A study on the performance of barrier‐screw extruders

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Abstract
An experimental and theoretical study was conducted on the performance of barrier‐screw extruders. For the experimental study, an extruder (D = 63.5 mm and L/D of 24) with a Davis‐Standard barrier screw was used to extrude a low‐density polyethylene. The extruder had nine pressure transducers which were mounted, almost equally spaced along the extruder axis, on the wall of the extruder barrel, so that we could measure axial pressure profiles during extrusion. Also conducted were tracer experiments (i.e., screw ‘pushout’ experiments), which enabled us to examine the profiles of melt film thickness in the solid channel. For the theoretical study, the analysis of Elbirli, et at, was extended by making the following modifications: (a) the flow through the barrier flight clearance was treated separately from the flow of melt film above the solid bed; (b) the melt films surrounding the solid bed were assumed to circulate around the solid bed; and, (c) convective heat transfer was included in the energy equations for the melt channel and melt films surrounding the solid bed. Prediction of axial pressure profile was found to be in reasonable agreement with experiment. Predictions are presented of the effects of extrusion conditions on the thicknesses of the upper and lower films, and on the solid‐bed velocity. Practical implications of these results are discussed from the view of designing barrier screws and selenting barrier‐screw extruders.