The plastic melt into the mold cavity under high temperature and high pressure, and pressure forming, when the temperature drops, the melt solidified into the plastic parts, plastic parts size is smaller than the cavity size, the smaller size of the phenomenon is shrinking. The shrinkage of the following reasons.
1. plastic expansion and contraction, the vast majority of matter in thermal expansion and contraction, plastic molding, need the molten plastic raw material, melting temperature and reached two or three Baidu, plastic raw materials, thermal expansion, and the temperature in the cooling process of plastic molding, reduce the volume, it must shrink.
2.changes in chemical structure, some in the plastic molding process, its chemical structure will change, such as thermosetting plastic forming process, resin molecules by linear structure into the body structure, and the mass body structure than the volume quality of the linear structure, the overall product is reduced, resulting in shrinkage.
3.Change of residual stress, plastic parts in the molding, the molding pressure and shear force, and the effect of mold temperature heterogeneous anisotropic, additives, formed plastic parts in the residual stress, the residual stress will gradually become smaller and the new distribution, the results cause plastics to shrinkage, the shrinkage is generally referred to as post shrinkage.
4.The gate section size, different mold, the gate section size, large size gate is helpful to improve the cavity pressure and the extended gate closed time, facilitate more melt into the mold cavity, so the density is bigger also, from but reduce shrinkage, otherwise will improve the shrinkage rate.
5. varieties of plastic, plastic shrinkage of each are different, and the process factors vary. In general, crystalline plastics, such as PP, PA, non crystalline plastics, such as PC, PS, ABS shrink range and exhibit large die shrinkage and wide. Crystalline plastics shrinkage reason is: thermal expansion of plastics melt condensation when the formation coefficient and crystal structure.
6.The part thickness, thickness uniformity of thin wall plastic parts in the die cavity cooling speed, mould shrinkage rate to minimum wall thickness, the same thick plastic cooling in the cavity of the time more long, mould shrinkage is greater, if a plastic parts wall thickness thick with thin, in after demoulding will have different degrees of shrinkage, the wall thickness at the sudden change, shrinkage will suddenly change, this change will cause the produce larger internal stress.