injection molding 2.810 fall 2002 professor tim gutowski
Post on 16-Dec-2015
224 Views
Preview:
TRANSCRIPT
Injection Molding
2.810 Fall 2002Professor Tim Gutowski
Short history of plastics
1862 first synthetic plastic1866 Celluloid1891 Rayon1907 Bakelite1913 Cellophane1926 PVC1933 Polyethylene1938 Teflon1939 Nylon stockings1957 velcro1967 “The Graduate”
Outline
Basic operation
Cycle time and heat transfer
Flow and solidification
Part design
Tooling
New developments
Environment
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
30 ton, 1.5 oz (45 cm3) Engel
Injection Molding Machinefor wheel fabrication
Process & machine schematics
*
* Source: http://www.idsa-mp.org/proc/plastic/injection/injection_process.htm
*
Schematic of thermoplastic Injection molding machine
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
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
**
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; 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
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
Centerline, = 0.1, Fo = t/L2 = 1Temperature in a slab
Bi-1 =k/hL
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
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
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
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
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
* 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
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
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
Where would you gate this part?
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
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
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
Tooling Basics
Cavity Plate
Cavity
MouldingCore
Core Plate
Basic mould consisting of cavity and core plate
Runner
Cavity
Gate
NozzleSprue
Melt Delivery
Part
Cavity
Core
Stripper plate
Tooling for a plastic cup
Runner
Knob
Nozzle
Tooling for a plastic cup
Runner
Part
Cavity
Nozzle
Part
Cavity
Knob
Stripper plate
Runner
Part
Cavity
Nozzle
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
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
New developments- Gas assisted injection molding
New developments ; injection molding with cores
Cores and Part Molded in Clear Plastic
Cores used in Injection Molding
Injection Molded Housing shown in class
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
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
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.
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
top related