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Investment casting
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Investment casting, also called lost-wax casting, is one of the oldest known metal-forming techniques. From 5,000 years ago, when beeswax formed the pattern, to today’s high-technology waxes, refractory materials and specialist alloys, the castings allow the production of components with accuracy, repeatability, versatility and integrity in a variety of metals and high-performance alloys.
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Investment casting, also called lost-wax casting, is one of the oldest known metal-forming techniques. From 5,000 years ago, when beeswax formed the pattern, to today’s high-technology waxes, refractory materials and specialist alloys, the castings allow the production of components with accuracy, repeatability, versatility and integrity in a variety of metals and high-performance alloys. Lost foam casting is a modern form of investment casting that eliminates certain steps in the process.
The process is generally used for small castings, but has produced complete aircraft door frames, steel castings of up to 300 kg and aluminium castings of up to 30 kg. It is generally more expensive per unit than die casting or sand casting but with lower equipment cost. It can produce complicated shapes that would be difficult or impossible with die casting, yet like that process, it requires little surface finishing and only minor machining.
Process
Casts can be made of the wax model itself, the direct method; or of a wax copy of a model that need not be of wax, the indirect method. The following steps are for the indirect process, which, in total, can take two days to one week to complete.
- Produce a master pattern: An artist or mold-maker creates an original pattern from wax, clay, wood, plastic, steel, or another material.
- Moldmaking: A mold, known as the master die, is made of the master pattern. The master pattern may be made from a low-melting-point metal, steel or wood. If a steel pattern was created then a low-melting-point metal may be cast directly from the master pattern. Rubber molds can also be cast directly from the master pattern. The first step may also be skipped if the master die is machined directly into steel.
- Produce the wax patterns: Although called a wax pattern pattern materials also include plastic and frozen mercury. Wax patterns may be produced in one of two ways. In one process the wax is poured into the mold and swished around until an even coating, usually about thick, covers the inner surface of the mold. This is repeated until the desired thickness is reached. Another method is filling the entire mold with molten wax, and let it cool, until a desired thickness has set on the surface of the mold. After this the rest of the wax is poured out again, the mold is turned upside down and the wax layer is left to cool and harden. With this method it is more difficult to control the overall thickness of the wax layer.
If a core is required, there are two options: soluble wax or ceramic. Soluble wax cores are designed to melt out of the investment coating with the rest of the wax pattern, whereas ceramic cores remain part of the wax pattern and are removed after the workpiece is cast. - Assemble the wax patterns: The wax pattern is then removed from the mold. Depending on the application multiple wax patterns may be created so that they can all be cast at once. In other applications, multiple different wax patterns may be created and then assembled into one complex pattern. In the first case the multiple patterns are attached to a wax sprue, with the result known as a pattern cluster, or tree; as many as several hundred patterns may be assembled into a tree. Foundries often use registration marks to indicate exactly where they go. The wax patterns are attached to the sprue or each other by means of a heated metal tool. The wax pattern may also be chased, which means the parting line or flashing are rubbed out using the heated metal tool. Any other imperfections are also addressed at this point so that the wax now looks like the finished piece.
- Investment: The ceramic mold, known as the investment, is produced by three repeating steps: coating, stuccoing, and hardening. The first step involves dipping the cluster into a slurry of fine refractory material and then letting any excess drain off, so a uniform surface is produced. This fine material is used first to give a smooth surface finish and reproduce fine details. In the second step, the cluster is stuccoed with a coarse ceramic particle, by dipping it into a fluidised bed, placing it in a rain tower sander, or by applying by hand. Finally, the coating is allowed to harden. These steps are repeated until the investment is the required thickness, which is usually . Note that the first coatings are known as prime coats. An alternative to multiple dips is to place the cluster upside-down in a flask and then liquid investment material is poured into the flask. The flask is then vibrated to allow entrapped air to escape and help the investment material fill in all of the details.
- Common refractory materials used to create the investments are: silica, zircon, various aluminium silicates, and alumina. Silica is usually used in the fused silica form, but sometimes quartz is used because it is less expensive. Aluminium silicates are a mixture of alumina and silica, where commonly used mixtures have a alumina content from 42 to 72%; at 72% alumina the compound is known as mullite. During the primary coat(s), zircon-based refractories are commonly used, because zirconium is less likely to react with the molten metal. Other refractory materials that have been used include molochite and chamotte. Prior to silica, a mixture of plaster and fire-proof material, such as chamotte, was used.
- The binders used to hold the refractory material in place include: ethyl silicate (alcohol-based and chemically set), colloidal silica (water-based, also known as silica sol, set by drying), sodium silicate, and a hybrid of these controlled for pH and viscosity.
- Dewax: The investment is then allowed to completely dry, which can take 16 to 48 hours. Drying can be enhanced by applying a vacuum or minimizing the environmental humidity. It is then turned upside-down and placed in a furnace or autoclave to melt out and/or vaporized the wax. Most shell failures occur at this point because the waxes used have a thermal expansion coefficient that is much greater than the investment material surrounding it, so as the wax is heated it expands and induces great stresses. In order to minimize these stresses the wax is heated as rapidly as possible so that the surface of the wax can melt into the surface of the investment or run out of the mold, which makes room for the rest of the wax to expand. In certain situations holes may be drilled into the mold beforehand to help reduce these stresses. Any wax that runs out of the mold is usually recovered and reused.
- Burnout & preheating: The mold is then subjected to a burnout, which heats the mold between 870 °C and 1095 °C to remove any moisture and residual wax, and to sinter the mold. Sometimes this heating is also as the preheat, but other times the mold is allowed to cool so that it can be tested. If any cracks are found they can be repaired with ceramic slurry or special cements. The mold is preheated to allow the metal to stay liquid longer to fill any details and to increase dimensional accuracy, because the mold and casting cool together.
- Pouring: The investment mold is then placed cup-upwards into a tub filled with sand. The metal may be gravity poured, but if there are thin sections in the mold it may be filled by applying positive air pressure, vacuum cast, tilt cast, pressure assisted pouring, or centrifugal cast.
- Removal: The shell is hammered, media blasted, vibrated, waterjeted, or chemically dissolved (sometimes with liquid nitrogen) to release the casting. The sprue is cut off and recycled. The casting may then be cleaned up to remove signs of the casting process, usually by grinding.
History
Investment casting dates back thousands of years. 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 in India's Harappan Civilisation (2500–2000 BC) idols, Egypt's tombs of Tutankhamun (1333–1324 BC), in Mesopotamia, Mexico, and the Benin civilization in Africa where the process produced detailed artwork of copper, bronze and gold.
The earliest known text that describes the investment casting process (Schedula Diversarum Artium) was written around 1100 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 (1500–1571), 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 Dr. D. Philbrook of Council Bluffs, Iowa in 1897. Its use was accelerated by Dr. William H. Taggart of Chicago, whose 1907 paper described his development of a technique. He also formulated a wax pattern compound of excellent properties, developed an investment material, and invented an air-pressure casting machine.
In the 1940s, 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. Sturm, Ruger, founded in 1949, rose to dominance in firearms manufacturing by using the new technology to reduce labor-intensive machining.
Modern investment casting techniques stem from the development in the United Kingdom of a shell process using wax patterns known as the investment X process. This method resolved the problem of wax removal by enveloping a completed and dried shell in a vapor degreaser. The vapor permeated the shell to dissolve and melt the wax. This process has been evolved over years into the current process of melting out the virgin wax in an autoclave or furnace.
Applications
Investment casting is used in the aerospace and power generation industries to produce turbine blades with complex shapes or cooling systems. Blades produced by investment casting can include single-crystal (SX), directionally solidified (DS), or conventional equiaxed blades. It is also widely used by firearms manufacturers to fabricate firearm receivers, triggers, hammers, and other precision parts at low cost. Other industries that use standard investment-cast parts include military, medical, commercial and automotive.
Investment casting offers high production rates, particularly for small or highly complex components, and extremely good surface finish (CT4-CT6 class accuracy and Ra1.6-6.3 surface roughness) with very little machining. The drawbacks include the specialized equipment, costly refractories and binders, many operations to make a mold, and occasional minute defects.
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