Page:Advanced Automation for Space Missions.djvu/166



Casting is a process by which a fluid melt is introduced into a mold, allowed to cool in the shape of the form, and then ejected to make a fabricated part or casing (Lindberg, 1977; Yankee, 1979). Four main elements are required in the process of casting: pattern, mold, cores, and the part. Pattern, the original template from which the mold is prepared, creates a corresponding cavity in the casting material. Cores are used to produce tunnels or holes in the finished mold, and the part is the final output of the process.

Substitution is always a factor in deciding whether other techniques should be used instead of casting. Alternatives include parts that can be stamped out on a punch press or deep-drawn, items that can be manufactured by extrusion or by cold-bending, and parts that can be made from highly active metals.

The casting process is subdivided into two distinct subgroups: (1) expendable and (2) nonexpendable mold casting.

4B.1 Expendable Mold Casting

Expendable mold casting is a generic classification that includes sand, plastic, shell, and investment (lost-wax technique) moldings. All of these involve the use of temporary and nonreusable molds, and need gravity to help force molten fluid into casting cavities - either by artificial gravity or pressure-feeding of molds in a zero-g SMF. Lack of atmosphere should be beneficial to some processes since molten fluids need not displace air.

(a) Sand Casting

Sand casting requires a lead time of days for production at high output rates (1-20 pieces/hr-mold), and is unsurpassed for large-part production. Green (wet) sand has almost no part weight limit, whereas dry sand (more likely with extraterrestrial materials) has a practical part mass limit of 2300-2700 kg. Minimum part weight ranges from 0.075-0.1 kg. Sand in most operations can be recycled many times and requires little additional input. The only serious restriction is the necessity for gravity-feeding the molten liquid. A general manufacturing facility using sand casting might require centrifugal force feeding instead.

Preparation of the sand mold is fast and requires a pattern which can "stamp" out the casting template in a few days. Typically, sand casting is used for processing low-temperature steel and aluminum, magnesium, and nickel alloys. It is by far the oldest and best understood of all techniques. Consequently, automation may easily be adapted to the production process, somewhat less easily to the design and preparation of forms. These forms must satisfy exacting standards as they are the heart of the sand casting process - creating the most obvious necessity for human control.

(b) Plaster Casting

Plaster casting is similar to sand molding except that plaster is substituted for sand. Plaster compound is actually comprised of 70-80% gypsum and 20-30% strengthener and water. Generally, the form takes less than a week to prepare, after which a production rate of 1-10 units/hr-mold is achieved with a capability to pour items as massive as 45 kg and as small as 30 g with very high surface resolution and fine tolerances.

The plaster process requires carbon, a relatively rare substance in nonterrestrial materials, for the gypsum binder. Once used and cracked away, normal plaster cannot easily be recast. The water used in mold production may be recycled during the baking process. Plaster casting is normally used for nonferrous metals such as aluminum-, zinc-, or copper-based alloys. It cannot be used to cast ferrous material because sulfur in gypsum slowly reacts with iron. Also, the plaster process requires gravity or centrifugal injection of casting fluid into the mold. (Prior to mold preparation the pattern is sprayed with a thin film of parting compound to prevent the mold from sticking to the pattern. The unit is shaken so plaster fills the small cavities around the pattern. The form is removed after the plaster sets.)

Plaster casting represents a step up in sophistication and required skill. The automatic functions easily are handed over to robots, yet the higher-precision pattern designs required demand even higher levels of direct human assistance. Another research issue with particular relevance to an extraterrestrial facility is plaster recyclability, so that