Since the screw pitch decreases towards the outlet, this compacts the material as it passes through the barrel. This is sufficient to propel the plug through the pivoted non-return valve at the end of the barrel and into a chamber into which air is continuously supplied through a series of nozzles. A pressure drop of about 0.5 bar must be allowed for the air across these nozzles, which adds significantly to the power requirement. A screw having a decreasing pitch does, however, require a very high power input for a given feed rate.