PET plastic injection molding is an extremely sensitive process to process parameters; even a slight deviation in any parameter can lead to the entire batch of products being defective. The entire closed-loop process begins with the drying and conveying of PET raw materials. Precisely metered, dried PET pellets are fed into the injection molding machine hopper. Specialized PET servo injection molding machines are typically used, with screws designed for low shear and shallow grooves to prevent excessive shear heat from degrading the raw material molecules. The control of process parameters for PET plastic injection molding mainly revolves around four systems: temperature, pressure, speed, and time.
Temperature control is crucial. It is divided into several stages: the barrel temperature needs to be precisely gradient-increased from back to front (from the feed section to the nozzle), typically within the range of 260-290℃. Excessive temperature leads to thermal degradation, producing acetaldehyde (AA), affecting the beverage's taste; insufficient temperature results in poor plasticization and poor melt flowability. The nozzle temperature needs independent and precise control to prevent drooling. PET preform mold temperature, especially the cooling water temperature of the cores and cavities, is crucial in determining the crystallinity and appearance of the PET preforms. The temperature of cooled water needs to be maintained at a low temperature (typically 10-18℃) to ensure rapid cooling and solidification of PET preforms, fully solidified before ejection.
Injection pressure and speed control directly affect melt filling behavior. The injection pressure is set based on factors such as preform weight, neck size, etc to ensure the melt fills the cavity without causing flash or excessive internal stress. Injection speed is typically controlled in a slow-fast-slow pattern: a slow initial stage to expel air and prevent trapped air; high-speed filling in the middle to ensure uniform temperature at the melt front and avoid cold runner marks; and a final deceleration to prevent overfilling and ejection. Holding pressure and time settings compensate for melt cooling shrinkage, preventing shrinkage marks and voids, but excessive holding pressure increases internal stress. Time control includes injection time, holding time, cooling time, and cycle time. Cooling time accounts for the majority of the cycle and must ensure the preform's central layer is fully solidified before ejection.
The PET plastic injection molding parameters are an interconnected, dynamically balanced system. The PET servo injection molding machine adopts a PLC closed-loop control system to monitor and adjust these parameters in real time. It ensures that each PET preform has consistent weight, size, and internal quality in large-volume, long-cycle production.