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Injection molding process

update:19-02-2021
abstract:

The injection molding process of plastic parts mainly i […]

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

1. The filling stage

Filling is a step in the entire injection cycle process. The time starts from when the mold is closed and the injection is started, until the mold cavity is filled to about 95%. In theory, the shorter the filling time, the higher the molding efficiency, but 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 (densification) of the plastic to compensate for the shrinkage behavior of the plastic. During the pressure holding process, the back pressure is relatively high because the mold cavity is already filled with plastic. During the packing and compaction process, the screw of the injection molding machine can only move forward slightly slowly, and the flow speed of the plastic is relatively slow. The flow at this time is called the packing flow. In the pressure holding stage, the plastic is cooled and solidified by the mold wall faster, and the melt viscosity increases quickly, so the resistance in the mold cavity is very large. In the later stage of pressure holding, the material density continues to increase, and the plastic parts are gradually formed. The pressure holding stage should continue until the gate is cured and sealed. At this time, the cavity pressure in the pressure holding stage reaches a high value.

3. Cooling phase

In the injection molding mold, the design of the cooling system is very important. This is because the molded plastic product can only be cooled and solidified to a certain rigidity, and then the plastic 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, increase injection molding productivity, and reduce costs. An improperly designed cooling system will lengthen the molding time and increase costs; uneven cooling will further cause warpage and deformation of plastic products.

4. Demoulding stage

Demoulding is the latter part of an injection molding cycle. Although the product has been cold-formed, demolding still has a very important impact on the quality of the product. Improper demolding method may cause uneven force on the product during demolding, and product deformation during ejection. There are two main ways of demolding: ejector pin demoulding and stripping board demolding. When designing the mold, choose the appropriate demoulding method according to the structural characteristics of the product to ensure the product quality.