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Mechanical behaviour of wet ground clay during the extrusion of ceramic products

The flow of wet ground clay through an extrusion die during the production of ceramic products is simulated in an experimental ram extrusion device and by a FEM numerical model, where the mechanical behaviour of the wet clay is described by the von Mises criterion for the flow stress, the associative flow rule and the rigid-viscoplastic constitutive equation. The application of the Tresca boundary condition for the prediction of the factional stresses between the plastically deforming clay and the rigid die provides good agreement with the experimental results. The values of the material parameters of the constitutive equation and the Tresca friction factor for a certain type of clay are determined by fitting the analytical with the experimental results for the extrusion force at different extrusion speeds. As the frictional stresses account for a major part of the extrusion force, the applied boundary condition for the friction between clay and die is particularly important for the numerical results. The better suitability of the Tresca over the Coulomb friction boundary condition is confirmed by alternative calculations with the use of the Coulomb boundary condition, which provide results inconsistent with the experimental measurements and grossly overestimate the frictional stresses. The developed numerical simulation model is applied to calculate the distributions of clay sliding velocity, equivalent strain rate and the von Mises equivalent stress on the surface of the die.