Analysis of the spring-back and water effect on the coring point during direct extrusion

The present work proposes to come up with theoretical analyses and equations describing the extrusion pressures for direct ceramic paste extrusion through a circular die considering the coring point. For example, in a ceramic paste, the coring point is evidenced by a significant rise in the extrusion pressure beyond the steady state. This increase in the extrusion pressure may be linked to a change in the paste behavior during the unsteady state. Furthermore, regarding the feedstock properties’ variations, it may be accounted to water loss, densification of the clay paste, and shift of the friction coefficient. In practice, when the coring point is forecast during extrusion, die damages, lubrication problems, product defects, and a raise in the production cost can be minimized and even avoided. This work proposes two theoretical analyses and equations considering the effect of the coring point on the extrusion pressure. For instance, one analysis considers the effect of the spring-back phenomenon on the pressure, while the second considers the influence of water loss (water migration) during extrusion. The two proposed approaches demonstrated a satisfactory correlation with an experimental curve. In addition, the water loss approach seemed to be more conservative when compared to the spring-back phenomenon. In conclusion, the theoretical analyses seemed to be very useful to aid the construction of new equations. Thus, it was possible to fill a gap in the direct extrusion of ceramic, where publications about the unsteady state regime and equations which acknowledge it are very rare.