Due to high light transmittance of transparent plastics, it is inevitable that surface quality of plastic products must be strict, and there must not be any spots, pores, whitening. Halo, black spots, discoloration, poor gloss and other defects, so the entire injection molding process requires raw materials, equipment.
Molds, and even product design, must pay great attention and put forward strict or even special requirements. Secondly, since transparent plastics are mostly high melting points and poor fluidity, in order to ensure surface quality of product, it is often necessary to make subtle adjustments to process parameters such as machine height temperature, injection pressure, and injection speed, so that injection plastic can fill mold without generating internal stress and causing product deformation and cracking.

 

Following is about its raw material preparation. Let's talk about the matters that should be paid attention to in terms of equipment and mold requirements, injection molding process and raw material processing of products.
Preparation and drying of raw materials
Since any impurities in plastics may affect transparency of product, they should be stored and transported. During feeding process, attention must be paid to sealing to ensure that raw materials are clean.
In particular, raw materials contain moisture, which will cause raw materials to deteriorate after heating, so they must be dried, and during injection molding, feeding must use a dry hopper. Another point to note is that during drying process, input air should preferably be filtered and dehumidified to ensure that raw materials will not be contaminated.
Cleaning of barrels, screws and their accessories
In order to prevent raw material contamination and presence of old materials or impurities in depressions of screws and accessories, especially presence of resins with poor thermal stability, screw cleaning agents should be used to clean all parts before use and after shutdown to prevent impurities from sticking. When there is no screw cleaning agent, PE, PS and other resins can be used to clean screw.
When machine is temporarily shut down, in order to prevent raw materials from staying at high temperature for a long time and causing degradation, temperature of dryer and barrel should be lowered. For example, barrel temperature of PC, PMMA, etc. should be reduced to below 160℃. (Hopper temperature for PC should be reduced to below 100℃)

 

Mold design (including product design)
In order to prevent poor reflux or uneven cooling from causing poor plastic molding, surface defects and deterioration, following points should be noted when designing mold:
Wall thickness should be as uniform as possible, and demolding slope should be large enough;
Transition part should be gradual. Smooth transition to prevent sharp corners. Sharp edges, especially PC products must not have gaps;
Gate. Runner should be as wide and short as possible, gate position should be set according to shrinkage and condensation process. If necessary, a cold well should be added;
Mold surface should be smooth and low in roughness (preferably less than 0.8);
Vent hole. Groove must be sufficient to exhaust air and gas in melt in time;
Except for PET, wall thickness should not be too thin, generally not less than 1 mm.
Injection molding process and injection molding machine
In order to reduce internal stress and surface quality defects, following issues should be noted in injection molding process:
Special screws and injection molding machines with separate temperature-controlled nozzles should be selected;
Injection temperature should be as high as possible under premise that plastic resin does not decompose;
Injection pressure: Generally higher to overcome defect of high viscosity of melt, but too high pressure will produce internal stress, causing demolding difficulties and deformation;
Injection speed: Under condition of meeting mold filling, it is generally better to be low, and it is best to use slow-fast-slow multi-stage injection;
Pressure holding time and molding cycle: Under condition of meeting product mold filling and not producing dents and bubbles; it should be as short as possible to minimize residence time of melt in barrel;
Screw speed and back pressure: Under premise of meeting plasticization quality, it should be as low as possible to prevent possibility of decompression;
Mold temperature: Cooling of product has a great impact on quality, so mold temperature must be able to accurately control its process. If possible, mold temperature should be higher.

 

Other issues
In order to prevent deterioration of upper surface quality, mold release agent is generally used as little as possible during injection molding; when recycled material is used, it should not exceed 20%.
Except for PET, all products should be post-processed to eliminate internal stress. PMMA should be dried in 70-80t hot air circulation for 4 hours; PC should be heated at 110-135℃ in clean air, glycerin, liquid paraffin, etc. Time depends on the product, and it takes up to more than 10 hours. PET must undergo a two-way stretching process to obtain good mechanical properties.