metalcasting 101 M
Investing in Lost Wax Casting Cast Metals Institute, Schaumburg, Illinois
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nvestment casting, also known as lost wax, is a casting process in which a wax pattern is cov- ered with a ceramic shell mold and melted out prior to the
introduction of metal to the mold. Investment castings are used in a
wide variety of industries, such as aero- space and aircraft, surgical tools, dental and joint implants, golf clubs, sculptures, jewelry and weapons. T e process has been used for thousands of years, but World War II greatly accelerated its advancement when the machine tool industry could not meet the increased demand for precision aircraft and armament components. T ough most investment castings tend to weigh a few pounds, they can be made as small as fractions of an ounce or as large as more than 1,000 lbs. Virtually any alloy can be used in the process. Most investment casting patterns are
made from wax in an aluminum die, but plastic or any other heat-disposable ma- terial can be used. Additives are blended
into the wax to minimize pattern shrink- age and increase strength for handling and storage. More complex patterns can be assembled piece by piece. Complete investment casting pat-
terns are glued to a wax gating system, solution-cleaned and etched in prepara- tion for shell buildup. A primary coat of liquid slurry yields a smooth surface fi nish and must be free of air bubbles. A solid coating of stucco is applied to the tree following the slurry dip. T ese solid refractory granules (special sands) are what give the shell its thickness and strength. After the coating is dried suffi ciently, additional, coarser coatings are applied in the same fashion, until a suffi cient shell thickness is achieved. Solid investment molds also can be made by pouring the slurry mixture into a fl ask and allowing it to cure. T e fi nished molds still containing
wax are placed in a high temperature autoclave or fl ash-fi red to melt out the patterns and gating system. T e shell then is placed into a higher tempera-
ture oven to facilitate a transforma- tion of the ceramic shell, achieving the strength needed before pouring the molten metal. After the metal has solidifi ed, the shell is knocked off the castings by vibratory, high pressure water jets and media-blasting cabinets. Castings are cut off the tree, lightly dressed to remove the gates, and gauged to ensure mold or wax distor- tion has not occurred. T e investment casting process has
some advantages over other casting processes. T e smooth surface of the mold yields excellent casting sur- face fi nish, and because the patterns are not created in two parts, no line is formed where the halves come together. T is eliminates cleaning and fi nishing requirements. For alloys diffi cult to machine, investment cast- ing can create a near-net-shape and closely-toleranced part. T e hot shell also gives the metal the ability to fi ll thin sections and produce sharp detail.
A pattern technician applies the gating system to the wax pattern of an investment casting tree. After the gating and rigging is applied, the wax pattern will be dipped into liquid slurry to build a shell.
48 | METAL CASTING DESIGN & PURCHASING | Jul/Aug 2012
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