Injection Molding Warpage Analysis
Theory: warpage comes from shrinkage differences
Overview
If a moulding shrank uniformly it wouldn't warp. Where do the differences come from?
Differences arise through the thickness, between regions, and by direction. Injected resin freezes from the mould surfaces; if the two faces cool differently, the earlier- and later-cooled sides shrink differently and the plate bends. Thick regions cool slowly and receive less packing pressure, so they shrink more and pull on thinner surroundings. And when molecules or fibres align with the flow, shrinkage differs along and across it. Warpage analysis takes temperature and pressure histories and orientation from filling, packing and cooling analyses, computes residual strain (shrinkage) from them and then finds the deformation of the whole part by structural analysis. Because parts are thin, through-thickness differences act strongly as bending.
Shrinkage difference, curvature and bow
Δs is the shrinkage difference between faces, h thickness, κ curvature, δ the mid-span bow of a plate of length L, α_eff an effective expansion coefficient and ΔT the temperature difference between fixed and moving mould halves.
Knowing the warp before cutting steel
Warpage used to be fixed by moulding with a trial tool, recutting it and repeating. Each tool correction took weeks and cost heavily, so filling and cooling analysis was commercialised from the 1980s, and in the 1990s software solving fibre orientation through to warpage spread. Today simulation is also used to pre-compensate tool geometry in the opposite direction of the predicted warp.
Worked examples
κ = Δs/h assumes a free homogeneous thin plate with a linear through-thickness eigenstrain gradient; a two-layer model has a different coefficient. The 1.4 mm directional shrinkage difference is a dimensional difference, not a bow height.
Example 1: bow of a 2 mm plate (effective expansion 1×10⁻⁴/K assumed)
| Mould temperature difference | Shrinkage difference | Curvature | Bow, L = 100 mm | Bow, L = 200 mm |
|---|---|---|---|---|
| 5 K | 0.05% | 0.00025 /mm | 0.31 mm | 1.25 mm |
| 10 K | 0.10% | 0.00050 /mm | 0.62 mm | 2.50 mm |
| 20 K | 0.20% | 0.00100 /mm | 1.25 mm | 5.00 mm |
Bow scales with length squared, so a part twice as long bows four times as much for the same shrinkage difference. Shrinkage difference is not set by temperature alone; this is a simple estimate to show the trend.
Example 2: orientation shrinkage difference in glass-filled material (200 mm)
| Material / direction | Shrinkage | Shrinkage on 200 mm |
|---|---|---|
| Filled, flow direction | 0.3% | 0.6 mm |
| Filled, transverse | 1.0% | 2.0 mm |
| Unfilled PP (reference) | 1.5% | 3.0 mm |
Filled material shrinks less overall but the directional difference is a large 1.4 mm, appearing in box-shaped parts as twist or leaning walls. Shrinkage values are typical.
The warpage result shows only about 1 mm of deformation, yet assembly leaves a 3 mm gap. Is the analysis wrong?
Check how deformation is displayed and constrained. Warpage results look very different depending on which points are assumed fixed. Referenced to the part centre, edge deformation looks small; re-displayed with constraints at the mounting holes or bosses used in assembly, the far edge lifting 3 mm is not unusual — twist especially so. Some materials also warp differently right after moulding than after days of moisture uptake and stress relaxation. Compare deformations constrained at the same datum points as the measurement and under the same timing. If the gap persists, revisit fibre orientation prediction and packing and cooling conditions (the actual mould temperature distribution).
Practical workflow
- Prepare the material's PVT (pressure-volume-temperature) data, viscosity, thermal properties and, for filled grades, orientation-related data.
- Check weld lines and how packing pressure reaches each region in filling and packing analysis.
- Run cooling analysis including cooling circuits and check the temperature difference between mould halves.
- In warpage analysis, separate contributions by cause (cooling, region, orientation).
- Compare deformation constrained at assembly datums with trial measurements.
“Separate the causes”
Many warpage tools can split warp into 'differential cooling', 'differential shrinkage (regional)' and 'orientation' contributions. Changing mould temperature while watching only the total does little if orientation dominates. Knowing the dominant cause tells you whether to fix the cooling circuit, move the gate to change flow direction, or even out wall thickness.
Common mistakes
Mistakes and fixes
| Mistake | Effect | Fix |
|---|---|---|
| Constraints differ from measurement | Warp comparison fails | Constrain at assembly datums |
| Uniform mould temperature | Through-thickness difference missed | Analyse cooling circuits |
| Ignoring fibre orientation | Twist missed | Couple orientation analysis |
| PVT data from another grade | Wrong shrinkage | Use data for the actual grade |
| Packing differs from the machine | Wrong shrinkage in thick sections | Enter actual machine settings |
I'd like to learn related topics.
Related pages include injection molding cooling analysis, fiber orientation analysis, injection molding filling analysis and springback analysis.
Frequently asked questions
Why do injection-moulded parts warp?
Because shrinkage differs by region and direction. Three main causes: (1) uneven cooling through the thickness, for example from a temperature difference between the fixed and moving mould halves; (2) different cooling and shrinkage between regions of different wall thickness; and (3) direction-dependent shrinkage from molecular or fibre orientation. Warpage analysis takes shrinkage from filling, packing and cooling results and computes deformation by structural analysis.
How much does a mould temperature difference warp a part?
A 0.1% shrinkage difference between the faces of a 2 mm plate gives curvature 0.0005/mm: 0.62 mm of bow over 100 mm and 2.5 mm over 200 mm. With an effective expansion coefficient of 1×10⁻⁴/K, a 10 K difference between mould halves can produce such a difference, so cooling-circuit design ties directly to warpage.
How is warpage different for fibre-filled materials?
Shrinkage depends strongly on fibre direction. A glass-filled material might shrink 0.3% along the flow and 1.0% across it — 0.6 mm versus 2.0 mm on a 200 mm part, a 1.4 mm difference. Predicting fibre orientation and passing directional shrinkage and stiffness to the structural analysis is essential for warpage prediction.
References and scope of verification
Numbers are educational reference calculations under the stated assumptions. Sources document theory and setup, not proof that these numerical values were reproduced in each package. Engineering decisions require validation for the actual conditions.
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