Machinery's Handbook, 31st Edition
CASTING OF METALS 1489 Permanent metal molds are commonly made of steel or cast iron, with metal or sand cores, though it is desirable and generally more economical to use permanent steel cores to form cavities. When the casting has re-entrant surfaces or cavities from which one- piece permanent metal cores cannot be withdrawn, destructive cores made of sand, shell, plaster, or other materials may be used. This process is called semipermanent mold cast - ing . Sectional steel cores also may be used in some instances. An advantage of permanent metal molds is that they heat up and expand during the pour, so the cavity does not need to be expanded as much as in sand castings. Therefore, the cavity, with the gating system included, can be machined into halves that produce more precise parts with closer dimensional tolerances and smoother surfaces. Permanent mold castings also usually have better mechanical properties than sand castings, as solidifica - tion is more rapid and fill is more laminar. Permanent mold casting is used mostly for aluminum, copper alloys, magnesium, and gray iron, because of their generally lower melting points. Typical parts include automo- bile pistons, cylinder heads, gears, and kitchenware. Parts that can be made economically generally weigh less than 55 lb (25 kg), though special castings weighing a few hundred kilograms have been made using this process. It may not be economical for small, unique production runs (due to the cost of permanent molds) and generally is not suitable for cast- ing intricate shapes (because of difficulty of removing the part from the mold). Disadvantages of different types of castings include poor finish; wide tolerance (sand casting); limited workpiece size (shell molds and ceramic molds); patterns with low strength (expendable-pattern casting); expensive, limited shapes (centrifugal casting); porosity (all types); and environmental problems (all types). Heating and Pouring the Metal Pouring is the process by which molten metal is transferred to the mold for cooling and solidification to be converted into the intended shape. Pouring temperature ( T p ) is the temperature to which the molten metal must be heated before being poured into molds for cooling and setting. The heat energy required for heating metal to a pouring temperature is the sum of 1) the heat needed to raise the temperature of a unit mass solid to its melting point; 2) the heat of fusion required to convert it from a solid at its melting point to a liquid without an increase in temperature (see Fig. 1); and 3) the heat needed to raise the molten metal to the desired temperature for pouring. The pouring temperature must take into account heat lost in the transfer of metal through ladles, due both to the heat absorbed by the ladles and to the distance between the furnace and the mold. Molten metal must be poured carefully to avoid casting defects. For example, too rig- orous a stream could cause mold erosion; highly turbulent flows could result in air and inclusion entrapments; and relatively slow filling might generate cold shuts. Thus, design of the gating and venting overflow systems must take into consideration proper control of the liquid metal as it fills the mold.
Fig. 1. Phase Change Diagram of Process of Heating Metal Casting to Pouring Temperature.
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