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Metal Casting Technologies : March 2007
www.metals.rala.com.au 38 TECHNICAL FEATURE Using Fluid Flow Analysis to Improve Casting Quality Q By David C Schmidt, Finite Solutions Inc uality requirements for commercial investment castings have increased in recent years, virtually requiring the use of simulation tools to insure that the casting process will result in castings of appropriate quality. As these requirements change, more attention needs to be paid to the rigging system and its' role in providing hot metal to the casting cavities. The investment casting process is unique in that most gating arrangements also serve to provide risering, or feed metal, to the castings. In sand casting, for example, the gating and risering functions are typically considered separately. The gating system is designed to get the metal into the mold, and the risering system is designed to feed shrinkage in the castings. The tree structure in an investment casting normally combines both functions into a single gating/risering system. One of the byproducts of this combination is that the gating aspects are often overlooked, and the metal is literally dumped into the shell, making many types of filling-related defects more likely. Fluid flow analysis is an easy, visual way to directly see the implications of a given rigging system on overall casting quality. Changes to a system can be quickly evaluated, and directly compared with alternative approaches. Assumptions can be proved or disproved, without the need to create patterns, shells or pour metal. Simple changes to a casting layout can be tested and shown to improve casting quality. Two examples will be shown to demonstrate how these comparisons can be made, and the overall quality differences that can occur. Please note that the screen shots presented in this article cannot truly show the dynamics of the filling process. Windows AVI animation files from which these figures were taken are available by contacting the author. EXAMPLE 1 -- SINGLE VALVE BODY CASTING When a single part is cast, the main variable is the part orientation in the shell. The longer the part is, compared to its' width or thickness, the more desirable it is to cast the part horizontally. This is because the metal will have a shorter drop, with a reduced likelihood of mixing, splashing and air entrainment. An example of this is shown in Figure 1. In this example, the gating system is filled mainly by metal from the casting, which splashes 3 or 4 times on the way down, then flows out of the bottom two gates into the feeder bar! The distance dropped is several times greater than a horizontal orientation, and the temperature gradients developed during filling are opposite to those desired for directional solidification. In order to achieve good feeding, the runner/feeder bar should be hotter than the casting, and solidification should progress from the casting, through the gates and into the feeder bar. But if filling comes through the casting out into the feeder bar, the casting becomes hotter and solidification is delayed. Now contrast this scenario with the model shown in Figure 2. This is the same casting, but the rigging has been designed to fill horizontally. The runner bar is horizontal, bringing metal to two side risers, then into the casting. Using this arrangement, the risers become hotter than the casting, providing the appropriate temperature gradients for good directional solidification. With the shorter vertical drop, there is almost no splashing at all, and mixing and air entrainment are reduced to a minimum. This arrangement produces a sound casting, where the vertical rigging produced a high percentage of scrap. A byproduct of the horizontal arrangement was a higher casting yield, resulting in a better quality casting at a lower cost. Figure 1. Vertical orientation for the valve body casting results in large metal drop, with lots of splashing. Figure 2. Horizontally cast valve body shows quiescent filling, without splashing.