Production process of PET
2019-06-30

The rheology of PET is better in melting state, and the influence of pressure on viscosity is greater than that of temperature. Therefore, the melt fluidity is mainly changed by pressure.

1. Because of the hydrophilicity of PET macromolecule, the treatment of plastics is sensitive to water at high temperature. When the moisture content exceeds the limit, the molecular weight of PET decreases and the products become 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. It can be checked whether the material is completely dry by air-jet method. Generally, the proportion of recycled materials should not exceed 25%, and the recycled materials should be thoroughly dried.

2. Because of the short time of stabilization after melting point and the high melting point, the injection system with more temperature control section and less self-friction heat generation during plasticization should be selected for injection moulding machine, and the actual weight of the product (water-cut material) should not be less than 2/3 of the injection volume of the machine. Based on these requirements, Huameida has developed a small and medium-sized series of special plasticizing system for PET in recent years. The clamping force is selected according to the force greater than 6300t/m2.

3. Mold and gate design PET bottle embryo is usually formed by hot runner mould. It is better to have a heat insulation plate between the mould and the injection machine template. Its thickness is about 12 mm, and the heat insulation plate can withstand high pressure. The exhaust must be sufficient to avoid local overheating or fragmentation, but the exhaust outlet depth should not exceed 0.03mm, otherwise it is easy to produce flash.

4. The melt temperature can be measured by air-jet method. Enhanced grade GF-PET can be set to 290-315 (?)

5. Injection speed is generally faster to prevent premature coagulation during injection. But too fast and high shear rate make the material fragile. The projectile is usually completed in 4 seconds.

6. The lower the back pressure, the better to avoid wear and tear. Generally not more than 100 bars. Usually no need to use.

7. Do not use too long retention time to prevent molecular weight from decreasing. Avoid temperatures above 300 degrees Celsius as far as possible. If the downtime is less than 15 minutes. Only air-jet treatment is required; if it lasts more than 15 minutes, it should be cleaned with viscous PE and the barrel temperature should be lowered to PE temperature until the machine is restarted.


Production process of PET