PET chemical name is polyethylene terephthalate, also known as polyester. The most widely used among customers is GF-PET, mainly Pet Preform. The rheology of PET in the molten state is better, and the effect of pressure on the viscosity is greater than the temperature. Therefore, the fluidity of the melt is mainly changed from the pressure.
1, plastic processing
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 finished product is colored and brittle. In this case, it is necessary to dry the material before processing, and the drying temperature is 150 ° C, more than 4 hours, generally 170 ° C, 3-4 hours. It is not completely boring to use the air-launch method to check the data. The share of the recovered material should generally not exceed 25%, and the recovered material should be completely boring.
2, injection molding machine selection
Because PET has a short time after the melting point and the melting point is high, it is necessary to select an injection system with more temperature control sections and less self-conflict heat generation during plasticization, and the finished product (aqueous mouth material) practical component can not be smaller than the machine injection. 2/3 of the amount. Based on these requirements, Ramada has developed a small and medium-sized series of PET plasticizing systems in recent years. The clamping force is selected to be greater than 6300t/m2.
3, mold and gate planning
The PET preform is generally formed by a warm runner mold. There must be a heat shield between the mold and the injection molding machine. The thickness of the PET preform is about 12 mm, and the heat shield must be able to accept high pressure. Exhaust is necessary to prevent partial overheating or chipping, but the depth of the exhaust port should generally not exceed 0.03mm, otherwise the flash will occur simply.
4, the melting temperature
It can be measured by air-to-air method. Between 270 and 295 ° C, the enhanced grade GF-PET can be set to 290-315 ° C and the like.
5, injection speed
Generally, the injection speed is fast, which prevents premature coagulation when the needle is injected. But too fast, the high shear rate makes the material brittle. The shot is usually completed within 4 seconds.
6, back pressure
The lower the better, the prevention of wear. Generally do not exceed 100bar. Generally no need to use.
7, staying moments
Do not use excessive residence times to prevent molecular weight degradation. Try to prevent temperatures above 300 °C. If the shutdown is less than 15 minutes. It only needs to be air-aired; if it exceeds 15 minutes, it should be cleaned with viscosity PE and the barrel temperature will be lowered to PE temperature until it is turned on again.
8, matters needing attention
(1) The material to be recycled should not be too large, otherwise it will easily occur at the place of the material to “bridge” and affect plasticization.
(2) If the mold temperature is not well controlled or the material temperature is not properly controlled, it is prone to “white fog” and not clear. The mold temperature is low and uniform, the cooling rate is fast, and the crystallization is less, the finished product is bright.
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