Call: 708-425-9080
Using Castings in Vacuum Service
Centrifugally cast stainless steel cylinders prior to machining.
Historically, castings have been avoided in vacuum systems due to their porosity. While aluminum castings are particularly prone to porosity, it is a phenomenon which occurs in all metals. Porosity in castings can come from a number of sources. Oxidized metal entrapped in the casting, gas generated by the mold material or the binder as it is heated and dissolved, and gas coming out of solution as the metal cools and solidifies all contribute to porosity. In principle many, if not all, of these sources of porosity can be eliminated. However, in a real foundry operation it may not be practical to do so.
Advances in casting technology have resulted in the availability of centrifugally cast materials and aluminum sand cast materials now being suitable for vacuum applications.
Meyer Tool has successfully used centrifugally cast stainless steel and aluminum materials in vacuum and cryogenic components requiring helium leak test acceptance rates of less than 1 x 10-9 std-cc/sec. In one situation we successfully worked with our customer to redesign a cylindrical component with wall thickness ranging from .035" to 1" over its ~30" length from a multi-component weldment with four (4) electron beam welds to a single piece, machined from a centrifugal cast cylinder, with a single GTAW welded end cap, reducing cost, lead-time and the risk of leaks.
Advances in casting technology have resulted in the availability of centrifugally cast materials and aluminum sand cast materials now being suitable for vacuum applications.
Meyer Tool has successfully used centrifugally cast stainless steel and aluminum materials in vacuum and cryogenic components requiring helium leak test acceptance rates of less than 1 x 10-9 std-cc/sec. In one situation we successfully worked with our customer to redesign a cylindrical component with wall thickness ranging from .035" to 1" over its ~30" length from a multi-component weldment with four (4) electron beam welds to a single piece, machined from a centrifugal cast cylinder, with a single GTAW welded end cap, reducing cost, lead-time and the risk of leaks.
Machining of sand cast aluminum rectangular vacuum vessel, 50" square x 8" high.
We have also supplied over 40 vacuum chambers leak tested to 1 x 10-8 std-cc/sec; machined from near net shape, sand cast aluminum. The use of these vacuum quality sand cast aluminum vessels is common in certain segments of the semiconductor industry.
Centrifugal casting, while not as widely practiced as other casting methods, is able to produce castings suitable for high vacuum service in both stainless steel and aluminum. To produce a centrifugal casting, the mold is rotated about its axis. The centrifugal force holds the molten metal against the outer wall of the mold, compressing the metal and causing lighter material such as oxide to be deposited on the inner surface of the casting where it can be removed by machining. The high forces acting on the liquid metal result in a dense casting which is free of voids. Intricate details on a mold can be accurately reproduced. Cooling the mold results in solidification from the outside inward so that the effects of shrinkage appear on the inner diameter.
There are obvious limitations to the shapes that can be produced by this method. A centrifugal casting must be close to cylindrical in shape. In order to overcome the effects of gravity and fill a mold uniformly the centrifugal force must be many times larger than the gravitational force. Typically centrifugal casting is performed with accelerations on the order of 70 g. At a radius of 0.1m (4") this requires 790 RPM while at a radius of 1m (39.4") only 250 RPM are required.
The reader is cautioned that not all centrifugal casting suppliers are familiar with the procedures required to produce helium leak tight castings.
Rectangular and similarly shaped aluminum vacuum vessels can be machined from sand castings. Very few foundries specialize in providing these high vacuum compatible castings. Through strict process controls and attention to detail, these foundries are able to consistently supply aluminum castings that are helium leak tight and dimensionally stable. Due to initial tooling costs to develop the molds, this material is best suited for production applications.
The engineering department at Meyer Tool strives to identify manufacturing methods which will provide our customers with the highest possible value while not compromising quality. This is just one of the ways in which we work to Reduce Project Risk to achieve the lowest total cost of ownership.
Centrifugal casting, while not as widely practiced as other casting methods, is able to produce castings suitable for high vacuum service in both stainless steel and aluminum. To produce a centrifugal casting, the mold is rotated about its axis. The centrifugal force holds the molten metal against the outer wall of the mold, compressing the metal and causing lighter material such as oxide to be deposited on the inner surface of the casting where it can be removed by machining. The high forces acting on the liquid metal result in a dense casting which is free of voids. Intricate details on a mold can be accurately reproduced. Cooling the mold results in solidification from the outside inward so that the effects of shrinkage appear on the inner diameter.
There are obvious limitations to the shapes that can be produced by this method. A centrifugal casting must be close to cylindrical in shape. In order to overcome the effects of gravity and fill a mold uniformly the centrifugal force must be many times larger than the gravitational force. Typically centrifugal casting is performed with accelerations on the order of 70 g. At a radius of 0.1m (4") this requires 790 RPM while at a radius of 1m (39.4") only 250 RPM are required.
The reader is cautioned that not all centrifugal casting suppliers are familiar with the procedures required to produce helium leak tight castings.
Rectangular and similarly shaped aluminum vacuum vessels can be machined from sand castings. Very few foundries specialize in providing these high vacuum compatible castings. Through strict process controls and attention to detail, these foundries are able to consistently supply aluminum castings that are helium leak tight and dimensionally stable. Due to initial tooling costs to develop the molds, this material is best suited for production applications.
The engineering department at Meyer Tool strives to identify manufacturing methods which will provide our customers with the highest possible value while not compromising quality. This is just one of the ways in which we work to Reduce Project Risk to achieve the lowest total cost of ownership.
