Identify, analyze and fix common plastic injection molding defects
Sink marks appear as depressions on thick wall sections due to uneven shrinkage. Cause: insufficient holding pressure or pack time. Solution: increase holding pressure and time; reduce wall thickness to under 4 mm. Use ribbed structures instead of thick walls. Semi-crystalline polymers (PP, PE, POM) are more prone.
Warpage is dimensional distortion from differential shrinkage. Cause: non-uniform cooling, unbalanced fill, or molecular orientation. Solution: optimize cooling channels; balance gate locations. Maintain mold temperature within ±10°C. Glass-filled materials have anisotropic shrinkage.
Flash is excess plastic at the parting line or ejector pin locations. Cause: low clamp force, mold damage, or deep venting. Solution: increase clamping force; polish parting line. Vent depth: 0.02-0.05 mm for semi-crystalline. Reduce injection speed near switch-over.
Cavity incompletely filled. Cause: insufficient material, low pressure, or blocked gate. Solution: increase shot size and pressure; raise melt temperature. Check L/T ratio. Ensure adequate venting.
Weld lines form where flow fronts meet; strength 10-50% lower. Increase melt/mold temperature. Burn marks from trapped air compression — improve venting at last-to-fill locations.