Injection Molding Defects

Common injection molding defects can be prevented by understanding their root causes and implementing appropriate solutions. This guide covers the most frequent defects and how to fix them.

Defect Reference Table

Defect	Appearance	Primary Causes	Solutions
Sink marks	Localized depression on thick sections	Insufficient packing, thick walls, hot mold	Increase packing pressure/time, reduce wall thickness, lower mold temp
Flash	Thin protrusion at parting line	Low clamp force, high injection pressure, worn mold	Increase clamp force, reduce injection speed, repair mold
Warpage	Part distortion or bending	Uneven cooling, high internal stress	Balance mold temp, reduce packing, add cooling channels
Short shot	Incomplete fill	Low injection pressure/speed, insufficient material	Increase shot size, raise injection pressure, increase melt temp
Burn marks	Discolored, charred areas	Trapped air overheating, excessive melt temp	Improve venting, reduce injection speed, lower melt temp
Flow lines	Streaks or rings on surface	Variable flow speed, low mold temp	Increase injection speed, raise mold temp, optimize gate location
Weld lines	Visible line where flows meet	Insufficient melt temperature at merge point	Increase melt/mold temp, raise injection speed, move gate

Prevention Strategies

• Design for molding: Uniform wall thickness, generous radii, proper draft angles
• Mold design optimization: Correct gate type and location, adequate venting, balanced runner system
• Process window validation: Design of experiments (DOE) to find optimal parameters
• Mold flow simulation: Identify potential issues before steel cutting
• Regular maintenance: Mold cleaning, vent inspection, cooling channel maintenance