The chief consideration is that the shape of the casting should allow for orderly solidification by which the solidification front progresses from the remotest parts toward the points where molten metal is fed in. Whenever possible, section thickness should be uniform. Large masses of metal lead to hot spots, where freezing is delayed, and a shrinkage cavity is produced when the surrounding metal freezes first. Figure 13.27 illustrates some design features that can eliminate the shrinkage cavity problem. A transition between two sections of different thicknesses should be made gradually (a). As a rule of thumb, the difference in thickness of adjoining sections should not exceed 2 to 1. Wedge-shaped changes in wall thickness should not have a taper exceeding 1 to 4. The thickness of a boss or pad (b) should be less than the thickness of the section the boss adjoins, and the transition should be gradual. The local heavy section caused by omitting the outer radius at a corner (c) should be eliminated. The radius for good shrinkage control should be from one-half to one-third of the section thickness. A strong hot spot is produced when two ribs cross each other (d).These areas solidify after the thinner sections surrounding the junction so that the shrinkage cannot be fed with liquid metal, resulting in a shrinkage cavity. This problem can be eliminated by offsetting the ribs as shown in (d). A good way to evaluate where hot spots brought about by a large mass of molten metal, occur is to inscribe a circle in the cross section of the part. The larger the diameter of the circle, the greater the thermal mass effect, and the more the concern with shrinkage cavity formation. Castings must be designed to ensure that the pattern can be removed from the mold and the casting from a permanent mold. A draft , or taper, of from 6 to 3 degrees is required on vertical surfaces so the pattern can be removed from the mold. Projecting details or undercuts should be avoided, as these require extra cores. Molds made with extensive use of cores cost more money, so castings should be designed to minimize the use of cores. Also, provisions must be made for placing cores in the mold cavity and holding them in place when the metal flows into the mold.
Solidification stresses can occur when different sections of the casting solidify at different times and rates. If this happens while the alloy is cooling through the temperature range where both liquid and solids coexist, it can result in internal fracture called hot tearing. Uneven cooling as the temperature continues to drop can result in severe distortion or warping of the casting.
Some casting processes like die casting, permanent mold casting and investment casting produce parts with excellent dimensional accuracy and smooth surfaces. Parts made with these processes are net shape parts that require no machining before using. Sand-cast parts always require machining after casting in order to attain the required dimensions and surface finish. Therefore, it is necessary to provide extra material in the casting as a machining allowance.
study note of FDM : Guidelines for the Design of Castings