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 Video)
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.
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.
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.
The basic CLA process has, in the years since its development, lent itself to many adaptations designed for specific applications. A few of these processes are described below:
CLV: the Countergravity Low pressure Vacuum process 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. 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.
CLI: The Countergravity Low-pressure Inert gas process blankets the melt with an inert gas in order to cast reactive alloys quickly and efficiently.
C3: The C-cubed process uses centrifugal force to aid in mold fillout in thin sections. (View Video)
SSCLA: A version of CLA which supports the mold with sand. Hitchiner has deployed a high-volume, fully automated multi-station rotary SSCLA casting machine at its main production facility
SLIC (several layer investment casting): The latest fuel-efficient, clean Hitchiner countergravity casting process. SLIC reduces shell material usage by about 70%, floor space requirements by up to 70%, and mold-heating energy by 87.5% while eliminating the need for large burnout furnaces. The water-based process achieves these goals by sigificantly reducing the number of shell layers needed to make molds.