The rheology of PET in the molten state is better, and the pressure has a greater influence on the viscosity than the temperature. Therefore, the pressure is mainly used to change the fluidity of the melt.
Because PET macromolecules contain fat base and have certain hydrophilicity, the pellets are more sensitive to water at high temperature. When the moisture content exceeds the limit, the molecular weight of PET decreases during processing, and the product becomes colored and brittle. Because of this, the materials must be dried before processing. The drying temperature is 150°C for more than 4 hours, generally 170°C for 3-4 hours. The air shot method can be used to check whether the material is completely dry.
2. Selection of injectionMolding Materials
Since PET has a short stable time after melting point and high melting point, it is necessary to choose an injection system with more temperature control sections and less self-friction heat generation during plasticization, and the actual weight of the product cannot be less than 2/3 of the injection volume of the machine . Based on these requirements, Ramada has developed a series of small and medium-sized PET special plasticizing systems in recent years. The clamping force is selected as greater than 6300t/m2.
3. Mold and gate design
PET preforms are generally formed by hot runner molds. It is better to have a heat shield between the mold and the injection molding machine template, the thickness of which is about 12mm, and the heat shield must be able to withstand high pressure. Exhaust must be sufficient to avoid local overheating or chipping, but the depth of the exhaust port should generally not exceed 0.03mm, otherwise it is easy to plastic machining fla4. Melting temperature
It can be measured by air shot method. The temperature ranges from 270-295°C, and the enhanced GF-PET can be set at 290-315°C.
5. Injection speed
Generally, the injection speed should be fast to prevent premature coagulation during injection. But too fast and high shear rate make the material fragile. Injection is usually completed within 4 seconds.
6. Back pressure
The lower the better, so as to avoid wear. Generally not more than 100bar. Usually don’t need to be used.
7. Detention time
Do not use an excessively long residence time to prevent the molecular weight from decreasing. Try to avoid temperatures above 300°C. If the downtime is less than 15 minutes. Only need to do air injection treatment; if it exceeds 15 minutes, clean with viscosity PE, and reduce the barrel temperature to PE temperature until the machine is turned on again.
8. Matters needing attention
If the mold temperature is not well controlled or the material temperature is not properly controlled, it is easy to produce “white fog” and opacity. The mold temperature is low and uniform, the cooling rate is fast, and the product is transparent with less crystallization.