mold insurance

Review of the pressure/volume/temperature (PVT) relationship of a polymer, as presented in Chapter 2 will show that the volume change of the polymer as it cools is very significant. The volumetric shrinkage at zero pressure can be as much as 20 to 30%. An example of the PVT relationship of a polypropylene is shown in Fig. 3.7. As the material is cooled from a melt temperature of 240 °C to room temperature (23 °C), the specific volume of the material at 0 MPa pressure is reduced by nearly 20%. This shrinkage should not be confused with the linear in-mold shrinkage values used in mold construction. Figure 3.7 also shows that at the maximum pressure of 200 MPa, the specific volume of the material (at 240 °C) is reduced to approximately 88% of its original volume. The PVT relationship characterizes the primary volumetric shrinkage behavior of a polymer as it cools under varying pressure throughout the molding cycle. The shrinkage of the plastic in the mold can cause many problems in addition to its effects on warpage, residual stresses, and predicting part size. Shrinkage is also the reason, for high ejection forces required to remove the part from the core of a mold. The shrinkage over any core of a mold leaves a residual tension stress in the part, which applies a normal load to the side surfaces of the core section. This normal force increases the friction experienced by the part when it is ejected, sometimes causing excessive ejection forces, which can damage a part. Another problem introduced by polymer shrinkage is the tendency to pull away from the mold’s cavity wall, thereby losing intimate contact, which will increase the cooling time for the part. The general tendency on a simple core/cavity part would be for the polymer to shrink away from the cavity wall but maintain good contact with the core wall. However, if adequate pressure is maintained on the part as the frozen layer thickness builds during the packing phase, good contact with the cavity wall can be improved, which will help uniformity of cooling and minimize cooling time.

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