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Metal Casting Technologies : March 2007
47 gating system can also prevent the formation of oxides after the stream passes through the filter. The nature of ceramic foam filters is such that turbulent flow before the filter is changed to a lamellar flow pattern thus reducing the possibility of reoxidation of the metal stream. APPLICATION EXAMPLE Filters are typically inserted into the gating system of the casting such as shown in the following example of a steel valve body. The number and size of filters required is determined by the weight of metal required to pass through the filters and the alloy under consideration. The deoxidation practice utilised will also have an effect on the capacity of the filters. Figure 3 shows filters inserted into the drag half of a mould. Figures 4 and 5 show the drag and cope sides of the filtered valve casting, note the absence of inclusions. CONCLUSION The use of ceramic foam filters can dramatically improve the aesthetics and structural quality of castings. It is important to ensure the flow of metal after the filter is non-turbulent by avoiding or minimising free fall and high stream velocities as these actions can undo some of the benefits of filtration. It should also be noted that filters are primarily intended to remove micro inclusions from liquid steel and not to provide a license for poor practice. The performance of filters will be greatly impaired if the metal is dirty as they will quickly block and stem the flow of metal. Filters can only be successfully used in combination with good melting, pouring, moulding and gating practice. ● METAL Casting Technologies March 2007 Fig. 2. Steel Valve Body showing filters either side of the downsprue. Fig. 3. Filters inserted into mould. Fig. 4. Drag side of filtered Valve Body casting. Fig. 5. Cope side of filtered Valve Body casting.