Some issues about PET preform injection molding
1. Plastic material
Because PET plastic material has strong moisture absorption, the material must be dried before processing, and the drying temperature is 150°C for 4 hours or more; generally 170°C for 3-4 hours. The air shot method can be used to check whether the material is completely dry. The proportion of recycled PET preforms should generally not exceed 25%, and the recycled materials should be thoroughly dried.
2. Selection of injection molding machine
Because the preform molds are generally multi-cavity, the weight is heavy, so pay attention to the injection weight when choosing an injection molding machine.
The product total weight should be 2/3 of the injection weight
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, and the heat shield must be able to withstand high pressure. The 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, ranging from 270-295℃, and for enhanced GF-PET can be set at 290-315℃.
5. Injection speed
The injection speed should be fast to prevent premature solidification during injection. But too fast, the shear rate is high, making the material fragile. Injection is usually completed within 4 seconds.
6. Back pressure
The lower the better, so as to avoid wear. Generally, it does not exceed 100bar and usually does not need to be used.
7. Detention time
Do not use too long a residence time to prevent the molecular weight from decreasing, 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 lowered to the PE temperature until the machine is turned on again.
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