IPDB › China Industry › Manufacturing & Processing Machinery
China, Chinese Shrinkage and Warpage of Injection Moulding1 Industrial Products Supplier Manufacturer Details, price list catalog:
China Industrial Products Supplier Manufacturer List Catalog
Other MouldsMouldMold
Shenzhen Meibang Precision Technology Co., Ltd.
Preview:
Product Description Why do they occur?Shrinkage is inherent in the injection molding process. Shrinkage occurs because thedensity of polymer varies from the processing temperature to the ambient temperature(see Specific volume (pvT diagram)). During injection molding, the variation in shrinkage both globally and through the cross section of a part creates internal stresses. These so-called residual stresses (see Residual stress) act on a part with effects similar to externally applied stresses. If the residual stresses induced during molding are highenough to overcome the structural integrity of the part, the part will warp upon ejectionfrom the mold or crack with external service load.ShrinkageThe shrinkage of molded plastic parts can be as much as 20 percent by volume, when measured at the processing temperature and the ambient temperature. Crystalline and semi-crystalline materials are particularly prone to thermal shrinkage; amorphous materials tend to shrink less. When crystalline materials are cooled below their transition temperature, the molecules arrange themselves in a more orderly way, forming crystallites. On the other hand, the microstructure of amorphous materials does not change with the phase change. This difference leads to crystalline and semi-crystalline materials having a greater difference in specific volume ( ) between their melt phase and solid (crystalline) phase. This is illustrated in Figure 1 below. We'd like to point out that the cooling rate also affects the fast-cooling pvT behavior of crystalline and semi-crystalline materials.FIGURE 1. The pvT curves for amorphous and crystalline polymers and the specific volume variation between the processing state (point A) and the state at room temperature and atmospheric pressure (point B). Note that the specific volume decreases as the pressure increases.Causes of excessive part shrinkageExcessive shrinkage, beyond the acceptable level, can be caused by the following factors. The relationship of shrinkage to several processing parameters and part thickness is schematically plotted in Figure 2.- low injection pressure- short pack-hold time or cooling time- high melt temperature- high mold temperature- low holding pressure.Problems caused by part shrinkageUncompensated volumetric contraction leads to either sink marks or voids in the molding interior. Controlling part shrinkage is important in part, mold, and process designs, particularly in applications requiring tight tolerances. Shrinkage that leads to sink marks or voids can be reduced or eliminated by packing the cavity after filling. Also, the mold design should take shrinkage into account in order to conform to the part dimension. Part shrinkage predicted by C-MOLD offers a useful guideline for proper mold design.
Purchasing Request
Note: Send your purchase request, and we will screen suppliers or manufacturers for you free of charge...