Machinery's Handbook, 31st Edition
1490 Fluid Flow Whether the casting process takes place by expendable-mold casting (such as sand cast- ing) or nonexpendable-mold casting (using a permanent metal mold), the basic terminol- ogy of the casting process is the same. Fluid Flow Fluid flow is very important in casting. The molten metal is poured through a pour - ing basin. It then flows through the gating system (comprising sprue, risers, runners, and gates) to fill the mold cavity. (See the gravity casting system shown in Fig. 2.) The sprue is the vertical part of the gating system that connects the pouring cup or pour - ing basin and runners; liquid metal enters the mold through the sprue. The cope is the top half of the mold. The runners, which are cut in the drag (the lower part of the mold), form the horizontal portion of the gating system that connects the sprue to the gates. The gate is the portion of the runners through which molten metal enters the mold cavity; there may be one or more gates. The core is designed to leave an unfilled space in the part. Risers are the part of the gating system used to identify when enough liquid material has been poured to fill the mold; it also acts as a reservoir of extra material to compensate for shrinkage during solidification. The flask is the molding box or outside of the mold. Fig. 2. Cross Section of Typical Two-Part Sand Mold Gating Design: Successful casting requires proper design and control of the solidifi - cation process to ensure adequate fluid flow in the system. For example, an important function of the gating system in sand casting is to trap contaminants (oxides and other inclusions) in the molten metal by having such contaminants adhere to the walls of the gat- ing system, thereby preventing them from reaching the mold cavity. A properly designed gating system also avoids or minimizes problems such as premature cooling, turbulence, and gas entrapment. Two basic principles of fluid flow are relevant to gating design: Ber - noulli’s theorem and the law of mass continuity. Bernoulli’s theorem states that the sum of the energies in a flowing liquid is constant at any two points. This can be written as: (1) where h = elevation above a certain preference plane (datum); p = pressure at that eleva- tion; v = velocity of the liquid at that elevation; ρ = density of the fluid; and g = gravitation constant. This concept of conservation of energy requires that (between two different elevations) the following relationship be satisfied as: (2) where f = frictional losses in the liquid as it travels downward through the system. The law of mass continuity states that for incompressible liquids and in a system with impermeable walls, the volume rate of flow remains constant throughout the liquid. So, considering two different locations in the system: g 2 + + = + + + ρ ρ g g g 2 h p g + + = ρ 2 2 g v constant h p v h p v f 1 1 1 2 2 2 2 2
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