1. Treatment of plastics
Because PET macromolecules contain fat base and have certain hydrophilicity, pellets are more sensitive to water at high temperature. When the moisture content exceeds the limit, the molecular weight of PET decreases during processing, and the product becomes colored and brittle. Because of this, the materials must be dried before processing. The drying temperature is 150°C for more than 4 hours, generally 170°C for 3-4 hours. Air shot method can be used to check whether the material is completely dry. Generally, the proportion of recycled materials should not exceed 25%, and the recycled materials must be thoroughly dried.
2. Selection of injection molding machine
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.
3. 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.
4. Melting temperature
It can be measured by air shot method. The temperature ranges from 270-295°C, and the enhanced GF-PET can be set at 290-315°C.
5. Injection speed
Generally, the injection speed should be fast to prevent premature coagulation during injection. But too fast and high shear rate make the material fragile. Injection is usually completed within 4 seconds.
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