warpage

ENGINEERING POLYMERS: THE ‘TOP TEN’INJECTION MOULDING PROBLEMS

R. Wilkinson, E.A. Poppe, K. Leidig and K. Schirmer

Part 9: 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.

unreinforced fibre reinforced

What can be done when warping has occurred?

The most important step, especially in the case of glass-fibre-reinforced materials, is to carry out a mould filling study, i.e. by partly filling the mould in several stages. By studying the melt front profile, fibre orientation can be reconstructed. By referring to the shrinkage curve for the reinforced material, measures can be taken to reduce warpage, e.g. by incorporating flow aids or flow restrictors. These alter the melt front profile, thereby influencing warpage.

This method requires a great deal of practical experience and, at the same time, increases the amount of knowledge to enable precautionary measures to be taken in future. It also has its limitations, due to the properties of the raw material and physical conditions. For crystalline polymers, it is not possible to obtain the same flatness as for amorphous polymers. In this connection, mention should be made of low-warpage, semi-crystalline polymer blends. These represent a compromise between properties and warpage, due to chemical modification or the combination of different reinforcing components. The last, and also the most expensive method consists of modifying the mould. If there already is experience with similar mouldings, correctable inserts are the best solution for critical parts.