(78b) Modelling the Ram Extrusion Force of a Frictional Plastic Material

Extrusion is a current forming process for a wide range of materials such as polymers, metals, foods and ceramics pastes. For those materials, the extrusion stress and the flow typology are well known and have been the subject of numerous studies. The extrusion process is not a common way to form cement-based materials in an industrial context. This is explained by the heterogeneity of such material and the difficulty to formulate the right composition that enables to flow. We developed a model to predict the ram extrusion force of frictional plastic materials such as cement based pastes. The extrusion of cement based material has already been studied but interaction between shaping force and paste behaviour still have to be highlighted. Our model is based on the plastic frictional behaviour of cement based materials and integrates the physical mechanisms that govern material extrusion flow and extrusion force increasing. When extrusion process begins, a pressure gradient is created in the extruder due to wall frictions of the paste which is submitted to plug flow. It induces a consolidation of the material. As a result, a large increasing of extrusion force appears. This consolidation phenomena is studied and localized during the ram extrusion of cement pastes. This is done with a simple experimental technique that enables the local hardness evaluation in the billet paste. Our experimental modelling takes into account the evolution of the billet paste flowing in the extruder which becomes heterogeneous with the ram advance. A Coulomb law is used to model the frictional behaviour of the cement based materials which is considered as consolidating granular media. Such model results are compared with experiments performed with extrudible adjuvanted cement pastes. It is finally shown that our model and experiments are in good agreement.