Views: 2 Author: Site Editor Publish Time: 2023-07-20 Origin: Site
Transparent plastics due to high light transmittance, it is necessary to require strict surface quality requirements of plastic products, can not have any markings, pores, white. Fog halo, black spots, discoloration, poor luster and other defects, so in the entire injection process to raw materials, equipment. Mold, and even product design, should pay great attention to and put forward strict and even special requirements. Secondly, because transparent plastics are mostly high melting point and poor fluidity, in order to ensure the surface quality of the product, it is often necessary to make subtle adjustments in the process parameters such as the temperature of the barrel, the injection pressure, and the injection speed, so that the injection of plastic can be filled with mold, and will not produce internal stress and cause product deformation and cracking.
(1) Preparation and drying of raw materials
Due to any impurities contained in the plastic, it may affect the transparency of the product, so storage and transportation. During the feeding process, attention must be paid to sealing to ensure that the raw materials are clean. In particular, the raw material contains water, which will cause the deterioration of the raw material after heating, so it must be dry, and in the injection molding, the feeding must use the drying hopper. It should also be noted that during the drying process, the input air should preferably be filtered and dehumidified to ensure that it will not pollute the raw materials.
(2) Cleaning of barrel, screw and accessories
In order to prevent raw material pollution and old materials or impurities in the depression of the screw and accessories, especially the existence of resin with poor thermal stability, so before use and after shutdown, screw cleaning agent is used to clean each part, so that it is not stuck with impurities, when there is no screw cleaning agent, PE, PS and other resins can be used to clean the screw. When the temporary shutdown, in order to prevent the raw material from staying at high temperature for a long time, causing degradation, the temperature of the dryer and the barrel should be reduced, such as PC, PMMA and other barrel temperatures should be reduced to below 160 ° C.
(3) Problems that should be paid attention to in mold design
In order to prevent poor back flow, or uneven cooling caused by poor plastic molding, resulting in surface defects and deterioration, generally in mold design, the following points should be paid attention to.
a) The wall thickness should be as uniform as possible, and the demoulding slope should be large enough;
b) The transition should be gradual. Smooth transition to prevent sharp corners. Sharp edge generation, especially PC products must not have gaps;
c) Gate. The runner should be as wide and short as possible, and the gate position should be set according to the shrinkage condensation process, and the cooling well should be added if necessary;
d) Mold surface should be smooth, low roughness (preferably less than 0.8);
e) Exhaust holes. The tank must be sufficient to discharge air and gases from the melt in a timely manner;
f) Except for PET, the wall thickness should not be too thin, generally not less than lmm.
(4) Problems that should be paid attention to in the injection molding process
In order to reduce internal stress and surface quality defects, the following aspects should be paid attention to in the injection molding process.
a) The injection molding machine with a special screw and a separate temperature control nozzle should be selected;
b) Under the premise that the plastic resin does not decompose, higher injection temperature should be used;
c) Injection pressure: generally higher to overcome the defect of large viscosity of molten material, but too high pressure will produce internal stress resulting in difficult demording and deformation;
d) Injection speed: In the case of mold filling, it is generally low, and it is best to use slow-fast-slow multi-stage injection;
e) Pressure holding time and molding cycle: under the condition that the product is filled without dents and bubbles; It should be as short as possible to minimize the residence time of molten material in the barrel;
f) Screw speed and back pressure: under the premise of meeting the plasticizing quality, it should be as low as possible to prevent the possibility of degradation;
g) Mold temperature: the cooling of the product has a great impact on the quality, so the mold temperature must be able to accurately control the process, if possible, the mold temperature should be higher.
In order to prevent the deterioration of the quality of the upper surface, the release agent is generally used as little as possible during injection molding. Not more than 20% when using recycled materials. In addition to PET, products should be post-treated to eliminate internal stress, PMMA should be dried at 70-80℃ hot air circulation for 4 hours; PC should be glycerin in clean air. Liquid paraffin is heated at 110-135 ° C, the time depends on the product, and the maximum need is more than 10 hours. PET must undergo a bidirectional drawing process in order to obtain good mechanical properties.