000)
Toxic Mat'ls (lb/$1000)
EPA 2001, DOE 2001
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The estimated environmental performance of various mfg processes (not including auxiliary
requirements)
*Energy per wt. normalizedby the melt energy
** total raw mat’l normalizedby 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