Requirements for raw materials for injection molding
Injection molding material does not meet the application requirements will also lead to the appearance of poor luster. The causes and treatment measures are as follows:
1. The original guess of the injection molding process is that the content of water or other evaporative substances is too high, and the evaporative components condense between the cavity wall of the mold and the molten material when forming, resulting in the appearance of the plastic parts with poor luster. The material should be pre-boring.
2, material or colorant differentiation discoloration resulting in poor luster. Materials and colorants with high temperature resistance should be selected.
3, the activity function of the material is too poor, so that the appearance of plastic parts is not dense, resulting in poor luster. Should be replaced with a better function of the resin or increase the use of appropriate lubricants and improve the processing temperature.
4, the original guess mixed with different materials or insoluble materials. New materials should be used.
5, the material particle size is not uniform. Materials with large particle size differences should be screened.
6, crystallized resin because of uneven cooling resulting in poor luster. The mold temperature should be reasonably controlled for injection molding. On thick wall plastic parts, if cooling is lacking, it will also make the appearance of plastic parts hairy and dim luster. The solution is to remove the plastic parts from the mold and immediately put them into the cold pressing mold dipped in cold water to cool down.
7. The reuse share of recycled materials in raw materials is too high, which affects the uniform plasticization of molten materials. The amount should be reduced.
In injection molding, the control of injection pressure is usually divided into one injection pressure, two injection pressure (pressure retention) or more than three injection pressure control. Whether the pressure switching opportunity is appropriate, it is very important to avoid too high pressure in the mold and avoid material overflow or lack of material. The specific volume of moulds depends on the molten pressure and temperature at the gate closure stage. If the pressure and temperature are the same each time from the pressure holding to the cooling stage of the product, the specific volume of the product will not change.
In the stable molding temperature, the important parameter to determine the size of the product is the pressure holding, the important variables affecting the size of the product is the pressure holding and temperature. For example: after the end of the filling, the pressure holding pressure immediately drops, when the surface layer forms a certain thickness, the pressure holding pressure rises again, so that the low sealing force can be used to form large products with thick walls, to eliminate the collapse pit and flying edge.
The pressure holding pressure and speed are usually 50%~65% of the high pressure and speed when the plastic cavity is filled, that is, the pressure holding pressure is about 0.6~0.8MPa lower than the injection pressure. Because the holding pressure is lower than the injection pressure, the load of the oil pump is low within the considerable holding time, the service life of the solid oil pump is extended, and the power consumption of the oil pump motor is also reduced.
Three-stage pressure injection molding can not only make the workpiece smooth filling, but also will not appear weld line, sag, flying edge and warping deformation. On the thin wall parts, long small parts, long process of large parts of the molding, even the cavity equipment is not too balanced and not too tight mold molding are good.
