Transparent plastic PC + gf products due to high transmittance, so PC + gf plastic surface quality requirements must be strict, can not have any streaks, pores, white, fog halo, black spots, discoloration, poor gloss and other defects, Therefore, in the whole process of injection molding of raw materials, equipment, molds and even product design, we must pay great attention to and put forward strict or even special requirements.
In addition, due to the high melting point of transparent PC + gf plastic, poor fluidity, so to ensure the quality of the product surface, often require higher temperatures, injection pressure, injection speed and other process parameters also need to make minor adjustments, Can fill the mold, but will not produce internal stress and cause product deformation and cracking.
Therefore, from the raw material preparation, equipment and mold requirements, injection molding process and raw material handling of the product should be carried out in several aspects.
1. Raw material preparation and drying
Because PC + gf plastic contains any impurities, may affect the transparency of the product, so in the storage, transportation, feeding process must pay attention to the seal, to ensure clean raw materials.
In particular, raw materials contain moisture, heating will cause the deterioration of raw materials, so be sure to do the drying process. In injection molding, feeding must use a dry hopper. Also note that the drying process, the best input air Should be filtered, dehumidified, so as not to pollute the raw materials.
2. Cylinder, screw and accessories cleaning
In order to prevent PC + gf contamination of raw materials and the old materials or impurities in the screw and accessories depression, especially in the poor thermal stability of the resin, so before use, after the shutdown are applied screw cleaning agent to clean the pieces, Sticky impurities, when there is no screw cleaning agent, available PE, PS and other resin cleaning screw.
When the temporary shutdown, in order to prevent the raw material to stay at high temperatures for too long, causing the solution down, the dryer and barrel temperature should be reduced, such as PC + gf, PMMA cylinder temperature dropped to 160 ℃ or less (hopper The temperature should be reduced to below 100 ℃ for PC).
3. Mold design should pay attention to the problem
In order to prevent the poor return flow of raw materials, or uneven cooling caused by poor plastic forming, resulting in surface defects and deterioration, generally in the mold design, should pay attention to the following points:
Wall thickness should be as uniform as possible, the draft should be large enough slope;
The transition part should be smooth and gradual transition to prevent sharp corners, sharp edges produce, especially PC + gf products must not be gap;
Gating, runner as large as possible, stubby, and should be based on condensation condensation process to set the gate location, if necessary, add cold material wells;
Mold the surface should be smooth, low roughness (preferably less than 0.8);
Vents. Slot must be sufficient to allow timely discharge of air and melt gases;
In addition to PET, the wall thickness is not too thin, generally not less than lmm.
4. Injection molding process should pay attention to the problem
In order to reduce the internal stress and surface quality defects, the injection molding process should pay attention to the following aspects:
Injection pressure: generally higher to overcome the melt viscosity of the defects, but the pressure is too high will produce internal stress caused by demoulding and deformation;
Injection speed: In the case of filling to meet the general low, it is best to use slow a slow multi-level injection;
Dwell time and molding cycle: in the product to meet the filling, does not produce depression, the case of bubbles; should be as short as possible to minimize the melt in the barrel stay time;
Screw speed and back pressure: plasticizers in the premise of meeting the premise, should be as low as possible to prevent the possibility of solution;
Mold temperature: the cooling of the product is good or bad, 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 and better.
5. Other aspects of the problem
In order to prevent deterioration of the surface quality, the general injection molding time should be minimal use of mold release agent; when used back to the material shall not exceed 20%.
In addition to PET, the products should be post-processed to eliminate internal stress, PMMA should be 70-80t hot air circulation drying 4 hours; PC should be clean air, glycerol, liquid paraffin and other heating 110-135 ℃, time by product However, the maximum required more than 10 hours, while the PET must be biaxially stretched in order to get good mechanical properties.