Things you need to be careful doing in the clear plastic injection molding
Posted on : Aug 4 , 2022 By GREFEE
As the light transmittance of clear plastic is high, so there is a strict requirement for the surface quality of plastic products, such as no spots and patterns, porosity, whitening, fringing, black spots, discoloring and uneven gloss. Therefore, during the whole injection molding process, there should be a rigorous and special requirements for the raw material, equipments, molds and even the design of the product.
Second, as the clear plastic generally has a high melting point but poor flowability, the adjustment of the coefficient of temperature of machine, injection pressure, injection speed to ensure the surface quality, which not only can fill out the injection part, but also reduce the possibilities of producing internal stress and cause deformation and cracking. Hence, strict operations should be carried out in terms of raw material preparation, equipment, such as the clear plastic molding kit and injection mold requirements, injection molding process and raw material treatment of products. How to make the clear plastic molds? The following aspects you should be aware of.
1.Preparation and drying of raw material
As even a little impurities in plastic will affect the transparency of products, the products should be sealed properly to make sure it is clean during the storage, transportation, and feeding processes, especially the raw material is likely to be deteriorated after being heated when there is little moisture, so it should be dried. Besides, the drying hopper will be required when feeding. Moreover, during the drying process, the input air should be filtered and dehumidified to ensure that it will not pollute the raw materials. Drying of the clear plastic: material/processing drying temperature (℃) drying time (h) material layer thickness (mm) remarks PMMA 70 ~ 80 2 ~ 4 30 ~ 40 Pc 120 ~ 130 >6 <30 hot air circulation drying pet 140 ~ 180 3 ~ 4 continuous drying feeding device is recommended.
2. Cleaning of barrel, screw and other accessories
To prevent the pollution of raw martial and the residues or impurities hidden in the depression areas of accessories, especially resin with poor thermal stability, it is essential to clean mold off plastic and the screw bar of the machine with the detergent to remove the impurities completely, or use pe, ps and other resin to clean the screw bar when there is no screw bar detergent. When the machine shut down suddenly, to prevent the raw material decomposed due to the long time under high temperature, decrease the temperature of drying machine and barrel, such as the barrel temperature of pc and pmma should be below 160 degrees (The hopper temperature should be reduced below 100 ℃ for PC).
3. Things should pay attention to in designing of injection mold (include design of products)
The following points should be paid attention to in the design of injection mold in case the occurrence of poor plastic molding, surface defects and deterioration caused by poor reflux or uneven cooling.
a)The wall thickness should be as constant as possible, the draft angle should be as large as possible.
b) The transition should be gradual and smooth to prevent the generation of the sharp corners and sharp edges, especially for pc products, there should not be gaps.
c) Gate: the runner should be wide and short as possible, and the position of gate should be according to the shrinkage and condensing process. The cold slug well is needed when it is necessary.
d)The surface of injection mold should be smooth with low roughness (maximum 0.8)
e)The number of venting holes and grooves should be enough for the gas exhaustion in the air and molten.
f)In addition to pet, the wall thickness should not be too thin, no less than 1mm generally.
4.Things should be paid attention to in injection molding process (including requirements of injection molding machine)
To reduce the internal stress, surface quality defects, the following issues should be aware of during the injection molding processing.
Select special screw and injection molding machine with separate temperature control nozzle.
a)High injection temperature is recommended if the plastic resin is not decomposed.
b)Injection pressure: generally the injection pressure is high to overcome the high viscosity of melt. However, if the injection pressure is too high, it will produce the internal stress and cause hard to release mold and deformation.
c)Injection speed: when it meets the condition to fill the mold, the injection speed should be slow and It is best to adopt slow fast slow multi-stage injection.
d)Time of holding pressure and molding cycle; when it meets the molding filling of products without depression and bubbles, shortens the time and cycle as much as possible to reduce the time that melt stays inside the barrel.
e)Rotating speed of screw and back pressure: when it meets the conditions of plasticizing quality, lower the rotating speed and back pressure as much as possible to prevent the possibilities of decomposition.
f)Temperature of injection mold: the cooling effect of products impacts hugely to quality, so the mold temperature ought to be controlled accurately. If is possible, high mold temperature is better.
5. Issues from other aspects:
In order to prevent the deterioration of the upper surface quality, it is not recommend to use mold release agent, and the recycled material should not be higher than 20%. The products should accept the post-treatment with an exception of pet to reduce the internal stress. The pmma should be dried for 4 hours under the 70-80t hot wind. PC should be heated at 110-135 ℃ in clean air, glycerin, liquid paraffin, etc. The time depends on the product, and it takes more than 10 hours at most. Pet must undergo biaxial stretching process to obtain good mechanical properties.
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