Because of the high light transmittance of transparent plastics, the surface quality of plastic products must be required to be strict, there can be no markings, pores, whitening, haze, black spots, discoloration, poor luster and other defects, and thus raw materials throughout the injection molding process, equipment.
The molds and even the design of the products must be paid attention to and put forward strict or even special requirements. Secondly, because transparent plastics are mostly of high melting point and poor fluidity, in order to ensure the surface quality of the products, the temperature at the machine is often high, the injection pressure, and the injection Fine adjustments are made to the process parameters such as the speed, so that the injection molding can not only fill the mold, but also cause no internal stress and cause product deformation and cracking.
Here are some things to note in terms of raw material preparation, equipment and mold requirements, injection molding process and product raw material handling.
Raw material preparation and drying
Since any impurities in the plastic may affect the transparency of the product, and during storage, transportation, and feeding processes, care must be taken to ensure that the material is clean.
In particular, water is contained in the raw material, and it will cause deterioration of the raw material after heating. Therefore, it must be dried. In the injection molding, a dry hopper must be used for the addition. Note that during the drying process, the input air should preferably be filtered and dehumidified. , in order to ensure that it will not contaminate the raw materials.
Barrel, screw and accessories cleaning
In order to prevent contamination of raw materials and presence of old materials or impurities in the screw and attachment recesses, special resins with poor thermal stability exist. Therefore, before use, all parts shall be cleaned with a screw cleaning agent so that they do not adhere to impurities. When there is no screw cleaning agent, PE, PS and other resins can be used to clean the screw.
When temporary shutdown, in order to prevent the raw materials from staying at high temperature for a long time and cause the solution to drop, the temperature of dryer and barrel should be reduced. For example, PC, PMMA and other barrel temperatures should be reduced to below 160°C. (Hopper temperature for PC Should be reduced to below 100°C
Mold design (including product design)
In order to prevent backflow, or uneven cooling, causing plastic molding defects, 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 should be large enough;
The transition should be gradual. Smooth transition to prevent sharp corners. Sharp edges, especially PC products must not have gaps;
Gates. The runners should be as wide and short as possible, and the gate location should be set according to the contraction condensation process. If necessary, cold material wells should be added.
The mold surface should be smooth, with low roughness (preferably lower than 0.8);
Vents. Slots must be sufficient to discharge air and gases from 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:
Should use special screw, injection molding machine with separate temperature control nozzle;
Injection temperature should not be decomposed under the premise of plastic resin, it is better to use higher injection humidity;
Injection pressure: Generally higher, to overcome the defects of large melt viscosity, but the pressure is too high will cause internal stress caused by mold release and deformation;
Injection speed: In the case of filling the mold, it should be generally low. It is better to use slow, fast, and slow multi-stage injection.
Pressure holding time and molding cycle: In the case of satisfying product filling, no depressions or bubbles, it 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 quality of plasticization, it should be as low as possible to prevent the possibility of dropout;
Mold temperature: The cooling of the product is very good, which has a great influence 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
Because it is necessary to prevent the quality of the upper surface from deteriorating, injection molding agents should be used as little as possible during the injection molding process; when using recycled materials, they should not be greater than 20%.
In addition to PET, the products should be post-processed to eliminate internal stress, PMMA should be dried in 70-80t hot air circulation for 4 hours; pc should be in clean air, glycerin. Liquid paraffin and other heating 110-135 °C, time by product However, it takes more than 10 hours. PET must undergo a biaxial stretching process to obtain good mechanical properties.