Because PET macromolecules contain a fat base and have certain 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 product becomes colored and brittle.
Therefore, the materials must be dried before processing, and the drying temperature is 150°C for more than 4 hours; generally, 170°C for 3-4 hours. The air shot method can be used to check whether the material is completely dry. Generally, the proportion of recycled PET preforms should not exceed 25%, and the recycled materials should be thoroughly dried.
Injection molding machine selection
Since PET has a short stable time after the melting point and high melting point, it is necessary to choose an injection system with more temperature control sections and less self-friction heat generation during plasticization, and the actual weight of the product (water cut material) cannot be less than the machine injection 2/3 of the amount.
Mold and gate design
PET preforms are generally formed by hot runner molds. It is better to have a heat shield between the mold and the injection molding machine template, the thickness of which is about 12mm, and the heat shield must be able to withstand high pressure. Exhaust must be sufficient to avoid local overheating or chipping, but the depth of the exhaust port should generally not exceed 0.03mm, otherwise it is easy to produce flash.
It can be measured by air injection method, ranging from 270-295℃, and for enhanced GF-PET can be set at 290-315℃.
Generally, the injection speed should be fast to prevent premature solidification during the injection. But too fast, the shear rate is high, making the material fragile. An Injection is usually completed within 4 seconds.
The lower the better, so as to avoid wear. Generally, it does not exceed 100bar and usually does not need to be used.
Do not use too long a residence time to prevent the molecular weight from falling, and try to avoid temperatures above 300°C. If the machine is shut down for less than 15 minutes, only air injection is required; if it exceeds 15 minutes, it should be cleaned with viscosity PE, and the temperature of the barrel should be reduced to the PE temperature until the machine is turned on again.
• The recycled material should not be too large, otherwise, it is easy to cause "bridging" at the feeding place and affect plasticization;
• If the mold temperature is not well controlled or the material temperature is not properly controlled, it is easy to produce "white fog" and opacity; the mold temperature is low and uniform, the cooling speed is fast, and the crystallization is less, the product is transparent.