Abstract
In Europe, clay roof tiles are manufactured by pressing in 'open' moulds whereas in South East Asia 'closed' moulds are commonly used. The European products are more complex, having a greater degree of detail. Closed mould pressing could, however, be advantageous as it would minimise scrap recycling but the products would need to have equivalent or superior durability to existing tiles. The aim of this work was to investigate the feasibility of using a closed mould for the manufacture of European tiles by examining the relationship between the type of pressing and subsequent durability, in terms of resistance to repetitive freeze-thaw cycles. One specific clay type, the Marseille rose blend, was investigated over a range of forming water contents. Preliminary data relating to plasticity and friction were obtained through an empirical (Pfefferkorn) plasticity test, uniaxial compression of cylinders and friction ring experiments. Clay cylinders containing 16-21 wt % water were deformed at a compression rate of 240 mm/min. The yield stress was found to increase with decreasing moisture content. The plastic ranges of the stress-strain curves were well represented by a plastic deformation equation of uniaxial compression under sticking friction conditions. A laboratory-scale pressing rig was designed to make profiled specimens which would reproduce the essential features of a European tile. Comparison of the microstructures of the laboratory and factory samples showed that there was sufficient resemblance to validate the replication of the Marseille products in the laboratory tests. Bats of three moisture contents of 16.4, 18.4 and 20.6 wt % and different geometries were pressed in open, partially closed and closed rubber lined resin moulds in a mechanical testing machine using a cross-head speed of 240 mm/min. This was lower than a typical pressing operation but the speed of pressing had been found to not be a significant variable over the range commonly encountered. Clay was trapped in the features as the mould closed and flowed mainly within the flat part of the samples during pressing. For the open and partially closed moulds excess material was extruded through the gaps at the sides, a process referred to as flashing. The shape of the load-displacement curves was characteristic of the stages of the pressing process. The stress-strain curves for the pressing in open moulds showed good qualitative agreement with the results from uniaxial compression of cylinders. The open porosity of the samples after firing was strongly related to the forming moisture content of the clay with the open porosity increasing with increasing water content. Comparison of extruded bats and pressed bars showed that the influence of the pressing processes was not significant. Likewise, in freeze-thaw testing, the effect of the moisture content was again the overriding parameter, with acceptable durability always being obtained from the lowest open porosity samples. Given the marginal differences, the pressing processes investigated in this study were assumed to be equivalent in terms of the quality of the samples produced. Thus, closed mould pressing is feasible but does not lead to product improvement and hence may not be economically viable.