Degassing

Representation of the degassing key figures

The degassing key figures can be displayed via ‘Graphic > Filling degree and degassing’.

Black curve: The black curve represents the filling level. A single-screw extruder is fully filled as standard (filling level = 100%). However, this does not apply to degassing extruders. Here, the degassing zone must be depressurised so that the screw is only partially filled in this area.
Degassing surface area: This value represents the surface area of the plastic melt with the air/vacuum.
Renewal time: The renewal time is a measure of how long the melt takes to exit and re-enter the degassing zone before the pushing web and is therefore a measure of how quickly the degassing surface mixes.
Specific surface renewal: This value is the quotient of the degassing surface and the renewal time. The larger the degassing surface and the shorter the renewal time, the higher this value.
Specific degassing index: This value is a measure of the degassing performance of the degassing zone. The specific surface renewal increases approximately proportionally to the speed, whereas the specific degassing index is almost independent of the speed.

Filling level: The filling level should be well below 100% in the degassing zone and sufficiently long to prevent flooding of the degassing opening.
For a high degassing performance, the degassing surface should be large, the renewal time short and thus the specific surface renewal large. The longer the partial filling, the larger the degassing surface area. The most important measure is the specific degassing coefficient, which also takes into account the residence time of the melt in the degassing.