Centrifugal Casting 101
The county fair and metalcasting have little in common. But somewhere beyond the midway in nearly every traveling amusement park, there’s a demonstration of the principles of true centrifugal casting happening aboard the Gravitron, a ride in which a spinning room propels riders up its walls, seemingly defying gravity.
When riders enter the Gravitron, they are the molten metal. When the ride begins to spin on a vertical axis, the laws of physics at work are similar to those operating in a centrifugal casting machine. (In centrifugal casting, the metal can be fed into an already-spinning machine. This is not advisable on the Gravitron.) Riders are forced outward and upward against the walls of the ride with centrifugal force just as molten metal would fill the often intricately designed sidewalls of the centrifugal casting mold.
Luckily, the fairgoers won’t be stuck in that position. When the ride stops spinning, they will be lowered back to a standing—though perhaps nauseated—position. By the time the ride stops in a casting facility, the molten metal will be solidified against the walls of the mold, producing a hollow metal component.
And while the Gravitron unsettles the stomach, the process of true centrifugal casting produces the opposite effect, settling the insides of the casting as it solidifies and leaving it with mechanical properties that cannot be achieved through other processes. The perceived downside is that the process occupies a niche for only cylindrical parts, but in reality it serves many end uses.
Several important comparisons should be made between the Gravitron, a saucer-like contraption with padded walls, and a true centrifugal casting machine, essentially a casting mold atop a spinning table.
First, the spinning axis of a true centrifugal casting machine can be oriented vertically or horizontally. The latter would produce lawsuits if used at the fair. Also, just as the amusement park ride forces riders up its walls, a true centrifugal casting machine pushes metal outward under forces many times that of gravity, where it hits the mold wall and spreads relatively evenly over the surface. Horizontal machines achieve good uniformity throughout the casting. When the axis of symmetry is vertical, though, the spread is not entirely even.
“In the vertical [true centrifugal casting] process, there is the effect of gravity, so it’s a parabola that’s thicker at the bottom,” said Jack Lilley, VP of engineering and quality assurance for Wisconsin Centrifugal, Waukesha, Wis. “You must take that into account when you design the part.”
The settling of the metal in the centrifugal casting process occurs on two levels. The first is in physical terms. It seems obvious, but when considering the way a casting forms, remember that matter in the liquid state solidifies only when it loses its retained heat. And it loses heat through contact with objects cooler than itself. As molten metal strikes the surface of a spinning mold in the centrifugal casting process, it transfers heat into that surface more quickly than into the air on the opposite side and cools and solidifies only on the mold face. In effect, a true centrifugal casting starts as a thin layer as its outside diameter and builds layer by layer toward its hollow center.
This eliminates the possibility of air and gas being trapped in the casting. It also eliminates shrinkage defects, which can occur in all other metalcasting processes because molten material contracts as it solidifies. When this occurs in centrifugal casting, more molten material moves in directly behind it to fill the void.
The true centrifugal casting process also limits inclusion defects. Because impurities are generally lighter than metalcasting alloys, they will be forced to the inside diameter of the casting, where they can be machined away. Fewer defects means fewer scrapped castings for the metalcaster and metal casting buyers.
The second way a true centrifugal casting is settled is on a chemical, or microstructural, level. Building a metal component in the centrifugal method is like building a house; you start with the foundation and go up. Of course, the foundation of a house is built not with irregularly shaped stones but various identically shaped blocks. The grain structure of a centrifugal component has the same traits. According to Nathan Janco, author of Centrifugal Casting, a reference book on the subject, the process “obtains perfect directional solidification from the outside inward, and grain growth is typically columnar.”
So if you’re in the market for a hollow component with heightened material properties and low scrap rates, centrifugal casting might be the most cost effective casting process for you, even if you’re not a pipe maker.
More Than Just Pipe
For most people in the casting industry, true centrifugal casting is associated with the mass production of gray and ductile iron pipe. And for people just learning about the process, a tubular component makes the most sense.
However, some components other than perfectly round cylinders can be suited for the true centrifugal casting process—particularly when used in the vertical attitude. Nearly every metal that can be cast can be used in the centrifugal method, and geometric features such as flanges and tapers can be designed in, but the finished casting must be no more than three times longer than its diameter, which can reach more than 10 feet. Geometrically intricate castings, such as combustor cases, furnace fittings, hub bodies, bearing cages, flow meter bodies and decorative pieces, all can be produced in the process.
“Anything that’s hollow and has an axis of symmetry for rotation is a candidate for the centrifugal process,” Lilley said. He added that having an axis of symmetry does not mean the casting mold has to be perfectly symmetrical. “We have the capability to do a wide range of geometry beyond cylindrical by varying the die material or mold material. The die can vary from permanent mold steel to graphite. You go from cylindrical in steel, to more complex in graphite, to sand, to investment [ceramic shell], which is the most complex on the [outside diameter].”
As centrifugal parts stray from perfectly symmetrical, they can become a safety risk because of the out-of-balance rotating masses changing throughout the process. This must be considered when assessing the feasibility of a casting design. The high pressures exerted on the mold or die face are also exerted on the covers of vertical dies and holders and the end covers of horizontal dies. Die covers and fastening systems are engineered to withstand these high forces and safely contain the molten metal until it has completely solidified.
“We do have the capability to design parts with some asymmetry and extra features, but the whole design process, including die design and safety factors should be considered,” Lilley said. “It’s worth it. That’s the sweet spot; that’s where the process has a huge advantage. It creates a hollow part using rotation and centrifugal force to place the metal within the die or mold cavity without the use of cores.” Metal