DefinitionWarpage is the unintended distortion of a molded part, such as bowing, twisting or curling out of its designed shape, that appears once the part is released from the mold. It is caused by uneven shrinkage, usually from differences in wall thickness, uneven mold cooling or, in fiber-filled plastics, fiber orientation.
Explained
What Is Warpage in Injection Molding?
Every molded plastic part shrinks as it cools. If it shrank by exactly the same amount everywhere, it would simply come out slightly smaller than the mold, and the mold would be cut oversize to compensate. Warpage happens when shrinkage is not uniform. Internal stresses build up while the part is held in the mold, and once it is ejected and the pressure is released, those stresses bend or twist it.
The usual sources of uneven shrinkage are:
- Wall thickness differences. Thick sections cool more slowly and shrink more than thin ones.
- Mold temperature differences. If the cavity side and core side run at different temperatures, one face of the part shrinks more than the other and the part bows.
- Flow direction. Many plastics shrink differently along the flow and across it, so gate position changes how a part warps. In MIT's course example, a center gate with radial flow causes severe distortion, while end gates with linear flow give minimal warping.
- Fiber orientation. The glass fibers in a filled plastic are oriented by the flow, so the part shrinks by different amounts along and across the flow direction. Research on glass-fiber-reinforced plastics identifies that shrinkage difference as the cause of their warpage, which is typically worse than in the unfilled polymer.
- Insufficient packing. Too little packing pressure leaves thick areas free to shrink more.
Shrinkage vs Warpage
| Shrinkage | Warpage | |
|---|---|---|
| What it is | The overall reduction in size as the part cools | Distortion of shape caused by uneven shrinkage |
| Is it normal? | Yes, every molded plastic shrinks | No, it is a defect |
| How it is handled | The mold is cut oversize by the material's shrink rate | Design, gating, cooling and packing are changed |
| Typical values | About 0.3–0.8% for ABS, 0.5–0.8% for PC, 1.0–3.0% for PP, 0.6–2.0% for nylon | Measured as flatness or deflection on the part |
Semi-crystalline plastics such as polypropylene and nylon shrink more, and over a wider range, than amorphous plastics such as ABS and polycarbonate, one reason they are generally more prone to warping.
How to Reduce Warpage
- Keep wall thickness uniform, and add ribs rather than thick sections for stiffness.
- Choose a gate position that gives a straight, even flow path through the part.
- Balance the cooling lines so cavity and core run at similar temperatures.
- Increase packing pressure and allow a longer cooling time before ejection.
- Check ejector pin placement and draft so the part is not distorted on the way out.
- For glass-filled grades, plan fiber orientation with the gate and allow for anisotropic shrinkage in the tolerances.
Warpage usually shows up at the first mold trial. Every plastic injection molding project at NICE Rapid runs T1 and T2 trials and a pilot batch with a full dimensional inspection report before mass production.
Key takeaways
- Warpage is a molded part defect where the part bows, twists or curls after ejection.
- It is caused by uneven shrinkage from wall thickness differences, uneven mold cooling, flow direction, fiber orientation or insufficient packing.
- Shrinkage is normal and expected; warpage is distortion caused by non-uniform shrinkage.
- Semi-crystalline plastics such as polypropylene and nylon are generally more prone to warping than amorphous plastics.
- Warpage can be reduced through uniform wall thickness, balanced cooling, proper gate location, adequate packing and correct ejection design.