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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 courtesy of Hitchiner Manufacturing Inc..) 
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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 .
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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. You can obtain this software free of charge
from
Adobe Systems, Inc.
Download:
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