The rheology of PET in the molten state is good, and th […]
The rheology of PET in the molten state is good, and the pressure has greater influence on the viscosity than the temperature. Therefore, the fluidity of the melt is mainly changed from the pressure.
1, plastic processing
Because PET macromolecules contain lipid groups and have a certain degree of 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 products are colored and brittle. For this reason, the material must be dried before processing. The drying temperature is 150° C., more than 4 hours, which is generally 170° C. and 3-4 hours. Air ejection can be used to check if the material is completely dry.
2, injection molding machine selection
Because the PET has a short stabilization time after the melting point and a higher melting point, it is necessary to use an injection system with more temperature control segments and less self-heating during plasticization, and the actual weight of the product should not be less than 2/3 of the injection volume of the machine. . Based on these requirements, Ramada has developed a small and medium-sized PET special plasticizing system in recent years. The clamping force is greater than 6300t/m2.
3, mold and gate design
PET preforms are generally formed by hot runner molds. It is better to have insulation boards between the molds and the injection molding machine template. The thickness of the PET preforms is about 12 mm, and the insulation board must be able to withstand high pressure. Exhaust must be sufficient to avoid local overheating or fragmentation, but the depth of the exhaust port generally should not exceed 0.03mm, otherwise it is prone to flash.
4, melt temperature
Available air-ejection measurements. Different from 270-295°C, the reinforced grade GF-PET can be set to 290-315°C.
5, injection speed
The general injection speed is faster and prevents premature coagulation during injection. But too fast, the high shear rate makes the material fragile. The shot is usually completed within 4 seconds.
6, back pressure
The lower the better, so as not to wear out. Generally does not exceed 100bar. Usually do not need to use.
7, stay time
Do not use excessive residence time to prevent the decrease in molecular weight. Try to avoid temperatures above 300°C. If the stoppage is less than 15 minutes. Only air shot processing is required; if it exceeds 15 minutes, it is cleaned with viscosity PE and the barrel temperature is lowered to PE temperature until it is turned on again.
Learn more about pet material at http://www.petcup.cc