Due to the high light transmittance of transparent plastics, the surface quality of plastic products must be strict, and there must be no markings, pores or whitening. Fog, black spots, discoloration, poor gloss and other defects, so the raw materials, equipment throughout the injection process. Molds and even product designs must pay great attention to and impose strict or even special requirements. Secondly, because transparent plastics have high melting point and poor fluidity, in order to ensure the surface quality of the products, the process parameters such as machine temperature, injection pressure and injection speed are often finely adjusted so that the plastic can be filled with mold without Internal stress is generated to cause deformation and cracking of the product. Therefore, strict preparation must be carried out from the preparation of raw materials, equipment and mold requirements, injection molding processes and raw material handling of products.
First, the preparation and drying of the material due to the inclusion of any impurities in the plastic, may affect the transparency of the product, and therefore storage and transportation. During the feeding process, attention must be paid to the sealing to ensure that the raw materials are clean. In particular, the raw material contains moisture, which causes deterioration of the raw material after heating, so it must be dried, and in the injection molding, the drying hopper must be used for the feeding. It should also be noted that during the drying process, the input air should preferably be filtered and dehumidified to ensure that the raw materials are not contaminated. Drying process, such as drying process of transparent plastic: material process drying temperature (°C) drying time (h) material layer thickness (mm) Remarks pmma70～802～430～40pc120～130>6<30 using hot air circulation drying pet140～1803 ～4 is preferably used in continuous drying and feeding device
Second, the cleaning of the barrel, screw and its accessories to prevent contamination of raw materials and the presence of old materials or impurities in the recesses of the screw and accessories, especially the resin with poor thermal stability, so the screw cleaning agent is used before and after the shutdown. Clean all parts so that they do not stick to impurities. When there is no screw cleaning agent, clean the screw with pe, ps and other resins. When temporarily stopping, in order to prevent the raw material from staying at high temperature for a long time, causing the solution to fall, the temperature of the dryer and the barrel should be lowered. For example, the temperature of the barrel such as pc and pmma should be reduced to below 160 °C. (The hopper temperature should be reduced to below 100 °C for pc)
Third, the problems that should be paid attention to in the design of the mold (including the design of the product) In order to prevent the occurrence of poor back flow, or uneven cooling, resulting in poor plastic molding, surface defects and deterioration, generally in the mold design, should pay attention to the following points . a=The wall thickness should be as uniform as possible, the draft angle should be large enough; b=the transition part should be stepwise. Smooth transition to prevent sharp corners. Sharp edges are produced, especially pc products must not have a gap; c = gate. The flow path should be as wide and thick as possible, and the gate position should be set according to the shrinkage condensation process. If necessary, the cold well should be added; d = the mold surface should be smooth and the roughness is low (preferably less than 0.8); e = exhaust hole. The tank must be sufficient to discharge the air and the gas in the melt; f = except for pet, the wall thickness should not be too thin, generally not less than lmm.
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