EXTRUSION OF TIO2 CERAMIC POWDER PASTE

The extrusion of ceramics often requires the wetting of the powder wit h a binder in order to produce a paste. To control and optimise the pr oduction it is interesting to model behaviour of the paste under exter nal conditions that are closed to the actual extrusion conditions. The aim of this paper is to investigate the mechanical behaviour of a mix ture made of a ceramic powder (TiO2) and an aqueous acidic binder. The viscous behaviour is investigated using: (i) A capillary rheometry wh ich enables, for homogeneous materials, the determination of the shear viscosity as a function of the shear strain rate; (ii) a new rheomete r called 'cross apparatus', developed at LMT Cachan. The experimental results evidenced two specificities of this material: (i) Wall conditi ons play a major role during the how through a capillary and it can be assumed that the paste neither sticks to the wall nor that it slips w ith a viscous friction law; (ii) the cross apparatus confirms these re sults: since the paste behaves in a very different way than does a vis cous non-Newtonian fluid. There could be many reasons as to why these results are obtained, but probably the most important is some dissocia tion taking place in the material, indicating the inappropriateness of a homogeneous model. A biphasic interpretation of the flow has been d eveloped in a similar way to that for injection molding, which enable an explanation of the particular results of the capillary test. The or iginality of his paper lies in experimental results that are both char acteristic of pastes and different to those that are obtained with 'cl assical' non-Newtonian fluids such as homogeneous molten polymers. (C) 1997 Elsevier Science S.A.