Part 2: Additional casting processes
By George Genevro
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 continue to look at the major casting processes that manufacturers use to make aircraft and engines more efficient, safer, and durable.
Permanent mold casting
A common characteristic of the different types of permanent mold casting processes is that the metal mold may be reused many times. The molds are generally made of cast gray iron, ductile iron, or steel, but graphite molds may be used in some cases. Depending on the size, metal molds may be cast or machined from billets and in either case skilled hand finishing is generally required. Any imperfection in the mold will be reproduced in the mirror image in the casting and may make removal of the casting from the mold more difficult. Permanent molds are generally much more expensive than the tooling used for green sand or dry sand foundry processes, and when changes are made in the design of a casting costly rework of the mold is usually required.
In one of the two major subdivisions of the permanent mold casting process generally known as gravity permanent molding, the molten metal is poured into the heated mold through a sprue and gating system. Simple cavities in the casting may be formed by slightly tapered metal cores that can be withdrawn as soon as the casting has started to solidify. More complex cavities may be formed with segmented metal cores or with sand cores which must be replaced prior to making each new casting.
For casting most aluminum alloys the mold must be heated to about 800 F and maintained at that temperature in order for the metal to flow into all parts of the mold and for the castings to be free of porosity and cracks. The pouring temperature of the metal must also be closely controlled and will vary somewhat depending on the alloy being cast.
A release agent, usually a refractory material in an aqueous solution, is sprayed into the heated mold prior to making each casting. The refractory material remains on the mold surface after the water flashes off as steam. It protects the surface of the mold, makes removal of the casting easier, and generally improves the surface finish of the casting.
While the initial investment in permanent molds is quite high, if the cost can be amortized over a fairly large number of castings manufacturers will often choose it over sand casting because of the savings in labor and materials costs. While it is not generally used for very large parts, castings the size of a crankcase half for a typical six-cylinder aircraft engine can readily be made by the gravity permanent mold process.
Pressure die casting
The major difference between pressure die casting and the gravity permanent mold process is that in die casting the molten metal is forced into the mold cavity very rapidly at pressures as high as 30,000 psi. The mold is generally machined from steel and is locked in the closed position by either a hydraulic or mechanical system capable of exerting many tons of pressure to prevent leaks of hot metal past the mold faces. Mold temperature is very important because the cyclic rate of the process is in large part dependent on the time required for the casting to cool to a temperature at which it can be ejected from the mold cavity.
Because of the cost of the molds, heating equipment, the metal injection system, and related machinery, die casting is generally used for products made in fairly large quantities. The die castings found on aircraft engines are usually smaller and simpler items that are not highly stressed, such as rocker arm covers and parts for accessories. The heads, cylinder blocks, and crankcases of two-stroke cycle engines for light aircraft are almost always pressure die castings. Since sand cores cannot be used, cavities are formed by use of steel cores that are inserted and withdrawn hydraulically or mechanically. Complex cavities, particularly re-entrant shapes, are difficult to form without very expensive equipment.
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