First, the plastic thermal expansion and contraction
Plastic products company pointed out that the vast majority of substances will be thermal expansion and contraction, plastic parts in the molding, the need to melt the plastic raw materials, when the melting temperature of two or three hundred degrees, plastic raw materials by thermal expansion, and plastic parts in the cooling process , The temperature is reduced, its volume is bound to shrink.
Second, the gate cross-sectional size
Different molds, the gate cross-sectional size is different, large-size gate to help improve the cavity pressure and extend the gate closure time, to facilitate more melt into the cavity, and thus the density of plastic parts, so that Shrinkage decreased, otherwise it will increase the shrinkage.
Third, the residual stress changes
Plastic products company pointed out that the plastic parts in the molding, due to the molding pressure and shear force of the role of anisotropy, the additive mixture of non-uniformity and the impact of mold temperature, plastic parts after forming the existence of residual stress, The residual stress will gradually become smaller and redistributed, the result of plastic parts caused by retraction, this contraction is generally known as post-contraction.
Fourth, changes in chemical structure
Some plastic in the molding process, the chemical structure will change, such as thermosetting plastics in the molding process, the resin molecules from the linear structure into the body structure, and body structure volume volume quality than the linear structure of the mass quality, the total volume reduction , So the phenomenon of contraction.
Fifth, plastic varieties
The shrinkage of each plastic is different and varies with the process factors. In general, crystalline plastics, such as PP, PA, etc., than the non-crystalline plastic, such as PC, PS, ABS, etc. show a larger post-mold shrinkage and a wide range of contraction. Plastic products company pointed out that the shrinkage of crystalline plastic is due to: the impact of plastic thermal expansion coefficient and the formation of crystal structure when the melt cohesion.
Six, plastic wall thickness
Plastic products company pointed out that the thickness of thin-walled plastic parts in the mold cavity in the cooling rate is fast, the shrinkage after stripping the trend tends to be the smallest, thick wall thickness of the same plastic parts in the cooling time longer, stripping After the contraction of the larger, if a plastic wall thickness is thick and thin, after the demoulding will have a different degree of contraction, in this sudden change in wall thickness, the shrinkage will suddenly change, this change will Resulting in a greater internal stress.