An extruder screw for use
An extruder screw for use in the barrel of a plastic materials extruder is formed with a melt section which includes a melt flighting and a compaction channel and which also includes a barrier flight in which only smooth transitions are formed to prevent pressure surges or disruptions. The barrier flight rises radially outwardly of a forward wall of the melt channel, and defines a clearance space with the adjacent wall of the barrel which varies from a maximum at the beginning of the barrier flight to a minimum at the end of the barrier flight sheet production extruder. The varying clearance between the barrier flight and the barrel wall approximates the change in viscosity of molten plastic material flowing over the barrier flight from the solids channel into the melt channel, so that a relatively constant shear energy input is applied to the molten plastic material flowing over the barrier flight.
An extruder screw for use in the barrel of a plastic materials extruder, said screw having a drive end and having a forward end extended into said barrel, said screw comprising a core, a primary flighting extending along said core and having a generally radially extending rearwardly facing flank which conforms to a substantially uniform pitch throughout a substantial portion of the length of the screw, said screw having a fill section for receiving unmelted plastic material, a melt section adjoining said fill section, and a metering section adjoining said melt section, said melt section being defined by an unmelted plastic material compaction portion adjacent said fill section and by a barrier portion extending from said melt section to said metering section, said compaction portion being further defined by a melt flighting formed in adjacent relation to a forwardly facing flank of said primary flighting and having a radially outer surface forming a melt channel with said primary flighting, said melt flighting having a forwardly facing wall which defines with the adjacent rearward facing flank of said primary flighting a channel for unmelted plastic material, a barrier flight rising radially outwardly from said melt channel forwardly facing wall, and defining a clearance space with the adjacent wall of said barrel, said clearance space varying from a maximum at the junction of said compaction portion with said barrier portion and tapering toward said barrel wall to a minimum clearance at the junction of said barrier portion and said metering section, whereby the clearance between said barrier flight and said barrel wall approximates the change in viscosity of molten plastic material flowing over said barrier flight into said melt channel so that a relative constant shear energy input is applied to molten plastic material.
This invention relates to an extruder screw for use in the barrel of a plastics material extruder and is more particularly directed to the construction and design of the portion of the screw beginning with the initial fill with unmelted plastic material, through the initial melting and plastic material compacting stage and through and including the first dispersive mixing stage in which the molten plastic material traverses a melt barrier flight and runs into and along a channel separate from the channel for compacted unmelted plastic material.