the filter intercepts dirt emanating from a molten - polymer extruder . the filter is clamped between perforated support plates , and a rotor sweeps the holes in the upstream plate . the rotor defines a cavity which communicates with a small sector of the holes at any one time . the cavity is openable to drain , via a valve . the melt is under high pressure , to which the cavity is exposed . the rotor is rotated by a drive - shaft , and the drain is located co - axially with the drive - shaft .