The crystallinity and crystalline morphology of crystalline plastics affect the physical and mechanical properties of the products. If the cooling rate is slow, the crystallization can be improved. Therefore, it is very important to control the mold temperature to obtain products with good mechanical properties and good surface gloss. In order to slow down the cooling rate and to fully crystallize, the mold temperature must be raised, but the molding cycle will inevitably be prolonged. The specific volume (cm3/g) of crystalline plastics changed considerably near its melting point. All materials have a certain degree of shrinkage during cooling. Generally speaking, crystalline plastics have greater shrinkage than non crystalline plastics. Therefore, its products are prone to deformation, and its thick wall products are prone to dents, that is, large parts may be warped and form dents in thicker parts. In a word, it is important not only to take into account the temperature of the mold, but also to note that all parts of the product must be uniformly cooled and solidified (or uniformly crystallized).
(1) polyethylene
Polyethylene generally has good fluidity when it is formed. It is almost unnecessary to worry about its thermal stability. However, its molecular orientation is strong, so products are prone to deformation.
High density polyethylene (HDPE) has a sharp crystallization temperature. Generally speaking, higher injection pressure and injection speed are required. Especially for thick walled products, injection speed is very important, it can improve the surface gloss of products, and prevent products warpage and reduce molding shrinkage.
(2) polypropylene
A lot of polypropylene is similar to polyethylene, as shown in Figure 6, its fluidity is proportional to the temperature of the barrel, but as the resin begins to aging at about 280 C, it is necessary to control the temperature below 270. The orientation of its molecules is strong. If it is formed at low temperature, it is prone to warpage and distortion due to its molecular orientation. Therefore, temperature must be controlled.
(3) polyamide (nylon)
The viscosity of polyamide is extremely sensitive to temperature changes. Unlike other thermoplastics, the melting point of nylon is more obvious. Polyamide is molded on its melting point, so its molding temperature must be higher than that of ordinary materials. The relationship between the crystallinity of nylon and the mold temperature is shown in Figure 8.
Because nylon is hygroscopicity, it must be dried in advance. However, if dry at 90 degrees Celsius will change color, you must pay attention to it.
(4) Polyoxymethylene
Polyoxymethylene (POM) has homopolymer and copolymer, which are not good in fluidity. Such resins are prone to thermal decomposition. Attention must be paid to controlling the temperature during molding. The thermal stability of paraformaldehyde is better than that of paraformaldehyde (Figure 9). It can be processed at a slightly higher temperature, but the time of the material to stay in the barrel should not be too long. Otherwise, the thermal decomposition will make the products yellow and yellow.
(5) PBT resin
PBT resin (poly butylene terephthalate) and PET (polyethylene terephthalate) all belong to saturated polyester (thermoplastic polyester). PBT resin is characterized by low viscosity and good formability. It solidified quickly because it crystallized rapidly.
PBT and PET resins are generally enhanced by glass fiber reinforcement. Here, the key points of forming non reinforced resin substrate are introduced. Its heating cylinder is generally 230~270 degrees (flame retardant level is 250 degrees Celsius). The mold temperature is 40~90 C, although it can be molded at lower die temperature, higher mold temperature should be adopted for products with gloss. The injection pressure range was 50~130 MPa. Because the resin coagulating rapidly, the injection speed should be fast, so that the appearance of the product is good and beneficial to improve the performance. In addition, as the hygroscopic resin melted, the result of water splitting will result in brittle products, so resin pre drying is necessary before processing.
