EXTRUSION SCREWS

    Practically all thermoplastics can be processed by extrusion, but in order for the extrudate to maintain its shape until solidification, extrusion grades require a higher molecular weight associated with higher viscosity and melt strength.  Melt Index, which relates to strength and flow behavior, is lower for extrusion materials than for injection materials where the easier flowing material is necessary to fill cold molds.

    A major difference between injection molding and extrusion relates to the time that the material has to melt.  In injection processing, a pellet typically has 2 to 5 minutes from the time it enters the barrel from the hopper until it exits the nozzle end of the screw.  This residence time allows the resin to heat soak and absorb conductive heat energy (in addition to shear heat) as it works its way up the screw.

    In extrusion, a pellet is typically allowed less than 2 minutes (and in some cases, less than one minute) to travel from the feed pocket of the screw to the screen pack. 

    The most obvious difference between injection and extrusion screw designs is the length of the flighted surface, or the L/D ratio.  Where 20:1 ratio is common in injection, 24:1 is considered a standard L/D ratio for extruder screws and many have ratios of 30:1 or more.  Lacking residence time, extruder screws must compensate by keeping the material on the screw for a greater length.

    Extrusion screws tend to have greater shear capability, resulting from the increased use of barrier designs and the utilization of a grooved feed zone in the barrel.  A shallow feed zone channel in the screw in combination with a grooved feed section in the barrel also creates shear and a more aggressive feeding of the material.

    Westland has been successful in using the Eagle™ mixer in its extruder screw designs.  Frequently, the Eagle™ and the Bald Eagle are used in tandem.  The Bald Eagle is the standard Eagle™ mixer with the omission of the mixing notches.  This design further assures that no unmelt is present in the extrudate.