Plastics used for injection molding are divided into two categories

Thermoset


It is melted at low temperature (30-130) ℃, and cross-linked and solidified at high temperature (140-200) ℃ in the mold. The characteristic is that it can be transformed into a plastic melt at a certain temperature, but if the temperature continues to increase and the heating time is increased, the polymer will have a cross-linking effect and solidification.

The cured plastic can neither be dissolved in the solvent, nor can it be softened to its original state by heating, and will decompose if the temperature is continued to increase. This is an irreversible process and cannot be processed repeatedly.


Thermoplastic


It is divided into crystalline and amorphous. It is characterized by heating and softening, melting when the temperature continues to rise, and returning to the solid state when cooling. This process is reversible and can be repeated many times.


Crystallinity; polymer molecular chains are arranged in a regular and orderly state. Such as; PP, PE, POM, PA, PBT, PET ‘PC’


Amorphous; polymer molecular chains are arranged in a random and disordered state. , Such as PS, AS, ABS, PC+ABS, PMMA, PVC, PU, PPO, PPE, PEI, LCP, PPS, etc. The common difference between crystalline and amorphous polymers;


Crystalline materials are particularly sensitive to temperature during molding. They all have their own melting points. When the temperature is below the melting point, there is no fluidity, but once the temperature reaches the melting point, the fluidity increases dramatically. The molding shrinkage rate is large, the apparent gloss is good, and it has heat resistance and high mechanical strength.


High crystallinity polymer density, tensile strength, surface finish increased; impact strength, stiffness decreased. Rigidity is one of the conditions for product demoulding, and the improvement of crystallinity helps to improve the softening temperature and heat distortion temperature and heat resistance, which is conducive to demolding. The increase in crystallinity will reduce the volume of the product and increase the shrinkage. Because the product is cooled in the mold, due to the difference in crystallinity caused by the difference in temperature, the macromolecular chain produces an isotropic orientation. Uneven density and shrinkage are used.


Lead to higher internal stress and cause warpage, and reduce the resistance to stress cracking. Amorphous materials have no obvious melting point, the influence of pressure during molding is more sensitive than temperature, and the shrinkage rate is small. Temperature resistance, mechanical strength, chemical resistance, surface finish are not as good as crystalline polymers.


Both are affected by the mold temperature in the same way. That is, the higher the mold temperature, the greater the shrinkage rate, and the product size has a downward trend. The crystalline material has an optimal crystallization temperature, at which the product size is more stable and the deformation is small. However, the deformation of the two is affected by the mold temperature in opposite ways.


That is, crystalline products are deformed to the side with high mold temperature, and amorphous products are deformed to the side with low mold temperature.