Vented Screw Barrel for Feeding, Melting

A vented or two-stage screw is essentially two screws on the same shaft. The first works as a conventional screw by feeding, melting, and conveying the polymer forward. The second screw does the same, except it is melt-fed by the first screw. The first screw operates with no head pressure, while the second has to overcome any head pressure from the die and other downstream components. In order to keep the vent section of the screw from filling and developing vent flow, the second screw must have more output capacity than the first screw. Proper design of a two-stage screw requires knowledge of the polymer viscosity, the output capacity of both stages, and the head pressure. Simply using a ratio in channel depths between the first and second metering sections is risky.