Part 1: Wax and sand casting processes
By George Genevro
Aeronautical engineers face an imposing array of problems in designing aircraft that are safe, perform well, and can be built and sold at a competitive price. After the desired aerodynamic elements of the design have been established and the major type of airframe construction - aluminum, tube and fabric, composite, or some combination - has been chosen, the manufacturing and production engineers must make many choices. They must choose from a variety of processes that may include among others, welding, forming, riveting, forging, and casting. For certain parts with unusual or complicated shapes it is often more economically and technically feasible to cast the item and finish it by appropriate heat treating and machining rather than forging or fabricating it.
Casting is a process that dates from man's earliest work with metals. According to some historians, it was first used about 3000 BC in the area that is now Iraq and possibly was also known and used in China and India at about the same time.
Almost 100 years ago this ancient process helped mankind enter the era of powered, heavier-than-air flight when it was used to make the complex cast-aluminum crankcase and cylinder block of the engine that Charles Taylor built for the Wright Brothers. So, let's look at the major casting processes that manufacturers use to make aircraft and engines more efficient, safer, and durable.
The lost wax process
The earliest precision casting method, used primarily by silversmiths and goldsmiths, was the cire perdue, or lost wax process. It is still used extensively to make products such as dental appliances, jewelry, gas turbine blades, and cylinder heads, among others. This process is of intense interest to those in the aircraft and aerospace industries because of its versatility and adaptability to a wide variety of metals.
In its most elementary form, lost wax casting consists of making an exact wax model of the part to be cast and attaching sprues and gates, the passages through which the molten metal will flow into the mold. This assembly, either singly or in groups, is then placed in a container, or flask, and a plaster and water slurry is poured in and allowed to set up. The flask is then placed inverted in a furnace to start the burn-out process, first at relatively low temperatures to melt out the wax and then to slowly remove the water from the plaster without causing it to crack. Next, at temperatures well above 1,000 F, the wax residue is burned out and the mold is dried completely. Thorough drying of the mold is important because any moisture left in the plaster will turn to steam with explosive force and destroy the mold when hot metal is poured into it.
The molten metal is then introduced into the hot mold by one of two methods. Simply pouring it into the mold - the static process - works well with castings where the mold cavity is not very complex and the molten metal is quite fluid. The centrifugal process, widely used for unusually shaped, small, and complex parts, requires a machine that carries the hot plaster mold at the end of an arm with the sprue opening facing inward. An amount of hot metal slightly in excess of what is needed to fill the mold cavity is poured into a basin just below the sprue opening and the machine is started. As rotation begins, the rapid acceleration centrifuges the metal into the mold cavity with considerable force, thus completely filling it. The plaster mold is allowed to cool and is carefully broken to retrieve the casting, which is then cleaned and finished as needed.
Highly heat-resistant alloys may be cast by a variation of the lost wax process developed by those who manufacture jet and rocket engine parts and cutters for machine tools. Individual or grouped wax patterns with gates and sprues attached are repeatedly dipped in a highly heat-resistant plaster slurry and then coated with zircon sand. This produces a very durable shell around the wax pattern. The wax is then melted out and the burnout process heats the shell to temperatures of 2,000 F. The alloys cast by use of this method are usually poured at temperatures that may exceed 2,600 F.
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.
For castings such as cylinder heads for air-cooled engines, both the mold and the cores are usually made of baked or chemically bonded sand. These are generally referred to as "dry sand" molds and while they are more expensive, thinner sections can be cast and closer dimensional tolerances can be maintained. For the spaces between the cylinder head fins, thin cured sand segments are joined to form the mold, and cores are used for the intake and exhaust passages, spark plug holes, and other cavities.
A variation of the dry sand process sometimes used for making aircraft castings is shell molding, which requires heated aluminum tooling for forming both the cores and the mold segments. The sand used in shell molds and cores is coated with a resin that is partially melted when it comes in contact with the hot aluminum pattern or core box, causing the sand grains to adhere to each other and form the shell. The core or mold that is formed has a generally uniform wall thickness that follows the contour of the tooling and has a smooth surface. The sprue and gating system are incorporated in the tooling, along with alignment bosses and corresponding cavities on each half of the mold. When the cores are pasted in place and the mold is closed it is ready to pour. The heat of the metal degrades the binder, allowing the casting to shrink as it cools and making core removal easier. This is the most accurate of the sand molding processes and is used by both OEM and aftermarket makers of air-cooled engine cylinder heads.
George Genevro, a retired college professor at Cal State Univ. at Long Beach, CA, is an A&P mechanic, pilot, and aircraft owner.