Injection molding PET bottle blank adjustment machine process
1. Plastic processing
Because PET macromolecules contain lipid groups and have a certain degree
of hydrophili city, the pellets are sensitive to water at high temperatures. When the moisture content exceeds the limit, the PET
molecular weight decreases during processing, and the product becomes colored and brittle.
Therefore, before processing, the material must be dried, and the drying temperature is 150℃ for more than 4 hours; generally 170℃ for 3-4 hours. The air injection method can be used to check whether the material is completely dry. The proportion of recycled materials in PET preforms should generally not exceed 25%, and the recycled materials must be thoroughly dried.
2. Selection of injection molding machine
Because PET has a short stable time after the melting point and a high melting point, it is necessary to use an injection system with more temperature control sections and less self-friction heat during plasticization, and the actual weight of the product (including water-inlet material) cannot be less than 2/3 of the machine injection volume.
3. Mold and gate design
PET preforms are generally formed with hot runner molds. It is best to have an insulation board between the mold and the injection molding machine template. The thickness is about 12mm, and the insulation board 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 it is easy to produce flash.
4. Melt temperature
It can be measured by air injection method, ranging from 270-295℃, and the enhanced GF-PET can be set to 290-315℃, etc.
5. Injection speed
Generally, the injection speed should be fast to prevent premature solidification during injection. But too fast, the shear rate is high, making the material fragile. The injection is usually completed within 4 seconds.
6. Back pressure
The lower the better to avoid wear. Generally not more than 100bar, usually not necessary.
7. Residence time
Do not use too long a residence time to prevent the molecular weight from decreasing, and try to avoid temperatures above 300℃. If the shutdown is less than 15 minutes, only air injection is required; if it exceeds 15 minutes, it should be cleaned with viscosity PE and the barrel temperature should be reduced to PE temperature until it is restarted.
8. Notes
The recycled material should not be too large, otherwise, it is easy to "bridge" at the feeding point and affect plasticization;
If the mold temperature is not well controlled, or the material temperature is improperly controlled, it is easy to produce "white fog" and opacity; if the mold temperature is low and uniform, the cooling speed is fast, and there is less crystallization, the product will be transparent.
