Causes of warpage and how to prevent it

If the part comes out of the mold straight while hot but bends, closes up, or shrinks more on one face than the other after cooling, that's warpage. It's different from a sink mark: a sink mark is a localized surface defect, while warpage distorts the geometry of the entire part.

Root cause: uneven shrinkage in different places

The basic cause of warpage is that not every part of the piece shrinks at the same rate and in the same direction. If one region shrinks more, or in a different direction, than another, the difference remains inside the part as permanent residual stress, and the part bends to relieve that stress.

Three separate mechanisms usually drive this together:

1. Wall thickness difference. A thick region cools more slowly than a thin one and keeps shrinking longer. If the nominal wall thickness is 2 mm but one region suddenly jumps to 5 mm, that region keeps shrinking after the others have frozen and solidified, pulling the part toward itself.
2. Uneven distribution of cooling. The problem is usually not "slow cooling" overall but uneven cooling: if one region of the mold removes heat much more slowly than its surroundings, it stays hotter and keeps shrinking after the rest has set.
3. Material type and molecular/crystalline orientation. Shear stress during injection stretches and aligns molecules in the flow direction. In amorphous materials (ABS, PS, PC, etc.) this orientation relaxes while the material is hot; but if cooling happens at the same time as shear, the orientation freezes in and causes slightly more shrinkage along the flow direction — total shrinkage generally stays under 0.7%. In semi-crystalline materials (PP, PA, POM, etc.) noticeable crystallization is also added during cooling; shrinkage perpendicular to flow can be greater, and total shrinkage can reach the 1–3% range. This is why the same mold with the same geometry can be fine in PP but warp in ABS (or vice versa).

Where it shows up

Corners and thick-to-thin transitions are the highest-risk regions: at a corner, material accumulates thicker and the two surfaces cool at different rates. Gate location matters too; regions near and far from the gate experience a different pressure/temperature history.

Practical solution order

1. Keep wall thickness as constant as possible; make thickness transitions gradual (ramped), avoid sudden jumps.
2. Check cooling balance: do all channels get water at the same flow rate and temperature, is there a hot spot on the mold surface? Conformal cooling is one way to achieve this balance in complex geometries.
3. Equalize holding pressure and time until every region of the part solidifies in a balanced way; to do this by measuring instead of by "feel," the scientific molding approach and the viscosity curve test help.
4. If you're switching material (e.g. amorphous ↔ semi-crystalline), account from the start for the fact that shrinkage behavior will differ — don't "fine-tune" the mold for a single material only.

*This article draws on injection molding warpage articles from PTonline.com and related international sources.*