a small false cut error in an NC machining operation , proves to have a significant impact on the strength of the ABS plastic part

1. FEA stress analysis
defect in a Molded Plastic part
Solidworks Simulation
8/25/2016
Don Blanchet
3B Associates
dwb3298@verizon.net
Model from
Grabcad

2. 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.

3. 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.

4. Model
ABS plastic
1.55 inch dia

5. Model with defect
NC false cut
Manufacturing engineering “part will be o.k. – ship 5,000 parts”

6. Boundary Conditions
Fixed base
Worst case static
Loads
+ Fy = 25lbs
- Fz = 20lbs

7. FEA adaptive mesh

8. Stress results original design
Max VonMises stress = 3,000 psi
Elastic deformation

9. Stress results flawed part
VonMises stress = 2,000 psi
In flaw

10. Stress results flawed part
Max VonMises stress = 6,000 psi
Same location as original part
Permanent plastic deformation here

11. Results
Analysis
case
Maximum
Stress
psi
Factor of Safety
Tensile
strength =
4500 psi
Original 3,000 1.5
With NC
Manufacturing
flaw
6,000 0.75
failure

12. 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.

13. 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.