Injection molding is a widely used forming technology in modern manufacturing, producing complex parts by injecting molten plastic into molds and cooling it for solidification. However, various defects often arise during production due to factors such as material properties, mold design, process parameters, or equipment conditions, affecting product quality and efficiency. This article analyzes common issues in injection molding and proposes corresponding solutions.

1. Common Problems and Causes

1.1 Surface Defects

• Sink Marks: Local depressions on product surfaces, typically in thick-walled or ribbed areas.
  Cause: Uneven material cooling, insufficient packing pressure, or short cooling time.
• Weld Lines: Linear marks where two melt flows converge, potentially reducing strength.
  Cause: Poor mold runner design, slow injection speed, or low material temperature.
• Flow Marks: Silver or white streaks on product surfaces, compromising aesthetics.
  Cause: Material degradation, excessive injection speed, or low mold temperature.

1.2 Dimensional Deviations

• Warping: Product bending or twisting after cooling, failing assembly requirements.
  Cause: Uneven cooling, insufficient packing time, or varying material shrinkage rates.
• Dimensional Out-of-Tolerance: Actual dimensions deviating from design specifications.
  Cause: Mold wear, process parameter fluctuations, or material batch differences.

1.3 Filling Issues

• Short Shot: Incomplete mold cavity filling due to insufficient melt flow.
  Cause: Low injection pressure, low material temperature, or blocked runners.
• Air Traps: Bubbles forming inside or on product surfaces, weakening structure.
  Cause: Poor venting, excessive injection speed, or material moisture.

1.4 Mold-Related Problems

• Mold Wear: Scratches or corrosion on mold cavity surfaces, causing rough product finishes.
  Cause: Corrosive materials, improper mold maintenance, or excessive usage cycles.
• Mold Sticking: Difficulty in product ejection, risking mold or product damage.
  Cause: Insufficient draft angles, poor ejector design, or high material adhesion.

2. Solutions and Optimization Measures

2.1 Surface Defect Mitigation

• Sink Marks: Increase packing pressure/time; optimize cooling system design; adjust wall thickness uniformity.
• Weld Lines: Improve runner layout; raise injection speed/material temperature; add venting slots at convergence points.
• Flow Marks: Reduce injection speed; ensure proper material drying; optimize mold temperature control.

2.2 Dimensional Stability Improvements

• Warping Control: Extend cooling time; balance mold cooling water circuits; symmetrize product design.
• Dimensional Accuracy: Regularly calibrate molds; stabilize process parameters; select materials with consistent shrinkage rates.

2.3 Filling Problem Resolution

• Short Shot Prevention: Raise injection pressure/speed; check runner patency; increase material temperature.
• Air Trap Elimination: Add venting channels; use multi-stage injection (slow-fast); strictly dry materials.

2.4 Mold Maintenance Enhancements

• Mold Wear Repair: Polish cavity surfaces periodically; use corrosion-resistant steel/coatings; limit mold usage cycles.
• Sticking Reduction: Increase draft angles; optimize ejector mechanisms; apply release agents.

3. Conclusion

Problems in injection molding stem from interrelated factors like materials, molds, processes, and equipment. Systematic analysis is key to identifying root causes. Solutions include optimizing parameters, improving mold design, enhancing equipment maintenance, and strict material management. Simulation software (e.g., Moldflow) can further reduce trial costs by predicting issues early. Future advancements in smart manufacturing will drive injection molding toward greater precision and efficiency.