Common Injection Molding Defects and Solutions

Injection molding defects typically arise from four factors: material, process, mold, and machine. This guide covers 28 common defects, prioritized for quick on-site troubleshooting.

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Part 1: Surface/Appearance Defects

Defect	Description	Key Solutions
1. Sink Marks	Local surface depression	Increase pack/hold pressure & time; enlarge gate; optimize rib thickness (≤0.6× wall)
2. Charred Streaks	Silver/brown streaks	Reduce injection speed; avoid sharp corners; check material drying
3. Moisture Streaks	Wavy silver streaks	Pre-dry material thoroughly; increase mold temp; lower melt temp
4. Air Streaks	Grooves, dots or lines on surface	Add/deepen vents; adjust injection speed; change gate location
5. Colored Streaks	Inconsistent color	Increase back pressure; lower melt temp; use mixing screw
6. Glass Fiber Streaks	Rough surface, exposed fibers	Raise mold/melt temp; inject faster; use shorter glass fibers
7. Gloss Differences	Matte or shiny spots	Increase and balance mold temp; raise pack pressure; optimize transfer point
8. Jetting	Snake-like pattern near gate	Reduce injection speed; enlarge gate; raise melt/mold temp
9. Record Grooves	Wavy ripples on surface	Increase injection speed/pressure; raise melt/mold temp
10. Dull Spots Near Gate	Shadow near gate	Reduce speed near gate; enlarge gate; add radius transition

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Part 2: Internal/Filling Defects

Defect	Description	Key Solutions
11. Blistering	Round/oval internal voids	Vacuum: lower speed, increase pack pressure; Gas: dry material, increase back pressure
12. Voids	Irregular cavities in thick sections	Locate gate at thickest wall; avoid sudden wall thickness changes
13. Short Shot	Incomplete filling	Increase injection pressure/speed/temp; enlarge gate; improve venting
14. Weld Line	Visible line where flows meet	Raise melt/mold temp/speed; add venting; use overflow tab
15. Cold Slugs	Hardened material lumps	Check nozzle temp; add cold well; adjust decompression
16. Dark Spots	Black spots on light parts	Clean screw/barrel; check material for contaminants; lower melt temp

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Part 3: Deformation/Mechanical Defects

Defect	Description	Key Solutions
17. Warpage	Overall part distortion	Balance cooling; optimize gate location; adjust packing; balance ejection
18. Deformation During Demoulding	Bending/collapse on ejection	Increase cooling time; lower pack pressure; add ejector area; check undercuts
19. Visible Ejector Marks	White marks at ejector pins	Increase ejector count/area; add draft angle; lower pack pressure
20. Ejection Grooves	Scratch lines on side walls	Increase cooling time; increase draft angle; polish in ejection direction
21. Stress Whitening/Cracks	White marks or cracks	Lower pack pressure; optimize ejection; add radii to part design
22. Broken Film Hinge	Hinge fracture	Move gate away from hinge; raise mold temp; increase hinge thickness moderately
23. Flaking of Surface Layer	Layer separation	Check material purity/compability; optimize plastication; reduce injection speed

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Part 4: Other/Process Defects

Defect	Description	Key Solutions
24. Diesel Effect (Burner)	Local burning (yellow/black)	Reduce injection speed; add venting; change flow pattern
25. Flash	Excess plastic at parting line	Increase clamp force; reduce injection/pack pressure; repair mold
26. Filamentation	Stringy plastic near gate	Lower nozzle temp; increase cooling time; check hot runner
27. Plate Out	Deposits on mold/vent surfaces	Lower melt temp; shorten cycle time; check additive compatibility
28. Electroplated Part Defects	Blisters/cracks after plating	Reduce molded-in stress; dry material thoroughly; avoid sharp corners; use high-gloss mold finish

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Quick Troubleshooting Checklist

1. Material – Is it properly dried? Free of contaminants?
2. Mold – Are vents sufficient? Is gate design appropriate?
3. Process – Adjust in this order: temperature → speed → pressure. Change only one parameter at a time.
4. Machine – Is screw non-return valve worn? Is barrel temperature stable?