What are the reasons for the shrinkage of plastic products during injection molding?wall charger who

　　With the continuous improvement of injection molding processing accuracy requirements, in addition to correct selection of materials and modification of selected plastics, how to correctly select injection molding process conditions and reasonably analyze the relationship between the comprehensive shrinkage rate of the product and the parameters of the processing process , And the various reasons for shrinkage and the influence of processing factors on it are a very important topic. Let’s take PP plastic as an example to analyze and analyze some factors that affect the deformation of plastic products during injection molding.

　　Shrinking process and mechanism

　　Carefully observe the injection of the PP sample under the combination of process conditions, it can be found that the shrinkage rate of the entire product is 1%-3%, and this shrinkage is composed of three parts, namely, shrinkage before gate solidification, cooling shrinkage and demolding After the free contraction.

　　Product shrinkage before gate solidification

　　The shrinkage of PP products before the gate solidifies to a large extent depends on the compensation ability of the melt. As soon as the material enters the mold, because the mold temperature is low at this time, the melt temperature drops rapidly, while the density and viscosity of the melt continue to increase. The compensation capability of the melt depends on the holding pressure and the entire filling time for the melt to continue to be transferred into the mold under this pressure. This process continues until the gate is solidified and closed. Therefore, it can be considered that the holding pressure and holding time have a great relationship with the contraction at this stage. If the holding pressure is large and the holding time is long, the shrinkage rate of the product is relatively low.

　　Product shrinkage during cooling

　　The whole stage from the gate solidification to demolding is the cooling stage. At this stage, no other new melt enters the mold cavity, and the weight and density of the PP product no longer change. During the cooling process, the shrinkage of the product is related to the mold temperature, cooling rate, and cooling time. The cooling rate is fast, and the product is too late to relax, and the shrinkage rate may be small; if the mold temperature is high and the cooling rate is slow, the molecules have sufficient relaxation time and shrink The rate may increase.

　　Free shrinkage of the product after demolding

　　The demolded PP products need to be allowed to stand for a period of time at room temperature, because the demolded products still have a process of free shrinkage. If the process conditions are not set properly, some deformation after natural stress release may occur during this period.

　　Therefore, the total shrinkage measured from the product is the sum of the above three parts. It is normal for plastic materials to shrink, but how to control and master the relationship between product shrinkage and process factors is a more complex research topic. It is necessary to analyze and study the process factors that affect product shrinkage.

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