What is Investment casting?
In investment casting, a wax or suitable polymer pattern is coated by dipping into the refractory material slurry. Once the refractory material coating is hardened then this dipping process is repeated several times to increase the coating thickness and its strength. Once the final coating is hardened the wax is melted out and molten metal is poured into the cavity created by the wax pattern. Once the metal solidifies within the mold, metal casting is removed by breaking the refractory mold.
As shown in figure 1, Investment casting process can be summarized in following steps – Master pattern making, Master die manufacture, Pattern making, Pattern assembly, refractory mold making, dewaxing, casting, and part removal.
Investment casting known as the lost wax casting is a precision casting process as it is capable of making high accuracy and intricately detailed castings compared to similar manufacturing processes such as sand casting.
Typical parts include turbine rotors, gears, electronics enclosures, valves, jewelry and dental fixtures. Parts up to 1.5 m in diameter and weighing up to 1140 kg have been cast successfully in the past by this process, although typical part would be on average less than 100 mm in size and weighs less than 500g.
Figure 2 shows a gas turbine rotor casting at different stages. (Wax pattern, ceramic shell coating around the wax pattern, molten metal fill stage & final investment casting)
History of Investment casting
Although Investment casting has been used in various forms since it was first used by ancient Egyptians between 4000 and 3000 B.C, it was only after the World War II its industrial importance and popularity grew due to the growing demand for parts in aircraft engine and air frame sector.
Investment casting has changed from the days where clay was packed around the bee’s wax pattern to use of ceramics shell and the introduction of special dewaxing ovens etc.
The high manufacturing cost of master dies has traditionally limited investment casting to large production quantities, but the introduction of additive manufacturing or 3D printing of wax patterns in recent years has eliminated the manufacturing cost of dies and enable the use for shorter runs.
Advantages of Investment casting
- Parts with extremely complex shapes and intricate features can be cast as a single piece using investment casting
- With short length or shallow depth feature, thin sections down to 0.40 mm (0.015 in) can be cast without cold shut defects
- Lost wax casting has excellent dimensional accuracy and tighter tolerances of 0.075 mm (0.003 in) are easily achievable.
- Compared to similar manufacturing processes, Investment casting can achieve excellent surface finish without any post-processing. Typically around 1.3 – 0.4 microns RMS Ra
- It offers almost unlimited freedom in terms of investment cast materials, but most common materials used include Aluminium alloy, cast iron and non-ferrous alloys. The process is particularly attractive for high-temperature alloys.
- Draft on walls are not required but if a master die is used to make wax patterns then draft on the face would help the pattern making process easier.
- Since there are no parting lines, the cast would not have any flash. But the wax patterns might have parting lines from the master die.
- Additional machining can be eliminated or reduced and allowance of as little as 0.4 to 1 mm (0.015 to 0.040 in.) usually enough.
- Excellent dimensional precision can be achieved in combination with very smooth as-cast surfaces. These capabilities are especially attractive when making products from the high-melting temperature, difficult-to-machine metals that cannot be cast with plaster- or metal mold processes.
- The wax used can usually be recovered for reuse.
Disadvantages of Investment casting
- Compared to other methods of metal casting, investment casting involves many complex steps making the process relatively expensive. But some of the steps can be automated for certain products. It can be more expensive than die casting or sand casting, but per-unit costs decrease with large volumes.
- The high cost of dies to make patterns has traditionally limited investment casting to large production quantities
- The high cost is also due to specialized equipment requirement, costly refractory material, and high labor cost
- Parts are difficult to cast if they require cores, got holes smaller than 1.6 mm or deeper than 1.5 times the diameter