1. Treatment of plastics
Because PET macromolecule contains lipid group, it has certain hydrophilicity. Particles are 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, before processing, the material must be dried, its drying temperature is 150 C, more than 4 hours; generally 170 C, 3-4 hours. It can be used to check whether the material is completely dry. The proportion of recycled PET bottle blank should not exceed 25%, and the recycled PET bottle blank should be thoroughly dried.
2. Selection of Injection Molding Machine
Because of the short time of stabilization after melting point and the high melting point of PET, the injection system with more temperature control section and less self-friction heat generation during plasticization should be selected, and the actual weight of the product (water-containing orifice) should not be less than 2/3 of the injection volume of the machine.
3. Mould 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. The thickness of the heat insulation plate is about 12 mm, and the heat insulation plate must be able to withstand high pressure. 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. Melt temperature
It can be measured by air irradiation method, ranging from 270 to 295 degrees C, and the enhanced grade GF-PET can be set to 290 to 315 degrees C.
5. Injection speed
Generally, the injection speed should be fast 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. back pressure
The lower the better, so as not to wear. Usually not more than 100 bars, usually do not need to use.
7. Residence time
Do not use too long detention time to prevent the molecular weight from decreasing, and try to avoid the temperature above 300 C. If the shutdown time is less than 15 minutes, only air-jet treatment is required; if it is 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.
(1) Recycled materials should not be too large, otherwise, it is easy to “bridge” at the discharging site and affect plasticization;
(2) If the mould temperature is not well controlled or the material temperature is not properly controlled, it is easy to produce “white fog” and opaque; if the mould temperature is low and uniform, the cooling speed is fast and the crystallization is small, the product is transparent.
1. Treatment of plastics