Advanced Automation for Space Missions/Appendix 4B
Appendix 4B Review Of Casting Processes
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 each mold (or the materials used to make it) need not be thrown away after just a single use.
(c) Shell Molding
Shell molding is also similar to sand molding except that a mixture of sand and 3-6% resin holds the grains together. Set-up and production of shell mold patterns takes weeks, after which an output of 5-50 pieces/hr-mold is attainable. Aluminum and magnesium products average about 13.5 kg as a normal limit, but it is possible to cast items in the 45-90 kg range. Shell mold walling varies from 3-10 mm thick, depending on the forming time of the resin.
There are a dozen different stages in shell mold processing that include: (1) initially preparing a metal-matched plate; (2) mixing resin and sand; (3) heating pattern, usually to between 505-550 K, (4) investing the pattern (the sand is at one end of a box and the pattern at the other, and the box is inverted for a time determined by the desired thickness of the mill); (5) curing shell and baking it; (6) removing investment; (7) inserting cores; (8) repeating for other half; (9) assembling mold; (10) pouring mold; (11) removing casting; and (12) cleaning and trimming. The sand-resin mix can be recycled by burning off the resin at high temperatures, so the only SMF input using this technique is a small amount of replacement sand and imported resin.
(d) Investment Casting
Investment casting (lost-wax process) yields a finely detailed and accurate product. After a variable lead time, usually weeks, 1-1000 pieces/hr-mold can be produced in the mass range 2.3-2.7 kg. Items up to 45 kg and as light as 30 g are possible for unit production.
To make a casting, a temporary pattern is formed by coating a master mold with plastic or mercury. The pattern is dipped in refractory material (typically a ceramic mixture of Zircon flour and colloidal silicate) leaving a heavier coating 3-16 mm thick. The process requires a constant input of Zircon flour because the mold is expendable, although mercury is recycled by processing in a pressurized positive-gravity environment. The mold is baked and mercury or plastic collected and recycled. The old is filled, then broken away after hardening.
Investment casting yields exceedingly fine quality products made of all types of metals. It has special applications in fabricating very high-temperature metals, especially those which cannot be cast in metal or plaster molds and those which are difficult to machine or work.
4B.2 Nonexpendable Mold Casting
Nonexpendable mold casting differs from expendable processes in that the mold need not be reformed after each production cycle. This technique includes at least four different methods: permanent, die, centrifugal, and continuous casting. Compared with expendable mold processes, nonexpendable casting requires relatively few material inputs from Earth in the context of an orbital SMF.
(a) Permanent Casting
Permanent casting requires a set-up time on the order of weeks, after which production rates of 5-50 pieces/hr-mold are achieved with an upper mass limit of 9 kg per iron alloy item (cf., up to 135 kg for many nonferrous metal parts) and a lower limit of about 0.1 kg. Hot molds are coated with refractory wash or acetylene soot before processing to allow easy removal of the workpiece. Generally, gravity is unnecessary since forced-input feeding is possible. Permanent molds have a life of 3000 castings after which they require redressing. Permanently cast metals generally show 20% increase in tensile strength and 30% increase in elongation as compared to the products of sand casting.
The only necessary terrestrial input is the coating applied before each casting. Typically, permanent mold casting is used in forming iron-, aluminum-, magnesium-, and copper-based alloys. The process is highly automated and state-of-the-art easily could be adapted for use in an extraterrestrial manufacturing facility. The main disadvantage is that the mold is not easy to design or produce automatically. More research is needed on robot production of delicate molds.
(b) Die Casting
In die casting fluid is injected into a mold at high pressures. Set-up time for dies is 1-2 months, after which production rates of 20-200 pieces/hr-mold are normally obtained. Maximum mass limits for magnesium, zinc, and aluminum parts are roughly 4.5 kg, 18 kg, and 45 kg, respectively; the lower limit in all cases is about 30 g. Die injection machines are generally large (up to 3 X 8 m) and operate at high pressures - 1000 kg/cm2 and higher, although aluminum usually is processed at lower pressure. A well-designed unit produces over 500,000 castings during the production lifetime of a single mold. The major production step is die construction, usually a steel alloy requiring a great deal of skill and fine tooling to prepare. Only non-ferrous materials are die cast, such as aluminum-, zinc-, and copper-based alloys.
The only serious difficulty in applying die casting to an SMF is unit cooling. In terrestrial factories, die machines are water- or air-cooled, both difficult in space. There is little water in the system since flash is removed and remelted, but care must be taken to prevent cold welding of parts or dies in a vacuum manufacturing environment. Die casting is readily automated (Miller and Smith, 1979). Present technology already permits semi-automation, but more research is required on machine design and automatic die mold preparation for space applications.
(c) Centrifugal Casting
Centrifugal casting is both gravity- and pressure-independent since it creates its own force feed using a temporary sand mold held in a spinning chamber at up to 90 g. Lead time varies with the application. Semi- and true-centrifugal processing permit 30-50 pieces/hr-mold to be produced, with a practical limit for batch processing of approximately 9000 kg total mass with a typical per-item limit of 2.3-4.5 kg. A significant advantage of the centrifugal force method is that no external gravity is required, making it ideal for space applications. Sand is easily recycled, so centrifugal processing depends only to a small degree on terrestrial resupply. There is no limit to the types of metals that can be fabricated.
Automation can be utilized in centrifugal casting. The only requirement is the advent of spin-functional robots, research of which should lead to the broader synergistic advancement of other processes normally dependent on gravity to function properly, such as investment casting.
(d) Continuous Casting
Continuous casting, much like centrifugal molding, produces sheets or beams which may undergo further fabrication. Continuous casting was discussed briefly by an MIT study group in the context of SMF design (Miller and Smith, 1979), and involves forcing a melted metal through an open-ended mold. Heat is extracted and metal exits the mold as a solid fabricated sheet. The MIT study suggested that SMF molds, as those on Earth, might be made of graphite. Unfortunately, carbon is rare in space.
Gravity plays no irreplaceable role in continuous casting on Earth - gravity feeds are used, but manufacturing facility casting machines can rely on pressure to feed liquid metal. Molds or "dies" last several weeks, after which graphite must be reworked to original specifications. Metal melting points impose severe restrictions on mold design. Consequently, iron is difficult while aluminum and its alloys are relatively easy to process. The technique already is well-automated and is used to fabricate aluminum and copper alloys, but only on very special applications for iron.
4B.3 Casting in Space Manufacturing
Casting has its limitations in space. Gravity is a major problem but can be overcome with development of centrifugal systems which work in concert with other systems. The cold-welding effect is also of major concern. To overcome this, it is suggested that fabrication should take place within a closed atmospheric unit.
Lunar basalt molds possibly may replace iron molds. But basalt has a low coefficient of thermal conduction and more research is needed to ensure feasibility of the concept. Lunar basalt should provide adequate molds for aluminum alloys as the former melts at 1753 K (1480°C) and the latter around 873 K (600°C).
These problems are hardly intractable. In the long term, the issues of fully autonomous production, refurbishing of patterns and molds, automatic process control systems, and the application of robotics and other advanced automation techniques to casting technology, must all be addressed.
Lindberg, Boy A.: Processes and Materials of Manufacture. Second Ed. Allyn and Bacon, Boston, 1977.
Miller, R. H.; and Smith, D. B. S.: Extraterrestrial Processing and Manufacturing of Large Space Systems, NASA CR-161293, 1979.
Yankee, H. W.: Manufacturing Processes. Prentice-Hall, Engiewood Cliffs, New Jersey, 1979.