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Defect analysis in molded plastic part
A molded plastic part experienced a defect from post-machining, leading to an analysis of its impact on strength and safety. Finite Element Analysis (FEA) revealed that while the original part had a factor of safety of 1.5, the flawed part dropped to 0.75, indicating significant risk in shipment. Ultimately, FEA proved essential for assessing potential failure not directly at the defect site.
Defect analysis in molded plastic part
- 1. FEA stress analysis defectin a Molded Plastic part Solidworks Simulation 8/25/2016 Don Blanchet 3B Associates dwb3298@verizon.net Model from Grabcad
- 2. CAUTION Don’t assumethat 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 highvolume 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.
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- 5. Model with defect NCfalse cut Manufacturing engineering “part will be o.k. – ship 5,000 parts”
- 6. Boundary Conditions Fixed base Worstcase static Loads + Fy = 25lbs - Fz = 20lbs
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- 8. Stress results originaldesign Max VonMises stress = 3,000 psi Elastic deformation
- 9. Stress results flawedpart VonMises stress = 2,000 psi In flaw
- 10. Stress results flawedpart 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 isa 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 startedstructural 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.