Laminations in ceramic forming – mechanisms revealed by MRI

In the forming of materials by the extrusion of gels and pastes, the mechanical integrity and strength can be severely compromised by the development of defects, termed laminations. These may be formed at the confluence of flow after passage over an internal obstruction, a process examined here using magnetic resonance imaging. The structural weakness is controlled by the development of a void space immediately downstream of a point of confluence, but this void space can be closed in subsequent processing. The increased distance after the obstruction and enhanced velocity of flow both serve to eliminate the void, effects that are logically linked to increased pressure at the point of confluence. This has been established for two gel materials, precursors in the manufacture of ceramics, but the behaviour is likely to be equally relevant to a wide class of gels and pastes relevant in many areas of practice.