Because of the high light transmittance of transparent plastics, it is inevitable that the surface quality of plastic products must be strict and there must be no markings, air holes, and whitening. Fog haze, dark spots, discoloration, poor gloss and other defects, and thus raw materials and equipment throughout the injection molding process.
The design of molds and even products must pay great attention to and put forward strict or even special requirements. Second, because transparent plastics are mostly of high melting point and poor flowability, in order to ensure the surface quality of products, fine adjustments are often made to the process parameters such as machine height temperature, injection pressure, injection speed, etc., so that injection molding plastics can be filled with molds, but not Internal stress can cause deformation and cracking of the product.
Here are some things to note regarding raw material preparation, equipment and mold requirements, injection molding process, and raw material handling of products.
Raw material preparation and drying
Due to the presence of any impurities in the plastic, it may affect the transparency of the product, and therefore storage and transportation. During the feeding process, we must pay attention to sealing to ensure that the raw materials are clean.
In particular, water is contained in the raw materials, and the raw materials are deteriorated when heated. Therefore, the raw materials must be dried and the dry hopper must be used for feeding. It is also important to note that during the drying process, the input air should preferably be filtered and dehumidified to ensure that it does not contaminate the material.
Cleaning of barrels, screws and accessories
In order to prevent the contamination of raw materials and the existence of old materials or impurities in the recesses of the screw and accessories, there are special resins with poor thermal stability. Therefore, the screw cleaning agent should be used to clean all parts before use and after shutdown to prevent them from being contaminated with impurities. When there is no screw cleaning agent, the screw can be cleaned with resins such as PE and PS.
When temporarily shut down, in order to prevent the raw material from staying at a high temperature for a long time, causing the solution to drop, the temperature of the dryer and the barrel should be reduced. For example, the barrel temperature of the PC, PMMA, etc. should be reduced to below 160°C. (The hopper temperature should be lower than 100°C for PC)
Mold design (including product design)
In order to prevent back flow, or uneven cooling caused by poor 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 as uniform as possible, and the draft should be large enough;
The transition should be gradual. Smooth transition to prevent sharp corners. Sharp edges, especially PC products must not have gaps;
Gate. The runners should be as wide and short as possible, and the gate location should be set according to the contraction condensation process. If necessary, cold material wells should be added.
The mold surface should be smooth, with low roughness (preferably less than 0.8);
Vent. The tank must be sufficient to discharge the air and gases in the melt.
Except for PET, the wall thickness should not be too thin, generally it should not be less than 1 mm.
Injection molding process and injection molding machine
In order to reduce internal stress and surface quality defects, the following aspects should be noted in the injection molding process:
Should use special screw, injection molding machine with separate temperature control nozzle;
Injection temperature should not be decomposed under the premise of plastic resin, it is better to use higher injection humidity;
Injection pressure: generally high to overcome the defects of large melt viscosity, but the pressure is too high will cause internal stress caused by demoulding and deformation;
Injection speed: In the case of filling the mold, it should generally be low, it is best to use a slow multi-level slow injection;
Holding time and molding cycle: in the case of filling the product to meet the mold, without sinking, bubbles; it should be as short as possible to minimize the residence time of the melt in the barrel;
Screw speed and back pressure: In the premise of satisfying the quality of plasticization, it should be as low as possible to prevent the possibility of solution.
Mold temperature: The cooling of the product is good or bad. It 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.
Since it is necessary to prevent the deterioration of the upper surface quality, the mold release agent should be used as little as possible during the injection molding; when using recycled materials, it should not be greater than 20%.
In addition to PET, the products should be post-processed to eliminate internal stress, PMMA should be dried in 70-80t hot air circulation for 4 hours; pc should be in clean air, glycerin. Liquid paraffin and other heating 110-135 °C, time is determined by the product, the highest need more than 10 hours. PET must undergo a biaxial stretching process to obtain good mechanical properties.