The Investment Casting Process Beneath a brief
explanation about the investment casting process.
This article is meant for those who are not familiar with
'the lost wax casting". |
| Modern investment casting is the equivalent of the ancient "lost wax" casting. By definition, casting is one of the oldest of industries. This industry ended the Stone Age and began the Bronze Age during the third millennium before Christ in our western World. Here ended man's virtually exclusive vocations of "hunter-gatherer" and early agrarian. Here began the metal ages and the infancy of industry. |
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Wax Pattern Tools The casting process begins with a customer's (you) part requirement. A design engineer (you) then develops a set of design criteria for the part. These criteria or specifications are sent to a foundry and its tool maker (you) to design and build a tool that will produce a wax pattern of the part. The tool is usually made from a block of aluminum. A cavity is machined into the block and an injecting port from the surface to the cavity is added. |
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Wax Pattern Injection Wax is injected into the tool either as a paste or as a liquid and allowed to cool slightly. The wax is removed and sent to an assembly area. |
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Wax Assembly The wax patterns are "melted" (welded) onto a runner system. This system serves several purposes. One, it has to be structurally strong enough to withstand the rest of the manufacturing process. Two, the runner system serves as a conduit for molten metal to fill the part. Finally, the system should provide for the metal's fluid flow with the greatest filling speed and the least turbulence possible. |
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Investing and Stuccoing These wax patterns and runners, often called clusters or trees, are cleaned and dipped into a ceramic slurry (investing). This slurry is a mixture of water, silica or zirconia (sand) and a clay like binder such as dentists' plaster. Thus wetted the clusters are stuccoed with silica sand or zircon sand. As many as 9 and as few as 6 coats will be put on the clusters with appropriate drying time between coats. |
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Dewaxing The stuccoed cluster, now weighing perhaps twice what the bare wax weighed, is put into an autoclave and heated to 180 degrees F. @ 300 psi. This may seem like overkill but wax like the metals expand about 5% volumetrically near its melting point. The 300 psi of the autoclave keeps the wax from expanding and cracking the shell while the wax is melting and trying to run out. |
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Firing The empty shells are fired in an oven at 1800 degrees F. for about 1 hour. This serves two purposes. One, the dry clay and sand mixture of the shell turns into a ceramic capable of withstanding high temperatures. Two, this firing gets rid of small amounts of wax that are left in nooks and crannies. |
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Casting The cast metal is melted on an induction hearth or in a furnace and poured or ladled into a hot ceramic shell. Typically for aluminum, the shell temperature is about 600 degrees F. and the metal is 1100 degrees F. For steel, the shell is about 1800 degrees F. and the metal is 2900 degrees F. |
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Knockout Upon cooling, the remaining ceramic is removed by a variety of methods. Physically removing it with a hammer and chisel, water blaster, caustic soda, sand blaster and others. |
