Introduction
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. Defects such as fog, black spots, discoloration, and poor gloss require great attention and strict or even special requirements for raw materials, equipment, molds, and even product design during the entire injection molding process.

Secondly, since most transparent plastics have high melting points and poor fluidity, in order to ensure the surface quality of the product, it is often necessary to make fine adjustments to process parameters such as barrel temperature, injection pressure, and injection speed, so that the mold can be filled with plastic while not filling the mold. Internal stress will be generated, causing product deformation and cracking.

Common issues that should be paid attention to during transparent plastic injection molding process

Preparation and drying of raw materials: Any impurity contained in the plastic may affect the transparency of the product. Therefore, during storage, transportation, and feeding, attention must be paid to sealing to ensure that the raw materials are clean. In particular, the raw materials contain moisture, which will cause deterioration of the 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 the drying process, the input air should preferably be filtered and dehumidified to ensure that it does not contaminate the raw materials.

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

The wall thickness should be as uniform as possible, and the draft angle should be large enough;

The transition part should be gradual. Smooth transition to prevent sharp corners. Sharp edges are generated, especially PC products must not have gaps;

Gate. The flow channel should be as wide and short as possible, and the gate position should be set according to the shrinkage and condensation process. If necessary, a cold well should be added;

The mold surface should be smooth and clean, with low roughness (preferably less than 0.8);

Exhaust hole. The tank must be sufficient to discharge air and gases in the melt in a timely manner;

Except for PET, the wall thickness should not be too thin, generally not less than 1 mm.

An injection molding machine with a special screw and a separate temperature-controlled injection nozzle should be used;

Injection temperature. On the premise that the plastic resin does not decompose, a higher injection temperature should be used;

Injection pressure. Generally higher to overcome the defect of high melt viscosity, but too high a pressure will produce internal stress, causing difficulty in demolding and deformation;

Injection speed. When the mold filling is satisfied, it should generally be low, and it is best to use slow-fast-slow multi-stage injection;

Pressure holding time and molding cycle. Under the condition that the product is fully filled and no dents or bubbles are produced, it should be as short as possible to minimize the residence time of the molten material in the barrel.

Screw speed and back pressure. On the premise of meeting the plasticizing quality, it should be as low as possible to prevent the possibility of degradation;

Mold temperature. The cooling quality of the product has a great impact on the quality, so the mold temperature must be able to accurately control the process. If possible, the mold temperature should be higher.

In order to prevent the deterioration of the 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 hot air circulation at 70-80°C for 4 hours; PC should be dried in clean air and glycerin. Liquid paraffin, etc. are heated to 110-135°C. The time depends on the product, and it takes more than 10 hours at most. PET must undergo a biaxial stretching process to obtain good mechanical properties.