InjectionMolding1

0.1cm 10cm

2.5
For Die casting of aluminum
Flow rate Heat xfer rate
~
1 4
10cm / s 0.1cm 0.3cm2 / s

0.1cm 10cm
102
* Very small, therefore it requires thick runners
Where would you gate this part?
Weld line, Sink mark
Gate
Mold Filling
Weld line
Solidified part
Sink mark
* Source: http://www.idsa-mp.org/proc/plastic/injection/injection_design_7.htm
Flow rate ~ V L2z 1 VLz Lz
Heat xfer rate 4 Lx 4 Lx
Small value => Short shot
For injection molding
Flow rate Heat xfer rate
~
1 4
10cm / s 0.1cm 103cm2 / s
Conduction kT
Brinkman number
For injection molding, order of magnitude ~ 0.1 to 10
Non-Isothermal Flow
Flow rate: 1/t ~V/Lx
v
Heat transfer rate: 1/t ~a/(Lz/2)2
Center gate: radial flow – severe distortion
Air entrapment Gate
Edge gate: warp free, air entrapment
Diagonal gate: radial flow – twisting
End gates: linear flow – minimum warping
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
For Die casting
Re

3103 10 1 10 3 10 3
300
* Source: http://www.idsa-mp.org/proc/plastic/injection/injection_process.htm
Re 104
Viscous Shearing of Fluids
Gate Location and Warping
Sprue
2.0
60
Shrinkage
Direction of flow – 0.020 in/in
1.96
60.32
Perpendicular to flow – 0.012
2.0
Before shrinkage
1.976
After shrinkage
Effects of mold temperature and pressure on shrinkage
Shrinkage Shrinkage
0.030 0.025 0.020 0.015 0.010 0.005 0.000
LDPE
PP Acetal
Nylon 6/6
PMMA
100
120 140 160 180 200 220 240
F v
h
Fv Ah
F/A
1
v/h
Newtonian Viscosity
v
h
Generalization: : shear rate


( )
Injection molding
“Shear Thinning”
~ 1 sec-1 for PE

Typical shear rate for Polymer processes (sec)-1
Basic rules in designing ribs to minimize sink marks
Injection Molding
* *
* Source: http://www.idsa-mp.org/proc/plastic/injection/injection_design_2.htm
Temp.
Processing window
Thermal degradation
Flash
Shortshot
Melt
Pressure
Typical pressure/temperature cycle
*
*
Time(sec)
Cooling time generally dominates cycle time
Shrinkage distributions
sample V=3.5cm/s V=8cm/s
Transverse direction
* Source: G. Menges and W. Wubken, “Influence of processing conditions on Molecular Orientation in Injection Molds”
* 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
Where is injection molding?
Ltotal = Lmold + Lshrinkage
qx
qx + qx
Fourier’s law
t
(
cp
T )xy


qx x
xy
T qx k x
cp
T t
k
2T x 2
or
T t

2T x 2
Boundary Conditions: 1st kind
T (x x') constant
Re L

V L2
inertia VL viscous
For typical injection molding
1g cm3 103 N m4 s2 ; LZ 103 m thickness
V Part length 101 ; Fill time 1s
103 N s m2
Rate of Temperature rise
cp
dT dt
v 2
h
or
dT


v
2


dt cp h
Rate of Conduction out
dT dt

k
cp
d 2T dx2
~
k
cp
T h2
Viscous heating v2
Ti TW
x 1;
L
FO

t
L2
Dimensionless equation:
Initial condition Boundary condition
2 FO 2
FO 0 1 0 0 2 0
Separation of variables ;
O来自百度文库tline
Basic operation Cycle time and heat transfer Flow and solidification Part design Tooling New developments Environment
Readings
Tadmore and Gogos
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”
Mold Temperature (F)
0.030 0.025 0.020 0.015 0.010 0.005 0.000
LDPE
PP with flow
PP across flow
Acetal
Nylon 6/6
PMMA
6000
10000
14000
18000
8000
12000
16000
Pressure on injection plunger (psi)
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
1-dimensional heat conduction equation :
2nd kind 3rd kind
k T (x x') constant x T
k x (x x') h (T T )
The boundary condition of 1st kind applies to injection molding since the tool is often maintained at a constant temperature
* Source: http://islnotes.cps.msu.edu/trp/inj/inj_time.html
half thickness2
tcool

103 cm3 sec for polymers
Time(sec)
Calculate clamp force, & shot size
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”
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
Heat transfer
Tii Let Lch = H/2 (half thickness) = L ; tch = L2/ ;
t
Tch = Ti – TW (initial temp. – wall temp.)
-L
x
TW +L
Non-dimensionalize: T TW ;
30 ton, 1.5 oz (45 cm3) Engel
Injection Molding Machine for wheel fabrication
Process & machine schematics
*
*
Schematic of thermoplastic Injection molding machine
Extrusion
102~103
Calendering
10~102
Injection molding 103~104
Comp. Molding 1~10
Viscous Heating
Rate of Heating = Rate of Viscous Work
P

F
v

F

v


v
2
Vol Vol A h h