Evaluation of slip in capillary extrusion of ceramic pastes

In this study, the capillary extrusion behaviour of an alumina paste materi al, made using submicron alumina powder, hydroxy ethyl cellulose (HEC), a p olyelectrolyte dispersant, poly (methacrylate), and water is described. It confirms that the computed shear stress, obtained using estabilished analyt ical procedures, depends upon both the diameter and the length of the capil lary tube. Additionally, fur a given nominal shear rate, the shear stress s o calculated is less for the smaller diameter capillaries than for the corr esponding larger diameter capillaries. These trends are rationalised, along established lines; in terms of the preponderance of a material process sli p at the wall of the capillary as opposed to cohesive or fully developed un iform flow. Moreover, for a given diameter of the capillary tubes the calcu lated shear stress is greater for the smaller length capillary tubes than t he corresponding longer capillary tubes. II is concluded that these results are not amenable to meaningful analysis using classical capillary rheomete ry analytical procedures based upon the presumption of a fully developed fl ow. The extensive slip which inevitably occurs at the interface between the capillary and the extrudate negates the sensible application of these type of procedures. Instead, an attempt is made, using the same data, to interp ret the slip characteristics of the paste-wall interface in terms of a quan tity defined as the "reference slip thickness", which is seen to depend upo n the extrudate velocity (or the apparent shear rate) but is independent of the dimensions of the capillary tube. (C) 2001 Elsevier Science Ltd. All r ights reserved.