In comparison to other processes, lost wax casting is expensive since it is labor-intensive and the molds cannot be reused. The accuracy and versatility of this process makes it very valuable to manufacturers of aircraft gas turbine parts as well as some makers of aftermarket air-cooled piston engine cylinders.
Sand casting processes
Sand casting is also a very old process that can generally be divided into two major segments - the green sand and the dry sand procedures. In many cases, such as when cores are used to produce a cavity in a casting, elements of each method are used in the same mold. Green sand casting, in which the molding medium is one of several types of specially compounded sands bonded with a small amount of clay and water, is not as precise as some other processes. Nevertheless, it is widely used because of its versatility, molding material can be reused many times after reconditioning, and the relatively low initial cost of tooling. Changes in tooling, which basically consists of patterns, core boxes, and special flasks in some cases, are generally more easily made than in tooling for other casting processes.
Over the years, sand casting has evolved into a process that can be used for almost all metals, including alloy steels. It should be noted that some moving parts which are quite often cast of malleable iron or one of the various forms of ductile iron in automotive engines, such as crankshafts, connecting rods, and camshafts, are practically always made of alloy steel forgings or bar stock in aircraft engines.
After a complex casting such as an aircraft engine crankcase has been designed, the next major step is making a master pattern, usually from wood or a combination of wood and metal. The advent of computer-assisted drafting and its application to processes whereby a prototype pattern is formed of activated layers of some type of resin is eliminating the wood pattern in some applications. The wooden pattern is generally made about 1.5 percent larger to compensate for the shrinkage of the aluminum casting as it cools. This is referred to as single shrinkage and it applies if the pattern is used as prototype tooling for making experimental rather than production castings. In addition to the shrinkage allowance, machining allowances are added wherever surfaces are to be machined.
Production patterns for green sand molding are usually made of aluminum or a wear-resistant epoxy plastic. When the design of a complex casting has been finalized, a master pattern is made with about a 3 percent shrinkage allowance added and this is used to produce the aluminum production patterns. The design of the gating system through which the molten metal flows into the mold is a mixture of art, science, experience, and intuition and will have a major effect on the soundness of the castings.
In addition to being as light as possible the finished castings must be dimensionally and geometrically accurate and strong enough to withstand structural loads and vibration. Air-cooled engine castings must also withstand repeated heating-cooling cycles that are relatively severe as well as unusual thermal stresses generated by improper warm-up and overcooling during rapid descents. These factors, along with relatively low volume production, account in large part for the high cost of aircraft engine castings.
Practically all complex castings have cavities that are formed by cores. In green sand molding operations, a core is usually made of clean silica sand and a binder that is activated by heat, carbon dioxide gas, or some other chemical. Cores are produced by placing sand that has been coated with the binder in a core box, where the binder is activated by use of carbon dioxide gas or some other means when making "no-bake" cores. In the case of baked cores, the binder is activated by heat after the core has been transferred from the core box onto a baking plate.
Cores are placed in the drag, which is the lower half of the mold, and rest in indentations called core prints which were formed by bosses on the pattern. The cope, the upper half of the mold, is then placed in position on the drag and the two parts of the mold are accurately aligned by pins, thus forming the mold cavity into which the molten metal will flow. As the molten metal flows into the mold cavity and around the core or cores, the binder retains its strength until the mold is filled and initial solidification has taken place. It then degrades and the cores can be removed when the mold is opened and the casting is cleaned prior to inspection and heat treatment.
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