Warpage
The part distorts or bends after it leaves the mold.
Symptoms
- A surface that should be flat is curved
- Part dimensions differ from the mold dimensions
- Bending that increases over time
Tip: The direction of warpage shows which side shrinks more; the hotter half usually pulls inwards.
Order of solutions
- Support the part by increasing the cooling time or by holding it in a fixture after ejection
- Equalize the temperature of the mold halves
- Reduce the holding pressure and check the time
- Balance the cooling circuits
- If necessary, change the gate location and number or equalize the wall thickness
Likely causes
Process setting
- Cooling time short, part leaves hot
- Holding pressure high or uneven (internal stress)
- Temperature difference between the mold halves
- Melt temperature too high
Raw material
- Semi-crystalline material (PP, PA, POM)
- Flow/cross shrinkage difference in fiber-reinforced grades
Mold
- Cooling circuits unbalanced
- Gate location and number create the flow direction
Part design
- Uneven wall thickness
- Directional shrinkage difference in semi-crystalline material
- Fiber orientation in glass-fiber-filled material
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Related articles
Other defects
Short shot (incomplete fill)
The cavity does not fill completely; the farthest or thinnest area of the part stays incomplete.
Flash
Plastic escapes through the parting line or around the ejector pins.
Sink mark
A dent on the surface opposite a thick section; the surface is drawn inwards.
Void (internal vacuum void)
Internal cavity in a thick section of the part; the outer surface may look perfect.