What problems should we pay attention to in transparent plastic injection molding?

Time:2022-06-23 16:34:22 / Popularity: / Source:

Due to high light transmittance of transparent plastics, surface quality of plastic products must be strictly required, and there should be no defects such as streaks, pores, whitening, halo, black spots, discoloration, and poor gloss. equipment.
transparent plastic injection molding 
Mold, and even product design, must pay great attention and put forward strict or even special requirements. Secondly, since most of transparent plastics have high melting point and poor fluidity, in order to ensure surface quality of product, it is often necessary to make fine adjustments to process parameters such as high temperature, injection pressure, injection speed, etc., so that mold can be filled when plastic is injected, product will not be deformed and cracked due to internal stress.
Below is preparation of its raw materials. In terms of equipment and mold requirements, injection molding process and raw material handling of products, we will talk about matters that should be paid attention to.

Preparation and drying of raw materials

Since any impurity contained in plastic may affect transparency of product, therefore storage and transportation. During feeding process, attention must be paid to sealing to ensure that raw materials are clean.
In particular, raw material contains moisture, which will cause raw material to deteriorate after heating, so it must be dried, and a drying hopper must be used for feeding during injection molding. It should also be noted that in drying process, input air should preferably be filtered and dehumidified to ensure that raw materials will not be polluted.

Cleaning of barrels, screws and accessories

In order to prevent contamination of raw materials and existence of old materials or impurities in depressions of screw and accessories, especially resin with poor thermal stability, screw cleaning agent should be used to clean all parts before use and after shutdown, so that no impurities can be attached. When there is no screw cleaning agent, screw can be cleaned with pe, ps and other resins.
When temporarily shutting 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, such as barrel temperature of pc, pmma, etc. should be lowered to below 160℃. (Hopper temperature should drop below 100℃ for pc)

Mold design (including product design)

transparent plastic injection molding 
In order to prevent poor backflow or uneven cooling, resulting in poor plastic molding, surface defects and deterioration, following points should be paid attention to in mold design:
Wall thickness should be as uniform as possible, and demolding slope should be large enough;
Transition part should be gradual. Smooth transitions to prevent sharp corners. Sharp edges are generated, especially PC products must not have gaps;
Gate. Runner should be as wide and stubby as possible, gate position should be set according to shrinkage and condensation process, and a cold well should be added if necessary;
Surface of mold should be smooth and low in roughness (preferably below 0.8);
Vent. Tank must be sufficient to discharge 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 aspects should be paid attention to in injection molding process:
Special screw and injection molding machine with separate temperature control nozzle should be used;
Injection temperature should be higher than injection humidity under premise that plastic resin does not decompose;
Injection pressure: generally high to overcome defect of high melt viscosity, but too high pressure will cause internal stress to cause difficulty in demolding and deformation;
Injection speed: In the case of satisfying mold filling, it should generally be low, and it is best to use slow, fast, and slow multi-stage injection;
Pressure holding time and molding cycle: under condition that product is filled with mold without dents and bubbles; it should be as short as possible to minimize residence time of molten material in barrel;
Screw speed and back pressure: under premise of satisfying plasticization quality, it 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.

Other issues

In order to prevent quality of upper surface from deteriorating, generally use as little release agent as possible during injection molding; when using recycled materials, it should not exceed 20%.
Except for pet, all products should be post-treated to eliminate internal stress, pmma should be dried in 70-80t hot air circulation for 4 hours; pc should be in clean air and glycerin. Heating liquid paraffin at 110-135 ℃, time depends on product, and it takes more than 10 hours at most. Pet must go through process of biaxial stretching in order to obtain good mechanical properties.

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