Quantitative Analysis of Structural Transformation in Extrusion Processing

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Author Last Name: 
PhD: Tim Althaus, Prof. Dr.-lng. Erich J. Windhab
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Research Area: 
Wet Systems
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Partially saturated powders consist of a bulk solid whose cavities are to a certain extent lled with binder liquid. The presence of air signicantly aects the ow behavior of such systems due to interparticle frictional forces showing a strong normal force dependency. The role of the binder liquid is ambiguous as it increases normal forces between particles by capillary suction pressures and surface tensions at the contact line between liquid and solid but on the other hand reduces the coecient of internal friction between the particles by acting as a lubricant on the particle surface. Additionally interlocking eects between particles can signicantly hinder shear ow when partially saturated powders are in a densely packed state. Thus the extrusion of wet powders with minimised binder liquid fractions is often impeded by blocking of the material in the die inlet region. However, under certain process and geometrical conditions shear and pressure forces in the die inlet region can transform the 3-phase-system (3P) into a state where increased saturation as well as an orientation of the particles in shear layers reduce the resistance against ow. The reduction of the entrapped air cells ideally results in a local saturation of the system making it a 2-phase-suspension (2S) with viscous friction instead of Coulomb solid friction dominating its ow behavior. Small-scale ram extrusion combined with visual observations can be used in order to quantify critical stresses which have to be surmounted before reaching steady-state ow. Critical stresses are in uenced by particle properties (size/size distribution, shape and surface), binder liquid viscosity and particle/binder liquid interfacial properties (contact angle, interfacial tensions). An understanding of the mechanisms of wet powder ow and the interdependencies of process and material parameters during extrusion can be used for the design of product specic tailor-made processes and allow for processing of powder/binder systems with minimized binder fraction, thus reducing potential drying costs.