The main reasons for the production shrinkage are the following:
1. Plastic thermal expansion and contraction. In the plastic mold molding, the need to melt the plastic raw materials, when the melting temperature of 200 to 300 degrees, plastic raw materials heated expansion, and in the plastic mold cooling process, the temperature decreases, its volume is bound to shrink.
2. Changes in chemical structure. Such as thermosetting plastics in the molding process, the resin molecules from the linear structure into the body structure, and body structure volume quality than the linear structure of the mass quality, the total volume smaller, so the phenomenon of contraction.
3. Residual stress changes. Plastic mold in the molding, due to the molding pressure of the role of shear force, anisotropy, additive mixture of non-uniformity and the impact of mold temperature,
There is residual stress in the plastic mold after molding, and the residual stress will gradually become smaller and redistributed. As a result, the shrinkage of the plastic mold occurs. This shrinkage is generally called post-contraction.
4. Gate section size. Different molds, the intersection of different cross-sectional size, large-size gate helps to improve the cavity pressure and extend the gate closure time, to facilitate more melt into the cavity,
So the density of plastic parts is also larger, so that the reduction in shrinkage, and vice versa will increase the shrinkage.
5. Plastic varieties. In general, crystalline plastics, such as PP, PA, etc., than the crystalline plastic, such as PC, PS, ABS and other show a larger post-mold shrinkage and a wide range of contraction.
Crystalline plastic shrinkage is due to the effect of plastic thermal expansion coefficient and the agglomeration of the melt at the time of crystal structure formation.
6. Plastic mold wall thickness. The thickness of the thin-walled plastic mold in the mold cavity in the cooling speed, stripping after the shrinkage rate tends to be the smallest wall thickness of the same thick plastic mold cooling in the cavity for a long time,
Mold shrinkage after large, if the plastic mold 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 rate will suddenly change, this change Will cause the Department to produce a greater internal stress.