Comprehensive guide to common injection molding defects: sink marks, weld lines, flash, short shots, warpage, burn marks, and jetting.
Defects in injection molded parts waste material, time, and money. Understanding root causes is the first step to eliminating them.
Appearance: Depressions on thick sections. Causes: Insufficient packing pressure, insufficient cooling time. Solutions: Increase holding pressure and time. Redesign thick ribs to be 60 percent of nominal wall thickness.
Appearance: Lines where two melt flow fronts meet. Causes: Low melt temperature, low injection speed, obstructions splitting flow. Solutions: Raise melt/mold temperatures. Increase injection speed. Add vents at weld locations.
Appearance: Thin plastic layer at the parting line. Causes: Clamping force too low, mold wear. Solutions: Increase clamp force. Reduce injection pressure. Repair parting line surfaces.
Appearance: Incomplete cavity fill. Causes: Insufficient material, blocked gate, trapped air. Solutions: Increase shot size. Clean nozzle and gates. Add venting.
Appearance: Distorted part, does not lie flat. Causes: Non-uniform cooling, differential shrinkage. Solutions: Balance cooling channels. Increase cooling time. Use lower melt temperature.
Appearance: Black/dark brown streaks. Causes: Trapped air compressing and igniting. Solutions: Add venting at air trap locations. Reduce injection speed.
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