No matter what injection molding products, they are inseparable from plastic injection mold tooling. But sometimes the plastic injection mold tooling will shrink. So what are the reasons for the shrinkage of plastic injection mold tooling?
1. Thermal expansion and cold contraction of plastics
When the plastic mold is formed, it is necessary to melt the plastic raw materials. At this time, the melting temperature reaches 200 ~ 300 degrees, and the plastic raw materials are heated and expanded. During the cooling process of the injection mold, the temperature decreases, and its volume potential must shrink.
2. Changes in the chemical structure of plastics
For example, in the molding process of thermosetting plastics, resin molecules change from linear structure to body structure. The volume mass of body structure is larger than that of linear structure, and its total volume becomes smaller, so it shrinks.
3. Residual stress change
During the plastic injection mold tooling, it is affected by the shear force of molding pressure, anisotropy, uneven mixing of additives and mold temperature.
There is residual stress in the injection mold after molding. This residual stress will gradually decrease and redistribute. As a result, the injection mold will shrink again. This shrinkage is generally called post shrinkage.
4. Gate section size
Different injection molds have different cross-section sizes. Large gate helps to improve cavity pressure and prolong gate closure time, so as to facilitate more melt to flow into the cavity.
5. Plastic variety
Generally speaking, crystalline plastics such as PP and PA have larger shrinkage after demoulding and wider shrinkage range than non crystalline plastics such as PC, PS and ABS.
The reasons for the large shrinkage of crystalline plastics are the influence of the coefficient of thermal expansion of plastics and the condensation of melt during the formation of crystalline structure.
The above five points are the reasons for the shrinkage of injection molds. Therefore, for injection molding plants, the quality of injection molds must be checked frequently when producing injection products.