The phenomenon of bubbles in the injection molding process of PC plastics is very serious. The problems that should be paid attention to in the mold design (including product design) are as follows: in order to prevent poor backflow, poor plastic molding, surface defects and deterioration caused by uneven cooling, the following points should be noted in mold design:
A = the wall thickness should be as uniform as possible, and the stripping slope should be large enough;
B = the transition part should be gradual and smooth to prevent sharp corners. Sharp edge, especially PC products must not have a gap;
C = gate. The runner should be as wide and short as possible, and the gate position should be set according to the shrinkage condensation process. If necessary, the cold material well should be added;
D = die surface should be smooth and low roughness (preferably less than 0.8); E = vent hole. The tank must be sufficient to discharge the air and the gas in the melt in time; F = except for pet, the wall thickness should not be too thin, generally not less than LMM.
Material preparation and drying due to the presence of any impurities in the plastic, may affect the transparency of the product, therefore and storage, transportation. In the process of feeding, attention must be paid to sealing to ensure that the raw materials are clean. In particular, the raw material contains moisture, which will cause deterioration of the raw material after heating, so it must be dried, and during injection molding, the drying hopper must be used for feeding.
It should also be noted that in the drying process, the input air should be filtered and dehumidified to ensure that the raw materials will not be polluted. For example, the drying process of transparent plastics: material process drying temperature (℃) drying time (H) material layer thickness (mm) remarks pmma70-802-430-40pc120-130 > 6 < 30 hot air circulation drying pet140-1803-4 using continuous drying and feeding device is preferred.
Cleaning of barrel, screw and accessories in order to prevent raw material pollution and the existence of old materials or impurities in the depression of screw and accessories, especially the existence of resin with poor thermal stability, so screw cleaner should be used to clean all parts before use and after shutdown, so that no impurities can be stuck. When there is no screw cleaning agent, PE, PS and other resins can be used to clean the screw.
In case of temporary shutdown, in order to prevent raw materials from staying at high temperature for a long time and causing degradation, it is necessary to reduce the temperature of dryer and barrel, such as PC, PMMA, etc., to below 160 ℃. (the hopper temperature should be reduced to below 100 ℃ for PC).
Due to the high transmittance of transparent plastics, the surface quality of plastic products is required to be strict, without any stripes, pores and whitening. Fog halo, black spots, discoloration, poor gloss and other defects, so in the entire injection molding process of raw materials, equipment. Strict and even special requirements should be paid attention to and put forward in the design of molds and even products.
Secondly, due to the high melting point and poor fluidity of transparent plastics, in order to ensure the surface quality of products, it is often necessary to adjust the process parameters such as high temperature, injection pressure and injection speed, so that the injection molding can not only fill the mold, but also will not produce internal stress, which will cause deformation and cracking of products. Therefore, strict operation should be carried out from the aspects of raw material preparation, equipment and mold requirements, injection molding process and product raw material treatment.
