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Investment Casting
The process is suitable for series production of high quality castings particularly of aluminum, steel and special high performance alloys. The process is generally used for small castings, often less than 500gm, although steel castings up to 300kg and aluminum castings of 30kg are being produced. Compared to other casting processes such as sand or die casting, investment casting is an expensive process, but it has versatility approached by few other metals forming processes. Intricate or re-entrant contours can be incorporated so that great freedom of design is possible. In many cases components are cast near to net shape so that little or no machining is necessary.
Brief description
Patterns of the castings to be made are molded by injection of a special wax into a metal die. Cores of pre-formed ceramic may be incorporated into the wax patterns as they are molded. The patterns are assembled into a cluster (often comprising tens or even hundreds of patterns) around a wax runner system. The 'tree' of patterns is then coated with 8-10 layers of a refractory material, each layer being dried or chemically cured before the next layer is applied.
The assembly is heated to remove the wax, then fired at high temperature to bond the refractory mold strongly. The hot mold is cast and when cool the mold material is removed by impact, vibration, grit blasting, high pressure water blasting or chemical dissolution leaving the castings, which are then removed from the runner system.
Various post-casting operations may be carried out to meet customer requirements.
Special characteristics
- High
production rates, particularly for small components
- High
dimensional accuracy and consistency
- High
integrity castings
- Extremely
good surface finish obtained
- Complex
shapes can be cast
- Long or
short runs can be accommodated
- Machining
can be eliminated
- Minimum
shot blast and grinding needed
- Almost
any alloy can be cast
- Environmentally
good
- Specialized
equipment needed
- New
technology must be learned and great attention to detail is
required
- The
process is expensive because costly refractories and binders
are used and many operations are needed to make a mold
- Because of the exceptional surface finish possible and expected, minute defects can cause rejection of castings and scrap rates can be high.
Principles of the investment casting process
The investment casting process is distinguished by the use of an expendable pattern. A metal die is normally used to produce the pattern, now almost universally of wax. The injection dies are made from materials such as duralumin or brass.
Pre-formed ceramic or water-soluble cores are used to give precision internal cavities in the casting and these cores are located in the wax die prior to injection or into a pre-formed cavity in a wax pattern. Patterns are mounted onto a runner system to give an assembly ready for subsequent coating with refractory.
The covering of the wax assembly with a refractory is known as investing - hence the modern name of the process. Mold production originally used the block mould process in which the wax pattern assembly is firstly coated with a refractory material, typically zircon, and allowed to dry. Around this is poured a ceramic slurry based on a low refractory 'grog' and a binder of sodium silicate or hydrolysed ethyl silicate.
The refractory mass, contained in a steel can or tube, is allowed to set (typically for 48 hrs) and is then de-waxed using hot air. On removal from the container, the block is slowly fired to about 950ºC prior to casting.
The majority of industrial investment casting is based on the ceramic shell process, introduced in the 1950's where the wax assembly is dipped into a thin refractory slurry and after draining, fine grains of refractory are deposited onto the damp surface, providing a primary refractory coating. The primary coat typically contains a zircon based refractory while the binders used are either alcohol-based (ethyl silicate) or water based (silica sol) or a hybrid of these.
When the primary coat has hardened or set, subsequent cycles of 'wet' dipping and dry 'stuccoing' build up the thickness of the invested material to provide a refractory shell that, when fully hardened, is sufficiently strong to hold the liquid metal during casting. At the end of the investing process, the wax pattern material is removed by thermal means, steam autoclaving being usual. The mold is then heated to a high temperature to eliminate any residual wax and to induce chemical and physical changes in the refractories that will ensure maximum strength and stability combined with minimal reaction between the mold surface and the metal to be poured into it.
The majority of investment casting foundries has air-melting facilities and cast a wide range of ferrous and non-ferrous materials. Steel casting furnaces tend to be of the rollover or tilt induction-melting type with solid state power units. For many of the more advanced nickel super alloys, vacuum melting/casting is essential, promoting the installation of vacuum facilities and their growing use for quality alloys in other materials. In the case of aluminum investment castings, a wide range of melting and pouring methods is used. Melting may be by gas or electricity, while various methods of pouring the molten metal are in use (e.g. Gravity pouring, bottom pouring, vacuum or pressure assisted).
When the mold has cooled sufficiently, the mould material is removed by impact, vibration, grit-blasting, high pressure water blasting or chemical dissolution, to leave the castings which are then removed from the runner system. Various post-casting operations (e.g. Heat treatment, surface treatment or coating, HIPping) may be carried out, to meet customer requirements.