Transparent plastics due to high transmittance, must require strict surface quality of plastic products, there can not be any mottling, porosity, whitening, halo, black spots, discoloration, poor luster and other defects, and therefore in the entire injection molding process of raw materials, equipment, molds, and even product design, we must pay close attention to and put forward strict and even special requirements.
Secondly, due to the high melting point of transparent plastics, poor fluidity, so in order to ensure the surface quality of the product, often in the barrel temperature, injection pressure, injection speed and other process parameters for fine-tuning, so that the injection of plastics can be full of mold, and will not produce internal stresses caused by product deformation and cracking.
Requirements for equipment and molds, injection molding process and product raw material handling aspects, talk about matters that should be noted:
First, the preparation and drying of raw materials
As the plastic contains any little impurity, may affect the transparency of the product, so in the storage, transportation and charging process, must pay attention to sealing, to ensure that the raw material is clean. Especially the raw material contains water, heating will cause deterioration of raw materials, so it must be dry, and in the injection molding, feeding must use dry hopper. Also note that the drying process, the input air is best filtered, dehumidification, in order to ensure that the raw material will not be contaminated.
Second, the barrel, screw and its accessories cleaning
In order to prevent contamination of raw materials and in the screw and accessories at the depression of the old material or impurities, especially the existence of poor stability of the resin, therefore, before use, after the shutdown should be cleaned screw cleaner, so that it shall not adhere to impurities, when there is no screw cleaner, available PE, PS and other resins to clean the screw.
When the temporary shutdown, in order to prevent the raw materials at high temperatures to stay a long time, causing degradation, should be the dryer and barrel temperature reduction, such as PC, PMMA and other barrel temperature should be reduced to below 160 ℃. (Hopper temperature for PC should be reduced to below 100 ℃)
Third, in the mold design should pay attention to the problem (including product design)
In order to prevent the emergence of poor reflux, or uneven cooling caused by poor plastic molding, surface defects and deterioration; generally in the mold design, the following points should be noted:
1, the wall thickness should be as uniform and consistent as possible, and the demolding slope should be large enough;
2, the transition part should be gradual. Round and smooth transition, to prevent sharp corners. Sharp edges are generated, especially PC products must not have a gap;
Sprue. Runner as wide as possible, thick and short, and should be based on the shrinkage condensation process to set the location of the gate, if necessary, should be added to the cold material well;
3, mold surface should be smooth, low roughness (preferably less than 0.8); exhaust hole slot must be enough to discharge air and gas in the melt in a timely manner; in addition to PET, the wall thickness should not be too thin, generally not less than 1mm.
Fourth, the injection molding process should pay attention to the problem (including the requirements of the injection molding machine)
In order to reduce the internal stress and surface quality defects, in the injection molding process should pay attention to the following aspects of the problem:
1, should choose a special screw, with a separate temperature-controlled injection nozzle of the injection molding machine;
2, the injection temperature in the plastic resin does not decompose under the premise, it is appropriate to use a higher injection temperature;
3, injection pressure: generally higher, in order to overcome the defects of the viscosity of the melt, but the pressure is too high will produce internal stress caused by demolding difficulties and deformation;
4, injection speed: in order to meet the case of mold filling, generally low, it is best to use slow – fast – slow multi-stage injection;
5, holding time and molding cycle: in order to meet the product mold filling, no depression, bubbles; should be as short as possible, in order to minimize the melt in the barrel residence time;
6, screw speed and back pressure: in order to meet the premise of plasticizing quality, should be as low as possible to prevent the possibility of degradation;
7, mold temperature: the cooling of the product is good or bad, the quality of the great impact, so the mold temperature must be able to accurately control its process, if possible, the mold temperature should be higher good.
V. Other aspects
In order to prevent deterioration of surface quality, the general injection molding as little as possible with the release agent, when using the reuse of materials shall not be greater than 20%.
For products other than PET should be post-treatment to eliminate internal stress, PMMA should be 70-80 ℃ hot air cycle drying 4 hours; PC should be in the clean air, glycerin, liquid paraffin, such as heating 110-135 ℃, the time depends on the product, the highest need for more than 10 hours; and PET must be after the bidirectional drawing of the process, to get good mechanical properties. process to get good mechanical properties.
