Vent design for single-screw extruders

Bausano single-screw extruder vent design

Bausano manufactures high-performance, customised single-screw extruders to meet any needs. From the very beginning, we have ensured the highest standards, been constantly open and keen to improve and stay up to date with advances in the industry, always with a focus on the future. For Bausano, air vent design for screw extruders is an important aspect, which we will look at more closely here.

The starting point to properly design a vented screw for a single-screw extruder is to understand how Bausano’s extruder vents can work for the devolatilization of polymers. The process starts with the first metering section, which is filled with polymer (I), and as it reaches the vent section (II), the channel volume increases. This eliminates any pressure in that area and clears an open space. A deflector must be placed in the barrel opening to avoid any vent flow that would clog the hole to atmosphere or vacuum. This means that the deflector design is critical, because without it, no screw design will vent properly.

The deflector is configured so that the amount of polymer that squeezes out as the screw flight passes the vent opening (III) will be scraped off and accumulate in the relief area. The amount must be small enough to fill only a portion of the relief area, without pushing any polymer back into the vent hole. The amount left in the relief area is pushed out by the next flight and refilled. If the relief is the right size for the amount being scraped off, the vent to atmosphere stays open and it becomes a steady-state process allowing volatiles to exit.

Proper vent design requires the right knowledge of polymer flow in the screw channels.

The molten polymer is attached to both the screw and the barrel, and from the perspective of a stationary screw, the motion of the polymer in the channel is equal to the rotational speed of the barrel on the barrel wall and zero at the root of the screw. Across the channel depth, the polymer is transported forwards by the combination of the rotating shear stress and the angle of the screw flight.