Injection Molding Defects: Causes, Solutions, and Prevention

Comprehensive reference guide to common injection molding defects including warpage, sink marks, burn marks, weld lines, short shots, flash, and their root causes and solutions.

Common Injection Molding Defects

Warpage

Cause: Uneven cooling, non-uniform shrinkage, molecular orientation, or residual stress in the molded part.

• Solutions: Optimize cooling channel design for uniform temperature distribution, reduce mold temperature differential, adjust packing pressure and time, use fillers to reduce shrinkage anisotropy

Sink Marks

Cause: Localized shrinkage in thicker sections where the surface skin cools and solidifies before the core material.

• Solutions: Increase packing pressure and time, reduce melt temperature, redesign thick sections with uniform wall thickness, add ribs for structural reinforcement

Burn Marks / Dieseling

Cause: Trapped air or gases that become compressed and heated to combustion temperature, or excessive shear heating at high injection speeds.

• Solutions: Improve mold venting, reduce injection speed at the filling stage, reduce melt temperature, use vacuum venting for difficult applications

Weld Lines / Knit Lines

Cause: Two or more melt flow fronts meet and fail to fully bond, often caused by flow around cores or through multiple gates.

• Solutions: Increase melt and mold temperatures, optimize gate locations, increase injection speed, use overflow wells at weld line locations

Short Shot

Cause: Insufficient material to fill the cavity completely, often due to insufficient shot size, inadequate injection pressure, or restricted flow.

• Solutions: Increase shot size and injection pressure, increase melt temperature for better flow, check for blocked gates or nozzles, improve venting

Flash

Cause: Molten plastic escapes from the cavity through the parting line or around moving components, typically due to insufficient clamp force or worn mold surfaces.

• Solutions: Increase clamp force, reduce injection pressure, check and repair parting line surfaces, reduce melt temperature for higher viscosity