Investment casting
Overview
 
Investment casting is an industrial process based on and also called lost-wax casting, 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.
Encyclopedia
Investment casting is an industrial process based on and also called lost-wax casting, 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 been used to produce complete aircraft door frames, steel
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...

 castings of up to 300 kg (660 lbs) and aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....

 castings of up to 30 kg (66 lbs). It is generally more expensive per unit than die casting
Die casting
Die casting is a metal casting process that is characterized by forcing molten metal under high pressure into a mold cavity. The mold cavity is created using two hardened tool steel dies which have been machined into shape and work similarly to an injection mold during the process...

 or sand casting
Sand casting
Sand casting, also known as sand molded casting, is a metal casting process characterized by using sand as the mold material.It is relatively cheap and sufficiently refractory even for steel foundry use. A suitable bonding agent is mixed or occurs with the sand...

, but has lower equipment costs. 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 can take two days to one week to complete.
  1. Produce a master pattern: An artist or mould-maker creates an original pattern from wax
    Wax
    thumb|right|[[Cetyl palmitate]], a typical wax ester.Wax refers to a class of chemical compounds that are plastic near ambient temperatures. Characteristically, they melt above 45 °C to give a low viscosity liquid. Waxes are insoluble in water but soluble in organic, nonpolar solvents...

    , clay
    Clay
    Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...

    , wood
    Wood
    Wood is a hard, fibrous tissue found in many trees. It has been used for hundreds of thousands of years for both fuel and as a construction material. It is an organic material, a natural composite of cellulose fibers embedded in a matrix of lignin which resists compression...

    , plastic
    Plastic
    A plastic material is any of a wide range of synthetic or semi-synthetic organic solids used in the manufacture of industrial products. Plastics are typically polymers of high molecular mass, and may contain other substances to improve performance and/or reduce production costs...

    , steel, or another material.
  2. Mouldmaking: A mould
    Molding (process)
    Molding or moulding is the process of manufacturing by shaping pliable raw material using a rigid frame or model called a pattern....

    , 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
    Rubber
    Natural rubber, also called India rubber or caoutchouc, is an elastomer that was originally derived from latex, a milky colloid produced by some plants. The plants would be ‘tapped’, that is, an incision made into the bark of the tree and the sticky, milk colored latex sap collected and refined...

     moulds 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.
  3. Produce the wax patterns: Although called a wax pattern, pattern materials also include plastic and frozen mercury
    Mercury (element)
    Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...

    . 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 3 mm (0.118110236220472 in) thick, covers the inner surface of the mould. This is repeated until the desired thickness is reached. Another method is filling the entire mould with molten wax, and let it cool, until a desired thickness has set on the surface of the mould. After this the rest of the wax is poured out again, the mould 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.
  4. Assemble the wax patterns: The wax pattern is then removed from the mould. 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
    Sprue (molding)
    A sprue is the passage through which liquid material is introduced into a mold. During casting or molding, the material in the sprue will solidify and need to be removed from the finished part.-Casting:...

    , 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
    Parting line
    A parting line in moldmaking is the place where two or more parts of the mold meet. At times, either because the mold halves do not meet with enough precision or because injection pressure is high, material will creep into the space between the molds. This material is generally called molding...

     or flashing
    Molding flash
    Flash, also known as flashing, is excess material attached to a molded, forged, or cast product, which must usually be removed. This is typically caused by leakage of the material between the two surfaces of a mold or between the base material and the mold...

     are rubbed out using the heated metal tool. Finally it is dressed, which means any other imperfections are addressed so that the wax now looks like the finished piece.
  5. Investment: The ceramic mould, 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 rainfall-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 5 to 15 mm (0.196850393700787 to 0.590551181102362 in). 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
    Refractory
    A refractory material is one that retains its strength at high temperatures. ASTM C71 defines refractories as "non-metallic materials having those chemical and physical properties that make them applicable for structures, or as components of systems, that are exposed to environments above...

     materials used to create the investments are: silica, zircon, various aluminium silicate
    Aluminium silicate
    Aluminium silicate has the chemical formula 2SiO3. It has a density of 2.8 to 2.9 g/cm³, a vitreous lustre, a refractive index of 1.56, a Mohs hardness of 4.5-7.5 , and can have orthorhombic crystallography...

    s, and alumina. Silica is usually used in the fused silica form, but sometimes quartz
    Quartz
    Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...

     is used because it is less expensive. Aluminium silicates are a mixture of alumina and silica, where commonly used mixtures have an alumina content from 42 to 72%; at 72% alumina the compound is known as mullite
    Mullite
    Mullite or porcelainite is a rare silicate mineral of post-clay genesis. It can form two stoichiometric forms 3Al2O32SiO2 or 2Al2O3 SiO2. Unusually, mullite has no charge balancing cations present...

    . During the primary coat(s), zircon
    Zircon
    Zircon is a mineral belonging to the group of nesosilicates. Its chemical name is zirconium silicate and its corresponding chemical formula is ZrSiO4. A common empirical formula showing some of the range of substitution in zircon is 1–x4x–y...

    -based refractories are commonly used, because zirconium
    Zirconium
    Zirconium is a chemical element with the symbol Zr and atomic number 40. The name of zirconium is taken from the mineral zircon. Its atomic mass is 91.224. It is a lustrous, grey-white, strong transition metal that resembles titanium...

     is less likely to react with the molten metal. Chamotte is another refractory material that has been used. Prior to silica, a mixture of plaster and ground up old molds (chamotte) was used.
    The binders used to hold the refractory material in place include: ethyl silicate (alcohol-based and chemically set), colloidal silica
    Colloidal silica
    Colloidal silicas are suspensions of fine amorphous, nonporous, and typically spherical silica particles in a liquid phase.-Properties:Usually they are suspended in an aqueous phase that is stabilized electrostatically...

     (water-based, also known as silica sol, set by drying), sodium silicate
    Sodium silicate
    Sodium silicate is the common name for a compound sodium metasilicate, Na2SiO3, also known as water glass or liquid glass. It is available in aqueous solution and in solid form and is used in cements, passive fire protection, refractories, textile and lumber processing, and automobiles...

    , and a hybrid of these controlled for pH
    PH
    In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

     and viscosity
    Viscosity
    Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...

    .
  6. 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
    Humidity
    Humidity is a term for the amount of water vapor in the air, and can refer to any one of several measurements of humidity. Formally, humid air is not "moist air" but a mixture of water vapor and other constituents of air, and humidity is defined in terms of the water content of this mixture,...

    . It is then turned upside-down and placed in a furnace
    Furnace
    A furnace is a device used for heating. The name derives from Latin fornax, oven.In American English and Canadian English, the term furnace on its own is generally used to describe household heating systems based on a central furnace , and sometimes as a synonym for kiln, a device used in the...

     or autoclave
    Autoclave
    An autoclave is an instrument used to sterilize equipment and supplies by subjecting them to high pressure saturated steam at 121 °C for around 15–20 minutes depending on the size of the load and the contents. It was invented by Charles Chamberland in 1879, although a precursor known as the...

     to melt out and/or vaporize 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.
  7. 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 used 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.
  8. 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
    Vacuum casting
    Vacuum casting is a casting process for elastomers using a vacuum to draw the liquid material into the mold. This process is used when air entrapment is a problem, there are intricate details or undercuts, or if the material is fiber or wire reinforced....

    , tilt cast, pressure assisted pouring, or centrifugal cast
    Centrifugal casting
    Centrifugal casting in silversmithing is a casting technique where a small mould is poured, then spun on the end of an arm. The centrifugal force thus generated encourages a successful pour.-Processes:Centrifugal casting, or centrifuging,...

    .
  9. Removal: The shell is hammered, media blasted, vibrated, waterjeted
    Water jet cutter
    A water jet cutter, also known as a waterjet, is a tool capable of slicing into metal or other materials using a jet of water at high velocity and pressure, or a mixture of water and an abrasive substance. The process is essentially the same as water erosion found in nature but greatly accelerated...

    , or chemically dissolved (sometimes with liquid nitrogen
    Liquid nitrogen
    Liquid nitrogen is nitrogen in a liquid state at a very low temperature. It is produced industrially by fractional distillation of liquid air. Liquid nitrogen is a colourless clear liquid with density of 0.807 g/mL at its boiling point and a dielectric constant of 1.4...

    ) 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
    Grinding (abrasive cutting)
    Grinding is an abrasive machining process that uses a grinding wheel as the cutting tool.A wide variety of machines are used for grinding:* Hand-cranked knife-sharpening stones * Handheld power tools such as angle grinders and die grinders...

    .

Counter-gravity pouring

A variation on the pouring technique is to fill the investment upside-down. A common form of this is called the Hitchiner process, which is named after the Hitchiner Manufacturing Company that invented the technique. In this technique the investment shell is placed in a vacuum tight mold chamber and then lowered into a pool of molten metal. A vacuum is then created, which draws the metal up into the investment shell. After the casting has solidified the vacuum is released, which allows any remaining liquid metal to flow back into the pool.

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 as 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. Plus, because the metal is drawn from below the top of the pool the metal is free from dross and slag, as these are lower density (lighter) and float to the top of the pool. The pressure differential helps the metal flow into every intricacy of the mold. Finally, lower temperatures can be used, which improves the grain structure.

This process is also used to cast refractory ceramic
Ceramic
A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...

s under the term vacuum casting.

Vacuum pressure casting

Vacuum pressure casting (VPC) uses gas pressure and a vacuum to improve the quality of the casting and minimize porosity. Typically VPC casting 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 molds.

Details

Investment casting is used with almost any castable metal, however aluminium alloys, copper alloys, and steel are the most common. In industrial usage the size limits are 3 g (0.105821886315336 oz) to about 5 kg (11 lb). The cross-sectional limits are 0.6 mm (0.0236220472440945 in) to 75 mm (3 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.

The advantages of investment casting are:
  • Excellent surface finish
  • High dimensional accuracy
  • Extremely intricate parts are castable
  • Almost any metal can be cast
  • No flash or parting lines


The main disadvantage is the overall cost. 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. However, the cost is still less than producing the same part by machining from bar stock
Bar stock
Bar stock, also colloquially known as billet, is a common form of raw purified metal, used by industry to manufacture metal parts and products....

; for example, gun manufacturing has moved to investment casting to lower costs of producing pistol
Pistol
When distinguished as a subset of handguns, a pistol is a handgun with a chamber that is integral with the barrel, as opposed to a revolver, wherein the chamber is separate from the barrel as a revolving cylinder. Typically, pistols have an effective range of about 100 feet.-History:The pistol...

s.

History

The history of lost-wax casting dates back thousands of years. Its earliest use was for idols
Cult image
In the practice of religion, a cult image is a human-made object that is venerated for the deity, spirit or daemon that it embodies or represents...

, ornaments and jewellery
Jewellery
Jewellery or jewelry is a form of personal adornment, such as brooches, rings, necklaces, earrings, and bracelets.With some exceptions, such as medical alert bracelets or military dog tags, jewellery normally differs from other items of personal adornment in that it has no other purpose than to...

, using natural beeswax
Beeswax
Beeswax is a natural wax produced in the bee hive of honey bees of the genus Apis. It is mainly esters of fatty acids and various long chain alcohols...

 for patterns, clay for the moulds and manually operated bellows
Bellows
A bellows is a device for delivering pressurized air in a controlled quantity to a controlled location.Basically, a bellows is a deformable container which has an outlet nozzle. When the volume of the bellows is decreased, the air escapes through the outlet...

 for stoking furnaces. Examples have been found across the world in India
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...

's Harappa
Harappa
Harappa is an archaeological site in Punjab, northeast Pakistan, about west of Sahiwal. The site takes its name from a modern village located near the former course of the Ravi River. The current village of Harappa is from the ancient site. Although modern Harappa has a train station left from...

n Civilisation (2500–2000 BC) idols, Egypt
Egypt
Egypt , officially the Arab Republic of Egypt, Arabic: , is a country mainly in North Africa, with the Sinai Peninsula forming a land bridge in Southwest Asia. Egypt is thus a transcontinental country, and a major power in Africa, the Mediterranean Basin, the Middle East and the Muslim world...

's tombs of Tutankhamun
Tutankhamun
Tutankhamun , Egyptian , ; approx. 1341 BC – 1323 BC) was an Egyptian pharaoh of the 18th dynasty , during the period of Egyptian history known as the New Kingdom...

 (1333–1324 BC), Mesopotamia
Mesopotamia
Mesopotamia is a toponym for the area of the Tigris–Euphrates river system, largely corresponding to modern-day Iraq, northeastern Syria, southeastern Turkey and southwestern Iran.Widely considered to be the cradle of civilization, Bronze Age Mesopotamia included Sumer and the...

, Aztec and Mayan Mexico
Mexico
The United Mexican States , commonly known as Mexico , is a federal constitutional republic in North America. It is bordered on the north by the United States; on the south and west by the Pacific Ocean; on the southeast by Guatemala, Belize, and the Caribbean Sea; and on the east by the Gulf of...

, and the Benin
Benin
Benin , officially the Republic of Benin, is a country in West Africa. It borders Togo to the west, Nigeria to the east and Burkina Faso and Niger to the north. Its small southern coastline on the Bight of Benin is where a majority of the population is located...

 civilization in Africa
Africa
Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...

 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
Theophilus Presbyter
Theophilus Presbyter is the pseudonymous author or compiler ofa Latin text containing detailed descriptions of various medieval arts, a text commonly known as the Schedula diversarum artium or De diversis artibus , probably first compiled between 1100 and 1120...

, a monk who described various manufacturing processes, including the recipe for parchment
Parchment
Parchment is a thin material made from calfskin, sheepskin or goatskin, often split. Its most common use was as a material for writing on, for documents, notes, or the pages of a book, codex or manuscript. It is distinct from leather in that parchment is limed but not tanned; therefore, it is very...

. This book was used by sculptor and goldsmith Benvenuto Cellini
Benvenuto Cellini
Benvenuto Cellini was an Italian goldsmith, sculptor, painter, soldier and musician, who also wrote a famous autobiography. He was one of the most important artists of Mannerism.-Youth:...

 (1500–1571), who detailed in his autobiography the investment casting process he used for the Perseus with the Head of Medusa
Perseus with the Head of Medusa
Perseus with the Head of Medusa is a recurring theme in artwork, depicting an event from Greek mythology in which the hero Perseus holds up the severed head of the gorgon Medusa....

 sculpture that stands in the Loggia dei Lanzi
Loggia dei Lanzi
The Loggia dei Lanzi, also called the Loggia della Signoria, is a building on a corner of the Piazza della Signoria in Florence, Italy, adjoining the Uffizi Gallery. It consists of wide arches open to the street, three bays wide and one bay deep. The arches rest on clustered pilasters with...

 in Florence
Florence
Florence is the capital city of the Italian region of Tuscany and of the province of Florence. It is the most populous city in Tuscany, with approximately 370,000 inhabitants, expanding to over 1.5 million in the metropolitan area....

, Italy
Italy
Italy , officially the Italian Republic languages]] under the European Charter for Regional or Minority Languages. In each of these, Italy's official name is as follows:;;;;;;;;), is a unitary parliamentary republic in South-Central Europe. To the north it borders France, Switzerland, Austria and...

.

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
Council Bluffs, Iowa
Council Bluffs, known until 1852 as Kanesville, Iowathe historic starting point of the Mormon Trail and eventual northernmost anchor town of the other emigrant trailsis a city in and the county seat of Pottawattamie County, Iowa, United States and is on the east bank of the Missouri River across...

 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
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

 increased the demand for precision net shape manufacturing and specialized alloy
Alloy
An alloy is a mixture or metallic solid solution composed of two or more elements. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may or may not be homogeneous in distribution, depending on thermal history...

s 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.

Applications

Investment casting is used in the aerospace
Aerospace
Aerospace comprises the atmosphere of Earth and surrounding space. Typically the term is used to refer to the industry that researches, designs, manufactures, operates, and maintains vehicles moving through air and space...

 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. Investment casting 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.

External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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