Under the action of the screw thrust, the melted material has flowed through the barrel, nozzle, runner, gate, etc. at a certain rate and then injected into the cavity, and the injection pressure is gradually decreased by overcoming the flow resistance. Plastic filling process and molding quality. In addition to the injection pressure, it depends on the melt injection speed, melt and mold temperature, and runners, gates, and molds. Generally, the higher the melt pressure and the faster the speed, the longer the distance that can flow. Using the cavity pressure, it is possible to objectively describe the melt flow and its state changes, and to control the quality of the product. The filling process is divided into four stages. At the same time, the pressure values obtained by different pressure measuring points are different in the flow length of the molten material in the cavity, but the pressure changes have similar laws.

(1) Filling and compacting stages. At this stage, the pressure increases as the melt inflow path becomes longer and eventually reaches a maximum. At the same time, the injection speed drops rapidly, and the melt in the cavity is compacted. Since the flow state of the melt in the cavity directly affects the surface quality, molecular orientation, internal stress of the product, etc., in order to adjust the filling process, according to the characteristics of the plastic product and the mold structure, a multi-stage injection speed can be adopted, that is, The speed is lower when the melt flows through the gate and at the end of filling, and other processes use high-speed injection.

(2) Pressure holding and densification stage. At this stage, the mold cools and the specific volume of the melt changes, causing the product to shrink. It is necessary to apply a certain holding pressure to the screw to compensate and thicken the melt. The holding time and pressure are related to the stress of the product. The higher the pressure, the smaller the shrinkage of the product, but the pressure is too large, and it is easy to generate large residual stress, which makes the demoulding difficult.

(3) Backflow phase. At this stage, the cavity pressure is higher than the pressure of the melt from the gate to the screw, the plastic in the cavity is not fully solidified, and the inner plastic also has a certain fluidity, which may cause a slight backflow to the gate, causing the product Defects such as shrinkage cavities and hollows are produced. The use of multi-stage holding pressure, switching by time, can eliminate residual stress. If the pressure holding switch is too early, it will cause the plastic in the cavity to flow back, causing defects such as shrinkage holes and hollows; the holding time is too long, and the gate is solidified and then filled, so that stress is formed around the gate.

(4) Product cooling stage. At this stage, the product continues to cool in the cavity, so that the product has sufficient rigidity when demolding, and the length of the cooling time is related to the residual stress of the product.