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Understanding the Shell Making of Investment Casting Process

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Understanding the Shell Making of Investment Casting Process

Investment casting is a sophisticated casting process which produces intricate shapes while still maintaining high dimensional accuracy and surface quality. Investment casting can be used to cast almost any metal, satisfying application requirements in diverse end-markets.

This metal forming process generally involves using a wax pattern surrounded by a ceramic shell to form a ceramic mold. Once the ceramic shell is dry, the wax is melted out, and the ceramic mold is formed. Molten metal is then poured into the ceramic mold to form a casting component.

The ceramic mold is subsequently removed, and the casting components are created. Some investment castings require a secondary machining process after casting. This article looks at the ceramic shell making part of the process.

How is the Shell Made?

Typically, the process of making the shell starts with a liquid wax mixture that is injected into a die. After cooling, the wax is removed from the die, and will serve as a pattern for the shells. Several wax patterns are made, which are then attached to a sprue pole, forming a sort of tree.

The tree with the wax patterns is dipped in a ceramic slurry mixture (composed of fine grain silica, water, and some form of binding agent), and then coated with sand or stucco. As layers dry, the dipping and coating steps are repeated until the shell has attained the thickness needed for the casting size and configuration.

Upon completion, some type of heating process is used to remove the wax from the mold. One common heating procedure is the use of an autoclave, or a steam heating device. The temperature for dewaxing has to be sufficient to melt the wax, and preheat the mold for casting.

The shell is then fired in an oven to strengthen it and remove any remaining wax residue. The fired shell, with a cavity in the shape of the desired part, is strong enough to be able to withstand the rigors of the casting process. It is placed in a bed of sand, and the molten material is poured into the shell.

The shell is removed and the part is ready for finishing. Methods used to remove the shell can include a hammer, vibration table, high pressure water blasts, chemical solvents, or a knockout machine. The shell is held in position to ensure uniformity of the removal process.

Inconsistencies in shell thickness can wreak havoc and cause casting defects, but fortunately these processes are now accomplished by highly-programmable robotic machines. They are extremely reliable, and eliminate coating and stability errors which can sometimes be associated with manual operations. The robotic equipment is capable of performing 360-degree automatic coating, stuccoing, and suspension line hanging with minimal human intervention, and can operate 24 hours a day, seven days a week.

Benefits of Shell Molds

The silica sand used for shell molding is very fine. Instead of using a clay binder, the sand is given a thin coating of phenolic resin which is what melts to form the shell. The major benefit of this finer sand is that it conforms more closely to fine surface features in the pattern, and provides a smoother surface for the metal. Other benefits include:

  • Works for ferrous and non-ferrous metals
  • Reproduces fine surface detail
  • Can maintain tight tolerances
  • Leaves a smooth surface finish

Learn More About Shell Making for Investment Casting

Careful shell making is an essential part of the investment casting process. The strength of the shell has to be sufficient to endure all of the casting procedures, yet malleable enough to allow the molten material to take on the desired end shape. The multiple processes of dipping and stucco coating are necessary to ensure the shell’s stability and durability for casting.

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