1. The thermal expansion and contraction of plastic. Wh […]
1. The thermal expansion and contraction of plastic. When the plastic mould is moulded, the plastic raw material needs to be melted. At this time, the melting temperature reaches 200 to 300 degrees, and the plastic raw material is thermally expanded, and during the cooling process of the plastic mould, the temperature is lowered, and its volume is required to shrink.
2. Changes in chemical structure. For example, in the process of forming a thermosetting plastic, the resin molecules change from a linear structure to a bulk structure, and the volumetric mass of the bulk structure is larger than the volumetric mass of the linear structure, and the total volume thereof becomes smaller, so that shrinkage occurs.
3. Residual stress changes. When the plastic mould is formed, due to the shearing force of the moulding pressure, the anisotropy, the mixing unevenness of the additive, and the influence of the mould temperature,
There is residual stress in the plastic mould after moulding, and the residual stress will gradually become smaller and redistributed, and as a result, the plastic mould will re-contract, and this shrinkage is generally called post-shrinkage.
4. Gate section size. Different moulds have different cross-section dimensions. Large-size gates help to increase cavity pressure and extend gate closure time, allowing more melt to flow into the cavity.
Therefore, the density of the plastic part is also large, so that the shrinkage rate is lowered, and conversely, the shrinkage rate is increased.
5. Plastic varieties. In general, crystalline plastics, such as PP, PA, etc., exhibit greater shrinkage after demoulding and a wider range of shrinkage than crystalline plastics such as PC, PS, ABS, and the like.
The reason for the large shrinkage of the crystalline plastic is the influence of the thermal expansion coefficient of the plastic and the coagulation of the melt when the crystalline structure is formed.
6. Plastic mould wall thickness. The thin-walled plastic mould with uniform thickness has a fast cooling speed in the mould cavity, the shrinkage rate after demoulding tends to be the smallest, and the thick plastic mould with the same wall thickness cools in the cavity for a long time.
The shrinkage rate after demoulding is large. If the wall thickness of the plastic mould is thick and thin, there will be a degree of shrinkage after demoulding. At this sudden change in wall thickness, the shrinkage rate will also suddenly change. Will cause a large internal stress at this place.