When a defect appears, isolate the variable:
1. Is it consistent? (mold/tooling issue)
2. Is it occasional? (process parameter drift)
3. Is it material-related? (moisture, contamination, batch variation)
4. Is it machine-related? (clamp pressure, injection unit wear)
Most Chinese molders keep a defect log — ask for it when troubleshooting.
Defect | Root Cause | Solution
--- | --- | ---
Sink Marks | Insufficient packing | Increase holding pressure by 20-30%, increase hold time
Weld Lines | Melt fronts too cool | Raise mold temp by 10-15°C, increase injection speed
Short Shot | Insufficient material volume | Increase shot size 5%, check check ring wear
Flash | Clamp pressure too low | Increase clamp force 10%, check mold parting line for damage
Warpage | Uneven cooling | Balance cooling channels, use conformal cooling
Jetting | Gate too small | Enlarge gate or change to fan/tab gate
Material | Common Defects | Prevention
--- | --- | ---
ABS | Sink marks, splay | Increase back pressure, dry 80°C/2h
Nylon PA66 | Short shot, flash | Dry 80°C/4h, increase mold temp to 80°C
PC | Cracking, voids | Dry 120°C/4h, slow injection speed
PP | Sink marks, warpage | Increase packing, use 40°C mold temp
POM | Gas burning | Reduce melt temp 10°C, improve venting
PMMA | Splay, bubbles | Dry 90°C/4h, slow speed
Typical scrap rates in Chinese molding shops:
Consumer parts: 2-5%
Automotive/interior: 1-3%
Medical: 0.5-1.5%
Electronics: 1-3%
If your scrap rate is above 5%, start with these fixes:
1. Check material drying — inadequate drying causes 40% of defects
2. Verify mold temperature uniformity (±5°C across cavity)
3. Run a Design of Experiments (DOE) for temp/pressure/speed
4. Inspect gate and runner balance (multi-cavity tools)
5. Review part design — sharp corners cause flow issues