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Metal Casting Technologies : December 2007
BacktoBASICS "hot-spot" and subsequent porosity at the casting contact. The exact width to thickness ratio should be determined by the solidification time of the casting, as a rule of thumb, the thickness of ingates should be less than one-third the thickness of the casting at the point of contact. Fig.5. FILTRATION Filters have been in common use in copper-base foundries for many years. They take many forms, from simple strainers and woven cloth, to ceramic blocks of various types. Ceramic block filters come in two main types: pressed or extruded, which have straight parallel openings; and ceramic foam, which consists of a skeleton of ceramic material of foam structure having no preferred pore direction. Fig. 7. The benefits associated with ceramic block filtration of copper base alloys are well documented; they are very effective in removing dross and inclusions and have a positive effect on mechanical properties. Ceramic foam type filters have a distinct advantage over the pressed or extruded variety in that there is no separation of the initial metal stream as it passes through them. This reduces the possibility of reoxidation at the filter exit. The capacity of ceramic foam filters, or amount of metal which will pass before blockage, varies according to the dross-forming tendency of the alloy being cast. Capacities can be less than 2kg/cm2 of effective filter area for an aluminium bronze to more than 20kg/cm2 for phosphor bronze. Filter capacity is also influenced by the effectiveness of the gating system upstream of the filter, i.e. if the metal reaching the filter is heavily drossed, the filter capacity may be dramatically reduced. Filters are not a licence for sloppy practice, for effective results, sound gating practice needs to be used in conjunction with them. CONCLUSION The copper base casting alloys cover the full spectrum of gating sensitivities, many of which are very unforgiving of less than ideal gating design. It is only by having a thorough understanding of the thermal and fluid characteristics of the alloy in question can the copper- base foundryman hope to obtain consistently successful results. Figure 7. A selection of ceramic foam filters. Figure 6. The completed gating design Reference: 1. J. Campbell: Castings. Butterworth- Heinemann Ltd., 1991 2. R. W. Ruddle: The Solidification of Castings. Institute of Metals. 1957 METAL Casting Technologies December 2007 60 Figure 5. The gates