Troubleshooting · Defect guide
Warped, bowed or twisted parts
Find the cause before you change the recipe
Warpage has one root cause: different parts of the moulding shrink by different amounts. The hard part is that the difference can come from many places: uneven walls, uneven mould temperature, flow direction, fibres, even the colour you add. This guide helps you trace where yours comes from before you spend money on a material change.
Marketing Department, ADVANCEDCHEM SOLUTIONS CO., LTD. · 22 October 2026 · about 7 min read
Identify the type of warp
Put the part on a flat table or surface plate and look at how it deforms. The shape of the warp points fairly directly at the cause.
- Bow: the whole part curves one way. Usually the two faces cooled at different rates; the part bows towards the hotter side.
- Twist: opposite corners lift. Usually asymmetric flow or fibre orientation.
- Corners pulling in or walls leaning inwards in box-shaped parts: the inside walls cool more slowly than the outside, because the core is hard to cool.
- Only some colours warp: same mould and machine, one colour warps and another does not. That points to the pigment in PP.
Also note whether the part warps at ejection or after sitting for a while, because the two are fixed differently.
Mechanism: uneven shrinkage = warpage
Every plastic shrinks as it cools. If every region shrinks equally, the part gets smaller but stays flat. Trouble starts when one region shrinks more than another and pulls the rest out of shape. The difference comes in three forms: by location (thick versus thin walls), through the thickness (one face cools faster than the other) and by direction (along versus across the flow).
The practical takeaway: find which kind of difference you have. A difference between the two faces is fixed in cooling; a difference by direction is fixed in gating, flow or material. Simply adding cooling time often just lengthens the cycle without removing the cause.
Four groups of causes
1. Part design
Uneven wall thickness within one part: thick areas cool later and shrink more than thin ones. Wide flat panels without ribs to stiffen them, and box shapes whose inside corners are hard to cool. If the wall has local thick spots, see Sink marks and voids as well, because the root cause is similar.
2. Mould
Unbalanced cooling circuits between the two halves, channels partly blocked by scale, cores that coolant cannot reach, and gate locations that make the melt flow in several directions or leave uneven pressure across the part.
3. Process
Different surface temperatures on the two halves, ejecting parts while they are still too hot, uneven packing between the gate area and the end of flow, and stacking parts while they are still warm. The underlying settings are explained in Injection molding.
4. Material
Semi-crystalline resins such as PP and HDPE shrink more and are more sensitive to cooling rate than amorphous ones. Glass fibre makes shrinkage strongly direction-dependent. Some pigments in PP, such as the phthalocyanine blues and greens, act as nucleating agents, so parts in one colour warp while others do not. Shrinkage by resin type is covered in Polymer basics.
Step-by-step checks, cheapest first
- Measure surface temperature on both mould halves at several points during normal running. If one half is clearly hotter and the part bows towards it, you have found the cause.
- Mould natural resin next to the coloured compound. If the natural part is flat and the coloured one warps, the problem is the pigment, not the machine.
- Measure parts twice: right after ejection and after sitting for at least a day. If warp grows later, there is high residual stress or high post-moulding shrinkage.
- Check flow rate and inlet/outlet temperature of each cooling circuit. A partly blocked channel makes one side hotter even with identical set points.
- Run a short-shot series to see how the melt travels from the gate. Asymmetric or multi-directional filling is a common source of twist.
- Extend cooling time step by step. If warpage drops clearly, parts were being ejected too early, but weigh that against the longer cycle.
- Review design and gating. If the steps above are not enough, wall thickness, ribs, gate position or the cooling layout must change. That is mould work and the most expensive step.
Cause → check → fix table
| Cause | Check | Fix |
|---|---|---|
| Mould halves at different temperatures | Measure surface temperature at several points on both halves | Balance coolant temperature; descale the channels |
| Core not reached by cooling | Inside walls of box parts lean in, corners pull in | Add baffles or bubblers; use a high-conductivity core insert |
| Parts ejected too hot | Warp drops when cooling time is extended | Set enough cooling time, or use a cooling jig as a supporting measure |
| Uneven wall thickness | Measure thickness at several points | Even out the walls; core out thick areas |
| Asymmetric flow | Short-shot series, watch the flow front | Move or add gates for a symmetric fill |
| Packing differs between gate and end of flow | Compare dimensions near the gate and at the end of flow | Adjust the holding pressure profile; enlarge the gate |
| Pigment acting as a nucleating agent in PP | Natural resin flat, coloured part warps | Ask the colour masterbatch supplier for pigments that do not drive directional crystallisation |
| Glass fibre orientation | Warp increased after moving to a glass-filled grade | Revise gating, or consider adding a mineral filler |
| Parts stacked while warm | Parts at the bottom of the stack warp more | Cool parts laid out on racks before packing |
How much can additives help?
Sometimes, and only once you know where the shrinkage difference comes from. If the cause is uneven mould temperature or uneven walls, a recipe change will hardly help. These are the material options that actually exist, including ones ACS does not sell:
- Nucleating agents in PP make crystallisation faster and more uniform. Some types reduce warpage; others increase the directional shrinkage difference and make it worse. Choose the type for the part and test side by side. See Cycle time and warpage for the details.
- Mineral fillers such as talc reduce overall shrinkage and, in many cases, the difference by direction. Stiffness, toughness, weight and surface change with them.
- Glass fibre adds strength but makes shrinkage more direction-dependent, so flat parts warp more easily. It is not a warpage fix.
- Changing the pigment in the colour masterbatch, once you have shown that the warp depends on the colour.
What additives cannot fix: unbalanced cooling circuits, blocked channels, uneven wall thickness, and ejecting parts while they are still hot.
Technical caution
Let-down ratios and technical data on this page are typical reference values, not a guarantee of properties. Confirm by trial in your own process before production use.
FAQ
Warpage questions we hear often
We changed the colour masterbatch and PP parts started warping, with the same mould and machine. Why?
Some pigments, especially the phthalocyanine blues and greens, act as nucleating agents in PP and make crystals grow more in one direction than the other. Shrinkage along and across the flow then differs more, so parts in that colour warp while other colours do not. Mould natural resin next to the coloured compound. If the warpage follows the colour, talk to your colour masterbatch supplier about the pigment, rather than only adjusting the machine.
Parts are flat at ejection but warp after sitting for a while. What causes that?
Plastics keep shrinking after leaving the mould, especially semi-crystalline resins such as PP, which continue to crystallise for some time. Residual stress in the part also relaxes over time or when the part is heated. If parts are ejected too hot, the hotter side shrinks more and pulls the part out of shape. Cooling parts in a jig can hide the problem, but if residual stress stays high the part may warp again in hot service conditions, so fix the cooling balance first.
Can we run the two mould halves at different temperatures to bend the part back?
Yes, and it is common practice on the shop floor. Parts usually bow towards the hotter side, so heating one half can offset the warp. But the working window is narrow, and the effect shifts with coolant temperature, mould condition and resin lot. Treat it as a temporary measure while you fix the real cause, such as an unbalanced cooling circuit or uneven wall thickness.
Why did warpage get worse after switching to a glass-fibre grade?
Glass fibres align with the flow, so the part shrinks little in the fibre direction and more across it. That difference causes warpage, especially in flat parts or parts filled from several directions. The usual fixes are gate location, so the flow runs in a more uniform direction, or adding a platy or particulate mineral such as talc to reduce the shrinkage difference, which must be confirmed on the actual part.
Parts still warping after tuning the machine?
Send a photo of the part on a flat surface, the surface temperatures of both mould halves and the colour or masterbatch you use on LINE. Our technical team will help you separate mould, process and material causes.