Degassing vent ports

Degassing is not done on all extruders some extruders are equipped with a vent port. The vent port allows volatiles or gasses to escape from the plastic melt. Degassing is also referred to as devolatilization. The fiinctions listed above are... 

Degassing is done on a vented extruder this is an extruder with a vent port in the barrel. A special screw design has to be used in a vented extruder to make sure that there is a zero pressure region under the vent port. Degassing is needed when the plastic contains volatile components at a level high enough to cause problems. Generally, volatiles cause voids or surface defects in the extruded product. 

In the case of flash degassing, the polymer solution is first heated under pressure to above the boiling point of the volatile components and decompressed directly into the ZSK. The polymer and solvent (monomer) spontaneously separate from each other inside the ZSK and the majority of the volatile components are released via the back venting system. Depending on the pressure and the temperature, up to 90% of the solvent can be removed in this way. Efficiency depends on the temperature of the polymer solution at the feed intake, the pressure drop in the back vent, and the material properties of the feeding system. The back vent is located upstream from the polymer or polymer solution feeding port (see Fig. 10.2). In this case, there is no melt in the screw channel so that the entire screw cross-section is available for the removal of gas or vapors. 

Fig. 14. Schematic design of the Cerex probe. This is a sensor for optical density and mounted vertically in situ. Suspension enters the side drain ports deliberately and can be trapped inside the sensor by powering the solenoid coils the magnetic plunger closes the side ports. In the meantime, the trapped dispersion degasses and bubbles disappear through the upper vent hole. After some time, the optical density reading is declared representative . The next cycle starts with opening the side drain ports...

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