Front page / What causes warpage in injection moulded parts?
What causes warpage in injection moulded parts?
Warpage is caused by uneven shrinkage: when one area or one side of a part shrinks more than another, the part bends or twists once it leaves the mould. The usual design causes are uneven wall thickness, large flat areas with nothing to stiffen them, and fibre-filled resins flowing in an unbalanced way; the usual mould cause is one half running hotter than the other.
Every thermoplastic shrinks as it cools. A little shrinkage is expected and is built into the mould size. Warpage is the visible result of shrinkage that is not the same everywhere, which locks stress into the part until it is released from the steel.
Published figures
- Fictiv's warpage guide gives semi-crystalline resins (such as polypropylene, acetal and nylon) shrinkage of about 1 to 3 percent, against typically under 0.7 percent for amorphous resins such as ABS and polycarbonate. The more a resin shrinks, the more any imbalance shows.
- Fictiv suggests keeping the temperature difference between the two mould halves within about ±2 °C to avoid a directional bow.
- In one example Fictiv describes, straight cooling channels left more than 10 °C of variation across a cavity; conformal cooling cut that below 2 °C and removed the post-mould warp.
- Protolabs' wall-thickness guidance says thermoplastic moulding is generally stable and repeatable between about 1.5 mm (0.060 in) and 2.5 mm (0.100 in), and that ribs and gussets should be no more than about 60 percent of the wall they support.
The main causes
| Cause | Why it warps | Typical fix |
|---|---|---|
| Uneven wall thickness | Thick areas cool later and shrink more than thin ones | Core out thick sections; keep walls uniform |
| Large flat panels | Nothing resists the internal stress | Add ribs, gussets or a slight crown |
| Uneven mould cooling | The hotter side shrinks more after ejection | Balance cooling channels and mould temperatures |
| Fibre orientation | Glass fibres resist shrinkage along their length, not across it | Move or add gates to balance the flow |
| Uneven packing | Areas far from the gate get less pressure and shrink more | Gate nearer thick areas; review gate size |
| Ejected too hot | The part is still soft enough to move | Longer cooling or better cooling, not just more time |
The causes in the table follow Fictiv's guide. Its point about fibres is worth repeating: in a glass-filled grade, warp depends heavily on how the fibres line up during filling, which depends on where the gate is.
A warpage check before the mould is cut
- Map wall thickness across the part and look for any area much thicker than its neighbours.
- Check that ribs are thinner than the wall they join, so the junction does not become a thick spot.
- Look for large flat areas, especially on long parts, and ask whether they need stiffening.
- For filled resins, ask the toolmaker how the gate position will orient the fibres.
- Ask for a fill, pack and warp simulation on any part with a flatness requirement. Fictiv notes that tools such as Moldflow and Moldex3D predict shrinkage and warp, but results still need checking on real parts.
- Agree how flatness will be measured, and on what fixture, before first samples.
The figures here are published planning values. Shrinkage and warp depend on the specific grade, part and mould, so the resin data sheet and toolmaker's review come first.
Questions buyers ask
Can the moulder fix warpage with process settings?
Partly. Packing pressure, mould temperatures and cooling time can reduce it, but if the cause is in the geometry or the cooling layout, settings alone usually cannot remove it.
Why did my prototype not warp but the production part does?
The resin, mould material, cooling and cycle time may all have changed. A 3D-printed or machined prototype does not shrink the way a moulded part does, so it cannot show moulded warp.
Is warpage the same as sink?
No. Sink is a local dip over a thick spot; warpage is the whole part bending or twisting. Both come from shrinkage, and both are made worse by thick sections.
Can a warped part be straightened afterwards?
Some moulders cool parts on fixtures to hold them flat, but that adds cost and the stress remains in the part. Designing out the cause is more reliable.