The action of the pressure holding is to continue to exert pressure to the melt plastic, in order to increase the density of plastic to compensate for the shrinkage of plastic. In the process of pressure holding, the mold cavity has been filled with plastic; its back pressure is high. In the compaction process, the screws of the injection molding machine can only move slowly forward, the plastic flow velocity is also relatively slow, which is called the pressure flow. In the holding period, the solidification is accelerated for the mold wall cooling, and the melt viscosity increases quickly. So the resistance of the mould cavity is very big. In the late period of the holding pressure, the density of the material increases continuously, and the plastic parts are gradually completed. The pressure of the mold is kept until the gate is sealed.
In the holding period, due to the high pressure, the plastic part can be found to be compressed locally. In the area of high pressure, the plastic is more compact, so the density is higher. While the pressure is lower for some other places, the plastic is loose and the density is lower. so the density distribution changes with position and time. in the holding process , plastic flow rate is very low, so the flow is no longer a leading role here; pressure is a main factor affecting the packing process. In the process of holding pressure, the mold cavity is filled with the melt plastic, which is gradually solidified at this time. In the cavity pressure transfer to the mold wall surface by means of solidifying plastic, this would open the plastic mould. Therefore, it is necessary to add appropriate clamping force. In normal circumstances the slightly opened mold would help for mold venting; but if this force is too large, it easily causes molding flash, material overflow and open of the mold. Therefore, to choose an injection molding machine, a large enough clamping force injection molding machine should be decided, in order to prevent the mold open and help pressure holding.