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 fired at high temperatures to strengthen the bonds between the refractory materials in the shell. So we now have a hollow ceramic shall in the shape of a sprue with multiple patterns attatched (as shown in part 3 of this illustration).
With the mould still in a heated state, the molten metal is poured in the top. In some cases we can use pressure of vacuum assisted pouring of the metal (for aluminum and some nickel alloy castings), but in most cases a tilt or rollover method is used to prevent any air pockets or defects in the casting.
Once cooled the outer shell is removed (see part 4 of this illustration) by one of the following methods: Impact, vibration, grit blasting, high pressure water blasting or it is dissolved by chemicals. This just leaves the parts to be cut from the sprue and any final processing to be done.
Investment casting is a relatively expensive method in comparison to others such as die-casting or sand casting, but its great versatility means it is still used today since intricate or re-entrant contours can be incorporated and give great freedom in design. Also machinery costs are lower than those for the aforementioned processes, and most casts made in this way are produced in a near complete state so very little machining and processing is needed for the final piece unlike many other methods.
High production rates for small parts
High dimensional accuracy
Extremely good surface finish
Complex shapes are possible
Machining can be completely eliminated
Almost any metal alloy can be cast in this way
Relatively environmentally friendly
Specialist equipment is needed
Expensive due to refractive materials and multiple processes involved
Due to the possible high quality finish many parts are scrapped due to defects