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
