Transparent plastic for the light transmittance requirements are very high, so in the entire injection molding work, any process needs to be very careful, once the appearance of pores, black spots, discoloration and other issues will have a great impact on the entire product, so we generally ensure the surface quality of the product at the same time also to the machine high temperature, injection pressure, Injection speed and other technical parameters for fine adjustment, so that the injection molding material can be filled with the mold, and will not produce internal stress caused by product deformation and cracking.
Let's talk about the issues that transparent plastics should be aware of during injection molding.
Preparation and drying of raw materials The inclusion of any impurity in the plastic may affect the transparency of the product and therefore storage and transport.
In the process of feeding, must pay attention to seal, ensure the raw material Clean. In particular, the material contains moisture, heating will cause deterioration of raw materials, so must be dry, and in the injection molding, The feed must use the drying hopper.
Also note that the drying process, the input of the best air should be filtered, dehumidification, in order to ensure that the raw materials will not be contaminated.
Cleaning of cylinders, screws and accessories
In order to prevent contamination of raw materials and in the screw and attachment in the presence of old materials or impurities, special thermal stability of the resin is poor, so before use, after the application of screw cleaning agent cleaning clean pieces, so that they do not stick with impurities, when there is no screw cleaning agent, The use of pe, PS and other resin cleaning screw. When the temporary stop, in order to prevent the raw material stay at high temperature for a long time, causing the solution drop, the dryer and the barrel temperature should be lowered, such as pc, PMMA and other barrel temperature should be reduced to 160 ℃ BELOW.
(the Hopper temperature should be reduced to 100 ℃ for Pc)
Mold Design (including Product Design)
In order to prevent the return flow is not smooth, or cooling uneven plastic molding, resulting in surface defects and deterioration, generally in the mold design, should pay attention to the following points:
The wall thickness should be uniform and consistent, the release angle should be large enough; The transitional part should be gradual. Smooth transition to prevent sharp corners.
Sharp edge production, Especially PC products must not have a gap; Gate.
The flow path is as lenient as possible, and the gate position should be set according to the shrinking condensation process, and the cold material well should be added if Necessary.
Mold surface should be smooth, low roughness (preferably less than 0.8); Vent Holes.
The trough must be sufficient to discharge the air and the gas in the melt in time;
In addition to pet, the wall thickness should not be too thin, generally not less than 1 mm.
Injection moulding and injection moulding machines
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:
Mayors with a special screw, with a separate temperature injection molding machine nozzle;
The injection temperature should be higher than the injection humidity under the premise of non-decomposition of plastic resin.
Injection pressure: generally high, in order to overcome the melt viscosity of large defects, but too high pressure will produce internal stress caused by the mold release due to difficulties and deformation;
Injection speed: in the case of filling mold, the general should be low, it is best to use a slow one fast multistage injection;
Holding time and molding cycle: in order to meet the product filling mold, do not produce dents, bubbles in the case, should be as short as possible to minimize the melt in the barrel dwell time;
Screw speed and back pressure: in the premise of satisfying the plasticizing quality, should be as low as possible to prevent the possibility of a solution drop;
Mold Temperature: Product Cooling good or bad, the quality of the impact of great, so the mold temperature must be able to accurately control its process, if possible, mold Wen Yicho some good.
Other aspects of the problem
In order to prevent deterioration of surface quality, general injection molding as far as possible to use less release agent, when the use of the material should not be greater than 20%. In addition to plastic raw materials pet, products should be processed to eliminate internal stress, PMMA should be in the 70-80t hot air circulation drying 4 hours; Liquid paraffin and other heating 110-135 ℃, time according to the product, up to more than 10 hours.