Machinery's Handbook, 31st Edition
Design Considerations for Casting 1517 contact. The placement of the parting line and orientation of the part determine the num- ber of cores needed, and it is preferable to avoid using cores when possible. Location and Design of a Gating System.—The gating system must be designed to ac- complish extremely careful transport of the liquid metal to the mold cavity. Gating system location is defined based on the geometry of the mold, method of casting, and economic aspects. The gating system includes the pouring cup, sprue, risers, runners, and gates. Fig. 21 shows a schematic illustration of a typical gating system in gravity casting. Pouring Cup Design: At the top of the sprue, a pouring cup or pouring basin is often used to minimize splash and turbulence as metal flows into the sprue. The design of the pour - ing cup, for an optimal casting process, needs to be such that it can keep the sprue full of molten metal throughout the pour. A constant level of molten metal in the pouring cup is important for a successful casting process; also, if the level of molten metal is maintained in the pouring cup during pouring, then any dross will float and will not enter the mold cavity. Fig. 21. Typical Riser-Gated Casting Sprue Design: The design of the pouring cup and sprue can affect turbulence. For best results, these should be designed so that the sprue is kept full of molten metal throughout pour. A sprue tapered to a smaller size at its bottom will create a choke, which will help keep the sprue full of molten metal. If a non-tapered sprue is used, a choke can be added when making the runners; it is necessary to have the choke as close as possible to the bot- tom of the sprue. A choke also will increase the speed of the molten metal, which is undesirable. To ad- dress this problem, an enlarged area at the bottom of the sprue, called a sprue base, can be added to decrease the speed of the molten metal. There are two basic types of sprue bases, enlargement and well . The general rules of thumb for enlargement bases are: the diameter is roughly 2.5 times the width of the runners; the depth is equal to the depth of the runners. The general rules for well bases are: the depth of a well base is twice that of the runners; the cross-sectional area of the base is 5 times the cross-sectional area of the sprue. The bottom of the sprue base should be flat, not rounded like a bowl, or it will cause turbulence in the metal. Runner Design: One of the most important things to keep in mind when designing runners and gates is to avoid sharp edges. Any changes in direction or cross-sectional area should make use of rounded corners. Also make sure the runners and gates are well rammed and smooth; this helps avoid sand erosion and turbulence. To ensure that metal is not flowing too fast in the runners, the cross-sectional area of the runners should be greater than the area of the choke. And the walls of the runners should be as smooth as possible to avoid causing turbulence. The runners should be filled with metal before the gates; one way to ensure this happens is to put the runners in the drag and the gates in the cope. If a choke is needed in the runner to restrict flow, it should be at least 6 in. (152 mm) from the first gate. The cross-sectional area of runners should decrease as the gates come off them to keep the same gating ratio. For example, a good gating ratio for aluminum is 1:4:4. The 1 is the cross-sectional area of the choke; the first 4 is the total cross-sectional area of the runners (measured after the choke but before the first gate); and the final 4 is total cross-sectional area of the gates. For example, for a tapered sprue with an exit area of 0.5 in 2 (322 mm 2 ) and two runners with
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