Warpage

1. Moisture in the granules
2. Feed system too small
3. Wrong gate position
4. Hold time too short
5. Wrong melt temperature
6. Wrong tool temperature
7. Poor surface finish
8. Problems with hot runners
9. Warpage
10. Mould deposit

Partially crystalline substances such as POM (acetal), PA (nylon), PBT and PET (polyesters) tend to warp far
more than amorphous ones. This point should be taken into consideration already when designing moulds and
mouldings. If this is not done, it is almost impossible to rectify at a later stage. This article discusses the
causes of warpage and steps that can be taken to prevent and reduce it.

What are the main causes of warpage?

Shrinkage is relatively high in partially crystalline materials and is influenced by a number of factors. In the
case of unreinforced materials, warpage is greatly influenced by wall thickness and mould surface temperature.
It follows that major differences in wall thickness and unsuitable mould temperatures will cause the moulding
to warp. Totally different shrinkage characteristics will be evident in the case of glass fibre reinforced
materials, due to orientation of the glass fibres. The effect of wall thickness differences on shrinkage is
relatively slight. Here, the main cause of warping is the difference between fibre orientation longitudinally and
at right angles to the direction of flow. Warpage is essentially due to wall thickness distribution, gate location,
flow restrictions and by-passes, as well as the inherent rigidity of the moulded part.
These different causes of warping, depending on whether the material is fibre-reinforced or not, frequently
result in contrary warping phenomena in the same part.

How can warpage be prevented?

Unreinforced materials require uniform wall thicknesses. Melt accumulations should be avoided as far as
possible. Multi-point gating can be used to achieve a high pressure gradient and thus reduce shrinkage
differences to a minimum. The mould heating system should be designed so that heat is dissipated as evenly as
possible
With glass fibre reinforced materials, the symmetry of the moulded part is as important as uniform wall
thickness. Asymmetrical parts hinder melt flow and thus orientation, and eventually cause warpage. In the case
of asymmetrical parts it is therefore necessary to achieve a balance by incorporating blind cores at the mould
planning and design stage. The position of the gate is also important – every by-pass and every weld line is a
potential cause of warping.

What possibilities are open to the moulder?

Assuming that the moulded part, the gate and the mould have all been correctly designed, the moulder can
control warpage up to a point via the holding pressure and mould temperature. The use of several heating
circuits to optimise heat dissipation is normal practice.
In the case of reinforced materials, changing the injection rate or lowering the mould temperature is a slight
help. If the possibility of subsequent warpage has not been foreseen at the mould and moulded part design
stage, this cannot be subsequently rectified by modifying moulding conditions.