Reasons for shrinkage of material handling box mould production
The main reasons for production contraction are as follows:
1. Thermal expansion and cold shrinkage of plastics. In material handling of box molding, need the molten plastic raw material, at this time of the melting temperature of 200 ~ 300 degrees, plastic raw material thermal expansion, and in the process of material handling cases of mold cooling, the temperature is reduced, the volume of it is bound to happen.
2. Changes in chemical structure. As thermosetting plastic in the process of forming, resin molecules by the linear structure into a shape structure, quality and size of the structure of the volume is larger than the volume of a linear structure quality, its total volume is smaller, thus causing shrinkage phenomenon.
3. Changes in residual stress. When the material handling box mold is formed, it is affected by the shear force under the forming pressure, the anisotropy, the inhomogeneity of additives and the mold temperature.
Material handling after molding box mold in the presence of residual stress, the residual stress will gradually become smaller and redistribution, the result form material handling box mold to shrink, the general called after contraction.
4. Gate section size. Different molds have different cross-section sizes. Large size gate helps to increase the pressure of the cavity and extend the closing time of the gate so that more melt can flow into the cavity.
As a result, the density of plastic parts is also large, so that the shrinkage rate will be reduced, and conversely, the shrinkage rate will be increased.
5. Plastic varieties. Generally speaking, crystallized plastics, such as PP and PA, show larger shrinkage and wider shrinkage range than crystalline plastics, such as PC, PS and ABS.