The freedom in shape or appearance allowed by the metalcasting process puts a few restraints on creative design of engineered castings. Complex curves and ribs or bosses can be combined with minimal process restriction. It is important to remember, in the process of designing castings, that a constant section shape or thickness is not necessary. In fact, varying sections and tapered wall thickness are likely to simplify casting production. In most castings there are junctions between intersecting components. It is good design practice to incorporate fillets at these junctions to eliminate sharp inside corners and the attendant problems of stress concentration. Almost any shape that is stress loaded in any manner, other than pure tension or simple shear, is amenable to increased design efficiency by constructing it to conform to the stress pattern. Those shapes usually decrease the stress concentration - a factor that must be considered in design for dynamic loading where failure by metal fatigue is of primary concern. The designer should be cognizant also of the casting production problems caused by junctions. All acceptable designs must strive for maximum soundness of the cast part, while also retaining compatibility with other casting requirements. This means that it is incumbent on the design engineer to develop a conceptual framework for cost-effective casting design and to reconcile often diverging viewpoints between himself and the metalcaster. Because, in general, metal in a casting solidifies from the mold surfaces toward the center of a section; thin sections will freeze before heavy sections. Consequently, if a thin section is located immediately adjacent to a heavy section, and the only path of feed metal for the heavy section is through the thin section, then the thin section will freeze first and shut off the source of liquid feed metal for the heavy section. This is a serious problem. Without a source of feed metal to replace the volume lost - because of inevitable shrinkage - the solidified heavy section will be porous. A second problem may be stress-related "hot tears": With the onset of freezing, the thinner section begins to contract, while the heavy section is still solidifying. This will cause stress to develop in the casting, eventually leading to cracks at the weakest section. The part must be scrapped. A hot tear is defined as a fracture formed in a metal during solidification because of hindered contraction. A hot crack forms in a cast metal because of internal stress developed upon cooling following solidification. A hot crack is less open than a hot tear and usually exhibits less oxidation and decarburization along the fracture surface.
|
| Inscribing circles in casting junction and section arms helps designers compare sizes of metal mass. Large differences in mass can cause casting defects. |
Different sections of a casting can be conveniently compared by a method utilizing "inscribed circles." Their area ratio can be considered an indication of relative freezing times. The large sections finish freezing proportionally later than the thinner sections. When junctions are being designed, the ability of a sand mold to absorb heat must be considered. Generally speaking, heat from the liquid metal is transferred into the mold in a direction perpendicular to the interface, or radially if the interface is curved. The directions of heat transfer in two different types of L-junctions are shown schematically in an accompanying line drawing for clarification. At the outer surface of the junction shown there is more mold material available to absorb heat from the molten metal than is available adjacent to the straight portions of the casting that are well away from the corner. Consequently, the metal at this part of the junction will cool at a somewhat faster rate than will the metal in the remote straight sections of the casting. However, the metal at the inside corner of the junction will cool and will be cut off from feed metal if the source is through the straight section of the casting. A shrinkage defect will result, as shown. When the corner is provided with a fillet and uniform wall thickness, the shrinkage cavity is reduced in size. When the fillet radius equals the wall thickness of the junctioning members of the casting, shrinkage voids usually are eliminated, even under the most adverse conditions, because the metal cools uniformly. Practical experience on the shop floor has demonstrated that the stresses developed in castings during solidification and subsequent cooling are more important in determining fillet size than the wall thickness of the casting in the fillet area. This is an important point and should not be overlooked. When fillets are required in an area where the junctions can serve as flow and feed paths for the molten metal, it is advantageous to taper the fillets so that they are largest in the area where the riser will be placed and smallest at the point farthest removed from the riser. This tapering assists in creating a beneficial freezing pattern of the molten metal. Freezing will begin at the areas farthest removed from the riser and will proceed uniformly toward the riser. The junctions, being heavier than the adjacent sections, will freeze after these adjacent sections and thus will serve as sources for feed metal. Because of the tapering of the fillet in the junctions, freezing will progress toward the riser; this assures adequate feeding of the junctions by the riser. Applying this principle judiciously often permits a reduction in casting weight. In designing to prevent shrinkage cavities, one of the most difficult problems - for the foundryman who has to do the rigging - is to know when and where risers should be placed. The designer should be cognizant of this. It is primarily a matter of critical judgement and experience. Moreover, other considerations, such as tooling point locations, may prevent riser placement in the most advantageous location on the pattern. Knowledge, available historical data, and ingenuity must then be combined.