Watch The Hitchiner Advantage
A ten-minute video about our countergravity processes.
Developed in the mid-1970s by Hitchiner's Technical Center, the basic countergravity process is known by the acronym "CLA" for Countergravity Low pressure Air melt. The driving force behind the development of this process was economics. However, a solid body of evidence has been generated to show that this process yields parts that are substantially better in quality than corresponding parts using the best ladle pouring procedures. View CLA illustration.
In the CLA process, the mold is placed in a vacuum chamber with an open snout, or fill pipe, facing down. The chamber is sealed and lowered a precise distance into the melt. Vacuum is created, which siphons the metal up into the sprue cavity, filling every section completely.
After a brief hold time has elapsed, allowing the parts and a portion of the gates to solidify, the vacuum is released and the residual metal in the central sprue flows back into the melt. Only a short gating stub remains on the casting to be removed by a mass production gate grinding machine. With CLA 60 - 94 percent of the metal is used to produce product, compared with 15 - 50 percent in gravity poured parts, where much of the cast weight is in the sprue and gating.
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(1.9 MB / 35 sec. / 320 x 240 QuickTime movie) |
Unlike gravity poured parts, which must be cut away from the central sprue, there is no need to leave room for the cut-off blade in the design of a CLA casting cluster. As a result, many more parts can be assembled on a CLA sprue. The increase in pattern population per sprue may be two or three times greater than conventional assemblies, depending on part size and configuration. View sprue loading comparison.
The CLA process provides the ability to cast sections as thin as .015 of an inch; allows the control of grain size; and, since the sprue is filled in a non-turbulent fashion from clean metal beneath the surface of the melt, castings with far less slag and non-metallic inclusions are produced. Typically, countergravity cast metal contains only 15 percent of the inclusions of poured metal of the same analysis. This cleaner metal has been shown to reduce tool wear by 100 to 500% in comparative machining tests done under controlled conditions, and shows very few after-polish defects in highly polished parts, such as golf clubs. View photomicrograph of CLA vs conventional.
Splatter and turbulence caused by the pouring of metal in open air (above) results in reoxidation macroinclusions in the casting. Countergravity casting (below) offers enhanced casting quality through increased control of fluid flow and reduced turbulence. (Still-frame sequence from the Hitchiner Advantage video.)
These claims are backed up by independent research by the Steel Founders Society and by the University of Northern Iowa and the University of Alabama, as reported in a recent University of Alabama Metal Casting Technology Center Newsletter.
The basic CLA process has, in the years since its development, lent itself to many adaptations designed for specific applications. One offshoot, the Countergravity Low pressure Vacuum process (CLV), employed by Hitchiner's Gas Turbine Division, applies counter gravity casting technology to reactive alloys that must be cast in an inert or vacuum atmosphere. View CLV illustration.
Briefly, metal is melted in a vacuum in the lower chamber of the casting machine. The hot mold is introduced in a separate upper chamber and a vacuum is created there. Both chambers are back filled with Argon. A valve is opened and the melt is raised until the snout of the sprue enters the molten metal. Additional vacuum is applied to the upper chamber to draw the metal up. The vacuum is released after the parts and gates have solidified.
Several highly advanced, computer controlled countergravity casting machines have been put into service at Hitchiner, including a multi-station rotary machine utilizing a supported shell version of CLA (SSCLA) and a multi-purpose machine (pictured at the top of this page) incorporating the Check Valve (CV) process. The CV process enables the countergravity casting of large configurations. These new generation casting machines increase production throughput and process efficiency dramatically and achieve previously unattainable levels of casting quality. View a rotary SSCLA machine.
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Take two clear drinking glasses and fill one with water. Pour the water from the full glass into the empty glass and observe the resulting splatter, turbulence and the mixing of air in the liquid. These same dynamics are in effect when molten metal is poured into a mold. Next, siphon water up out of the glass using a pipette or a straw. By controling fluid flow you are able to minimize turbulence. These same principles are the basis of Hitchiner's countergravity casting processes.
(1.3 MB / 30 sec. / 320 x 240 QuickTime movie) |
Available in Portable Document Format (pdf): The University of Alabama Metal Casting Technology Center Spring 1995 newsletter (MCTC.pdf), which reports on the advantages of countergravity casting. To view and print this document you will need an Adobe Acrobat Reader for your DOS, Windows, Macintosh or Unix system. You can obtain this software free of charge from Adobe Systems, Inc. Complete instructions are provided on how to configure your browser to use the Acrobat Reader as a helper application. Additional PDF files are available at the Hitchiner Media Library.
Download:
- MCTC.pdf (University of Alabama Metal Casting Technology Center, spring 1995 newsletter, 16k)
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