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TRANSCRIPT
Injection Molding
Mech 550UBC Vancouverdr. ing. Bart Buffel PhD
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Application: injection molding
• Process description
• Consequences of high viscosity for injection moldingequipment
• Calculation of pressure drop and clamping force
• Non isothermal phenomena: shear heating
• Representing the injection molding cycle in a PVT diagram
• Flow induced fiber orientation in injection molding
• Autodesk moldflow simulation on a technical part
• Autodesk moldflow: hands on
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IM: process descriptionClamping unit Injection unit
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• Important parts
IM: process description
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• Melting polymer granules
IM: process description
barrelscrew
polymer pellets
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• Melting polymer granules
Heating: 70% internal friction
30% electrical heating
IM: process description
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• Melting polymer granules
IM: process description
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IM: process description
Hydraulic pressure build up
• Pressure build up
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• Pressure build up
IM: process description
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Polymer viscosity
IM: consequences of high viscosity
Fluid Viscosity[Pa.s]
Water 0,001
Blood 0,003-0,004 (37°C)
Motor oil 0,06-0,5
Olive oil 0,08
Honey 2-10
Molten glass 10-1000
Chocolate syrup 10-25
Ketchup 50-100
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Polymer viscosity – shear thinning behaviour
IM: consequences of high viscosity
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Polymer viscosity – shear thinning behaviour
IM: consequences of high viscosity
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Polymer viscosity – shear thinning behaviour
IM: consequences of high viscosity
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Illustration: pressure drop through circular runner
• Hagen-Poiseuille
laminar flow, incompressible fluid, newtonian fluid
•
with and from the power-law model
IM: consequences of high viscosity
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Illustration: pressure drop through circular runner
L = 100mm R = 2,5mm
=25cm³/s m = 1274
n = 0,44
Moldflow
IM: consequences of high viscosity
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Illustration: pressure drop through circular runner
Filling simple geometries requires large pressures!
Large forces and pressures
Heavy equipment (clamping force up to 40000kN)
Processing technology -> valve gate cascade injection
IM: consequences of high viscosity
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IM: pressure drop and clamping force
Basic equipment characteristics
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IM: pressure drop and clamping force
Example:
• ABS injection of a disc Ø240mm and 2,1mm thick
• Disc: = 360°C
• Single central gating (N=1)
• Flow rate: 160cm³/s
• Tmold: 50°C
• Tmelt: 245°C
• Thermal conductivity : 0,174W/mK
• Thermal diffusivity a: 7,72.10-4 cm²/s
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IM: pressure drop and clamping force
Example:
Based on power law model:
= 0,2565
=3,05.104
Stevenson clamping force model:
Isothermal pressure drop
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IM: pressure drop and clamping force
Isothermal pressure drop for a disc
,
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IM: pressure drop and clamping force
Empirical correction by Stevenson for actual pressure drop
,
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IM: pressure drop and clamping force
Empirical correction by Stevenson for actual pressure drop
Compare to =
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IM: pressure drop and clamping force
Calculation of the isothermal pressure drop
.
The equations in this example are valid for discs.
Other geometries are discussed by Stevenson 1977, 1978, 1979
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• Calculation requires detailed materialinformation
In practice a more qualitative approach is usedfor first calculations and machine selection
Based on values and curves from experience
IM: Equipment selection
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• Clamping forceo Determine projected area
o Pressure
• Through (personal) experienceAccuracy ?
• Ratio flow path / wall thicknessexperience
+ safety factor for different materials
• Numerical simulations
IM: Equipment selection
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IM: Equipment selectionThrough (personal) experience
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IM: Equipment selection
bar
Ratio flow path / wallthickness
Ratio flow path / wall thickness
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IM: Equipment selection
safety factor for different materials
Material Factor
PE, PS, PP 1
POM, PA 1,2 - 1,4
CA, CAB 1,3 - 1,5
ABS, SAN 1,3 - 1,4
PMMA, PPE 1,5 - 1,7
PC, PVC 1,7 - 2
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• Determine size injection unit
o Shot volume between 15 an 85% of maximum volume
o Minimize residence time on the screw
volume in screw channels?
IM: Equipment selection
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• Cooling timeo Flat plates
o Formulae
IM: Equipment selection
S= wall thickness
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IM: Equipment selection
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• Dosing capacityo DosingtTime = cooling time - 10%
o RPM <-> required torque
o Parallel or serial system
IM: Equipment selection
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• cycle timeo Machine time (opening and closing of the mold)
Hydraulic movements are slower
o Injection speed
• Limits determined by material supplier
o Packing time
• Depends on product wall thickness
o Cooling time
• Consumes 80% of the total cycle time
IM: Equipment selection
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Caused by frictional forces between “layers” inside the polymer melt shear flow
Generated heat:
Using the general steady state heat energy balance:
IM: shear heating
Ux
²
²
²
²
²
²0
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IM: shear heating
H=3mm
T1 = T2 = 240°C
=20Pa.s
= 0,1W/mK
Ux = 0,5m/s
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Real through thickness profiles
Tmold = 40°C & Tmelt = 240°C
IM: shear heating
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Real through thickness profiles
Tmold = 40°C & Tmelt = 240°C
IM: shear heating
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Real through thickness profiles
Tmelt = 240°C
IM: shear heating
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IM: shear induced flow imbalance
x
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IM: shear induced flow imbalance
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IM: shear induced flow imbalance
V/P switchover when cavity is 95-99% filled
??
use melt flippers
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IM: shear induced flow imbalance
(Source: http://www.beaumontinc.com)
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IM: PVT diagram
Crystalline polymers (PP, PA) Amorphous polymers (PS, PC)
Tg
Tm
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IM: PVT diagram
injection
cooling
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IM: PVT diagram
injection
cooling