Technical Disscussion

The extruder with Double vacuum station enough to dry the flakes in the process, then no need pre-drying?

13.02.2019 By: Oude

In recent years, there are multi-screw extruder system have been established in the market as an alternative to Single – screw extruders with a pre-drying system. (Here we call multi-screw extrudering system including Twin-screw extruders, Planetary roller extruders etc.)

But we think it is necessary to have pre-drying system even you are using multi-screw extruder. Because:

1) Multi-screw extruders what they all have is very complex vacuum-degassing systems installed on the extruder to prevent a hydrolysis effect from occurring due to no pre-drying process being installed. Usually such kind of extruder distinct using condition:

Maximum allowable feed moistures should not be higher than 3000 ppm (0.3 %)

In fact, Bottle flakes show variations in purity, particle size, particle size distribution and thickness – and especially in humidity. Post-consumer flakes allow retaining up to roughly 5,000 ppm of humidity in the product and storing many times this quantity of water on its surface. In some countries, feed moistures can be up to 14,000 ppm even packed in the Big Bag.

Both the absolute level of water content and its variations, which are unavoidable, are the real challenge for the multi-screw extruder and the related degassing concept. This does frequently result in process fluctuations, which are discernible from extremely variable output pressures of the extruder.It is very possible that a significant amount of moisture still remains as it is reaching its melt phase in the extruder due to the initial moisture level in the resin, and the amount removed during vacuum

2) PET is highly hygroscopic and absorbs moisture from the atmosphere. Small amounts of moisture will hydrolyze PET in the melt phase, reducing molecular weight. PET must be dry just prior to processing, and amorphous PET requires crystallizing prior to drying so that the particles don't stick together as they go though glass transition.

Hydrolysis can occur due to moisture and this often can be seen as a reduction in the IV (Intrinsic Viscosity) of the product. PET is "semi-crystalline". When the IV is reduced, the bottles are more brittle and tend to fail at the "gate" (injection point) during blowing and filling.

In its "crystalline" state it has both crystalline and amorphous portions in its molecular structure. The crystalline portion develops where the molecules can align themselves in a very compact linear structure. In the non-crystalline regions the molecules are in a more random arrangement. By insuring that your crystallinity is high, prior to processing, the result will be a more uniform and higher quality product.

ODE Made IRD Infrared Rotary Drum Systems have performed these sub-functions in a considerably more energy-efficient way. Special designed shortwave Infrared radiation stimulates the molecular heat fluctuation in the dry material directly without taking the rather inefficient intermediate step of using heated air. Such heating way in heat-up and drying times reduced to the range of only 8.5 up to 20 minutes depending on the specific application, while several hours have to be calculated for conventional hot-air or dry-air systems.

Infrared drying can significantly improve the performance of a twin-screw extruder because it reduces the degradation of IV values and significantly improves the stability of the entire process.

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