Evaluation of slip in capillary extrusion of ceramic pastes

In this study, the capillary extrusion behaviour of an alumina paste material, made using submicron alumina powder, hydroxy ethyl cellulose (HEC), a polyelectrolyte dispersant, poly (methacrylate), and water is described. It confirms that the computed shear stress, obtained using estabilished analytical procedures, depends upon both the diameter and the length of the capillary tube. Additionally, for a given nominal shear rate, the shear stress so calculated is less for the smaller diameter capillaries than for the corresponding larger diameter capillaries. These trends are rationalised, along established lines, in terms of the preponderance of a material process slip at the wall of the capillary as opposed to cohesive or fully developed uniform flow. Moreover, for a given diameter of the capillary tubes the calculated shear stress is greater for the smaller length capillary tubes than the corresponding longer capillary tubes. It is concluded that these results are not amenable to meaningful analysis using classical capillary rheometery analytical procedures based upon the presumption of a fully developed flow. 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 interpret the slip characteristics of the paste-wall interface in terms of a quantity defined as the “reference slip thickness”, which is seen to depend upon the extrudate velocity (or the apparent shear rate) but is independent of the dimensions of the capillary tube.