Pressure Generation and Energy Input in the Melt

The previous chapters have shown how screw machines are comprised of successive zones that perform specific process tasks. The most common process task is a combination of transporting materials and generating pressure. At the head of the machine, higher pressure needs to be applied, in particular, where typically an extrusion die or a nozzle plate is attached however, most mixing and kneading zones are also pressure consumers that require pressurization of the melt. 

Once the throughput per revolution is so high that the affected screw element is filled completely, the inherent throughput of the screw element is reached. If the screw element is run with a throughput that is greater than its inherent throughput, a pressure drop establishes across the screw element. This operating condition is described as overrun . 

Kneading blocks with this configuration are described as neutral (non-conveying). They are always filled completely and create a pressure drop. 

Conveying screw elements can actively generate pressure, provided that they are filled completely. If there is a continuous gas phase present, there can naturally be no axial pressure gradient. This means that back pressure always forms upstream of an obstruction to flow in a conveying screw element. The conveying screw element fills up until the pressure needed to overrun the obstacle has built up. 

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