A thermoplastic material plasticizing screw for rotation within a plasticizer bore, said plasticizing screw comprising:
- a screw root having a longitudinal axis, said screw root being rotatable about said longitudinal axis;
- a metering section along said screw root and having a volume available for the flow of an extrudate through the metering section when the plasticizing screw is rotated within the plasticizer bore;
- and
- a mixing section along said screw root and immediately downstream from said metering section for receiving said extrudate from said metering section in an at least partially melted condition, said mixing section comprising:
- at least two elongated inlet channels extending along said mixing section helically about said longitudinal axis of the screw root, each inlet channel having an upstream end and a downstream end and being relatively open to the passage of the extrudate at its upstream end and relatively closed to said passage at its downstream end;
- at least two elongated outlet channels extending along said mixing section helically about said longitudinal axis of the screw root, each outlet channel having an upstream end and a downstream end and being relatively closed to the passage of the extrudate at its upstream end and relatively open to said passage at its downstream end, there being an outlet channel for each inlet channel on said screw section;
- an elongated wiping land carried by said screw root in the mixing section for each inlet channel, each wiping land being disposed along a leading edge of its corresponding inlet channel between said inlet channel and an adjacent outlet channel and extending radially outwardly a first distance from said screw root longitudinal axis;
- an elongated barrier land carried by said screw root in the mixing section for each outlet channel, each barrier land being disposed along a leading edge of its corresponding outlet channel between said outlet channel and an adjacent inlet channel and extending radially outwardly from said longitudinal axis a second distance which is less than said first distance to define an undercut, the wiping land and the barrier land extending helically along said mixing section at different helix angles;
- the helix angle for said wiping land being about 40.degree. and the helix angle for said barrier land being about 50.degree. whereby the width of the inlet channel is reduced in the downstream direction;
- at least two notches extending transversely through each barrier land to permit the flow of the extrudate from an inlet channel through said notch to the adjacent outlet channel;
- said screw root having a first diameter in said metering section and a second diameter is said mixing section, said second diameter being less than said first diameter, said second diameter and the sizes of said notches combined with the barrier land undercut areas being sufficient to accept substantially all (95% to 100%) of the volume of melted extrudate from the metering section immediately upstream from the mixing section;
- and
- the wiping lands and the barrier lands conjointly forming barrier dams at their respective ends over which the extrudate must flow on entering and exiting the mixing section.