Squeeze film testing of ceramic pastes

Abstract

Equipment is described in which pastes are squeezed out from the region between two parallel circular discs. The force on the upper disc is measured for different fixed approach speeds of the discs. Two disc diameters are used, in conjunction with two ceramic pastes and one model paste. Different surface finishes are used on the discs and the effects of adding glass fibre and alumina fibre to the pastes are examined. Industrial squeezing flows are found in such processes as filtration, pressing, rolling, and injection moulding. A theory for squeezing flow is given which shows that, under certain conditions, a plot of squeezing force against the reciprocal of gap will be a straight line. The conditions are that the shear stress is constant and that the flow is dominated by shear forces. This is shown to occur in many real cases, although the biaxial extension of the bulk material also affects the load. It is shown that, for ceramic pastes, plate velocity has little effect on the force. For all pastes, rougher surfaces increase, and lubrication reduces, the force required to squeeze the paste. Wall slip is shown to occur in some non-lubricated situations. Fibre addition usually increases the stiffness of the pastes, especially for longer, thinner, fibres. In one case, a paste is found to flow more easily when thick, short fibres are used. Rings of coloured paste are used to show that several deformation profiles are possible: bell shaped, triangular, and near parabolic. Tests are shown to be simple to perform and the shear stresses obtained in this geometry found to be higher than those for flow in dies.