10 Shrinkage and Warpage • Injection molding and shrinkage • Basic causes of shrinkage and warpage • Designing accurate parts considering warpage 10.1 Injection Molding and Shrinkage In this section the relationship between processing and shrinkage is considered. In particular, the effect of packing pressure on shrinkage is described. 10.1.1 What Are Shrinkage and Warpage? Part shrinkage may be thought of as a geometric reduction in the size of the part. If the shrinkage is uniform, the part does not deform and change its shape, it simply becomes smaller. Warpage results when shrinkage is not uniform. If regions of the part shrink unequally, stresses are created within the part which, depending on part stiffness, may cause the part to deform or change shape. In the long term parts can even crack. 10.1.2 Shrinkage and Machine Settings All molders know that shrinkage and consequently warpage is affected by processing conditions. Figure 10.1 shows some of the classic relationships between machine settings and shrinkage, also shown is the effect of wall thickness. These curves apply only to a particular mold and material combination. It is clear from Figure 10.1 that the final shrinkage of a component is a complex function of machine settings. Nevertheless, a major factor is the pressure and time history of the material as it fills, packs, and cools in the mold.
Shrinkage and Warpage
May 22, 2023
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