The investment casting process is a sophisticated process of producing metal castings that dates back to 4000 B.C. Investment casting is one of the oldest metal casting techniques and also one of the most advanced. The method was used around the world in Africa, Egypt, China and in Mexico 4000 B.C. when it was used by artisans and sculptors to create statues, jewelry and idols (Krar & Bill, 2003). The modern industry ignored investment casting until it was back by dental professionals to create teeth crowns. The term investment casting comes from the use of slurry or investment to form an extremely smooth surface with a tolerance for high dimension. During WWII, the technology was brought to the forefront when quickly producing precision parts became necessary. The process provided a shortcut for manufacturers for producing complex parts that were challenging to create using alternative methods.
Today, investment casting is used to produce automotive, aircraft, power and hand tool components, and other recreational products including the head of a golf club (Garg, 2005). Civilian and military jets rely on investment casting for the manufacture of engines and other parts. For automotives, investment casting is used to produce gears, splines, valve and fitting, and levers.
Proper handling of pattern wax during the pre-pattern production can eliminate a number of wax pattern defects. All materials used in the production of a casting are part of a system and thus must work together to ensure production of quality casting. The final casting is as good as the wax pattern produced (Beeley et al., 2008). Pattern wax is composed of products such as organic fillers, synthetic and natural resins, natural waxes and petroleum waxes. Microcystalline and paraffin waxes are produced from the distillation of crude oil. Paraffin is the commonly used petroleum wax because it is cheaper than other raw materials. Additionally, paraffin wax controls and enhances the rheological properties. In turn, the enhanced rheological properties affect the injection temperature. The fluidity of the pattern wax mix is also affected by the enhanced rheological properties.
Resins are extracted from natural sources such as crude oil, coal tar and pine trees. Resins can also be produced synthetically. They are used to add body during the formulation and as a result they affect tackiness, hardness, rigidity and shrinkage of the wax blend (Beeley, et al., 2008). On the other hand, candelilla and carnauba waxes are the natural waxes used. They are derived from shrubs and leaves in Mexico and Brazil. Natural waxes affect the set up properties, surface finish and hardness of the pattern wax blend. Sometimes, synthetic additives are used in the natural wax formulation. Synthetic products are more stable and reliable compared to natural raw materials. Fillers are also important to the development of the pattern wax. The fillers are selected according to these criteria: low ash content, organic, fine particle distribution and relative high melting point. Hydro-fill, Bisphenol A, Polystyrene and Isopthalic are the commonly used organic fillers (Beeley et al., 2008).
Soluble waxes are comprised of three main raw materials which include: Effervescing carbonate, filler and binder. The binder also known as PEG is available in various molecular weights and is used in different combinations to achieve the preferred melt point, hardness and viscosity characteristics. PEG consists of a fine powder substance that is commonly used to improve shrinking characteristics. The filler also helps improve the structural strength of the wax blend. To improve the elastic properties and strength of the wax, fibrous materials are used. The sodium bicarbonate is used as an effervescing agent to help break down the soluble wax during the discharge process. Sodium bicarbonate also adds to the bulk of the wax. Both the sodium bicarbonate and the fillers are inorganic materials thus foundries must adhere to recommended handling practices of proper heating.
Material suitable for investment casting
Both non-ferrous and ferrous materials can be investment cast. Metals considered for investment casting can be melted in a vacuum furnace or a regular furnace. Materials that are difficult to produce using a machine are also recommended for investment casting. Castability rating, shrinkage, fluidity and resistance to hot tearing are the main properties considered before selecting a metal for investment casting. In regard to ferrous metals, ductile iron and steel alloys are the most commonly poured. On the other hand, non-ferrous metals include copper based metals, magnesium and iron with aluminum being the most popular non-ferrous metal.
Aluminum alloys are expected to have a density of 2.7g/cm3 with the exception of A07130, A07120 which have a density of 2.8g/cm3. A05350 is also an exception with a density of 2.6g/cm3. All alloys of aluminum are hardenable with the exception of A05140 and A05350. A02010 has copper as the main alloying element. Therefore, it is a strong alloy with a relative weldability and excellent machinability. A33550 is a premium quality aluminum with cooper and silicon as it’s alloying elements. The alloy offers good castability, machinability and weldability. On the other hand, A 356 has poor brazability and good weld-ability. Its main alloying elements are silicon, magnesium and/or cooper.
Carbon steel alloys have poor resistance to corrosion and fairly good machinability. All alloys of carbon steel have a density of 7.8 g/cm3 and are therefore hardenable. Although 1010 and 1020 are hardenable, they have a density of 7.9 g/cm3. 1040 offers a poor resistance to corrosion, good weldability and medium strength. On the other hand, 1050 offers good machinability and medium strength. Both 1040 and 1050 have fairly good fluidity, resistance to hot tearing and shrinkage. As pertains castability, both 1040 and 1050 rate as good.
Up to 8700c, Cobalt 6 is oxidation resistant with a good castability that is coupled with resistance to corrosion. On the other hand, Cobalt 12 has a high resistance to corrosion and boast excellent wear properties. Both Cobalt 21 and Cobalt 31 have excellent shrinkage and fluidity, good resistance to hot corrosion and very good castability rating. The alloy of Monel provides good resistance to corrosion at both high and low temperatures. Monel 4020 has excellent fluidity. Good resistance to tearing and shrinkage with very good castability. Iconel 600 is resistant to corrosion but in the presence of Sulphur. Iconel also offers good machinability.
Copper-based alloys have a density of 8.3g/cm3 thus are not hardenable. The exception to the density rule is Navy G and Phosphor Bronze with a density of 8.8g/cm3 and 8.7g/cm3 respectively. Ductile irons have good machinability and poor resistance to corrosion. Tool steel alloys have a density of 7.8g/cm3 and are all hardenable. Among the tool steel alloys, A-2 has a good resistance to wear, resistance to corrosion under high temperature, good machinability, weldability and castability. On the other hand, H-13 has good resistance to corrosion, fair resistance to tear and poor toughness. H-13 is machine-able, cast-able and weld-able.
Only two forms of investment casting are commonly known and they are ceramic shell and solid mold. The two types of investment casting differ based on the way the mold is formed. For the solid mold process, the mold pattern is placed in a container and the mold material poured around the pattern. During the ceramic shell mold, the shell is dipped into mobile slurry. Eventually the pattern is taken out of the slurry and span around to ensure uniform coating. The coating is then allowed to dry and the dipping process is repeated to achieve the desires thickness. The mold is then exposed to heat to drain the pattern wax leaving a hollow cavity.
Investment casting begins with preparation of wax patterns for the casting. One or more patterns can be attached to the sprue depending on the complexity and size of the cast. The ceramic pour cup is attached at the end of the sprue bar (Krar & Bill, 2003). This arrangement of wax patterns is known as a tree because the casting patterns on the sprue are similar to the branches of a tree. The wax pattern is then dipped into slurry comprising of binders, silica and water until then desired thickness is achieved. Once the required hardness has been acquired, the refractory coat is left in dry in air so as to harden.
The next step is key to investment casting. After hardening of the ceramic mold, it is turned upside down with the funnel side facing down. The hardened mold is then heated to a temperature of 900C to 1750C (National Conference, 2003). This causes the wax that is inside to melt leaving a cavity for the investment casting. The ceramic mold unlike the wax dose not melt under severe heat. The mold is then heated further with the temperatures raised to between 5500C-1100C. Further heating of the mold makes it stronger while removing leftover wax. Metal casting is poured while the mold is hot. This allows the molten metal to flow without difficulty within the mold cavity. Pouring the molten metal into a hot mold gives better dimensional precision since both the mold and the cast will shrink at the same time. Finally, the ceramic mold is broken after the molten metal solidifies inside the ceramic mold. The end product of the investment casting process is the cast.
Investment casting is one of the oldest and most sophisticated metal casting processes. The process is suitable for mass production and for the production of complex metallic parts that would be impossible to manufacture using normal processes. Investment casting was initially used by ancient artisans and sculptors. It was later rediscovered by dentist professions. Today, the process is used to produce components for air crafts, hand and power tools, automotive and recreational products. A good cast depends on the quality of the casting process right from the formulation of the pattern wax to the casting itself. All processes during casting are part of a system thus they must work together to ensure that a quality cast is produced. On the other hand, not all metals can be cast and those that can be cast have specific properties that make them suitable. These properties include: fluidity, castability, shrinkage and resistance to casting.
Beeley, P. R., Smart, R. F., & Institute of Materials (Great Britain). (2008). Investment casting. Leeds, UK: Maney Publishing.
Garg, S. K. (2005). Comprehensive workshop technology: Manufacturing processes.
Krar, S. F., & Gill, A. R. (2003). Exploring advanced manufacturing technologies. New York, NY: Industrial Press.
Top of Form
National Conference on Investment Casting, Mondal, B., & Central Mechanical Engineering Research Institute. (2004). Proceedings of the National Conference on Investment Casting: NCIC 2003. New Delhi: Allied Publishers.
Bottom of Form