| Popis: |
Pastes or gels are used in injection moulding to produce ceramic items. An important aspect is the expansion of the paste or gel from a entry of small cross-section to fill a larger cavity downstream. Flow visualisation during expansion has been performed. Non-uniform flow patterns were found and these can cause fractures, such as voids and laminations. Uniformity was favoured by a larger plate separation, a stiffer paste or gel and a decreasing divergent angle. The pressure drop can be predicted using an established flow model and gives good agreement if the expanding flow pattern is uniform. The addition of a straight channel to the outlet of a divergent die allowed the production of a uniform tape. A method for imaging the gel structure by measurement of the water distribution of gels and fired pellets has been developed using Nuclear Magnetic Resonance. This facilitated an understanding of the formation of lamination in wet gel and its effect on the structure of the final ceramic item. The structural weaknesses caused by lamination have been linked to a lower strength in a ceramic item by use of a mechanical break test. It is shown that a break if it has no further processing in the extrusion flow path can weaken the final ceramic item. Phase migration in pastes and gels has been investigated and two methods of predicting phase migration have been used. A numerical solution gives a fair prediction while a similarity solution predicts moisture ratio as a function of position and time only for an initial time period. Nuclear Magnetic Resonance images gave much greater detail of the liquid distribution than experiments based on splicing, weighing and drying. A parameter based on theory from the similarity solution has been used in a simple test to assess phase migration in paste or gel. For example the parameter can rank the phase migration rates with changing formulation, thereby optimising paste or gel formulation, to control liquid movement. Catalyst surface area and catalyst porosity have been found to be independent of processing conditions and formulation for boehmite gels. |
