Like sand casting, investment casting is one of the oldest casting processes practiced already in ancient civilisations. Dating back to B.C. in the Levant region for making jewelry, ornaments, and idols, this casting technique was also used by Harappa, Mesopotamia, Aztec, Mayan, and Benin civilisations, and found in the tombs of Egyptian rulers.
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The founding principles of investment casting have remained the same over thousands of years. However, adding supporting technologies such as CAD/CAM, additive manufacturing, and real-time monitoring capabilities has significantly improved the final part quality, consistency, and affordability of this industrial process.
In this article, we shall explore the workings of modern investment casting processes, along with their advantages, disadvantages and use cases.
Investment casting, also sometimes known as precision casting or lost wax casting, is one of the most popular metal forming techniques. The term ‘investment’ originates from the archaic meaning of ‘invest,’ which refers to clothing or surrounding an object with a layer of material.
In this process, a wax pattern is encased in refractory material to form a ceramic shell mould. The pattern is slightly larger in dimensions than the final product to accommodate shrinkage when the material cools.
Since the shell mould has a hard surface, investment casting delivers smoother surface finishes than sand casting. The process can work with most metals. Some popular metals that are highly compatible with investment casting are bronze, magnesium, stainless steel alloys, glass, carbon steel, brass, aluminium and aluminium alloys.
The investment casting process is more intricate than the widely used sand casting process, thus involving more stages. Investment casting involves 8 stages and these are as follows:
Creation of the master pattern
Creation of the master die
Creation of the wax pattern
Creation of the shell mould
Removal of wax
Pouring of molten metal and subsequent cooling
Removal of casting from the shell mould
Post-processing operations
Let’s look closer at what is happening in each of the stages:
The master pattern is an exact duplicate of the product required. It may be made from wood, metal, plastic, clay, wax or other specialist alloys. Over the years, manufacturers have shifted to 3D printing master patterns due to their compatibility with CAD/CAM and improved dimensional accuracy.
The master pattern is used to create a master die. The master die’s creation technique depends on the master pattern’s material. For instance, if the master pattern is from steel, it can be placed in molten aluminium, since aluminium melts at 660°C (1,220°F), whereas steel melts at around °C (2,732°F). Thus, the molten aluminium will not dissolve the steel. Upon solidification, it will create an aluminum-based master die.
3D-printed master dies are increasingly being used because they offer a cost-effective alternative to metal dies when only a few castings are needed.
There are several ways to produce wax patterns from master dies. One method involves filling the die with a small amount of wax and shaking it until it evenly coats the inner surface of the die and solidifies. This process is repeated until the desired thickness is achieved, resulting in a hollow wax pattern.
The second method involves filling the master die completely with wax. The wax may be fed directly or through high-pressure injection into the die. The high pressures enable the wax to fill all of the cavity’s features in the case of complex components. The wax patterns thus produced are solid.
During this stage, cores can be added to create internal cavities in the wax pattern. Common materials for cores include soluble wax or ceramic. The soluble wax is removed in stage 5, while ceramic cores are removed after the final product has hardened.
Wax patterns cannot withstand the high temperatures of molten metal. Therefore, ceramic moulds are created from wax patterns to endure the heat of molten metal. The ceramic mould is formed by immersing the wax pattern into a slurry of refractory material and allowing it to solidify.
The process is essentially the same as coating vanilla ice cream with a layer of chocolate. When the cold vanilla ice cream comes into contact with hot melted chocolate, the chocolate sticks to the ice cream and solidifies almost immediately, creating a thin shell of chocolate covering the ice cream.
Similarly, when the ceramic slurry comes into contact with the wax pattern, it adheres to it and solidifies. Several passes of the wax pattern are made, starting with a finer slurry and then a coarser one to achieve the desired thickness of the ceramic mould. The mould is then left to cool down.
Once the slurry has hardened, it is placed in a furnace to remove the wax and for further sintering of the ceramic mould. Sintering is the process through which items are hardened through pressure and heat but without liquefaction. As the temperature rises, sintering hardens the mould while the molten wax is reclaimed for future use. This stage gives us the solid ceramic mould for the actual casting process.
After the wax removal stage, the mould is cooled for testing. If any cracks or other imperfections are detected, they can be rectified using ceramic slurry or special cement.
Upon successful test results, the mould is heated again before pouring liquid metal into it. This preheating ensures that the molten metal remains in the liquid state for a longer period until it uniformly fills the mould.
To pour the metal, the ceramic mould is inverted and placed in a sand-filled container. The molten metal is poured into the mould under gravity or external pressure.
Vacuum-assisted filling may also be used. It comes at a higher price point but offers significantly better resource efficiency compared to gravity and pressure-based filling.
The vacuum pulls the molten metal into the mould, and once all the crucial parts solidify, the vacuum is released, allowing the unused melt to drain out. This process minimises material solidification in the sprue and gates, resulting in up to a 95% material yield, compared to 15-50% in gravity pouring.
The material is then allowed to cool and solidify until it is hard enough to withstand separation from the ceramic shell mould.
The divesting is typically carried out by hammering the mould to release the casting. Other methods to separate the metal casting from the mould include waterjetting, media blasting, vibration and chemical dissolution of the mould. The sprue, runners and other gating system components are then separated and recycled.
The investment casting process generally does not require subsequent machining. However, if the results are not as expected, the casting may need surface finishing operations before use. Typically, surface grinding with machine tools is adequate for polishing and trimming any surface defects. Nevertheless, depending on the situation, other finishing processes such as hand tooling, welding, and hydraulic straightening may be necessary.
The investment casting process offers the following advantages:
Excellent dimensional accuracy even for complex components
Smoother surface finish
Works with a wide range of metals
Parts are created as a single casting with no parting lines
Compatible with low- and high-volume manufacturing
Minimal wastage of resources
Parts can be combined, eliminating the need for assembly operations downstream
90-degree angles can be cast without worrying about shrinkage allowance
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More environmentally friendly than other metal fabrication processes
The investment casting process also has certain limitations:
A very high number of variables that can affect part quality. A high level of process control is essential for producing good-quality parts
There is an upper limit on the size of investment castings, which is lower than that of some other casting processes such as sand casting and centrifugal casting
Casting objects with internal cavities can be more challenging than other casting processes
Longer production cycles and lead times
Investment casting techniques are more expensive than other processes, especially when the quantity is low and the manufacturing process involves the use of permanent tools
The industry uses investment casting to produce the following products, highlighting the versatility of modern casting services:
Turbocharger rotors and turbine blades
Firearm components such as triggers, hammers and receivers
Power generation and cooling system parts such as pumps, valves and combustor components
Intricate jewellery with delicate components and tight tolerances
Automotive parts including engine components, transmission components, brakes, door handles, gears, housings, brackets, rods and more
After learning about the investment casting process, let’s summarise our knowledge by noting the five scenarios where investment casting provides the best ROI:
Medium size range – Investment casting gives better results when the weight of the product ranges from a few grams up to 1,000 kg. Parts weighing more than 1,000 kg are better suited for the sand casting process.
Extremely complex geometry – Investment casting can create super complicated shapes such as turbine blades, medical equipment, dental crowns and inlays, power generation equipment, aerospace components and more.
High number of parts – Investment casting is not cost-effective when a small number of items need to be manufactured, as the process becomes more expensive. However, as the quantity of items increases beyond a certain threshold, the cost per piece can decrease below that of other methods, including sand casting.
Need for a high surface finish – Investment casting uses a hard mould with a very fine surface, allowing the process to achieve an exceptional surface finish.
Investment casting is an industrial process based on lost-wax casting, one of the oldest known metal-forming techniques.[1] The term "lost-wax casting" can also refer to modern investment casting processes.
Investment casting has been used in various forms for the last 5,000 years. In its earliest forms, beeswax was used to form patterns necessary for the casting process. Today, more advanced waxes, refractory materials and specialist alloys are typically used for making patterns. Investment casting is valued for its ability to produce components with accuracy, repeatability, versatility and integrity in a variety of metals and high-performance alloys.
The fragile wax patterns must withstand forces encountered during the mould making. Much of the wax used in investment casting can be reclaimed and reused.[2] Lost-foam casting is a modern form of investment casting that eliminates certain steps in the process.
Investment casting is so named because the process invests (surrounds) the pattern with refractory material to make a mould, and a molten substance is cast into the mold. Materials that can be cast include stainless steel alloys, brass, aluminium, carbon steel and glass. The cavity inside the refractory mould is a slightly oversized but otherwise exact duplicate of the desired part. Due to the hardness of refractory materials used, investment casting can produce products with exceptional surface qualities, which can reduce the need for secondary machine processes.[3]
Water glass and silica sol investment casting are the two primary investment casting methods currently in use. The main differences are the surface roughness and cost of casting. Water glass method dewaxes into the high-temperature water, and the ceramic mould is made of water glass quartz sand. Silica sol method dewaxes into the flash fire, and silica sol zircon sand makes the ceramic mould. Silica sol method costs more but has the better surface than the water glass method.[4]
The process can be used for both small castings of a few ounces and large castings weighing several hundred pounds. However, it is most suitable for small parts at large volumes.[5] It can be more expensive than die casting or sand casting, but per-unit costs decrease with large volumes. Investment casting can produce complicated shapes that would be difficult or impossible with other casting methods. It can also produce products with exceptional surface qualities and low tolerances with minimal surface finishing or machining required.
The technical and trade organization for the global investment casting industry is the Investment Casting Institute and the trade magazine for the industry is INCAST Magazine.
Castings can be made from an original wax model (the direct method) or from wax replicas of an original pattern that need not be made from wax (the indirect method). The following steps describe the indirect process, which can take two to seven days to complete.
The main disadvantage is the overall cost, especially for short-run productions. Some of the reasons for the high cost include specialized equipment, costly refractories, and binders, many operations to make a mould, a lot of labor is needed and occasional minute defects occur. However, the cost is still less than producing the same part by machining from bar stock; for example, gun manufacturing has moved to investment casting to lower costs of producing pistols.
Additionally:
The variation on the gravity pouring technique is to fill the mould using a vacuum. A common form of this is called the Hitchiner process after the Hitchiner Manufacturing Company that invented the technique. In this technique, the mould has a downward fill pipe that is lowered into the melt. A vacuum draws the melt into the cavity; when the important parts have solidified, the vacuum is released, and the unused material leaves the mould. The technique can use substantially less material than gravity pouring because the sprue and some gating need not solidify.[18][19]
This technique is more metal efficient than traditional pouring because less material solidifies in the gating system. Gravity pouring only has a 15 to 50% metal yield compared to 60 to 95% for counter-gravity pouring. There is also less turbulence, so the gating system can be simplified since it does not have to control turbulence. The metal is drawn from below the top of the pool, so the metal is free from dross and slag (which are lower density (lighter) and float to the top of the pool). The pressure differential helps the metal flow into every intricacy of the mould. Finally, lower temperatures can be used, which improves the grain structure.[18]
This process is also used to cast refractory ceramics under the term vacuum casting.[20]
Vacuum pressure casting (VPC), properly referred to as vacuum assist direct pour, uses gas pressure and a vacuum to improve the quality of the casting and minimize porosity. Typically VPC machines consist of an upper and a lower chamber—the upper chamber, or melting chamber, housing the crucible, and the lower casting chamber housing the investment mould. Both chambers are connected via a small hole containing a stopper. A vacuum is pulled in the lower chamber, while pressure is applied in the upper, and then the stopper is removed. This creates the greatest pressure differential to fill the moulds.[21] The most common materials for vacuum casting process are the high nickel-based alloy and super alloys. Turbocharger products are a common applications for this casting process,[22] though it is also regularly used in the manufacture of silver and gold jewellery.
Investment casting is used with almost any castable metal. However, aluminium alloys, copper alloys, and steel are the most common. In industrial use, the size limits are 3 g (0.1 oz) to several hundred kilograms.[23] The cross-sectional limits are 0.6 mm (0.024 in) to 75 mm (3.0 in). Typical tolerances are 0.1 mm for the first 25 mm (0.005 in for the first inch) and 0.02 mm for the each additional centimeter (0.002 in for each additional inch). A standard surface finish is 1.3–4 micrometres (50–125 μin) RMS.[14]
The history of lost-wax casting dates back thousands of years.[24] Its earliest use was for idols, ornaments and jewellery, using natural beeswax for patterns, clay for the moulds and manually operated bellows for stoking furnaces. Examples have been found across the world, such as in the Harappan Civilisation (– BC) idols, Egypt's tombs of Tutankhamun (– BC), Mesopotamia, Aztec and Mayan Mexico, and the Benin civilization in Africa where the process produced detailed artwork of copper, bronze and gold. By far, one of the earliest identified uses of the investment casting process was seen in objects found in the 'Cave of Treasure', discovered in Southern Israel. These items were identified as being made around BC using Carbon-14 dating techniques.[25]
The earliest known text that describes the investment casting process (Schedula Diversarum Artium) was written around A.D. by Theophilus Presbyter, a monk who described various manufacturing processes, including the recipe for parchment. This book was used by sculptor and goldsmith Benvenuto Cellini (–), who detailed in his autobiography the investment casting process he used for the Perseus with the Head of Medusa sculpture that stands in the Loggia dei Lanzi in Florence, Italy.
Investment casting came into use as a modern industrial process in the late 19th century, when dentists began using it to make crowns and inlays, as described by Barnabas Frederick Philbrook of Council Bluffs, Iowa in .[26] Its use was accelerated by William H. Taggart of Chicago, whose paper described his development of a technique[citation needed]. He also formulated a wax pattern compound of excellent properties, developed an investment material, and invented an air-pressure casting machine.
In the s, World War II increased the demand for precision net shape manufacturing and specialized alloys that could not be shaped by traditional methods, or that required too much machining. Industry turned to investment casting. After the war, its use spread to many commercial and industrial applications that used complex metal parts.
Investment casting is used in the aerospace and power generation industries to produce turbine blades with complex shapes or cooling systems.[14] Blades produced by investment casting can include single-crystal (SX), directionally solidified (DS), or conventional equiaxed blades.
Investment casting is also widely used by firearms manufacturers to fabricate firearm receivers, triggers, hammers, and other precision parts at low cost.[citation needed]
Karsten Solheim famously revolutionized golf club design through his company PING by incorporating investment casting for the first time for clubheads.[27] Quickly the process became an industry standard to allow weight distribution around the perimeter of the clubhead.
Other industries that use standard investment-cast parts include military, aerospace, medical, jewelry, airline, automotive and golf clubs especially since the start of 3D printing technology.
With the increased availability of higher-resolution 3D printers, 3D printing has begun to be used to make much larger sacrificial moulds used in investment casting. Planetary Resources has used the technique to print the mould for a new small satellite, which is then dipped in ceramic to form the investment cast for a titanium space bus with integral propellant tank and embedded cable routing.