Plastic Injection Molding Technology for Transparent parts, the surface quality of plastic products must be strictly required, and there should be no markings, pores, and whitening. Haze, black spots, discoloration, poor gloss and other defects, so in the entire injection molding process, raw materials, equipment.
Mold, and even product design, must pay great attention and put forward strict or even special requirements. Secondly, because most of the transparent plastics have high melting point and poor fluidity, in order to ensure the surface quality of the product, it is often necessary to make fine adjustments to the process parameters such as high machine temperature, injection pressure, injection speed, etc., so that the plastic injection can not only fill the mold, but also not. Internal stress will cause product deformation and cracking.
Below is the 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 the matters that should be paid attention to.
1. Preparation and drying of raw materials
Since any impurity contained within the plastic might have an effect on the transparency of the merchandise, and thus storage and transportation. throughout the feeding method, attention should be paid to protection to make sure that the raw materials square measure clean.
In specific, the material contains wet, which is able to cause the material to deteriorate once heating, therefore it should be dried, and a drying hopper should be used for feeding throughout injection molding. It ought to even be noted that within the drying method, the input air ought to rather be filtered and dehumidified to make sure that the raw materials won’t be impure.
Plastic Injection Molding Technology for Transparent parts
2. Cleaning of barrels, screws and accessories
In order to prevent the contamination of raw materials and the existence of old materials or impurities in the depressions of the screw and accessories, especially the resin with poor thermal stability, the screw cleaning agent should be used to clean all parts before use and after shutdown, so that no impurities can be adhered. If there has no screw cleaning agent, it can used PE, ps and other resins to cleaned the screw.
When temporarily shutting down, in order to prevent the raw materials from staying at high temperature for a long time and causing degradation, the temperature of the dryer and the barrel should be lowered, such as the barrel temperature of pc, pmma, etc. should be lowered to below 160 °C. (Hopper temperature should drop below 100°C for pc)
3. Mold design (including product design)
In order to prevent poor backflow or uneven cooling resulting in poor plastic molding, surface defects and deterioration, the following points should be paid attention to in mold design:
The wall thickness must be uniform , and the demolding slope should be large enough;
The transition part should be gradual. Smooth transitions to prevent sharp corners. Sharp edges will appeared, especially PC products can not have a little gap;
The runner should be as wide and stubby as possible, and the gate position should be set according to the shrinkage and condensation process, and a cold well should be added if necessary;
The surface of the mold should be smooth and low in roughness (preferably below 0.8);
Vent. The tank must be sufficient to discharge air and gas in the melt in time;
Except for pet material’s product, the thickness shouldn’t too thin, generally not less than 1 mm
4. Injection molding process and injection molding machine
In order to reduce internal stress and surface quality defects, the following aspects should be paid attention to in the injection molding process:
Special screw and injection molding machine with separate temperature management nozzle ought to be used;
The injection temperature should be higher than the injection humidity under the premise that the plastic resin does not decompose;
Injection pressure: generally high to overcome the 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 the condition that the product is filled with mold and no dents and bubbles are generated; It should be as short as possible,so to the greatest extent minimize the residence time of the melt in material barrel of machine.
Screw speed and back pressure: under the premise of satisfying plasticization 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. The mold temperature need be higher.
5. Other Issues of Plastic Injection Molding Technology for Transparent parts
In order to prevent the quality of the 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 ℃, the time depends on the product, and it takes more than 10 hours at most. The pet must go through the process of biaxial stretching in order to obtain good mechanical properties.
