The rheology of PET in molten state is better, the effe […]
The rheology of PET in molten state is better, the effect of pressure on viscosity is greater than temperature, and the production of pet material mainly changes the flowability of the melt from pressure.
1. Treatment of plastics Because PET macromolecules contain a lipid group and have a certain hydrophilicity, the 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 is colored. Become brittle. Therefore, the material must be dried before processing, and its drying temperature is 150 ° C, more than 4 hours, generally 170 ° C, 3-4 hours. Airborne methods can be used to verify that the material is completely dry. The proportion of recycled materials should generally not exceed 25%, and the recycled materials should be thoroughly dried.
2. The selection of PET for injection molding machine is short because of the short time after melting point, and the melting point is higher. Therefore, it is necessary to select an injection system with more temperature control sections and less self-friction heat during plasticization, and the actual product (water-containing material) The weight should not be less than 2/3 of the machine's injection volume.
3, mold and gate design PET preforms are generally formed by hot runner mold, between the mold and the injection molding machine template should have a heat insulation board, the thickness of about 12mm, and the insulation board must be able to withstand high pressure. Exhaust gas must be sufficient to avoid local overheating or chipping, but the depth of the exhaust port should not exceed 0.03mm, otherwise it will easily produce flash.
4, the melting temperature can be measured by the air-launch method. Between 270 and 295 ° C, the enhanced grade GF-PET can be set to 290-315 ° C and the like.
5, the injection speed is generally faster, to prevent premature coagulation during injection. But too fast, the high shear rate makes the material brittle. The shot is usually completed in 4 seconds.
6, the lower the back pressure, the better, so as not to wear. Generally no more than 100bar. Usually no need to use.
7. Do not use excessive residence time for residence time to prevent molecular weight loss. Try to avoid temperatures above 300 °C. If the shutdown is less than 15 minutes. Only need to be air-blasted; if it is more than 15 minutes, clean it with viscosity PE and reduce the temperature of the barrel to PE temperature until it is turned on again.
For PET bottles, the requirements for the production of plastic bottles are relatively high. Therefore, the company has undergone strict screening both in the selection of equipment and in the assessment of employees.