Common problems noticed in the process of transparent plastic injection molding

2021-03-23 10:59:30 admin

Common problems noticed in the process of transparent plastic injection molding




Because of the high light transmittance of transparent plastics, strict requirements on the surface quality of plastic products are bound to be required. There should be no defects such as markings, pores, whitening, haze, black spots, discoloration, and poor gloss. Therefore, the raw materials are treated in the entire injection molding process. , Equipment, injection molds, and even product design, we must pay great attention to and put forward strict or even special requirements. Secondly, because transparent plastics mostly have high melting point and poor fluidity, in order to ensure the surface quality of the product, a higher temperature is often required. The process parameters such as injection pressure and injection speed must also be fine-tuned so that the injection molded parts can be filled with the mold. , And will not produce internal stress and cause product deformation and cracking. Therefore, strict operations must be carried out in terms of raw material preparation, equipment and injection mold requirements, injection molding process and product raw material processing.

 (1) Preparation and drying of raw materials

 Since any impurity in the plastic may affect the transparency of the product, it must be sealed during storage, transportation, and feeding to ensure that the raw materials are clean. In particular, the raw materials contain water, which will cause the raw materials to deteriorate after heating, so they must be dried. When injecting parts, a dry hopper must be used for 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 will not be polluted. The drying process is as follows. The drying process of transparent plastic: Material drying temperature (℃) Drying time (h) Material layer thickness (mm) Remarks PMMA 70~80 2~4 30~40 PC 120~130 >6<30 Use hot air Circulating drying PET 140~180 3~4 It is better to use continuous drying feeding device (2) The cleaning of the barrel, screw and accessories is to prevent raw material pollution and there are old materials or impurities in the recesses of the screw and accessories, especially poor thermal stability Because of the existence of the resin, the screw cleaning agent should be used to clean the parts before use and after the shutdown, so that no impurities are attached. When there is no screw cleaning agent, the screw can be cleaned with PE, PS and other resins. When temporarily shutting down, in order to prevent the raw material from staying at high temperature for a long time, causing the solution to drop, the temperature of the dryer and the barrel should be lowered, such as PC, PMMA, etc., the barrel temperature should be reduced to below 160 ℃ (the hopper temperature is suitable for PC Down to below 100°C).

 (3) Issues that should be paid attention to in the design of injection molds (including product design) In order to prevent poor backflow, or uneven cooling, resulting in poor plastic molding, surface defects and deterioration, generally when designing injection molds, The following points should be noted.

 a) The wall thickness should be as uniform as possible, and the draft angle should be large enough;

 b) The transition part should be smooth and gradually transition to prevent sharp corners and sharp edges, especially PC products must not have gaps;

 c) The gate and runner should be as wide and short as possible, and the gate position should be set according to the shrinkage condensation process, and a cold well should be added if necessary;

 d) The surface of the injection mold should be smooth, with low roughness (preferably lower than 0.8); e) Vent holes. The tank must be sufficient to discharge air and gases in the melt in a timely manner;

 f) Except for PET, the wall thickness should not be too thin, generally not less than 1mm.

 (4) Issues that should be paid attention to in the process of injection molded parts (including the requirements of the injection molding machine) In order to reduce internal stress and surface quality defects, the following issues should be paid attention to in the process of injection molded parts.

 a) A special screw and an injection molding machine with a separate temperature control nozzle should be used; b) The injection temperature should be higher when the plastic resin does not decompose; c) Injection pressure: generally higher to overcome melting The defect of material viscosity is large, but too high pressure will cause internal stress to cause difficult demolding and deformation; d) Injection speed: In the case of satisfying mold filling, generally it should be low, and it is best to use slow-fast-slow multi-stage Injection; e) Holding pressure time and molding cycle: Under the condition of satisfying product filling without pits 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 quality of plasticization, it should be as low as possible to prevent the possibility of degrading; g) Mold temperature of injection molded parts: 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.

 (5) Other problems. In order to prevent the deterioration of the upper surface quality, generally use less release agent when injection molded parts; when using recycled materials, no more than 20%. Except for PET, 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 heated in clean air, glycerin, liquid paraffin, etc., at 110-135℃, and the time depends on the product. It may take up to more than 10 hours. However, PET must undergo a biaxial stretching process to obtain good mechanical properties. Modifier additives (PA toughener, PC toughener, ABS toughener, PBT toughener, grafted PP, grafted ABS, blown film and pipe brightener, etc.).