How to deal with transparent plastic material PMMA,PC and PET injection molding
Due to their high light transmittance, the requirement for transparent plastic surface need to be strict, it can not has any markings, voids, whitening, black spots, discoloration, poor gloss and other defects. Thus the whole injection molding process for raw materials, equipment, molds, and even product design, could be said very strict and sometimes even very special. Further more, because of most transparent plastic with high melting point, poor mobility, to ensure the surface quality of the products, it often needs make minor adjustments at high temperature, injection pressure, injection speed and other parameters, so that it could both fill plastic injection mold properly and will produce proper stress which would cause no deformation and cracking.
How to dry the raw material
Due to any impurities in plastics, they may affect the transparency of the product .Therefore on storage, transportation, feeding process, we must pay much attention to seal well to ensure raw material clean. Especially raw materials containing moisture will cause deterioration of the material when heated, so the material must be sure to dry. During the injection process, it must use a drying feed hopper. Note also that during the drying process, the air after filtration and dehumidification should preferably be used, which ensure no contaminated plastic materials.
Transparent plastic drying process as follows:
|
material
|
temperature（℃）
|
time（h）
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Material thickness（mm）
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remarks
|
|
PMMA
|
70～80
|
2～4
|
30～40
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heated air circulate drying
|
|
PC
|
120～130
|
>6
|
<30
|
heated air circulate drying
|
|
PET
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140～180
|
3～4
|
|
continuous drying feeding
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How to clean barrel, screw and accessories
In order to prevent pollution, previous raw materials or impurities existing in the screw and accessories recess, especially for poor thermal stability resin, before and after the operation, screw cleaning agent should be used to clean each components to prevent any sticky impurities. But when no screw cleaning agent nearby, plastic material PE, PS resin could replace the agent to clean them. When just temporary shutdown, it need prevent raw material to stay a long time inside high temperature barrel, which could cause deterioration. So it should decreases drier and barrel temperature. As to PC, PMMA processing, barrel temperature should be reduced below 160 ℃. (Hopper temperature should be reduced to below 100 ℃ for PC).
How to design mold and parts
To prevent back flow of melt plastic or uneven cooling which could invite plastic surface defects and deterioration, generally in the mold design, it should pay attention to the following:
a) Wall thickness should be uniform and demould draft should be large enough;
b) The transition of part should be gradual to prevent any sharp corners. especially prevent PC products to have any breach;
c) Gate and Runner should be possible large and short , and the location of the gate should be set according to shrink, when necessary, add cold slug;
d) The mold surface should be smooth, and low roughness (preferably below 0.
;
e) Vent: Groove must be sufficient to vent air;
f) Except PET, the wall thickness should not be too thin, generally not less than l mm.
How to process injection molding.
To reduce stress and surface quality defects in the injection molding process, it should pay attention to the following:
a) Special screw should be used for transparent plastics , and separate temperature control on the nozzle of the injection molding machine is required ;
b) It is preferred to adopt higher injection temperature until it could reach the deterioration limits;
c) Generally high injection pressure would overcome melt viscosity defect, but too much pressure will produce stripping difficulties which is caused by internal stress and deformation;
d) Injection speed: In the case of filling mold full, injection speed should be low, preferably slow – fast – slow, a multi-stage injection process;
e) Packing time and cycle time: In the case of filling plastic mold full, the time should be as short as possible to minimize the melt residence in the barrel to prevent plastic deterioration.
f) Screw speed and back pressure: Under the premise to meet the quality demand of plastic part, the pressure should be as low as possible to prevent the possible deterioration
g) Mold temperature: Cooling would take a great impact on the quality, so the plastic mold temperature must be able to be precisely controlled for the process, if possible, high mold temperature is preferred.
h)To prevent the surface deterioration ,the release agent should be used as little as possible, and the max usage for reground material is 20%
Except PET, the plastic article should be treated to eliminate the stress after molding, PMMA articles should be subjected to heated air circulate drying for 4 hours at 70-80 ℃; PC articles should be subjected to heated air circulate drying with the clean air, glycerol. Liquid paraffin at 110-135 ℃,and the longest time could be more than 10 hours.
1, the injection molding process for PMMA
PMMA: high viscosity, less mobility, therefore high temperature and high injection pressure are needed, while the injection temperature’s role is greater than that of injection pressure. While the high injection pressure would improve the shrinkage of the product. Injection temperature range window is wide, with melting temperature @ 160 ℃, decomposition temperature @270 ℃, .The wide adjustment range means easier processing. Thus improving mobility may be achieved by adjusting injection temperature. Poor impact resistance, poor wear resistance, easy to scratch are some deficiencies, it should raise the mold temperature and improve the cooling process to overcome these deficiencies.
2, the injection molding process for PC
PC: high viscosity, high melting temperature, poor mobility, hence higher temperatures must be taken for the injection (between 270-320 ℃). As for relatively narrow range of temperature regulation, its injection molding process is not as good as PMMA. Injection pressure has little effect on mobility, but the high viscosity need high injection pressure. Correspondingly to prevent stress, packing time should be as short as possible. With large shrinkage, good dimensional stability, large internal stress and easy to crack, it is appropriate to raise the temperature rather than the pressure to improve flowability. Raising mold temperature, after-treatment, improving the mold structure would reduce the possibility of cracking. Nozzle temperature should be controlled separately. Mold temperature need to be higher, so runner, gate resistance is smaller.
3, the injection molding process for PET
PET molding temperature is high, and the injection temperature adjustment range is narrow (260-300 ℃), but after melting, good liquidity make process adjustment poor, and often in the nozzle ,anti-droop device need. Precise mold temperature control is an important factor to prevent warping and deformation, it is recommended to use hot runner system.
