Warpage is uneven shrinkage. One area of the part shrinks more than another as it cools, and the part bows or twists to relieve the stress. Process changes help, but often the biggest lever is the material.
Why some resins warp more
- Semi-crystalline resins (polypropylene, polyethylene, nylon, acetal, PBT) shrink more, and less evenly, than amorphous resins (ABS, polycarbonate, polystyrene, PC/ABS). See typical values in our shrinkage chart.
- Glass fiber makes shrinkage uneven: parts shrink less along the fiber direction than across it, which pulls flat parts out of shape.
- Uneven wall thickness and cooling make any resin worse.
Material changes that reduce warp
| Change | Why it helps |
|---|---|
| Mineral-filled or glass + mineral grades | Mineral (talc, mica, glass bead) shrinks evenly in every direction. |
| Lower glass content, or shorter fiber | Less difference between flow and cross-flow shrinkage. |
| Switch to an amorphous resin | Lower, more uniform shrinkage, if the part doesn’t need chemical resistance. |
| A nucleated or low-warp grade | Some grades are formulated to crystallize more uniformly. |
| A consistent supplier lot to lot | Shrinkage that changes between lots often traces back to melt flow or filler variation. |
Process checks first
Before changing material, check mold temperature balance between halves, cooling time, packing pressure and gate location. If those are right and the part still warps, it’s time to look at the resin.
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