Modelling the ram extrusion force of a frictional plastic material: Fid-Bau Portal

Modelling the ram extrusion force of a frictional plastic material

Perrot, Arnaud / Lanos, Christophe / Melinge, Yannick / Estelle, Patrice

The extrusion of cement based material has already been studied but interaction between shaping force and paste behaviour still has to be understood. Also, when the process of such materials starts, a pressure gradient is created in the extruder due to wall friction of the paste undergoing plug flow. This induces a consolidation of the material. As a result, a large increase of extrusion load appears. Here, the authors develop a model to predict the ram extrusion force of cement based pastes. The proposed modelling is based on the frictional plastic behaviour of cement based materials and integrates the physical mechanisms that govern extrusion flow and extrusion force increase. A Coulomb law is used to model cement based materials which is considered as consolidating granular media. Such a modelling is compared with experimental results provided by firm cement pastes extrusion. The proposed model gives a simple way to predict the extrusion force of a frictional plastic material. The modelling is suitable to characterize the extrusion conditions of an extrudible firm cement paste. The modelling enables to understand the mechanism of increasing extrusion force and extrusion blockage. Friction at the extruder wall creates a pressure gradient that induces hardening and consolidation of the paste. As the process goes on, the material arriving in the die land becomes firmer and the friction force increases as the material becomes more and more consolidated. As a result, the extrusion force increases exponentionaly. The crucial role of wall friction force as a dominant part of the total extrusion force is highlighted. The model was confronted with tests on a firm cement pastes. Rheological modelling parameters are measured and are included in the model. The length of the friction zone is evaluated and tends to a critical value with the increase of initial billet length. Additional tests and investigations have to be performed in order to link the friction zone length to the paste behaviour. A hydro-mechanical coupling may be envisaged in order to complete the study of paste consolidation during extrusion process. This work has to be extended to optimize the extrusion process of such materials with a minimisation of the friction force. Tests with lubricated walls or flow coupling with vibration have to be performed in order to reduce wall friction effects. It is finally shown that modelling and experimental results are in good agreement.