Processing of plastics

Because PET macromolecules contain fat groups and have a certain degree of hydrophilicity, pellets are more sensitive to water at high temperatures. When the moisture content exceeds the limit, the molecular weight of PET decreases during processing, and the product is colored and brittle.

Therefore, before processing, the materials must be dried, the drying temperature is 150 ℃, more than 4 hours; generally, 170 ℃, 3-4 hours. The air shot method can be used to check whether the material is completely dry. The proportion of PET preform recycled materials should not exceed 25%, and the recycled materials should be thoroughly dried.


Injection molding machine selection

Because PET has a short stabilization time after melting point and a higher melting point, an injection system with more temperature control sections and less self-friction heat generation during plasticization needs to be selected, and the actual weight of the product (water-containing mouth material) cannot be less than that of machine injection 2/3 of the amount.


Plastic treatment 

Because PET macromolecules contain fat groups and have a certain degree of hydrophilicity, the pellets are more sensitive to water at high temperatures. When the moisture content exceeds the limit, the molecular weight of PET decreases during processing, and the products are colored and brittle. Therefore, before processing, the materials must be dried, the drying temperature is 150 ℃, more than 4 hours; generally, 170 ℃, 3-4 hours. The air shot method can be used to check whether the material is completely dry. The proportion of PET preform recycled materials should not exceed 25%, and the recycled materials should be thoroughly dried.


Mold and gate design 

PET preforms are generally formed by hot runner molds. It is best to have a heat insulation board between the mold and the injection molding machine template. Its thickness is about 12mm, and the heat insulation board must be able to withstand high pressure. The exhaust must be sufficient to avoid local overheating or cracking, but the depth of the exhaust port should generally not exceed 0.03mm, otherwise flash is likely to occur. 

Melting temperature 

It can be measured by the air-shot method, ranging from 270-295 ℃, and enhanced grade GF-PET can be set to 290-315 ℃, etc. 


Injection speed 

Generally, the injection speed should be fast to prevent premature coagulation during the injection. But too fast, the shear rate is high, making the material brittle. The shot is usually completed within 4 seconds.


back pressure the lower the better, to avoid wear. Generally, no more than 100bar, usually no need to use.


residence time

Do not use too long residence time to prevent molecular weight from decreasing. Try to avoid temperatures above 300 ℃. If the machine is shut down for less than 15 minutes, only air shot is required; if it is more than 15 minutes, it needs to be cleaned with viscosity PE, and the barrel temperature is reduced to PE temperature until it is turned on again.


Precautions

· The recycled materials should not be too large, otherwise, it will be easy to "bridge" at the cutting place and affect the plasticization;

· If the mold temperature control is not good, or the material temperature is not controlled properly, it is easy to produce "white mist" and opaque; the mold temperature is low and uniform, the cooling speed is fast, and the crystallization is less, then the product is transparent.





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