Due to the high light transmittance of transparent plastics, the surface quality of plastic products must be strict, and there must be no markings, pores or whitening. Fog, black spots, discoloration, poor gloss and other defects, so the raw materials, equipment throughout the injection process. Molds and even product designs must pay great attention to and impose strict or even special requirements. Secondly, because transparent plastics have high melting point and poor fluidity, in order to ensure the surface quality of the products, the process parameters such as machine temperature, injection pressure and injection speed are often finely adjusted so that the plastic can be filled with mold without Internal stress is generated to cause deformation and cracking of the product.
Therefore, strict preparation must be carried out from the preparation of raw materials, equipment and mold requirements, injection molding processes and raw material handling of products.
(1) Preparation of raw materials and flash dryers Due to the inclusion of any impurities in the plastic, the transparency of the products may be affected, and thus stored and transported. During the feeding process, attention must be paid to the sealing to ensure that the raw materials are clean. In particular, the raw material contains moisture, which causes deterioration of the raw material after heating, so it must be dried, and in the injection molding, the drying hopper must be used for the 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 are not contaminated. Its disc dryer process is shown in Table 2.
Dry plastic drying process:
Material Process Drying Temperature (°C) Drying Time (h) Material Layer Thickness (mm) Remarks
Pmma 70～80 2～4 30～40
Pc 120～130 >6 <30 using hot air circulation drying
Pet 140～180 3～4 It is better to use continuous spray drying equipment to dry the feeding device. (2) Cleaning of the barrel, screw and its accessories to prevent contamination of raw materials and old materials or impurities in the recesses of screws and accessories, especially heat stable The resin with poor properties exists. Therefore, the screw cleaning agent should be used to clean the parts before use and after shutdown, so that they do not stick to impurities. When there is no screw cleaning agent, the screw can be cleaned with pe, ps and other resins. When temporarily stopping, in order to prevent the raw material from staying at high temperature for a long time, causing the solution to fall, the temperature of the drum scraper dryer and the barrel should be lowered. For example, the temperature of the barrel such as pc and pmma should be reduced to below 160 °C. (The hopper temperature should be reduced to below 100 °C for pc)
(3) Problems that should be paid attention to in mold design (including product design)
In order to prevent the occurrence of poor back flow, or uneven cooling, resulting in poor plastic molding, surface defects and deterioration, generally in the mold design, should pay attention to the following points.
a) the wall thickness should be as uniform as possible, and the draft angle should be large enough;
b) The transition should be gradual. Smooth transition to prevent sharp corners. Sharp edges are produced, especially PC products must not have gaps;
c) Gate. The flow channel should be as wide and thick 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 mold should be smooth and the roughness is low (preferably less than 0.8);
e) Venting holes. The tank must be sufficient to vibrate the fluidized bed to vent the air and the gas in the melt;
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 (including the requirements of injection molding machines)
In order to reduce internal stress and surface quality defects, the following aspects should be noted in the injection molding process.
a) should be used as the special screw band dryer separate temperature control of the nozzle exit;
b) The injection temperature should be higher than the plastic resin without decomposition;
c) Injection pressure: generally higher to overcome the defect of large viscosity of the melt, but the pressure is too high, which will cause internal stress and cause deformation and deformation of the rotary kiln ;
d) injection speed: in the case of filling the mold, it is generally low, it is best to use slow, fast and slow multi-stage injection;
e) holding time and molding cycle: in the case of satisfying product filling, no dents or bubbles; should be as short as possible to minimize the residence time of the melt in the barrel;
f) screw speed and back pressure: under the premise of satisfying the plasticizing quality, it should be as low as possible to prevent the possibility of depreciation;
g) Mold hot blast stove temperature: The cooling of the product is good or bad, which has 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.
(V) Other problems Because in order to prevent the deterioration of the upper surface quality, the mold release agent should be used as little as possible during injection molding; when it is used, it should not be greater than 20%.
For products other than pet, the product should be post-treated to eliminate internal stress. pmma should be dried in 70-80t hot air for 4 hours; pc should be clean air, glycerin. Liquid paraffin and other heating 110-135 ° C, the time depends on the product, the most need more than 10 hours. And pet must pass the biaxially stretched hollow blade dryer process to get good mechanical properties.
