1.Drying Treatment
Because PET macromolecules contain ester groups, they have a certain degree of hydrophilicity. The granules are quite sensitive to water at high temperatures. When the moisture content exceeds the limit, the molecular weight of PET decreases during processing, resulting in discolored and brittle products. Therefore, the material must be dried before processing. The drying temperature should be 150℃ for at least 4 hours, generally 170℃ for 3-4 hours. The dryness can be checked using the air-jet method. The proportion of recycled material should generally not exceed 25%, and the recycled material must be thoroughly dried.
2.Injection Molding Machine Selection
PET can generally only be molded using screw-type injection molding machines. It is best to choose a variable-speed screw with a check ring at the top, which has high surface hardness and wear resistance, with a length-to-diameter ratio (L/D) of (15-20):1 and a compression ratio of approximately 3:1.
If the L/D is too large, the material will remain in the barrel for too long, and excessive heating can easily cause degradation, affecting the performance of the product. A low compression ratio results in insufficient shear heat generation, poor plasticization, and poor product performance. Conversely, a high compression ratio leads to more glass fiber breakage and decreased mechanical properties. When processing glass fiber reinforced PET, the inner wall of the barrel experiences significant wear; therefore, the barrel should be made of wear-resistant materials or lined with wear-resistant materials.
Short nozzles are preferable, with a polished inner wall and a large enough orifice diameter. Hydraulically braked valve nozzles are preferred. Nozzles must have insulation and temperature control measures to prevent freezing and clogging. However, the nozzle temperature should not be too high, otherwise, drooling will occur. Low-pressure PP material must be used before molding begins, and the barrel must be thoroughly cleaned.
3.Mold and Gate Design
PET preforms are generally formed using hot runner molds. A heat insulation plate, approximately 12mm thick, is ideal between the mold and the injection molding machine template, and the heat insulation plate must be able to withstand high pressure. Sufficient venting is essential to prevent localized overheating or breakage, but the vent depth should generally not exceed 0.03mm, otherwise, flash may occur.
4.Melt Temperature
Measured using the air-injection method, ranging from 270-295℃. For reinforced GF-PET, it can be set to 290-315℃, etc.
5.Injection Pressure
PET melt has good fluidity and is easy to mold. Medium pressure is typically used, ranging from 80 to 140 MPa. For glass fiber reinforced PET, the injection pressure is 90-150 MPa. The determination of injection pressure should consider the viscosity of PET, the type and quantity of fillers, the location and size of the gate, the shape and size of the molded part, mold temperature, and the type of injection molding machine.
6.Injection Speed
Generally, a fast injection speed is preferred to prevent premature solidification during injection. However, excessive speed results in a high shear rate, making the material brittle. Injection is usually completed within 4 seconds.
7.Back Pressure
The lower the better to avoid wear. Generally, it should not exceed 100 bar. Usually, it is not necessary.
8.Residence Time
Avoid using excessively long residence times to prevent molecular weight reduction. Avoid temperatures above 300℃. If the machine is stopped for less than 15 minutes, simply perform a dry run; if it is stopped for more than 15 minutes, clean with PE and lower the barrel temperature to the PE temperature until the machine is restarted.
