In the injection molding production process, almost every factory encounters injection molding defects to some extent. These defects not only affect the product’s appearance and performance but also increase scrap rates and raise costs.
- Short Shot (Short Shot / Incomplete Filling / Underfill) One of the most common defects: The molten plastic fails to completely fill the mold cavity, resulting in missing material or incomplete edges on the part. Common Causes: Insufficient injection pressure/speed, low melt temperature or mold temperature, small gate/runner size, poor material flowability. Solutions: Increase injection pressure and speed, raise barrel/mold temperature, optimize gate design, select materials with better flowability.
- Flash / Burrs / Overflow (Flash / Burr) Molten plastic seeps out from the parting line, forming thin edges or excess material. Common Causes: Insufficient clamping force, wear/dirt on mold parting surfaces, excessively high injection pressure, or overly long holding pressure. Solutions: Increase clamping force, clean/repair mold parting surfaces, appropriately reduce injection/holding pressure, check mold precision.
- Sink Marks (Sink Marks) Depressions appear on the product surface, especially at ribs, bosses, or thicker wall sections. Common Causes: Insufficient holding pressure, uneven cooling, large wall thickness variations, high material shrinkage rate. Solutions: Extend holding time, optimize uniform cooling system, design parts with as uniform wall thickness as possible, select low-shrinkage materials.
- Warpage / Distortion (Warpage / Distortion) The part becomes bent, twisted, or uneven after ejection. Common Causes: Uneven cooling rates, high residual stress, uneven wall thickness, improper gate location. Solutions: Balance mold cooling channels, optimize holding and cooling times, adjust gate position, use materials with more stable shrinkage rates.
- Weld Lines / Knit Lines (Weld Line / Knit Line) Obvious seam lines appear where two or more melt fronts meet, affecting appearance and strength. Common Causes: Low melt temperature, slow injection speed, poor venting, improper number/position of gates. Solutions: Increase melt temperature and injection speed, improve mold venting, optimize gate layout (prefer single-point gating or hot runners).
- Silver Streaks / Splay Marks / Jetting (Silver Streaks / Jetting) Silver-white streaks or snake-like traces appear on the surface. Common Causes: Moisture in the material (inadequate drying), overly fast injection speed causing turbulence, poor mold venting. Solutions: Enhance material drying (strictly control moisture content), reduce initial injection speed, add venting slots.
- Burn Marks / Black Spots / Scorch Marks (Burn Marks / Black Spots) Localized scorching or blackening appears. Common Causes: Compressed trapped air overheating, material thermal degradation from overheating, poor mold venting. Solutions: Improve venting design, reduce injection speed, check barrel temperature to avoid overheating.
- Bubbles / Voids (Bubbles / Voids) Bubbles or cavities appear inside or on the surface of the part. Common Causes: High material moisture, trapped air, insufficient back pressure, inadequate holding pressure. Solutions: Thoroughly dry the material, increase back pressure, optimize holding parameters, strengthen venting.
- Color Difference / Discoloration / Poor Gloss (Color Difference / Dull Surface) Uneven color, dull gloss, or yellowing. Common Causes: Material contamination, excessive temperature causing degradation, uneven mixing, dirty or poorly polished mold surface. Solutions: Clean the barrel, use fresh material, control temperature, improve mold polishing level.
- Sticking / Poor Ejection (Sticking / Poor Ejection) The part is difficult to eject, shows whitening, ejection marks, or residue. Common Causes: Insufficient draft angle, poor mold polishing, short cooling time, unbalanced ejection. Solutions: Increase draft angles, improve mold surface treatment, extend cooling time, optimize ejection system.
The above 10 common injection molding defects cover over 90% of production pain points. Many issues can be quickly resolved through a combination of “process optimization + mold improvement + material selection.” Conducting DFM analysis (Design for Manufacturability) and mold flow analysis before production can prevent more than 80% of defect risks in advance.