According to the capacity of moisture absorption or adhesion, plastics can be divided into two categories: hygroscopic plastics and non-hygroscopic plastics. Hygroscopic plastics are more prone to water degradation during the injection molding process, and defects such as bubbles, silver threads, and markings appear on the plastic parts after molding. Therefore, the plastic packaging barrel must be dried before molding.
If necessary, an infrared heating device should be installed in the hopper of the injection machine to prevent the dried plastic from absorbing moisture or sticking water again in the hopper before entering the barrel. Hygroscopic plastics include polyamide, ABS,
polycarbonate, polyphenylene ether and polyfengle, etc., and non-hygroscopic plastics include polyethylene, polypropylene, polystyrene and fluoroplastics. Heat sensitivity refers to the sensitivity of the chemical properties of plastics to heat. Plastics with strong heat sensitivity are called heat-sensitive plastics.
Commonly used heat-sensitive plastics include rigid polyvinyl chloride, polyvinyl chloride vinyl acetate copolymer, polyoxymethylene and polychlorotrifluoroethylene. During the molding process, heat-sensitive plastics are prone to thermal decomposition and thermal degradation at not too high temperatures and release some volatile gases, which will affect the performance, color and surface quality of the plastic parts to the human body and molds.
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