Production process of PET plastic

PET has better rheology in the molten state, and the influence of pressure on viscosity is greater than that of temperature. Therefore, the fluidity of the melt is mainly changed from the pressure.
1. Treatment of plastics Due to the lipid group contained in the PET macromolecule, it has a certain hydrophilicity. The 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 is colored and brittle. Therefore, the material must be dried before processing. The drying temperature is 150 ° C, more than 4 hours, generally 170 ° C, 3-4 hours. The air injection method can be used to test whether the material is completely dry. The proportion of recycled material generally should not exceed 25%, and the recycled material should be completely dried.
2. The injection molding machine chooses PET because the stable time after the melting point is short and the melting point is high. Therefore, 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 (including water nozzle material) cannot be less than 2/3 of the injection volume of the machine. Based on these requirements, Ramada has developed a special plasticizing system for PET for small and medium series in recent years. The clamping force should be greater than 6300t/m2.
3. Mold and gate design PET preforms are generally formed with hot runner molds. It is best to have a heat insulation plate between the mold and the injection molding machine template. Its thickness is about 12mm, and the heat insulation plate must be able to withstand high pressure. Exhaust must be sufficient to avoid local overheating or fragmentation, but the depth of the exhaust port generally should not exceed 0.03mm, otherwise it is easy to produce flash.
4. Melting temperature can be measured by air injection method. It varies from 270-295 ℃, and the enhanced grade GF-PET can be set to 290-315 ℃, etc.
5. The injection speed is generally fast, which can prevent premature solidification during injection. But too fast and high shear rate make the material fragile. The shot is usually completed within 4 seconds.
6. The lower the back pressure, the better to avoid wear and tear. Generally no more than 100bar. Usually no need to use.
7. Retention time Do not use too long retention time to prevent molecular weight drop. Try to avoid temperatures above 300 ° C. If the shutdown is less than 15 minutes. Only air injection treatment is required; if it exceeds 15 minutes, clean it with viscosity PE and reduce the temperature of the barrel to PE temperature until it is turned on again.