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Processing Technology of Crystalline Plastics
- Nov 28, 2018 -

The crystallinity and morphology of crystalline plastics affect the physical and mechanical properties of the products. If the cooling speed is slow during the forming process, the crystallinity can be improved and the crystallinity can be formed. Therefore, it is very important to control the die temperature to obtain products with good mechanical properties and low surface roughness.

In order to slow down the cooling rate and fully crystallize, the mould temperature must be increased, but the forming cycle will inevitably be prolonged. The specific heat capacity of crystalline plastics varies greatly near its melting point. All materials shrink to a certain extent when they are cooled. Generally, the shrinkage rate of crystalline plastics is higher than that of non-crystalline plastics. Therefore, its products are prone to deformation, and its thick-walled products are prone to dents, that is, large parts may warp, forming dents in the thicker part.

In a word, it is very important not only to consider the temperature of the die, but also to notice that all parts of the product must be cooled and solidified (or crystallized evenly).

Polyethylene:

Polyethylene generally has good fluidity during moulding. There is no need to worry about its thermal stability. However, its molecular orientation is strong, so the products are prone to deformation.

High density polyethylene (HDPE) has a sensitive crystallization temperature. Generally speaking, higher injection pressure and injection speed are required. Especially for thick-walled products, injection speed is very important. It can reduce the surface roughness of products, prevent warpage and reduce the shrinkage of products.

Polypropylene:

Polypropylene is similar to polyethylene in many places. Its fluidity is proportional to the temperature of the barrel. However, because polypropylene begins to age at about 280 C, the temperature should be controlled below 270 C. The orientation of the molecule is strong. If the molecule is formed at a lower temperature, it is easy to warp and twist because of its orientation, so we must pay attention to controlling the temperature.

Polyamide (nylon):

The viscosity of polyamide is very sensitive to temperature. Unlike other thermoplastics, its melting point is obvious. Polyamide is formed at its melting point, so its forming temperature must be higher than that of ordinary materials.

Polyamide must be fully dried beforehand because of its high hygroscopicity. However, if it is dried above 90 C, it will discolor. Attention should be paid to it.

POM:

Polyformaldehyde is divided into homopolymer and copolymer, which are all resins with poor fluidity. This kind of resin is prone to thermal decomposition, so attention must be paid to controlling the temperature of forming. Copolymer formaldehyde has better thermal stability than homopolymer formaldehyde. It can be processed at a slightly higher temperature, but the material should not stay in the barrel for too long. Otherwise, thermal decomposition will occur to yellowish the color of the product.

Thermoplastic polyester:

Polybutylene terephthalate (PBT) and polyethylene terephthalate (PET) belong to saturated polyester (thermoplastic polyester). PBT is characterized by very low melt viscosity and good formability. Because it crystallizes rapidly, it solidifies quickly.

PBT and PET resins are generally reinforced with glass fibers to improve their properties. The main points of forming non-reinforced resin matrix are introduced. Its heating barrel is generally 230 - 270 ("flame retardant level" is 250 (). The temperature of the die is 40-90 C. Although it can be formed at lower die temperature, higher die temperature should be adopted in order to obtain products with good surface gloss. The injection pressure is 50-130 MPa.

Because the resin solidifies quickly, the injection speed should be fast, so that the appearance of the product is good and the performance can be improved. In addition, because the hygroscopic resin will be decomposed by water during melting, the product will become brittle, so the resin must be pre-dried before processing.