Molding Process
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PP Part Stress Whitening
Fenómeno del problema
White spots or white streaks appear at ejection, bending, impact, or assembly interference locations—especially obvious on dark and high-gloss parts—affecting appearance and indicating local yielding.
Análisis de la causa
When local stress on PP exceeds yield strength, numerous microvoids/silver streaks form inside, scattering light to appear white; excessive and concentrated ejection force, excessive assembly interference, drop impact, and stress concentration from sudden local wall thickness changes are all common triggers.
Solución
①Enlarge the force-bearing area of the ejection system (add ejector pins/use stripper plate) to disperse ejection force; ②reduce holding pressure to decrease gripping force; ③structurally avoid local interference and sharp corners, and design snap deformation reasonably; ④structurally reinforce repeatedly whitening areas; ⑤select stress-whitening-resistant grades for high-requirement parts.
Conclusión clave
Stress whitening is a clear signal of PP 'local yielding'; the control approach is to disperse concentrated forces (starting from ejection, assembly, and structure), not to cover it up; it is a free 'stress indicator'—use it well to locate structural weak points.
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