defect analysis in molded plastic part
TRANSCRIPT
FEA stress analysisdefect in a Molded Plastic part
Solidworks Simulation
8/25/2016
Don Blanchet
3B Associates
Model fromGrabcad
CAUTION
Don’t assume that a defect will always result in a part failure at the location of the defect.
Engineering experience and judgement can mislead.
Complex parts often have complex load paths.
PROBLEM
A high volume molded plastic part underwent post NC machining which introduced a defect due to an error.
What impact does this false cut have on the strength and factor of safety in the part.
Worst case Static Loading was specified by the customer.
Model
ABS plastic
1.55 inch dia
Model with defect
NC false cut
Manufacturing engineering “part will be o.k. – ship 5,000 parts”
Boundary Conditions
Fixed base
Worst case staticLoads
+ Fy = 25lbs- Fz = 20lbs
FEA adaptive mesh
Stress results original design
Max VonMises stress = 3,000 psiElastic deformation
Stress results flawed part
VonMises stress = 2,000 psiIn flaw
Stress results flawed part
Max VonMises stress = 6,000 psiSame location as original part
Permanent plastic deformation here
Results
Analysiscase
MaximumStress
psi
Factor of SafetyTensile
strength = 4500 psi
Original 3,000 1.5
With NCManufacturing
flaw6,000 0.75
failure
Conclusion
There is a significant risk in shipment of the defective parts.
The potential failure location is not at the defect. (surprise ! ??)
FEA could save the company embarrassment time and money.
Postscript
I started structural analysis in 1967 right out of undergrad school with the help of a slide rule and great mentors.
I was lucky enough to be among the earliest adopters of the finite element method.
It continues to please and amaze.