The main reasons for the decrease in the output of Dustbin Mould are as follows:
1. Thermal expansion and contraction of plastics. When forming the dustbin mould, it is necessary to melt the plastic raw materials. At this time, the melting temperature reaches 200 to 300 degrees. Plastic raw materials expand due to heat. During the cooling process of the dustbin mould, the temperature drops and its volume must shrink.
2. Changes in chemical structure. For example, in the molding process of thermosetting plastics, resin molecules change from linear structure to integral structure, and the integral mass of the integral structure is larger than that of linear structure, and the total volume becomes smaller, so shrinkage occurs.
3. Residual stress changes. During the injection molding of the dustbin mould, due to the influence of shear force, anisotropy, uneven mixing of additives and mold temperature, residual stress exists in the molded dustbin mould. This residual stress will gradually decrease and redistribute. As a result, the dustbin mould will shrink again. This contraction is often referred to as post-contraction.
4. Gate section size. Different moulds have different cross-sectional dimensions. Large gate is helpful to increase cavity pressure and prolong gate closing time, so that more melt can flow into cavity. Therefore, the density of plastic parts is also higher, which will reduce the shrinkage, otherwise it will increase the shrinkage.
5. Plastic varieties. Generally, compared with crystalline plastics such as PC, PS and ABS, crystalline plastics such as PP and PA show greater shrinkage rate and wider shrinkage range after demoulding. The reason of large shrinkage of crystalline plastics is the influence of thermal expansion coefficient and melt aggregation when forming crystalline structure.
6. Wall thickness of dustbin mould. Thin-walled dustbin mould with uniform thickness are rapidly cooled in the cavity, and the shrinkage tends to decrease after demoulding, while thick dustbin mould with the same wall thickness are cooled in the cavity for a long time. The shrinkage after demoulding is large. If the thickness of the wall of the dustbin mould is thick or thin, it will shrink in different degrees after demoulding. Under this sudden change in wall thickness, the shrinkage will also change suddenly. This change will cause greater internal pressure at this location.