As we know, PET is an environmentally friendly material that can be recycled repeatedly. However, during its recycling and reuse, the degree of viscosity reduction is a critical parameter: excessive viscosity loss leads to serious deterioration of material properties. Since PET is prone to hydrolysis when exposed to water in the molten state, its molecular chains break, resulting in a decrease in molecular weight and a sharp drop in viscosity. Therefore, before processing, the moisture content must be controlled below 50 ppm through a drying process. In the traditional PET extrusion process, long-term crystallization and drying is required prior to extrusion and recycling to maintain PET viscosity and prevent attenuation. Meanwhile, crystallization and drying consume a large amount of energy.
We fully utilize the excellent venting そして devolatilization performance of the 二軸押出機, combined with the super vacuum system designed by Cowin, to remove moisture from PET during melting, thereby maintaining its viscosity.
Cowin’s super vacuum system adopts a unit consisted of two-stage Roots pump and one water ring vacuum pump, featuring extremely large pumping speed and high ultimate vacuum. This ensures that the IV drop of PET during processing is controlled within 2.5%.
Specific process flow is as below:
Heat exchanger — Vacuum tank unit — Filter — Roots vacuum pump A — Roots vacuum pump B — Water-ring vacuum pump
The heat exchanger is used together with the chiller to instantly cool high-temperature impurities or water vapor. After condensation, moisture turns into liquid water, and its volume decreases sharply, which further improves the pumping efficiency of the Roots pump.
This system can be applied not only to PET pelletizing そして molding on twin-screw extruders, but also to PLA molding.
Photos are shown below:
