What 'general details' should be paid attention to in the injection molding process of transparent plastic materials?
by：Vowin Rapid Prototyping2023-02-05
In industrial products, many products that use plastic instead of steel have appeared. In the future, more and more steel products will be replaced by plastic materials. The phenomenon of replacing glass with plastic is also advancing. As long as the transparency of plastic raw materials is good enough, Then the probability of replacing glass will become greater and greater. How to ensure the transparency of plastic raw materials, and what is the relationship with the injection molding process? Today we will focus on this issue.
Raw material preparation and drying
Since any impurity in the plastic may affect the transparency of the product, it is necessary to pay attention to sealing during storage, transportation and feeding to ensure that the raw materials are clean. Especially the raw material contains moisture, which will cause the raw material to deteriorate after heating, so it must be dried, and a drying hopper must be used for feeding during injection molding. 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 will not be polluted.
Cleaning of barrel, screw and its accessories
In order to prevent contamination of raw materials, and there are old materials or impurities in the depressions of the screw and accessories. Therefore, before use and after shutdown, all parts should be cleaned with a screw cleaner to prevent impurities from sticking. When temporarily shutting down, in order to prevent raw materials from staying at high temperature for a long time and causing degradation, the temperature of the dryer and barrel should be lowered to below 160°C; and for PC, the temperature of the hopper should be lowered to below 100°C.
Mold/Product Design Issues
In order to prevent poor reflow or uneven cooling, resulting in poor plastic molding, surface defects and deterioration, the following points should be paid attention to when designing molds:
The wall thickness should be as uniform as possible, and the demoulding slope should be large enough;
Smooth transition to prevent sharp edges, especially PC;
The gate and runner should be as wide and short as possible, and the position of the gate should be set according to the shrinkage condensation process, and a cooling well should be added if necessary;
The surface of the mold should be smooth and clean, with low roughness (preferably less than 0.8);
Vent slots must be adequate;
Except for PET, the wall thickness should not be too thin, generally not less than 1mm.
In order to reduce internal stress and surface quality defects, the following aspects should be paid attention to in the injection molding process:
Injection molding machine: An injection molding machine with a special screw and a separate temperature-controlled nozzle should be selected;
Injection molding temperature: under the premise that the plastic resin does not decompose, a higher injection temperature should be used;
Injection pressure: Generally higher to overcome the defect of high melt viscosity, but too high pressure will generate internal stress, resulting in difficulty in demoulding and deformation;
Injection speed: In the case of satisfying mold filling, it should generally be low, and it is best to use slow-fast-slow multi-stage injection;
compress time&Molding cycle: In the case of satisfying product filling without sags and bubbles; it should be as short as possible to minimize the residence time of the molten material in the barrel;
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 degradation;
Mold temperature: The mold temperature must be able to accurately control the process. If possible, the mold temperature should be higher.