Due to high light transmittance of transparent plastics, surface quality of plastic products must be strict, and there must be no defects such as markings, pores, whitening, halo, black spots, discoloration, poor gloss, etc. Therefore, raw materials and equipment are required throughout the entire injection molding process.
Design of molds and even products requires great attention and strict or even special requirements. Secondly, since most transparent plastics have high melting points and poor fluidity, in order to ensure surface quality of product, it is often necessary to make fine adjustments to process parameters such as machine temperature, injection pressure, and injection speed, so that mold can be filled with plastic when plastic is injected, internal stress will not be generated to cause product deformation and cracking.

 

Following is preparation of raw materials. Discuss matters that should be paid attention to in terms of equipment and mold requirements, injection molding process and product raw material handling.

Because any impurity contained in plastic may affect transparency of product, storage and transportation. 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 deterioration of raw materials after heating, so they must be dry, and during injection molding, a dry hopper must be used for feeding. It should also be noted that during drying process, input air should preferably be filtered and dehumidified to ensure that it does not contaminate raw materials.

In order to prevent raw material contamination and presence of old materials or impurities in the recesses of screw and accessories, especially 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 temporarily shutting down, in order to prevent raw materials from staying at high temperature for a long time and causing decomposition, temperature of dryer and barrel should be lowered. For example, barrel temperature of PC, PMMA, etc. should be lowered to below 160℃. (Hopper temperature should drop below 100℃ for PC).

 

In order to prevent poor backflow or uneven cooling resulting in poor plastic molding, surface defects and deterioration, generally following points should be noted when designing mold:
Wall thickness should be as uniform as possible, and draft angle should be large enough;
Transitions should be gradual. Smooth transition to prevent sharp corners. Sharp edges are generated, especially PC products must not have gaps;
Gate. Flow channel 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 clean with low roughness (preferably less than 0.8);
Vent. Tank must be sufficient to discharge air and gases in the melt in a timely manner;
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 aspects should be paid attention to in injection molding process:
An injection molding machine with a special screw and a separate temperature-controlled injection nozzle should be used;
As long as injection temperature does not decompose plastic resin, a higher injection humidity should be used;
Injection pressure: Generally high to overcome defect of high melt viscosity, but if pressure is too high, internal stress will be generated, causing difficulty in demolding and deformation;
Injection speed: When mold filling is satisfied, it should generally be low. It is best to use slow, fast and slow multi-stage injection;
Holding time and molding cycle: Under condition that product is fully filled and no dents or bubbles are generated, it should be as short as possible to minimize residence time of molten material in barrel;
Screw speed and back pressure: On the premise of meeting plasticizing quality, they should be as low as possible to prevent possibility of degradation;
Mold temperature: Cooling quality of product has a great impact on quality, so mold temperature must be able to accurately control process. If possible, mold temperature should be higher.

 

In order to prevent deterioration of upper surface quality, generally use as little release agent as possible during injection molding; when using recycled materials, it should not exceed 20%.
Except for PET, 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 to 110-135℃ in clean air, glycerin, liquid paraffin, etc. Time depends on product, and it takes up to more than 10 hours. PET must go through a biaxial stretching process to obtain good mechanical properties.