Plasticating extruder screw conveyors

Claim 1

Apparatus for use in a plasticating extruder of the type having an axially extending elongate barrel for preparing a plasticated material from solids introduced into the extruder, said apparatus comprising: a first screw conveyor for placement in the barrel, said first screw conveyor including a main flight defining a channel for continuously conveying the material in a downstream direction as the material is melted within the channel, said flight having a leading face facing downstream and a trailing face facing upstream, said channel extending between the leading face and the trailing face; a backfeed passage within the first screw conveyor, said backfeed passage communicating with the channel at a downstream location and at an upstream location to establish an extended flow circuit along which the material will move through the channel and the backfeed passage, the downstream location being in position to receive at least some unplasticated material from the channel, and the upstream location being spaced axially upstream of the downstream location a distance sufficient to enable plastication within the backfeed passage of at least some of the unplasticated material admitted to the backfeed passage; a drainage opening interconnecting the channel and the backfeed passage at the downstream location for enabling the flow of unplasticated material into the backfeed passage at the downstream location; a melt-return aperture interconnecting the backfeed passage and the channel at the upstream location for enabling the flow of plasticated material from the backfeed passage to the channel at the upstream location; a second screw conveyor located in said backfeed passage and arranged to convey material from the downstream location to the upstream location so as to enable recirculation of material along the extended flow circuit through the backfeed passage; and a solids-directing flight on the first screw conveyor extending along the channel between the melt-return aperture and the drainage opening, the axial distance between the solids-directing flight and the main flight being varied along the first screw conveyor for directing unplasticated material into the drainage opening while diverting unplasticated material away from the melt-return aperture for unobstructed return of plasticated material from the backfeed passage to the channel and segregation of at least some of the returned plasticated material from the unplasticated material in the channel such that the segregated already plasticated material will not flow through the extended flow circuit.