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Metal Casting Technologies : March 2011
FEATURE STORY Experimental procedure To validate the feasibility of producing castings by SMMC process, a sheet metal mould for gland casting was produced from 0.4 mm thick low carbon steel sheet, as shown in Figure 2. Steel sheet pieces were cut to required size as per the dimensions of Figure 2 and assembled manually to produce the mould of the casting including the gating system. The photograph of the mould is presented in Figure 3. The assembly of the sheet metal mould was placed in a mould box above 2-3" loose unbounded silica sand at the bottom. The box was filled further with same sand gradually up to the top level of pouring cup with as maximum packing as possible. Low carbon steel was melted in an induction furnace and poured into the mould at 16100C. On solidification and cooling, the casting was taken out of loose sand mould and was studied for various dimensions and defects, if any. To study the joining of sheet steel with the inner part of the casting, one piece was taken from it by sectioning. Results and discussion Pouring into sheet metal mould was observed to be safe. It was as smooth as pouring of conventional sand castings. The photograph of the gland casting is shown in Figure 4. The most important observation was that the shape of the gland casting was reproduced perfectly indicating the success of the newly proposed casting process. Dimensions of various parts of the casting was checked as per the drawing (Figure 2) of the gland casting. All the dimensions were found to be matching reasonably with the original dimensions of the drawing. The surface of the casting was mostly free from adhered sand, since loose sand was used in the mould. In case of a binder used in a sand mould, the compounds from binder and additives may react or fuse with the metal or sand. However these possibilities are eliminated when loose sand is used as mould material and the cleaning of casting becomes comparatively easier. The sectioned sample taken out of the casting is shown in Figure 5. A narrow gap was found between the sheet metal mould and the casting indicating lack of fusion of steel sheet over the surface of casting. In spite of high metal temperature (16100C), the non-fusion of sheet with casting can be attributed to the following: ● Quick drop in temperature at the sheet metal -- casting interface, since casting cools rapidly after pouring ● Shrinkage of casting directed towards the centre of sections of casting ● Presence of oxide layer over the inner surface of steel sheet mould or surface of liquid metal during filling Hence necessary steps are to be taken to ensure complete fusion of the sheet metal over the casting to become a part of the latter. Another possibility is that the sheet metal mould not fused with the casting is to be taken out and these pieces are either to be reused for mould production if possible or these are to be recycled. It should be pointed out here that although the sheet metal mould for the experimental casting of this work was prepared manually, in actual production the sheet metal mould is to be divided suitably into a number of pieces and each piece is to be produced in sheet metal working machines using properly designed dies. Assembling these pieces would enable mass production of castings. Advantages and Disadvantages/ Limitations of SMMC process Advantages The SMMC process is similar to LFC Process and it has the following advantages: ● No cores are required, since holes/ recesses in casting are parts of sheet metal mould. Hence castings can be produced without core binder and core making ● No binders are required in the moulding sand. The following advantages result from sand without binder: ● Cost of binder can be saved. ● Easy shake out of casting and easy removal of core sand Figure 2. Drawing of gland casting. All dimensions are in mm Figure 3. Photograph of Sheet Metal Mould for gland casting 24 www.metals.rala.com.au