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Common Injection Molding Defects and Solutions

Injection molding is a highly efficient manufacturing process, but it can produce defects that affect product quality, performance, and aesthetics. Common injection molding defects issues include short shots (incomplete filling), flash (excess material leakage), sink marks (surface depressions), warpage (uneven cooling distortion), and burn marks (overheating discoloration). Other problems like welding lines, flow marks, bubbles, and cracking may arise from material, mold design, or process parameter mismatches. Belowing is the common injection molding defects and solutions for your reference.

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Common Injection Molding Defects

Common Injection Molding Defects & Solutions
Defect Name Description Possible Causes Solutions
Short Shot Incomplete filling of the mold cavity, resulting in missing material. 1. Insufficient injection pressure/speed
2. Melt temperature is too low
3. Poor runner/gate design
4. Poor venting		 1. Increase injection pressure/speed
2. Raise melt temperature
3. Optimize gate design
4. Add venting channels
Flash Excess plastic leaks out of the parting line or ejector pins, forming thin edges. 1. Insufficient clamping force
2. Mold wear or excessive gap
3. Injection pressure too high		 1. Increase clamping force
2. Repair mold
3. Reduce injection pressure
Sink Mark Localized depression on the product surface, usually in thick sections. 1. Insufficient packing pressure/time
2. Uneven cooling
3. High material shrinkage		 1. Increase packing pressure/time
2. Optimize cooling system
3. Use low-shrinkage material
Burn Mark Black/brown discoloration, often in trapped gas areas. 1. Poor venting (gas burning)
2. Melt temperature too high		 1. Improve venting or reduce injection speed
2. Lower melt temperature
Warpage Product deformation after cooling. 1. Uneven cooling
2. Excessive internal stress
3. Non-uniform shrinkage		 1. Optimize cooling system
2. Adjust packing curve
3. Use fiber-reinforced material
Impurities Foreign particles on/in the product. 1. Contaminated material
2. Dirty mold
3. Residual material in barrel/screw		 1. Clean material & mold
2. Regularly purge injection machine
Drag Marks Surface scratches during ejection. 1. Insufficient draft angle
2. Poor ejector pin design
3. Rough mold surface		 1. Increase draft angle
2. Optimize ejector layout
3. Polish mold
Welding Line Visible lines where molten plastics merge with insufficient temperature. 1. Melt temperature too low
2. Unfavorable merging angle
3. Slow injection speed		 1. Increase melt temp/injection speed
2. Adjust gate location
Flow Mark Wavy patterns on the product surface. 1. Uneven melt temperature
2. Injection speed too fast/slow		 1. Increase melt temperature
2. Adjust injection speed
Ejector Mark White marks or protrusions at ejector pin locations. 1. Excessive ejection force
2. Insufficient cooling time		 1. Reduce ejection force or add more pins
2. Extend cooling time
Wiredrawing String-like residue at the gate 1. Mold opening too fast
2. High melt viscosity		 1. Reduce mold opening speed
2. Adjust melt temp or use lower-viscosity material
Bubble Air pockets inside or on the surface. 1. Moisture in material
2. Insufficient packing
3. Poor venting		 1. Pre-dry material
2. Increase packing pressure
3. Improve venting
Discoloration Color inconsistency. 1. Poor colorant dispersion
2. Material degradation due to high temp		 1. Improve mixing
2. Lower melt temperature
Silver Streaks Silvery streaks on the surface. 1. Moisture in material
2. Excessive volatiles		 1. Dry material thoroughly
2. Clean barrel or reduce temperature
Greasy Dirt Oil stains or dirt on the surface. 1. Excessive mold release agent
2. Dust contamination		 1. Reduce mold release use
2. Clean production environment
Structure Problem Dimensional or shape inaccuracy. 1. Mold machining error
2. Incorrect process parameters		 1. Modify mold
2. Adjust parameters (e.g., shrinkage compensation)
Shine Overly glossy spots inconsistent with surroundings. 1. Mold temperature too high
2. Excessive shear stress		 1. Lower mold temperature
2. Adjust injection speed
Cracking Cracks on the surface or inside the part. 1. High internal stress
2. Low material toughness
3. Over-packing		 1. Optimize packing curve
2. Use higher-toughness material
3. Reduce injection pressure
Black Streaks Black streaks on the surface. 1. Material degradation
2. Carbon buildup in barrel/screw		 1. Lower melt temperature
2. Clean injection machine
Broken Part fractures due to low mechanical strength. 1. Material aging
2. Weak design zones
3. Stress concentration		 1. Change material
2. Optimize part design

Notes:

  • Multiple Factors: Some defects may result from combined causes (e.g., sink marks relate to both packing and cooling).

  • Material Dependency: Different plastics (ABS, PP, PC, etc.) require tailored solutions.

  • Mold Maintenance: Regular cleaning/polishing prevents many defects.

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