Extrusion of highly unsaturated wet powders: Stress fields in extruder barrels

Wet glass bead powders with solid-to-liquid volume ratios between 2 and 3 were extruded through cylindrical dies by means of a ram extruder. The vertical stress exerted by the ram and the wall normal stress above the die were recorded. A continuum mechanical model was developed to calculate stress fields in the extruder barrel as a function of the load superimposed by the ram. The model predicts that the vertical stress, the wall normal stress and the wall shear stress decrease exponentially above the die entry region. A sudden increase in wall normal and wall shear stresses is observed where the powder in the die vicinity starts to be vertically accelerated. The stresses then decrease to zero from this point to the die which is in agreement with experimental observations. Calculated wall normal stresses are consistent with measured values.

Wet powder ram extrusion is impeded by particle interlocking and frictional forces in the die entry region. In the barrel and in the die the material flows nearly as a plug while normal shear in the die entry region essentially affects the extrusion pressure and the stress distribution.Figure optionsDownload as PowerPoint slideResearch Highlights
► A continuum mechanical model was developed to calculate stress fields in wet powders during ram extrusion.
► The model proposed is entirely based on physical meaningful parameters.
► The model is a useful tool for the design of wet powder extrusion processes.
► A good agreement was found between calculated and measured wall normal stresses.