Introduction to transparent plastic injection molding knowledge

Due to the high transmittance of transparent plastics, the surface quality of plastic products must be strict, and there shall be no spots, pores and whitening. Fog halo, black spot, discoloration, poor gloss and other defects, so the raw materials and equipment are damaged in the whole injection molding process. Mold and even product design should pay great attention to and put forward strict or even special requirements. Secondly, because most transparent plastics have high melting point and poor fluidity, in order to ensure the surface quality of products, it is often necessary to make subtle adjustments to the process parameters such as high temperature, injection pressure and injection speed, so that the mold can be filled during plastic injection without internal stress causing product deformation and cracking. Therefore, strict operation should be carried out in terms of raw material preparation, equipment and mold requirements, injection molding process and raw material treatment of products.

Preparation and drying of raw materials because there are any impurities in the plastic, which may affect the transparency of the product, attention must be paid to sealing during storage, transportation and feeding to ensure that the raw materials are clean. In particular, the raw materials contain water, which will cause the deterioration of the raw materials after heating, so they must be dried, and the drying hopper must be used for feeding during injection molding. It should also be noted that during the drying process, the input air should be filtered and dehumidified to ensure that it will not pollute the raw materials. Drying process of transparent plastics: material / process drying temperature (℃) drying time (H) material layer thickness (mm) remarks pmma70 ~ 802 ~ 430 ~ 40pc120 ~ 130 > 6 < 30 hot air circulation drying pet140 ~ 1803 ~ 4 continuous drying feeding device is preferred

Cleaning of barrel, screw and its accessories in order to prevent raw material pollution and the existence of old materials or impurities in the depression of screw and accessories, especially the existence of resin with poor thermal stability, screw cleaning agent shall be used to clean all parts before use and after shutdown, so that there shall be no impurities. When there is no screw cleaning agent, PE, PS and other resins can be used to clean the screw. In case of temporary shutdown, in order to prevent the raw materials from staying at high temperature for a long time and causing degradation, the temperature of dryer and barrel should be reduced, such as, PMMA and other barrel temperatures should be reduced below 160 ℃（ Hopper temperature should drop below 100 ℃ for PC)

（3） Problems needing attention in mold design (including product design) in order to prevent poor plastic molding, surface defects and deterioration caused by poor reflux or uneven cooling, the following points should be paid attention to in mold design.

a) The wall thickness shall be as uniform as possible, and the demoulding slope shall be large enough;

b) The transition part shall be gradually. Smooth transition to prevent sharp corners. Sharp edges, especially PC products, must not have gaps;

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

d) The die surface shall be smooth and low roughness;

e) Vent. The tank must be sufficient to discharge air and gas in the melt in time;

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

（4） Problems needing attention in injection molding process (including requirements of injection molding machine) in order to reduce internal stress and surface quality defects, the following problems should be paid attention to in injection molding process.

a) The injection molding machine with screw and separate temperature control nozzle shall be selected;

b) when the injection temperature is not decomposed, it is advisable to use higher injection humidity.

c) injection pressure: generally high, in order to overcome the defect of large viscosity, but too high pressure will cause internal stress, resulting in difficulty in mold release and deformation.

d) Injection speed: generally, it should be low when the mold filling is satisfied, and it is best to adopt slow fast slow multi-stage injection;

e) Pressure holding time and forming cycle: when the product filling is satisfied without depression and bubble; It should be as short as possible to minimize the residence time of molten material in the barrel;

f) Screw speed and back pressure: on the premise of meeting the plasticization quality, it should be as low as possible to prevent the possibility of degradation;

g) Mold temperature: the cooling of products 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) in other areas, in order to prevent deterioration of surface quality, mold release agents should be used as little as possible in general injection molding. When used for recycling, it must not exceed 20%. For pet, the products should be treated postprocessing to eliminate internal stress. PMMA should be dried for 4 hours at 70 to 80 ton hot air circulation. PC should be clean air and glycerin. Liquid stones and wax can be heated at 110-135 C for more than 10 hours depending on the product. Pet must go through the process of biaxial stretching in order to obtain good mechanical properties.