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. Fog, black spots, discoloration, poor gloss and other defects, so the raw materials, equipment throughout the injection process.
Molds and even product designs must pay great attention to and impose strict or even special requirements. Secondly, because transparent plastics have high melting point and poor fluidity, in order to ensure the surface quality of the products, the process parameters such as machine temperature, injection pressure and injection speed are often finely adjusted so that the plastic can be filled with mold without Internal stress is generated to cause deformation and cracking of the product.
Here are some things to be aware of in terms of raw material preparation, equipment and mold requirements, injection molding process and raw material handling of the product.
Preparation and drying of raw materials
Since it contains any impurities in the plastic, it may affect the transparency of the product, and therefore, storage and transportation. During the feeding process, attention must be paid to the sealing to ensure that the raw materials are clean.
In particular, the raw material contains moisture, which causes deterioration of the raw material after heating, so it must be dried, and in the injection molding, the drying hopper must be used for the feeding. It should also be noted that during the drying process, the input air should preferably be filtered and dehumidified to ensure that the raw materials are not contaminated.
Cleaning of the barrel, screw and its accessories
In order to prevent raw material contamination and the presence of old materials or impurities in the screw and attachment recesses, especially the resin with poor thermal stability exists, so the screw cleaning agent is used to clean before use and after shutdown.
Clean the parts so that they do not stick to impurities. When there is no screw cleaning agent, the screws can be cleaned with pe, ps and other resins.
When temporarily stopping, in order to prevent the raw material from staying at high temperature for a long time, causing the solution to fall, the temperature of the dryer and the barrel should be lowered. For example, the temperature of the barrel such as pc and pmma should be reduced to below 160 °C. (The hopper temperature should be reduced to below 100 °C for pc)
Mold design (including product design)
In order to prevent the occurrence of poor back flow, or uneven cooling, resulting in poor plastic molding, surface defects and deterioration, generally in the mold design, should pay attention to the following points:
The wall thickness should be as uniform as possible, and the draft angle should be large enough;
The transition should be gradual. Smooth transition to prevent sharp corners. Sharp edges are produced, especially PC products must not have gaps;
Gate. The flow channel should be as wide and thick as possible, and the gate position should be set according to the shrinkage condensation process, and a cold well should be added if necessary;
The surface of the mold should be smooth and the roughness is low (preferably less than 0.8);
Vent. The tank must be sufficient to vent the air and the gas in the melt;
Except for pet, the 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, the following aspects should be noted in the injection molding process:
Special screw and injection molding machine with separate temperature control nozzle should be used;
The injection temperature should be higher than the plastic resin without decomposition;
Injection pressure: generally higher, to overcome the defects of large viscosity of the melt, but the pressure is too high will cause internal stress and cause deformation and deformation;
Injection speed: in the case of filling the mold, it is generally low, it is best to use slow, fast and slow multi-stage injection;
Packing time and molding cycle: in the case of satisfying product filling, no dents or bubbles; should be as short as possible to minimize the residence time of the melt in the barrel;
Screw speed and back pressure: Under the premise of satisfying the plasticizing quality, it should be as low as possible to prevent the possibility of depreciation;
Mold temperature: The cooling of the product is good or bad, which has a great impact on the quality. Therefore, the mold temperature must be able to accurately control the process. If possible, the mold temperature should be higher.
Other issues
In order to prevent the deterioration of the upper surface quality, the mold release agent should be used as little as possible during injection molding; when it is used, it should not be greater than 20%.
For products other than pet, the product should be post-treated to eliminate internal stress. pmma should be dried in 70-80t hot air for 4 hours; pc should be clean air, glycerin. Liquid paraffin and other heating 110-135 ° C, the time depends on the product, the most need more than 10 hours. And pet must undergo a biaxial stretching process in order to obtain good mechanical properties.
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