Stress Cracking: Delayed Failure Is Not Coincidental

Parts appear normal at ejection but crack after several days or upon solvent contact, with cracks originating from the gate or corners. This is stress cracking.

This article covers the sources of residual stress, explains why delayed cracking occurs, and shows how to distinguish it from brittle fracture.

1. Location

Near the gate: The area with maximum packing pressure has the highest residual stress.

Corners and wall thickness transitions: Geometric stress concentration points.

Weld lines: Incomplete fusion results in low strength combined with stress.

2. Root Causes

Excessive holding pressure: Over-packing leads to high density and stress near the gate.

Excessive or uneven cooling: The surface layer cools and solidifies first, while the interior continues to shrink, trapping stress in the part.

Demolding forces: Mold sticking, insufficient draft angle, or unbalanced ejection cause localized stretching.

Structural discontinuities: Stress concentrations occur at sharp corners, abrupt wall thickness changes, and rib roots.

Material and drying: PC, PMMA, and PSU are stress-sensitive and become more brittle when moisture is present.

III. Differentiate from Brittle Fracture and Stress Whitening

Brittle fracture occurs instantly under load, producing a flat fracture surface.

Stress whitening is surface whitening without cracking.

Internal stress cracking appears with delay; cracks are slender and propagate along the stress direction.

IV. Responsibility Allocation

Injection molding machine: hold pressure and time, cooling time.

Mold: mold temperature, draft angle, ejection, structural fillets.

Hot runner: gate temperature and sealing timing affect packing volume near the gate.

V. Solutions

Reduce holding pressure; set holding time based on sealing moment; do not use extended holding to compensate for dimensions.

Increase mold temperature and slow down cooling to align internal and external shrinkage.

Improve demolding: increase draft angles, polish surfaces, balance ejection.

Use fillets and wall thickness transitions in your design to reduce stress concentration.

Stress-relieve molded parts per material requirements.

Clarify the boundary: stress cracking is due to stress release, not material failure; first reduce pressure and ensure uniform cooling before considering material changes.

Conclusion

Internal stress is a chronic issue. Reduce holding pressure, ensure uniform cooling, and modify fillet radii simultaneously.

For further verification, please provide the mold 2D layout drawing. We will check gate positions and cavity temperatures item by item based on the layout, then provide a solution and quotation after confirmation.

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