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.
4B.4 References
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.
