Machinery's Handbook, 31st Edition
Design Considerations for Casting 1515 the alloy into the mold quickly to avoid oxidization and dross formation, while avoiding turbulence, wall erosion, and core displacement that can occur at higher fluid speeds. Although not altogether preventable in the manufacturing process, turbulence can be re- duced by using a gating system that promotes a more laminar flow of the liquid metal. Turbulence caused by sharp corners and abrupt changes in sections within the casting may be mitigated by employing radii (Fig. 18). Fig. 18. Design Modification to Avoid Abrupt Changes in Sections within Metal Casting: a) Incorrect; b) and c) Correct The designer also must be aware that fluid flow can introduce unacceptable thermal gradients, particularly if metal flows around a core and rejoins elsewhere in the mold. Designing the casting geometry and running system at the same time can help identify and eradicate these issues. Heat Transfer Considerations: Management of heat within a mold is an important part of the geometry design. At high pouring temperatures, considerable heat must be trans- ferred into the mold in a way that avoids creation of localized hot spots, where the heat cannot efficiently dissipate. This can occur on narrow peninsulas or tight corners, where molten metal surrounds thin areas of the mold. The retained heat slows solidification in that area that can lead to hot tears or pulls—particularly if the geometry design has intro- duced stress buildup during solid shrinkage, as the softer metal in the hot spot will have lower tensile strength. As mentioned earlier, directional solidification is important to the manufacture of a part during the metal casting process, in order to ensure that no area of the casting is cut off from the flow of liquid material before it solidifies. To achieve directional solidification within the casting, it is important to control the flow of molten metal and the solidification rate of the different areas of the metal casting. Regulation of thermal gradients is key. Sometimes there is an area of a metal casting that needs to solidify at a faster rate to ensure proper directional solidification. Planning sections effectively and regulating flow rates within the mold may not be sufficient. To accelerate solidification of a particular sec - tion, it may be necessary to employ the use of chills, which act as heat sinks, increasing the cooling rate in the vicinity where they are placed. Chills are solid geometric shapes of material, manufactured for this purpose. Chills are of two basic types. Internal chills are located inside the mold cavity and usually are made of the same material as the casting. When the metal solidifies, internal chills are fused into the metal casting itself. External chills are located just outside the casting. They are made of a material that can remove heat from the metal casting faster than the surrounding mold mate- rial. Possible materials for external chills include iron, copper, and graphite. Fig. 19 demon- strates the use of the two types of chills to solve a hot-spot problem in cross and T junctions. Designers should remember that thermal inequalities also can occur in the opposite direction if a sharp corner is surrounded by mold material. In this case, rather than a hot spot forming, the area will cool rapidly, which can cause cold cracking and other issues.
Fig. 19. Two Types of Chills: a) Cross Junction with Internal Chill; b) “T” Junction with External Chill
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