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One of the shortest routes from raw material to finished part is a casting process. In casting, a molten metal is poured into a mold or cavity that approximates the shape of the finished part (Fig. 13.25). Heat is extracted through the mold (in this case a sand mold), and the molten metal solidifies into the final solid shape. The chief design issues for the mold are (1) to provide an entry for the molten metal into the mold that creates laminar flow through the sprue and runner, (2) to provide a source of molten metal, suitably located in the mold so that it stays molten until all of the casting has been filled, and (3) that cores are suitably placed to provide hollow features for the part.
This seemingly simple process can be quite complex metallurgically, since the metal undergoes a complete transition from the super-heated molten state to the solid state. Liquid metal shrinks on solidification. Thus, the casting and mold must be designed so that a supply of molten metal is available to compensate for the shrinkage. The supply is furnished by introducing feeder heads (risers) that supply molten metal but must be removed from the final casting (Fig. 13.25). Allowance for shrinkage and thermal contraction after the metal has solidified must also be provided in the design. Since the solubility of dissolved gases in the liquid decreases suddenly as the metal solidifies, castings are subject to the formation of gas bubbles and porosity.
The mechanical properties of a casting are determined during solidification and subsequent heat treatment. The grain structure of the casting, and thus its properties, are determined by how fast each part of the casting freezes. This cooling rate is roughly proportional to the ratio of the square of the surface area of the casting to the square of its volume. Thus, bulky castings freeze much more slowly than thin section castings and have lower properties. A sphere of a given volume will freeze more slowly than a thin plate of the same volume because the plate has much more surface area to transfer heat into the mold.
The casting must be designed so that the flow of molten metal is not impeded by solidified metal before the entire mold cavity fills with molten metal. The casting should freeze progressively, with the region farthest from the source of molten metal freezing first so that the risers can supply liquid metal to feed shrinkage that occurs during solidification. Designing the needed solidification pattern can be achieved with finite element modeling to construct temperature distributions as a function of time.[/size][/b]
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study note of FDM : DESIGN OF CASTINGS