Precautions during injection molding of transparent plastics
Click： Time：2020-10-08 11:45:21
Transparent plastics should have high light transmittance and strict requirements on the surface quality of plastic products, which should not have any mottling, porosity, whitening. Fog halo, black spots, discoloration, poor luster and other defects, and therefore in the entire injection molding process of raw materials, equipment, molds, and even product design, there are strict and even special requirements.
(A) the preparation and drying of raw materials
Feeding process, pay attention to sealing, to ensure that the raw material is clean, especially the moisture in the raw material, heating will cause deterioration of raw materials, so it must be dry. In injection molding, adding material must use dry hopper. Also note that the drying process, the input air filtered, dehumidified, to ensure that the raw material will not be contaminated.
(B) the barrel, screw and its accessories cleaning
In order to prevent contamination of raw materials and in the screw and accessories in the depression of the old material or impurities (especially the presence of poor thermal stability of the resin), so before use, after the shutdown should be cleaned components, so that it shall not stick impurities, when there is no screw cleaner, available PE, PS and other resins to clean the screw.
When the temporary shutdown, in order to prevent the raw materials at high temperatures to stay a long time caused by the solution of the drop, the dryer and barrel temperature should be reduced, such as PC, PMMA and other barrel temperature should be reduced to less than 160 ℃.
(C) in the mold design should pay attention to the problem (including product design)
In order to prevent the occurrence of poor flow, uneven cooling and other conditions that cause poor molding of products, surface defects and deterioration, generally in the mold design, the following points should be noted:
a) Wall thickness should be as uniform as possible, and the demolding slope should be large enough;.
b) Smooth transition, to prevent sharp corners, sharp edges, especially the PC products must not have a gap;
c) The gate runner should be as wide and short 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.
d) The surface of the mold should be smooth and low roughness (less than 0.8).
e) exhaust holes, slots must be enough to discharge air and gas in the melt in a timely manner;
f) In addition to PET, the wall thickness should not be too thin, generally not less than 1mm.
(D) should pay attention to the injection molding process (including the requirements of the injection molding machine)
In order to reduce internal stress and surface quality defects
In the injection molding process should pay attention to the following aspects of the problem:
a) should be equipped with a dedicated screw and with a separate temperature-controlled injection nozzle injection molding machine;
b) injection temperature in the plastic resin does not decompose under the premise of the higher injection temperature is desirable;
c) injection pressure is generally higher to overcome the defects of the melt viscosity, but the pressure is too high will produce internal stress caused by the release of the mold due to the difficulty and deformation.
d) injection speed to meet the case of mold filling, generally low, using [slow - fast - slow] multi-stage injection;
e) Holding time and molding cycle to meet the product mold filling, do not produce concave, bubbles, it is desirable to try to shorten the melt to reduce the residence time in the barrel.
f) screw speed and back pressure to meet the premise of plasticizing quality should be as low as possible to prevent the possibility of degradation;.
g) mold temperature products cooling good or bad on the quality of great impact, so the mold temperature must be able to control its process, if possible, the mold temperature should be higher.
(E) other aspects of the problem
As to prevent the deterioration of the surface quality, the general injection molding as little as possible with the release agent, when using the return material shall not be greater than 20%.
For in addition to PET, products should be post-treatment to eliminate internal stress:
PMMA should be dried at 70-80℃ hot air cycle for 4 hours;.
PC should be heated 110-135 ℃ by clean air, glycerin, liquid paraffin, etc., the time depends on the product, the high need for more than 10 hours;
And PET must go through the process of bi-directional stretching to get good mechanical properties.
