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PET chemical name is polyethylene terephthalate, also known as polyester. PET is highly hygroscopic. Granular materials are sensitive to water at high temperatures. When the water content exceeds the standard, hydrolysis will occur during molding and processing, which will greatly reduce the quality. Therefore, raw materials must be dried during PET injection molding.
Because PET macromolecules contain ester groups, they have certain hydrophilicity. Granules are sensitive to water at high temperatures. When the moisture content exceeds the limit, the molecular weight of PET decreases during processing, and the products become colored and brittle. Therefore, the materials must be dried before processing. The drying temperature is 150 ℃ for more than 4 hours, generally 170 ℃ for 3 to 4 hours. The air shot method can be used to check whether the material is completely dry. Generally, the proportion of recycled materials shall not exceed 25%, and the recycled materials shall be thoroughly dried.
Generally, PET can only be molded by a screw injection molding machine. It is better to select the mutant screw with a check ring at the top, which has high surface hardness and wear resistance.
If the L/D is too large, the material will stay in the barrel for too long. If it is heated excessively, it will easily cause degradation and affect the product's performance. The compression ratio is too small, the shear heat generation is less, the plasticization is poor, and the product performance is poor. Otherwise, the glass fiber will break more and the mechanical properties will decline. When processing glass fiber-reinforced PET, the inner wall of the barrel is severely worn, and the barrel is made of wear-resistant materials or lined with wear-resistant materials.
The nozzle should be short, the inner wall should be polished, and the aperture should be as large as possible. The hydraulic brake valve type nozzle is preferred. The nozzle shall be provided with thermal insulation and temperature control measures to ensure that the nozzle will not freeze and block. However, the temperature of the nozzle shall not be too high, or salivation may occur. Before forming, low-pressure PP material must be used to clean the barrel.
PET bottle embryo is generally molded by hot runner mold. It is better to have a thermal insulation board between the mold and the injection molding machine template, with a thickness of about 12mm, and the thermal insulation board must be able to withstand high pressure. The exhaust must be sufficient to avoid local overheating or fragmentation, but the depth of the exhaust port should not exceed 0.03mm, otherwise flash may occur.
It can be measured by air shot method, ranging from 270-295 ℃, and the enhancement grade GF-PET can be set to 290-315 ℃, etc.
PET melt has good fluidity and is easy to be molded. Generally, medium pressure is used, with a pressure of 80~140MPa. The injection pressure of glass fiber-reinforced PET is 90~150MPa. The determination of injection pressure shall consider the viscosity of PET, the type and quantity of filler, the location and size of the gate, the shape and size of plastic parts, mold temperature, the type of injection molding machine, etc.
Generally, the injection speed should be fast to prevent premature solidification during the injection. But too fast, a high shear rate makes materials fragile. The injection is usually completed in 4 seconds.
The lower the better, to avoid wear. Generally, it does not exceed 100bar.
Do not use too long a retention time to prevent the molecular weight from dropping. Try to avoid temperatures above 300 ℃. If the shutdown is less than 15 minutes. Only air launch treatment is required; If it takes more than 15 minutes, clean it with PE, and reduce the barrel temperature to PE temperature until restart.
For more information about PET injection molding, plz feel free to contact us.
