Warpage is the deviation of a moulded part from its required shape. Warpage is always the result of shrinkage differences with the exception of distortions caused by deformation during demoulding.
Figure 23.1
Warpage on box-shaped parts
Figure 23.2
Warpage due to glass fiber orientation
Figure 23.3
Warpage on box-shaped parts
Warpage on injection-moulded parts is a large problem, as it is impossible to precisely predict it. Modifications to injection moulds often cause unpredictable costs and delays. Warpage is always the result of different shrinkage values in a moulded part.
Shrinkage
Shrinkage is a decrease in the volune of the moulded part during the cooling phase. If a plastic melt (figure 23.1a) cools down its volume will decrease due to the molecule chains nearing eachother(23.1b). In the case of partially crystalline materials,the shrinkage is higher compared with amorphous materials as a result of the crystallisation process, during which the molecule chains assume a very high packing density(23.1c).
Shrinkage is classified into:
-Moulding shrinkage(dimensional changes after 16 hours,see DIN 16901)
-Aftershrinkage(dimensional changes between the end of the moulding shrinkage and after a long time)
- A-Total shrinkage(moulding shrinkage and aftershrinkage)