The need for improved investment castings at Marshall Space Flight Center's Rapid Prototyping Lab has led to the development of a simple machine for centrifugal investment casting of shells. This machine spins a radially symmetric shell about its own axis in a vertical orientation. This rotational process increases the strength of the investment casting by allowing the metal to move into even the most obscure crevices of the shell.
The major components of the vertical centrifugal investment-casting machine are a funnel, a turntable with a clamping mechanism, an electric motor, exterior shells and supports, a base plate, two semicylindrical ceramic heaters with insulation, a heat-conducting interior drip shield, and a heater-control device with feedback from a thermocouple. Although not part of the machine, a digital stroboscope and an accelerometer with a temperature-compensation and signal-amplification circuit are used during the vertical centrifugal investment-casting process.
The process of investment casting is not new. However, risers of novel designs were included in the vertical centrifugal investment-casting machine to accommodate dynamics of fluids in a rotational environment. For example, risers used in static casting are usually made funnel-shaped to assist foundry workers during metal-pouring processes. But a funnel-shaped riser increases the tendency for accelerated fluid to escape from the opening of the rotating shell. Therefore, for experiments conducted in the design of this machine, inverted funnel-shaped risers were attached to each shell. These inverted funnel-shaped risers provided back pressure to the spinning molten metal. The widest conical portions of the funnel-shaped risers also served as reservoirs for molten metal that escaped from a shell-molding cavity during centrifugal investment casting.
The process of dipping a wax pattern in a ceramic slurry and then applying a sand coat creates a shell with slight imbalances. When such a shell is rotated at high speed, these small imbalances can lead to significant vibrations and, consequently, a poor casting. The vibrations can also be hazardous.
To restore the center of gravity to the axis of rotation and thereby correct for these imbalances, a counterweight is put on the turntable. Holes around the perimeter of the turntable enable for the attachment of counterweights. For each shell being cast, it is necessary to test for vibrations, using a hand-held stroboscope, in order to determine the exact masses and locations of counterweights and the optimum rate of rotation.
Once the shell is properly balanced, the ceramic heaters and exterior shells are secured to the base plate. The shell is preheated, using a thermocouple to provide a temperature reading. Next, the heaters are turned off and the thermocouple removed. Rotation is begun immediately. Molten metal is poured into the funnel once full speed is reached. Within only a few seconds, the metal has been poured into the shell-molding cavity. After cooling, the machine is dismantled and the shell is extracted.
This work was done by Gary Lai and Floyd Roberts of Marshall Space Flight Center. For further information, access the Technical Support Package (TSP) Please contact the NTTC (National Technology Transfer Center) at 1-800-678-6882 or visit their website www2.nttc.edu/leads.
Inquiries concerning rights for the commercial use of this invention should be addressed to
the Patent Counsel
Marshall Space Flight Center; (205) 544-0021.
Refer to MFS-31181.