Factors affecting the injection molding process

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The characteristics of the plastic part are such that when the molten material comes into contact with the surface of the mold cavity, the outer layer cools down immediately, forming a low-density solid shell. Due to the poor thermal conductivity of plastics, the inner layer of the plastic part cools slowly, resulting in a highly dense solid layer with significant shrinkage. Therefore, those with greater wall thickness, slower cooling, and thicker high-density layers experience greater shrinkage. Furthermore, the presence of inserts, as well as their layout and quantity, directly affect factors such as the direction of material flow, density distribution, and the magnitude of shrinkage resistance; therefore, the properties of the plastic part have a significant impact on the extent and direction of shrinkage. Factors such as the shape, size, and distribution of the feed inlet directly affect the flow direction of the material, the density distribution, the effect of pressure maintenance for filling gaps, and the molding time. For direct feed inlets, those with a large cross-section (especially those with a thicker cross-section) exhibit less contraction but greater directionality, whereas inlets that are wide but short have less directionality. Those closer to the feed inlet or parallel to the flow direction experience greater contraction. A close examination of the injection process of the PP specimens under various process condition combinations reveals that the overall shrinkage rate of the finished product is 1%–3%. This shrinkage consists of three components: shrinkage before the gate solidifies, cooling shrinkage, and free shrinkage after demolding. Factor 1 affecting the injection molding process: Shrinkage of the product before the gate solidifies. Experimenters at a plastic products manufacturing plant in Beijing found that the shrinkage of PP products before the gate solidifies depends to a large extent on the compensating capacity of the melt. As soon as the material enters the mold, the mold temperature is low at this point, causing the melt temperature to drop rapidly, while the density and viscosity of the melt continue to increase. The compensation capacity of the melt depends on the holding pressure, as well as the entire filling time during which the melt is continued to be delivered into the mold under that pressure. This process must continue until the gate solidifies and seals. Therefore, it can be concluded that the holding pressure and holding time have a significant impact on the shrinkage during this stage; higher holding pressure and longer holding time result in a relatively lower shrinkage rate of the product. Factor 2 affecting the injection molding process: Product shrinkage during cooling. The entire period from when solidification begins at the gate until demolding is the cooling phase. At this stage, no more 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, as well as the cooling rate and cooling time; when the cooling rate is high, the product does not have enough time to relax, and the shrinkage rate may be lower ； If the mold temperature is high and the cooling rate is slow, the molecules have sufficient time to relax, which may result in an increased shrinkage rate. Factor 3 affecting injection-molded part processing: Free shrinkage of the part after demolding. PP parts after demolding need to be left at room temperature for a while, as they undergo a process of free shrinkage. If the process conditions are not set properly, some deformation due to the release of natural stresses may also occur during this period. Therefore, Suliang Plastics concludes that the total shrinkage measured in the product is the sum of these three components. It is normal for plastic materials to shrink, but understanding how to control and manage the shrinkage of products and the factors that affect the injection molding process is a complex research topic. Analysis must be carried out by examining the process factors that influence the shrinkage rate of products. For more information, please visit http://www.topjoychem.com