FEA stress analysisdefect in a Molded Plastic part

Solidworks Simulation

8/25/2016

Don Blanchet

3B Associates

dwb3298@verizon.net

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.