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strengthening the materials core of manufacturing enterprises
Challenges in the Modeling of
Plastics in Computer Simulation
Hubert Lobo President
DatapointLabs, Ithaca NY
strengthening the materials core of manufacturing enterprises
Impact of Simulation
• Massive benefit to injection-molding process
– Great improvement in part quality
– Productivity increases
– Reduction in scrap
• Significant benefit to plastic part design
– Understand how to use these complex materials
– Create novel parts and products
– Prevent in-the-field part failure
strengthening the materials core of manufacturing enterprises
Challenges
• Plastics are very complex
– Not all behavior is well understood
– Experimental artifacts accompany the data
– Mathematical material models have limitations
– Behavior is not correctly represented in simulation
• These limitations can cause errors
• With proper understanding, good design
decisions can be made
strengthening the materials core of manufacturing enterprises
What Makes Plastics Complex
• Non-Newtonian, non-isothermal flow
• Cooling rate- and shear-dependent crystallization
• Viscoelastic (time-based behavior)
• Non-linear elasticity
• Complex plasticity (pre-yield, post-yield)
• Properties change over product operational
temperature and environmental exposure
strengthening the materials core of manufacturing enterprises
Current Topics
• Injection-mold analysis
– Material model inconsistencies
– Fiber-orientation prediction
• Finite element analysis
– Non-linear elasto-plasticity (most plastics)
– Hyperelastic with plasticity (elastomers)
• Fiber orientation (fiber-filled plastics)
• The promise of validation
strengthening the materials core of manufacturing enterprises
Data for Injection-mold Analysis
• Viscosity vs. shear rate and temperature
• Thermal conductivity vs. temperature
• Specific heat vs. temperature
• Transition temperature
• PVT
• Shrinkage data
– Mechanical properties
– CRIMS (Moldflow)
?
?
?
strengthening the materials core of manufacturing enterprises
Transitions: No-flow Temperature
• When does the polymer
solidify in the mold?
• Different test methods
produce different transitions
• Transition inconsistency in
semi-crystalline polymers
– super-cooling
– cooling rate effect
– viscoelastic effects
• Impact on simulation unclear
1.E+3
1.E+4
1.E+5
100 120 140 160 180 200Temp (C)
Eta
(P
a.s
)
DMA cooling dynamic scan
(semi-crystalline polymer)
strengthening the materials core of manufacturing enterprises
How to Measure PVT
0.90
0.95
1.00
1.05
1.10
1.15
100 150 200 250 300 350
Temperature °C
sp.v
ol (c
m3/g
)
PVT 10 MPa, Isobaric Heating
PVT 10 MPa, Isobaric Cooling
PVT 40 MPa, Isobaric Cool
PVT 40 MPa Isothermal Cool
Initial PVT data with injection-
molded sample has lower solid
density
Subsequent slow-cooled isobaric
sample has higher, incorrect
density
Shrinkage predictions can be affected
strengthening the materials core of manufacturing enterprises
Accounting for Rate Effects in PVT
0.90
0.95
1.00
1.05
1.10
1.15
1.20
25 75 125 175 225 275 325
Temperature °C
sp.v
ol (c
m3/g
)
-10
0
10
20
30
40
50
60
100 C/min Kinetics
0 MPa
PVTmodel 0 MPa, 3°C/min
PVT 10 MPa, 3°C/min
DSC 0 Mpa, 100°C/min
• Possible to correct PVT data using DSC high-cooling-rate curves (H. Lobo, ANTEC 1999)
• Strategy is incorrect
– DSC is quiescent: high super-cooling effect
– Shear effects in molding mitigate super-cooling effect
(Kennedy, Janeshitz-Kriegl)
strengthening the materials core of manufacturing enterprises
Solid State Behavior of Polymers
400%
Brittle plastic
(linear-elastic)
Ductile plastic
(elasto-plastic) Elastomer
(hyper-elastic)
strengthening the materials core of manufacturing enterprises
Effect of Environment: Temperature
• Properties and dependencies
change with temperature
– Modulus
– Ductile-brittle
transitions
– Rate dependency
strengthening the materials core of manufacturing enterprises
Effect of Environment: Moisture
Nylon
DAM vs. 50% RH
strengthening the materials core of manufacturing enterprises
Effect of Environment: In-vivo
UHMWPE, 37C
(dry vs. saline soak, 30 days)
strengthening the materials core of manufacturing enterprises
Models for Ductile Plastics
• True stress-strain curves – UTM with extensometers
– Testing to yield or break
• Material model: elasto-plasticity – Reduce to elasto-plasticity based on yield point
– Bilinear
– Multilinear
• Usage – Large deformation
– von Mises yield
strengthening the materials core of manufacturing enterprises
Elasto-plasticity in Metals
• Evaluate a modulus
• Define elastic limit
• Calculate multi-point
plasticity
where,
E = elastic modulus (MPa)
σt = true stress (MPa)
pt E
strengthening the materials core of manufacturing enterprises
Comparing Metal to Plastic
Aluminum
Polycarbonate
strengthening the materials core of manufacturing enterprises
Polymer Elasto-plasticity
• Non-linear elasticity
• Elastic limit well below
classical yield point
• Significant plastic strains
prior to yield
• Post-yield with necking
behavior 0
5
10
15
20
25
30
35
0.0 5.0 10.0 15.0 20.0
Engineering Strain (%)
En
gin
ee
rin
g S
tre
ss (
MP
a)
data
Plastic Point
strengthening the materials core of manufacturing enterprises
Modeling Options
Fidelity to plastic point Fidelity to curve shape
strengthening the materials core of manufacturing enterprises
Applying Abaqus FeFp Model
• Non-linear hyper-
elasticity with pre-yield
plasticity
• Accurate representation
of elastic behavior
• Accurate representation
of plasticity
(Lobo & Hurtado, Abaqus 2006)
strengthening the materials core of manufacturing enterprises
A True Representation of Plasticity
plas10
0
0.5
1
1.5
2
2.5
0 2000 4000 6000 8000
time, s
ex
ten
sio
n, m
m
0
200
400
600
800
0 200 400 600 800 1000
time (sec)
Lo
ad
(N
)
0
0.5
1
1.5
2
0 200 400 600 800 1000
time (sec)
exte
nsio
n (
mm
)
strengthening the materials core of manufacturing enterprises
Post-yield Ductile Behavior
0
10
20
30
40
50
60
70
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
Strain mm/mm
Str
ess
MP
a
UTM1-39.62-1
UTM1-39.62-2
Yield point
Necking starts
Necking propagation
strengthening the materials core of manufacturing enterprises
Digital Image Correlation (DIC)
• Stereo camera system
(ARAMIS)
• Simultaneous XYZ
dimension change
• Complete surface is
measured
• Post-measurement
selection of region of
interest (Lobo et al., LS-DYNA 2013)
strengthening the materials core of manufacturing enterprises
Measuring Post-yield Stress-Strain
0
20
40
60
80
100
120
140
160
180
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
True Strain (unitless)
Tru
e S
tre
ss (
MP
a)
1 mm GL
2 mm GL
4 mm GL
25 mm GL
50 mm GL
y x
z
strengthening the materials core of manufacturing enterprises
Reasonable Post-yield Approximation
strengthening the materials core of manufacturing enterprises
Fiber-filled Plastics
• Spatial orientation of fibers
– Properties vary spatially
• Can be approximated
– Worst case: use cross-flow data
• NEW: fiber-orientation material modeling
– Perform injection-molding simulation
– Obtain fiber orientations
– Calculate local orientation-based properties
– Send to FEA
Source:e-Xstream
strengthening the materials core of manufacturing enterprises
Typical Test Protocol
• Mold long plaques
– Edge gated: short end
– Fully developed flow
– High fiber orientation
• Cut test specimens
– 0°, 90°, 45°, …
• Obtain true stress-strain data
• Calibrate material model
strengthening the materials core of manufacturing enterprises
Example: Airbag Housing
Source:e-Xstream
strengthening the materials core of manufacturing enterprises
Impact on Failure
Source:e-Xstream
With Fiber Orientation
Isotropic
strengthening the materials core of manufacturing enterprises
The Promise of Validation
• Open loop validation
– Carefully designed benchmark models
– Not real-life component
– Multi-mode case
– Well-defined boundary conditions
– Load cases reproducible in virtual and real life
strengthening the materials core of manufacturing enterprises
Dynamic FEA TestBench Model
• ASTM D3763 falling dart impact
• Multi-axial loading with well-defined boundary conditions
– Dart with a ½-inch rounded tip
– Dart weight of 22.68 kg
– Disk dimensions • Thickness = ~3 mm
• Diameter = 76 mm
– B.C.s: fixed edges
– I.C.s: initial velocity of 3.3m/s
strengthening the materials core of manufacturing enterprises
Model Complexities
• Stress modes
– Biaxial
– Shear
– Bending
• Rate-dependent plasticity
• Complex failure
strengthening the materials core of manufacturing enterprises
Simulation v. Test
• Percent error at peak force
• Time: -1.0%
• Distance: -0.52%
• Force: 1.3% Force vs Time
-500
0
500
1000
1500
2000
2500
3000
3500
0 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008
Time (s)
Fo
rce (
N)
Lab
LS-Dyna
Force vs Displacement
-500
0
500
1000
1500
2000
2500
3000
3500
0 5 10 15 20 25
Displacement (mm)
Fo
rce (
N)
Lab
LS-Dyna
strengthening the materials core of manufacturing enterprises
In Closing...
• Do not oversimplify
• Understand model limitations
• Use appropriate data
• Use self-consistent data
• Validate where possible
strengthening the materials core of manufacturing enterprises
Acknowledgements
• J. Hurtado, Abaqus – FeFp model
• Sylvain Calmels – e-Xstream Engineering
• Brian Croop, DatapointLabs
• Dan Roy, DatapointLabs – DIC
• Megan Lobdell, DatapointLabs – Validation
strengthening the materials core of manufacturing enterprises
Remembrance
• Dr. VW Wang (passed away Dec 10th 2014)
• Authored the first science-based injection-molding simulation code (Cornell University, PhD 1985)
• Founder of C-MOLD