In the field of polymer applications, shrinkage rate is a critical parameter that profoundly affects the dimensional accuracy, performance, and final application outcomes of polymer products.

Whether in everyday plastic items or precision components used in advanced industrial sectors, understanding and controlling the shrinkage of polymer materials is essential to ensuring product quality and functionality.

In this article, we will delve into the key factors influencing polymer shrinkage and explore effective methods to reduce shrinkage.

1. Definition of Shrinkage Rate in Polymer Materials
The shrinkage rate of polymer materials refers to the percentage difference between the dimensions of a plastic part at the molding temperature and its dimensions after being removed from the mold and cooled to room temperature. It directly reflects the degree of dimensional reduction of the plastic part after cooling outside the mold.

To illustrate simply, it’s like placing a plastic model at a high temperature into a mold for forming; once cooled, its size will be smaller than its size at the molding temperature inside the mold. This reduction ratio is the shrinkage rate.

The shrinkage rate is calculated by the formula:

where R before is the dimension at the molding temperature, and R after is the dimension after cooling to room temperature.

Due to differences in molecular structure, chemical composition, and other factors, various polymer materials have different shrinkage rates. This makes considering shrinkage rate especially important during material selection and molding process design.

2. Factors Affecting Shrinkage Rate of Polymer Materials
Influence of Raw Materials

1. Different Types of Raw Materials

The shrinkage rates vary significantly among different types of polymer materials.

For example, long fiber-reinforced modified materials typically have a shrinkage rate of approximately 1.2% to 1.5%.

PP homopolymer 40% Long Glass Fiber Reinforced

Crystallinity of Raw Materials
When a material has a high degree of crystallinity, its molecular chains are tightly and orderly arranged. During cooling, the chains change from a disordered molten state to an ordered crystalline state, causing significant shrinkage. The tightly packed chains in the crystalline regions reduce the material’s volume, resulting in a higher shrinkage rate. In contrast, amorphous (non-crystalline) materials generally have lower shrinkage.

A simple analogy is stacking wood: randomly piled wood takes up more space, while neatly stacked wood occupies less volume.