Plastic Injection Mold and injection molding, Plastic Mold Automotive Part Mold
Plastic Injection Mold and injection molding, Plastic Mold Automotive part mold
What is injection molding?
The plunger or screw starts from the metering position in the barrel, and the high pressure is applied through the injection cylinder and the piston. The process of quickly passing the plasticized plastic melt through the nozzle at the front end of the barrel and the casting system in the mold into the closed cavity is called Injection molding. It is divided into three stages: flow filling, pressure feeding and backflow.
Refers to the process of injection of a plasticized melt into the mold cavity by an injection machine.
During the melt injection process, the outer friction of the barrel, the nozzle, the mold cavity of the mold casting system on the melt, and the viscous internal friction generated inside the melt are encountered. In order to overcome these flow resistances, the injection machine must apply a very high injection pressure to the melt through a screw or plunger. In order to grasp the flow filling law of the melt, it is necessary to understand the variation characteristics of the injection pressure in the process and the melt temperature, flow rate and die-casting problems associated with it.
Injection pressure and melt chargeability
The melt filling flow pattern is related to the filling speed, and the filling speed is affected by the injection process conditions and the mold structure. In the injection molding, it is not desirable to have a high-speed jet flow during the filling period, and it is desirable to obtain a medium-speed or low-speed extended flow. For this reason, it is necessary to analyze the flow orientation of the filling period to understand the influence of the injection pressure on the melt filling model.
In practice, when expanding the flow, the low-temperature melt film on the leading edge of the material head has a greater retarding effect on the melt, and the melt temperature entering the cavity first drops rapidly, and the viscosity also increases, which exacerbates the subsequent melt feed mode. Flow resistance at the time. In this case, the injection pressure is not large, and it is easy to stop the filling flow, resulting in waste in injection molding. For this reason, it is often necessary to increase the injection pressure. When the injection pressure is increased, the shearing action in the melt is strengthened, and the flow orientation effect is increased, which may eventually lead to a relatively obvious anisotropy of the product and cause deterioration of thermal stability. In the case of a product produced in this case, if it is operated in an environment where the temperature changes greatly, cracks consistent with the orientation are likely to occur.
Note: Under certain mold structure conditions, as long as high-speed jet flow does not occur during filling, the filling speed should be as fast as possible, which not only avoids the use of large injection pressure, but also improves the productivity. . Pressure-preserving feeding
Pressure-preserving and compensating stage: refers to filling the cavity from the melt until the plunger or screw starts to retreat in the barrel
Packing pressure refers to the process in which the injection pressure continues to compact the melt in the cavity; during the pressure-holding process, the injection machine fills the voids which are gradually cooled by the injection molding due to the molding shrinkage. Material action.
Analysis: During the pressure-preserving and compensating stage, if the plunger or screw stops in the original position, the cavity pressure curve will decrease slightly;
Conversely, if the cavity pressure is to remain constant, the plunger or screw is required to continue moving a little forward during the holding process, and the pressure curve will be parallel to the time axis.
The effect of pressure and dwell time on cavity pressure. If the pressure is insufficient, the feeding flow is limited by the frictional resistance of the gate. The cavity pressure drops rapidly due to insufficient feeding; if the holding time is insufficient, the melt in the cavity will flow backward, which will cause the cavity pressure to drop rapidly. The holding time is long enough to completely cure the melt in the gate or cavity, and backflow is not easy to occur.
Plastic Injection Mold Show
Plastic Part Show
Plastic Injection Mold and injection molding, Plastic Mold