Plastic Shrinkage Overview
Shrinkage rate of plastic refers to percentage difference between size of plastic part at molding temperature and size after it is removed from mold and cooled to room temperature. It reflects degree of size reduction of plastic parts after they are removed from mold and cooled. Factors that affect shrinkage rate of plastics include plastic varieties, molding conditions, mold structures, etc. Shrinkage rates of different plastics are different. In addition, shrinkage rate of plastics is also related to shape of plastic part, complexity of internal structure, whether there are inserts, etc.
Factors that affect plastic shrinkage are as follows:

 

Different types of plastic raw materials have different shrinkage rates. See Table 1 for details.

Table 1 Shrinkage rates of different plastic raw materials
As can be seen from Table 1, shrinkage rate of thermoplastic performance plastics is the lowest with 40% reinforced PPS, and shrinkage rate of thermosetting plastics with epoxy resin is the lowest. Shrinkage rates of both plastics are 0.2%. The largest shrinkage rate is fluorine plastic, which can reach a maximum of about 6%; the second largest shrinkage rate is low-density polyethylene, with a maximum shrinkage rate of 5%.

 

Same raw material has different crystallinity, and its shrinkage rate is different. The smaller crystallinity, the smaller shrinkage rate. Among many influencing factors, crystallinity of resin has the greatest impact on shrinkage.

The greater molecular weight of same raw material, the smaller its shrinkage rate. Resins with same molecular weight have good fluidity and small molding shrinkage.

After resin is modified by adding other resins, elastomers, and fillers, its shrinkage rate decreases to varying degrees.
Shrinkage rates of resins are different, which greatly affects molding accuracy of its products during melt processing. If you want to process high-precision plastic products, you must use resin with low shrinkage. Taking PP resin as an example, its shrinkage rate is 1.8% to 2.5%, and the smaller molecular weight, the greater shrinkage rate. It is difficult to make high-precision products from PP. For this reason, major companies around the world are actively studying how to reduce shrinkage of resin while ensuring the performance of resin. For example, Basel has developed a low-shrinkage PP with a low shrinkage of only 1%, named Hifax CA207A.