For previous article, please refer to Plastic Life and Death Book - Part 1: Success or Failure Depends on Materials! How Do Plastic Raw Ma.
A perfectly designed blueprint can become a "life sentence" for a plastic product! When broken gears and exploding casings lie on dissection table, forensic tweezers pick up murder weapon: a 0.1mm oversight on blueprint, a missing fillet, or a 0.5° draft. Today, we'll take you to structural crime scene and see how structural traps can strangle a product's lifespan by ten years.

 

Trap 1. Sharp-Angled Gallows
Stress concentration is a major cause of failure in plastic parts under stress. As shown in figure below, stress concentration factor K (the larger factor, the more stress concentrated) is related to radius/wall thickness (R/T). It can be seen that the larger fillet R, the smaller stress concentration factor K.

 

▲ Graph showing relationship between stress concentration factor and radius/wall thickness (R/T)
Therefore, to avoid stress concentration at sharp corners, rounded corners are often necessary.

 

▲ Optimized corner design
Structural crime:
A bread basket with a right-angled interior caused stress concentration during stacking, leading to one million pieces breaking within three months. However, experiments have shown that when bread basket's R/T ratio is 0.5, its impact resistance is more than three times better than original right-angled bread basket.

 

Trap 2: Wall Thickness Guillotine
Uneven wall thickness causes shrinkage (shrinkage holes) in plastic parts. Thick areas cool unevenly, with melt near mold surface cooling faster and center of mold cooling slower. When center of mold shrinks, if surface strength is high, internal vacuum bubbles (shrinkage holes) will form. If surface strength is low, surface depressions (shrinkage) and warping will occur.

 

When plastic parts are uneven in thickness, differences in cooling rates lead to different volumetric shrinkage rates, resulting in residual stress. When stress exceeds strength of a component, it can cause thickness marks at best, warping, deformation, or even cracking at worst.

 

Wall Thickness Sudden Murder Case:
Crime Scene:
White coupling nut involved died suddenly after three years of service. An autopsy revealed that 1.6mm protrusion mutation area was fatal injury—a 33% drop in wall thickness created a stress noose, which, combined with thread end kill zone, resulted in an 18% local stress overload.
Mechanism of Attack:
Compression washers applied pressure day and night, forcing protrusion into a "creep kill cycle":
▶ 0-12 months: Plastic deformation incubation period
▶ 13-36 months: Ductile to brittle transition (rupture critical point eruption)
▶ Evidence: Circumferential cracks developed precisely along ridge, confirming structural homicide.
Key to Revocation of Sentence:
Death protrusion was raised 1/4 inch (approximately 152mm judicial exemption height) to remove the nodule stress area; simultaneously, it was replaced with PA66 creep-resistant armor (no abnormalities reported after six years of service).
Final Verdict:
For crime of design mutation coupled with time poisoning, sentence is three years in prison, with immediate implementation of optimization and renovation measures!

 

Is a single trap insufficient for fatality? Environmental accomplices reduce sentences by 10 times!

�� Forensic Notes: "1+1=10" effect is at play here—structural defects exponentially amplify environmental damage!

1. Lifespan Simulation Decides Life or Death
Sentence Your Product to Death at Drawing Stage!
CAE Virtual Gallows Test:
Finite Element Analysis: Stress Concentration Points
Mold Flow Analysis: Frozen Stress Distribution
Accelerated Aging Execution Chamber:
Temperature Cycling: High and Low Temperature Cycling Test -40℃ to 120℃ (Induced Thermal Stress)
Immersion Test in Disintegration Solvent (Gasoline/Detergent)
Fatigue Life Verdict:
S-N Curve: Predicting Lifespan of Millions of Cycles
Discover Potential Hinge Failure Points → Add Ribs in Advance!
2. "Fingerprint Comparison" in Design Checklist
"Questioning" items in DuPont Design Checklist must be checked:
End-use temperature
Environmental effects, such as moisture, chemicals, and weathering
Magnitude and duration of applied loads

▲DuPont Design Checklist
3. Amnesty: Structural Redemption
◾ Three Principles of Fatigue-Resistant Design
Load Reduction: Replacing solid walls with honeycomb structures improves stress distribution uniformity by three times.
Heat Dissipation: Designing heat dissipation fins in high-frequency load areas reduces delayed heating by 25℃.
Toughening: Adding 15% toughening agent to PC increases notched impact strength from 60 J/m to 120 J/m.
For further reading, please refer to Plastic Life and Death Book - Part 3: Dissecting Failure Scene! A Diagnostic Manual for Plastic Fail.