Transparent plastic, because of the high light transmission rate, must require strict surface quality requirements for plastic products, which cannot have any mottling, porosity, whitening. Fog halo, black spots, discoloration, poor luster, and other defects, and thus the entire injection molding process
of raw materials, equipment.
Therefore, in the whole process of injection molding, the raw materials, equipment, molds, and even product design should be very careful and put forward strict and even special requirements. Secondly, because of the high melting point and poor fluidity of transparent plastic, to ensure the surface quality of the product, it is often necessary to make fine adjustments in the machine temperature, injection pressure, injection speed, and other process parameters, so that when injecting plastic can fill the mold, and will not produce internal stress and cause product deformation and cracking.
The following is its preparation of raw materials. Equipment and mold requirements, injection molding process
, and product processing of raw materials in several aspects, to talk about the matters that should be noted.
Preparation and drying of raw materials
As any little impurity in the plastic may affect the transparency of the product, therefore and storage, transportation. During the process of adding materials, attention must be paid to sealing and ensuring that the raw materials are clean.
Especially the raw materials contain moisture, heating will cause the raw materials to deteriorate, so it must be dry, and in the injection molding, adding material must use a dry hopper. Another point to note is that in the drying process, the input air should preferably be filtered, dehumidified, to ensure that no pollution of raw materials.
Cleaning of barrel, screw, and accessories
To prevent the contamination of raw materials and in the screw and accessories in the recesses of the old material or impurities, the especially poor thermal stability of the resin, so before use, after the shutdown should be used to clean the screw cleaning agent to clean the parts so that they do not stick to impurities, when there is no screw cleaning agent, available PE, PS, and other resins to clean the screw.
When the temporary stop, to prevent raw materials in the high temperature for a long time, causing the solution to fall, the dryer and barrel temperature should be reduced, such as PC, PMMA and other barrel temperature should be reduced to 160 ℃ or less. (Hopper temperature for PC should be reduced to below 100 ℃)
Mold design (including product design)
To prevent poor return flow, or uneven cooling resulting in poor plastic molding, surface defects, and deterioration, generally, in the mold design, the following points should be noted.
The wall thickness should be as uniform as possible, and the slope of the mold release should be large enough;
The transition part should be gradual. Smooth transition, to prevent sharp corners. Sharp edges are produced, especially for PC products must not have notches;
Gating. The runner should be as wide and thick as possible, and the location of the gate should be set according to the shrinkage condensation process, and the cold material well should be added if necessary.
The Mould surface should be smooth and have low roughness (preferably below 0.8).
Venting hole. Slots must be sufficient to vent air and gas from the melt promptly.
Except for PET, the wall thickness should not be too thin, generally not less than 1 mm.
Injection molding process and injection molding machine
To reduce internal stress and surface quality defects, the following aspects should be noted in the injection molding process
Special screws and injection machines
with separate temperature-controlled nozzles should be used;
Injection temperature: it is advisable to use higher injection humidity provided that the plastic resin does not decompose.
injection pressure: generally higher to overcome the defects of high melt viscosity, but too high a pressure will produce internal stresses resulting in difficult release and deformation.
Injection speed: under the condition of satisfying the mold filling, it is generally advisable to below, and it is better to use a slow, fast, and slow multi-stage injection.
Holding time and molding cycle: under the condition that the product fills the mold and does not produce depressions or bubbles, it is advisable to keep it as short as possible to minimize the residence time of the melt in the barrel.
Screw speed and back pressure: under the premise of satisfying the plasticization quality, it should be as low as possible to prevent the possibility of uncoupling.
Mold temperature: good or bad cooling of the products has a great impact on the quality, so the mold temperature must be able to precisely control its process, if possible, the mold temperature should be the higher good.
As to prevent the deterioration of the upper surface quality, generally injection molding
with as little mold release agent as possible; when using reuse material shall not be greater than 20%.
For except PET, the products should be post-treated to eliminate internal stress, PMMA should be dried in 70-80t hot air cycle for 4 hours; PC should be in clean air, glycerin. Liquid paraffin etc. is heated at 110-135℃, the time depends on the product, the highest need is more than 10 hours. And PET must go through the process of bi-directional stretching to get good mechanical properties.