The large shrinkage molding of PP is a major drawback of PP itself, mainly due to its high crystallinity. After crystallization, the specific gravity of PP increases and the volume decreases. When the crystallinity is 0% and 100%, their specific gravities are 0.851 and 0.936, respectively. Therefore, the molding shrinkage of pure PP is generally between 1.7 and 2.2. Controlling the molding shrinkage rate of PP mainly involves controlling the crystallinity of its raw materials during molding: the smaller the crystallinity, the smaller the molding shrinkage rate; On the contrary, the higher the crystallinity, the greater the molding shrinkage rate.

In PP modified plastics, the addition of various modifiers has damaged the original crystallinity of PP to varying degrees, thereby changing the original molding shrinkage rate of PP. To control the shrinkage rate of modified PP, the following types of modifiers are mainly used:
1、 The effect of mineral filling on the shrinkage rate of PP modified materials:
1. Mineral additives themselves do not shrink, and their addition reduces the overall shrinkage rate of PP modified materials in proportion;
2. The addition of mineral additives inevitably affects the crystallinity of PP, thereby affecting the shrinkage rate;
3. After the addition of fine mineral agents, they act as nucleating agents, changing the structural state of PP, preventing the formation of large spherulites, and also affecting the shrinkage rate of PP during molding,
2、 The effect of glass fiber on the shrinkage rate of modified PP molding:
Glass fiber has a significant impact on the shrinkage rate of PP modified materials during molding. When the content of glass fiber reaches 30% or more, the forming shrinkage rate of the PP modified material decreases from 1.8 to 0.5, and the surface treated glass fiber has a greater impact on the forming shrinkage rate than the untreated glass fiber. The addition of fiberglass destroys the crystallinity of PP, affects the shrinkage rate, and more importantly, limits the crystallization shrinkage of PP.
3、 The effect of adding polyethylene on the shrinkage rate of PP molding:
The addition of polyethylene also affects the molding shrinkage rate of PP modified materials. Although polyethylene is also a highly crystalline plastic with a large molding shrinkage rate, its addition to PP damages each other's crystallinity to varying degrees, resulting in a decrease in overall molding shrinkage rate. The molding shrinkage rate of PP modified materials varies due to the different modifiers added. In the experiment, we found that several modifiers have different effects when used together, and there are many influencing factors. The dispersion degree of the modifier in P, the particle size of the mineral modifier itself, and the surface treatment of various mineral additives all have an impact on the molding shrinkage rate. In addition, the molding process of the injection molding machine also has a significant impact on the molding shrinkage rate during the molding process. The changes in injection temperature and injection pressure have an impact on the molding shrinkage rate.
Guangdong Shuju New Material Technology Co., Ltd. is a modified plastic enterprise that integrates research and development, production, sales, and service. The company has five major series of products, including flame retardant, enhanced flame retardant, flame retardant alloy, fiberglass reinforcement, and functional modification, with more than 30 varieties and more than 120 grades. The flame-retardant PP series products have passed UL certification in the United States, and all products have passed CQC quality management system certification, complying with EU RoHS and Reach standards. The products are widely used in fields such as automotive, smart home appliances, lighting, medical, and electronic appliances.
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