What factors affect the shrinkage of engineering modified plastics PP, PA and PVC? What factors affect the shrinkage of engineering modified plastics PP, PA and PVC?

May 12, 2022

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Crystallinity of polymer

What causes the shrinkage of polymer products? What factors affect the shrinkage of polymers? There are two specific reasons:

First, the shrinkage caused by polymer crystallization. Different polymers have different crystallinity, and the same raw material has different crystallinity under different conditions, so the shrinkage is different. The general rule is that the smaller the crystallinity, the smaller the shrinkage.

Second, the shrinkage caused by thermal expansion and cold contraction of polymer products. The temperature drop from the time when the polymer viscous melt fills the mold to the time when the polymer products are cooled, shaped and opened leads to the cooling shrinkage. Because the linear expansion coefficient of different polymers is different, the shrinkage caused by thermal expansion and cold contraction is also different.

Among many influencing factors, the crystallinity of resin has the greatest influence on the shrinkage.

High-gloss flame retardant modified PVC can replace ABS products.

Molecular weight of polymer

The higher the molecular weight of the same raw material, the smaller its shrinkage. Taking PP as an example, the melt flow index shrinkage of different sizes is completely different, as shown in the following table.

Shrinkage of PP with different molecular weights

Modification of polymer

After the resin is modified with other resins, elastomers and fillers, its shrinkage rate decreases in different degrees.

The shrinkage rate of resin is different, which greatly affects the molding accuracy of its products in the melting process. If you want to process high-precision plastic products, you must use low shrinkage resin.

Take PP resin as an example, its shrinkage rate is 1.8%-2.5%, and with the decrease of molecular weight, its shrinkage rate increases, so it is difficult for PP to make high-precision products. For this reason, major companies all over the world are actively studying how to reduce the shrinkage rate of resin under the condition of ensuring its performance. Jupang Plastics can debug products to customers according to their requirements to reduce the shrinkage rate of products.