Analysis of four major factors affecting shrinkage of plastic products during injection molding

In the actual injection molding of plastic products, shrinkage is a complex and variable parameter. Not only do different types of plastics have significant differences in shrinkage, but even in the same type of plastic, due to differences in molecular weight, fillers and their ratio, shrinkage as well as anisotropy will vary significantly. So, what are the factors that affect the shrinkage variation of plastic products? In this paper, we will make an in-depth analysis from four aspects: the type of plastic, the structure of the product, the design of the injection mold and the injection molding process conditions.

Influence of the type of plastic

Each type of plastic has its own unique shrinkage range, determined by the physical and chemical properties of the plastic itself. However, even in the same type of plastic, due to differences in molecular weight, fillers and their ratio, shrinkage as well as anisotropy will vary significantly. Therefore, when selecting a plastic, its shrinkage characteristics should be fully considered to ensure the accuracy and stability of the product dimensions.

Influence of product structure

The shape, dimensions, wall thickness, the presence of inserts, the number and position of inserts in the product have a significant effect on shrinkage. As a general rule, the thicker the walls of the product, the greater its shrinkage; complex shaped products often have less shrinkage than simple ones. In addition, the presence of inserts prevents plastic shrinkage and reduces shrinkage due to the cooling effect. Therefore, when designing the product structure, it is necessary to fully consider the influence of these factors on shrinkage.

Influence of injection mold design

The parting plane, pressure direction, and the design, location, and size of the injection mold filling system directly affect the material flow direction, density distribution, the effect of additional pressure to compensate for shrinkage, and molding time, having a significant impact on shrinkage and its directionality. Therefore, when designing an injection mold, it is necessary to carefully consider these factors to ensure that the product shrinks uniformly and achieves the expected size and shape.

Influence of injection molding process conditions

Injection molding process conditions such as mold temperature, injection pressure, and pressure holding time also have a significant effect on product shrinkage. When the mold temperature is high, the plastic material cools more slowly, the density is higher, which leads to an increase in shrinkage; especially for crystalline plastics, due to the large volume change, their shrinkage is even more significant. In addition, the uniformity of mold temperature distribution directly affects the magnitude and directionality of shrinkage of different parts of the product. When the injection pressure is high, the viscosity of the plastic material is less, after being pushed out of the mold, the elastic recovery is greater, which reduces shrinkage. And the increase of pressure holding time is conducive to shrinkage reduction.

In general, shrinkage is not a fixed value, but fluctuates within a certain range. The variation of shrinkage directly leads to the change of product dimensions, so when designing an injection mold, it is necessary to comprehensively consider factors such as plastic shrinkage range, product wall thickness, shape, type, size, inlet hole location and other molding factors to determine the shrinkage of different parts of the product. For products with high precision requirements, plastics with a small range of shrinkage variation should be selected and a reserve should be left for mold adjustment. After a test casting, adjust the injection mold gradually to ensure that the product achieves the expected dimensions and accuracy.