Processing method of pet plastics
The rheological properties of pet in molten state are better, and the effect of pressure on viscosity is greater than that of temperature. Therefore, the fluidity of pet is mainly affected by pressure.
I Treatment of plastics
Because pet macromolecules contain aliphatic groups and have certain hydrophilicity, it is more sensitive to water at high temperature. When the water content exceeds the limit, the molecular weight of pet decreases and the product is brittle. Therefore, the material must be dried after processing. The drying temperature is 150 ℃ for more than 4 hours, generally 170 ℃ for 3-4 hours. The air jet method can be used to check whether the material is completely dry.
II Due to the short stabilization time and high melting point after the melting point of pet, the injection system with more temperature control sections and less self friction and heating in the plasticization process should be selected, and the actual weight of the product should not be less than 23% of the injection volume of the machine. Based on these requirements, Ramada has developed a series of pet specific plasticizing systems in recent years. The clamping force is greater than 6300tm2.
III Mold and gate design
PET bottle embryo is generally formed by hot runner mold. It is better to have a heat shield between the mold and the injection molding machine template, with a thickness of about 12mm, and the heat shield 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 generally does not exceed 0.03mm, otherwise it is easy to produce flash.
IV Melt Temperature
It can be measured by air injection method. 270-295 ℃, enhanced GF pet can be set to 290-315 ℃.
V Injection speed
Generally, the injection speed should be fast to prevent premature solidification during the injection process. But the speed is too fast and the shear rate is high, which makes the material fragile. The injection is usually completed within 4 seconds.
Vi The lower the back pressure, the more wear can be avoided. Generally not more than 100 bar. Usually not used.
VII retention time
Do not use too long retention time to prevent molecular weight reduction. Try to avoid the temperature exceeding 300 ℃. If the shutdown time is less than 15 minutes. Only air jet treatment is required; If more than 15 minutes, wash with sticky polyethylene and reduce the barrel temperature to polyethylene until the machine starts again.
VIII matters needing attention
If the mold temperature is not controlled properly or the material temperature is not controlled properly, it is easy to produce "white fog" and opacity. The mold temperature is low and uniform, the cooling speed is fast, and the transparency of crystalline products is poor.
The recycling problem of efficient depolymerized PET plastic bottles may be solved
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