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Process issues that need to be paid attention to in injection molding PET preforms
Nov 25,2022
Process issues that need to be paid attention to in injection molding PET preforms
1. Handling of plastics
Since PET macromolecules contain lipid groups and have a certain degree of hydrophilicity, pellets are 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, before processing, the material must be dried, and the drying temperature is 150°C for more than 4 hours; generally 170°C for 3-4 hours. The complete dryness of the material can be checked by air shot method. Generally, the proportion of PET preform recycled materials should not exceed 25%, and the recycled materials should be thoroughly dried.
2. Selection of injection molding machine
Due to the short stable time of PET after the melting point and the 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-containing material) should not be less than machine injection. 2/3 of the amount.
3. Mold and gate design
PET preforms are generally formed by hot runner molds. It is best to have a heat shield between the mold and the injection molding machine template. The thickness of the heat shield is about 12mm, and the heat shield must be able to withstand high pressure. The exhaust must be sufficient to avoid local overheating or fragmentation, but the depth of the exhaust port should generally not exceed 0.03mm, otherwise flashing will easily occur.
4. Melt temperature
It can be measured by air injection method, ranging from 270-295°C, and the enhanced grade GF-PET can be set at 290-315°C, etc.
5. Injection speed
Generally, the injection speed should be fast to prevent premature coagulation during injection. But too fast, the shear rate is high, making the material brittle. Injection is usually done within 4 seconds.
6. Back pressure
The lower the better to avoid wear and tear. Generally not more than 100bar, usually do not need to use.
7. Residence time
Do not use too long residence time to prevent the decrease of molecular weight, and try to avoid the temperature above 300°C. If the machine is shut down for less than 15 minutes, it only needs to be treated with air injection; if it is more than 15 minutes, it must be cleaned with viscosity PE, and the temperature of the machine barrel should be lowered to the PE temperature until it is turned on again.
8. Precautions
Recycled materials should not be too large, otherwise, it is easy to "bridge" at the cutting place and affect the 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 opaque; if the mold temperature is low and uniform, the cooling speed is fast, and the crystallization is less, the product is transparent.
