An experimental investigation of metallic contact between the screws and barrels of extruders processing polymeric materials is described. Above a critical shear stress of the order of 105 N/m2, contact is obtained between screw flight and barrel under all operating conditions. This critical value is similar in magnitude to that associated with the onset of melt fracture in capillary flow. It is therefore suggested that slip in, or near, the clearance reduces or eliminates polymer hydrodynamic lubrication of the screw. Below the critical shear stress, the hydrodynamic lubrication associated with a continuous film is capable of supporting substantial lateral loads applied to the screw.
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