Shrinkage-based die design for drying ceramic parts

Abstract

Drying of ceramic parts with complex geometry has been studied with emphasis on the effects of shrinkage on their final dimensions. The first step was to understand the processes governing moisture loss from a porous medium through experimental measurements yielding the coefficient of moisture expansion. As a result, non-homogeneous dimensional changes (shrinkage) occur in three-dimensional artefacts with varying cross-section. The moisture diffusion problem governed by Fick’s laws was solved numerically by analogy to the heat conduction problem. To this end the correspondence was established between physical parameters, variables and boundary conditions of heat flow and diffusion, characteristically the coefficient of moisture expansion being analogous to the coefficient of thermal expansion. The numerical model established and solved using finite element analysis predicted moisture distribution inside the part as well as the resulting change in its shape. Validation of numerical predictions was first ensured in two dimensions by modelling a simple slab and comparing with experimental measurements. Validation for a fully three-dimensional shape required use of the well-established iterative closest point algorithm for surface point matching and subsequently the creation of an error map. The reverse of this numerical model was used to predict the appropriate die geometry starting with the shape of the desired part by taking into account the variable drying shrinkage allowance, and the relevant steps are outlined.