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Important Injection Moulding Process Stages

Important Injection Moulding Process Stages

The injection moulding process stages of plastic parts mainly include four stages: filling, pressure-holding, cooling and demolding. These four stages directly determine the molding quality of the products, and these four stages constitute a complete continuous process.

1. The filling stage

Filling is the first step in the cycle of the entire injection moulding process stages. The time is counted from the moment when the mold is closed and the injection is started to the moment when the mold cavity is filled to about 95%. In theory, the shorter the filling time is, the higher the molding efficiency will be. However, in practice, the molding time or injection speed is restricted by many conditions.   

2. Pressure holding stage 

The function of the holding pressure stage is to continuously apply pressure, compact the melt, and increase the density of the plastic (densification) to compensate for the shrinkage behavior of the plastic. During the pressure holding process, the back pressure is relatively high because the mold cavity has already been filled with plastic. During the packing and compaction process, the screw of the injection molding machine can only move forward by mini steps, and the flow speed of the plastic is relatively slow either. The flow at this time is called the pressure holding flow.

3. Cooling stage 

During the process of injection moulding, the design of the cooling system is very important. This is because only when the molded plastic product is cooled and solidified to a certain rigidity, the product can be prevented from deforming due to external force after being demolded. Since the cooling time accounts for about 70% to 80% of the entire molding cycle, a well-designed cooling system can greatly shorten the molding time, which can improve injection molding productivity and reduce costs.

4. Demoulding stage 

Demoulding is the last step in the cycle of injection moulding process stages. Although the product has been cold-formed, demoulding still has a very important influence on the quality of the product. Improper demolding method may cause uneven force on the product during demoulding, and it will also cause product deformation during ejection. There are two main ways of demoulding: ejector rod demoulding and stripping board demoulding. When designing the mold, you should choose the appropriate demoulding method according to the structural characteristics of the product to ensure the quality of the product.

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