There are ten basic steps in the investment casting process.
1. A metal die is used to produce an expendable pattern, now almost universally a complex blend of resin, filler and wax. Precision internal cavities in the casting are obtained using preformed ceramic or water soluble cores which are located in the wax die prior to injection or in a preformed cavity in a wax pattern.
2. Patterns are mounted onto a runner system to give an assembly ready for subsequent coating with refractory.
3. The wax assembly is then ‘invested’ (coated) with a refractory. The majority of industrial investment casting is based on the ceramic shell process, The wax assembly is dipped into a thin refractory slurry. After draining, fine grains of refractory are deposited onto the damp surface, providing a coating.
4. When the primary coat has hardened or set, cycles of dipping and ‘sanding’ build up the thickness of the invested material, providing a refractory shell which, when fully hardened, is strong enough to hold the liquid metal during casting. Robots are commonly used nowadays for this shelling operation.
5. After the investing process, the wax pattern is removed thermally or chemically. Steam autoclaving is usual.
6. Any residual wax is eliminated by heating the mould to a high temperature, which also completes necessary chemical and physical changes in the refractories.
7. Investment casting melting techniques are similar to standard foundry industry methods but, for many of the advanced nickel superalloys, vacuum melting/casting is essential. Increasingly, vacuum facilities are being installed and used for quality alloys.
8. When the mould is sufficiently cool, it is removed to leave the castings.
9. These can then be taken from the runner system and finished according to customer requirements.
10. Modern investment casting is a precision shaping technique producing metal components cast to precise shape, with accurate dimensions and excellent surface finish.
In essence, it is the archetype of near net shape casting, reducing metal wastage, conserving raw material and minimising the need for (usually costly) post-casting machining.
Benefits of The Investment Casting Process
• Freedom of design
• Choice of alloys
• Sizes from a few mm to over 1000 cubic mm
• Weights from a few grammes to over 250kg
• Consistent dimensions and properties
• Tolerances of ±0.13mm per 25mm, ±0.08mm per 25mm possible
• Minimum wall thickness of 1.5mm, 1mm possible
• Surface finish of 1.5-3.2 microns, 0.8 microns and better possible
• Prototype or mass production quantities possible