by Adam Feneley

Created on: October 29, 2009

Metal Casting Processes: What is Investment Casting?

Investment casting is probably the oldest known metal casting method known to man, predicted to have been used since around 4000 - 3000 BC in its very basic form. Also known as the lost wax process, it is a fairly versatile process and can be used to produce steel parts of 300 kilograms and above. Although its more commonly used for small parts of around 500 grams, as well as some aluminum castings of around 30 kilograms. The process is well suited to series production of high quality castings usually of aluminum, steel and various high performance metal alloys.

The process works by a creation of a wax pattern, to which we add molten metal and subsequently the wax is dissolved leaving the metal to take the shape the wax previously held. First of all an initial wax pattern is produced; this is often done simply by pouring molten wax into a mould (made of a low melting point metal, steel or even wood.) of the desired shape, this is the 'master die from which all wax patterns are produced. Modern methods use a wax runner system or 'tree' to connect multiple wax patterns and hence produce batches of identical parts (see the first part of this illustration). The tree consists of a sprue (if any one did airfix kits or warhammer models plastic parts usually came from a sprue for you to trim down, this is just a larger model of the same principle.) which consists of a runner and gating
system which is all made of wax and connects the multiple max moulds.

This is when processes can differ between manufacturer. Mould production originally used the block mould process. This meant this wax tree was firstly coated in a refractory material (usually zircon) and then allowed to dry before being dipped into the ceramic slurry (see part 2 of this illustration). But since the 50's most manufactures now use the ceramic shell process. This is where the wax is dipped into a thin refractory slurry and drained to provide a thin coating of what is usually zircon mixed with water or alcohol based binders. This dipping process is known as investing hence the modern name of the process.

This complete wax assembly complete with coating is then dipped into a thick ceramic slurry which is allowed to coat the whole mould and is then left to dry in open air, usually at room temperature (see part 2 of this illustration). The whole assembly is then inverted and heated to allow the wax to melt and exit the ceramic shell; the assembly is then

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