Metal casting

In metalworking and jewelry making, casting is a process in which a liquid metal is delivered into a mold (usually by a crucible) that contains a negative impression (i.e., a three-dimensional negative image) of the intended shape. The metal is poured into the mold through a hollow channel called a sprue. The metal and mold are then cooled, and the metal part (the casting) is extracted. Casting is most often used for making complex shapes that would be difficult or uneconomical to make by other methods. 

Casting processes have been known for thousands of years, and have been widely used for sculpture (especially in bronze), jewelry in precious metals, and weapons and tools. Highly engineered castings are found in 90 percent of durable goods, including cars, trucks, aerospace, trains, mining and construction equipment, oil wells, appliances, pipes, hydrants, wind turbines, nuclear plants, medical devices, defense products, toys, and more.

Traditional techniques include lost-wax casting (which may be further divided into centrifugal casting, and vacuum assist direct pour casting), plaster mold casting and sand casting.

The modern casting process is subdivided into two main categories: expendable and non-expendable casting. It is further broken down by the mold material, such as sand or metal, and pouring method, such as gravity, vacuum, or low pressure.

Theory

Casting is a solidification process, which means the solidification phenomenon controls most of the properties of the casting. Moreover, most of the casting defects occur during solidification, such as gas porosity and solidification shrinkage.

Solidification occurs in two steps: nucleation and crystal growth. In the nucleation stage, solid particles form within the liquid. When these particles form, their internal energy is lower than the surrounded liquid, which creates an energy interface between the two. The formation of the surface at this interface requires energy, so as nucleation occurs, the material actually undercools (i.e. cools below its solidification temperature) because of the extra energy required to form the interface surfaces. It then recalescences, or heats back up to its solidification temperature, for the crystal growth stage. Nucleation occurs on a pre-existing solid surface because not as much energy is required for a partial interface surface as for a complete spherical interface surface. This can be advantageous because fine-grained castings possess better properties than coarse-grained castings. A fine grain structure can be induced by grain refinement or inoculation, which is the process of adding impurities to induce nucleation.

All of the nucleations represent a crystal, which grows as the heat of fusion is extracted from the liquid until there is no liquid left. The direction, rate, and type of growth can be controlled to maximize the properties of the casting. Directional solidification is when the material solidifies at one end and proceeds to solidify to the other end; this is the most ideal type of grain growth because it allows liquid material to compensate for shrinkage.

Terminology

Metal casting processes uses the following terminology:

Pattern: An approximate duplicate of the final casting used to form the mold cavity.

Molding material: The material that is packed around the pattern and then the pattern is removed to leave the cavity where the casting material will be poured.

Flask: The rigid wood or metal frame that holds the molding material.

Cope: The top half of the pattern, flask, mold, or core.

Drag: The bottom half of the pattern, flask, mold, or core.

Core: An insert in the mold that produces internal features in the casting, such as holes.

Core print: The region added to the pattern, core, or mold used to locate and support the core.

Mold cavity: The combined open area of the molding material and core, where the metal is poured to produce the casting.

Riser: An extra void in the mold that fills with molten material to compensate for shrinkage during solidification.

Gating system: The network of connected channels that deliver the molten material to the mold cavities.

Pouring cup or pouring basin: The part of the gating system that receives the molten material from the pouring vessel.

Sprue: The pouring cup attaches to the sprue, which is the vertical part of the gating system. The other end of the sprue attaches to the runners.

Runners: The horizontal portion of the gating system that connects the sprues to the gates.

Gates: The controlled entrances from the runners into the mold cavities.

Vents: Additional channels that provide an escape for gases generated during the pour.

Parting line or parting surface: The interface between the cope and drag halves of the mold, flask, or pattern.

Draft: The taper on the casting or pattern that allow it to be withdrawn from the mold

Core box: The mold or die used to produce the cores.

Chaplet: Long vertical holding rod for core that after casting it become the integral part of casting, provide the support to the core.

Some specialized processes, such as die casting, use additional terminology.

Workpiece spectrum and areas of application

A wide range of workpieces can be produced by casting. Some small parts weigh only a few grams, the largest over 200 tons. The variety of producible shapes is almost unlimited, especially free-form surfaces, i.e. three-dimensional curved surfaces, are possible. Important products include bells (made by bell casting), implants and prostheses, bronze statues (made by bronze casting) and other art castings, housings for pumps, gears and electric motors, impellers, ship propellers and turbine blades for the aerospace industry made of titanium or nickel. For the foundry's most important customer, the automotive industry, wheels, chassis parts such as brake discs, hubs and control arms for wheel suspensions, engine blocks, crankshafts, cylinder heads, exhaust manifolds and many other parts are manufactured, often by sand casting (with cast iron) or die casting (with aluminum). 

Material spectrum – casting materials and castability

Materials used in foundries are called casting materials or casting alloys, and their suitability for casting is called castability.

By far the most important casting material, with a share of 75%, is cast iron, an iron alloy with at least 2% carbon (usually around 4.3%). At 1200°C, it has a much lower melting point than steel (1500°C), which contains less than 2% carbon. Cast iron is also very easy to cast: the melt is very fluid and has good mold filling properties. Shrinkage and contraction during cooling and solidification are low. Cast iron also has very good performance properties, including wear resistance and vibration damping. Most types of cast iron also contain around 2% silicon, which improves castability. Cast iron is preferably cast in sand molds (sand casting).

The second most important casting material, measured as a mass share of total production in foundries, is aluminum castings with aluminum alloys that also contain silicon, magnesium, or copper. These alloys melt at approximately 570 °C and are also very easy to cast. Aluminum casting alloys can also be used for delicate components whose molds would not be completely filled by other materials. Aluminum alloys are preferred for die casting.

Cast steel and various copper alloys (brass, bronze, gunmetal) also have single-digit percentage shares. Medical implants, but also aircraft parts, are partly cast from titanium, but the titanium casting share is listed under "other" in foundry statistics, and on the other hand, only about 2% of the titanium is processed by casting, since it has a very high melting point and its melt tends to absorb oxygen, which leads to embrittlement in the solid state.

Achievable accuracies and productivity

The achievable accuracies are generally low. ISO tolerances range from IT16 to IT11 (smaller ones are more precise), and with special measures even IT10. Accuracies in forging are comparable (precision forging up to IT8), while machining accuracies are significantly better at IT7 to IT6, which is why cast parts are often reworked. Advances in foundry technology attempt to keep this rework to a minimum. As with forging, surface roughness is relatively high, with average peak-to-valley heights of 63 µm to 1000 µm; with machining, they range between 10 µm and 0.25 µm.

Series casting processes such as die casting are very productive. In contrast, vacuum casting is a process for precision casting of individual pieces, small series, or prototypes made of plastic.

Energy balance and material utilization

Material utilization is very good in both casting and forging. Only about 10% of the material is lost, while machining can remove more than half of the raw part in the form of chips. Despite the large amounts of energy required for melting, casting, like forging, is very energy-efficient when considering the entire process chain leading to the finished component, whereas machining requires about three times as much energy. 

Expendable mold casting

Expendable mold casting is a generic classification that includes sand, plastic, shell, plaster, and investment (lost-wax technique) moldings. This method of mold casting involves the use of temporary, non-reusable molds.

Sand casting

Sand casting is one of the most popular and simplest types of casting, and has been used for centuries. Sand casting allows for smaller batches than permanent mold casting and at a very reasonable cost. Not only does this method allow manufacturers to create products at a low cost, but there are other benefits to sand casting, such as very small-size operations. The process allows for castings small enough fit in the palm of one's hand to those large enough for a train car bed (one casting can create the entire bed for one rail car). Sand casting also allows most metals to be cast depending on the type of sand used for the molds.

Sand casting requires a lead time of days, or even weeks sometimes, for production at high output rates (1–20 pieces/hr-mold) and is unsurpassed for large-part production. Green (moist) sand, which is black in color, has almost no part weight limit, whereas dry sand has a practical part mass limit of 2,300–2,700 kg (5,100–6,000 lb). Minimum part weight ranges from 0.075–0.1 kg (0.17–0.22 lb). The sand is bonded using clays, chemical binders, or polymerized oils (such as motor oil). Sand can be recycled many times in most operations and requires little maintenance.

Loam molding

Loam molding has been used to produce large symmetrical objects such as cannon and church bells. Loam is a mixture of clay and sand with straw or dung. A model of the produced is formed in a friable material (the chemise). The mold is formed around this chemise by covering it with loam. This is then baked (fired) and the chemise removed. The mold is then stood upright in a pit in front of the furnace for the molten metal to be poured. Afterwards the mold is broken off. Molds can thus only be used once, so that other methods are preferred for most purposes.

Plaster mold casting

Plaster casting is similar to sand casting except that plaster of paris is used instead of sand as a mold material. Generally, the form takes less than a week to prepare, after which a production rate of 1–10 units/hr-mold is achieved, with items as massive as 45 kg (99 lb) and as small as 30 g (1 oz) with very good surface finish and close tolerances. Plaster casting is an inexpensive alternative to other molding processes for complex parts due to the low cost of the plaster and its ability to produce near net shape castings. The biggest disadvantage is that it can only be used with low melting point non-ferrous materials, such as aluminium, copper, magnesium, and zinc.

Shell molding

Shell molding is similar to sand casting, but the molding cavity is formed by a hardened "shell" of sand instead of a flask filled with sand. The sand used is finer than sand casting sand and is mixed with a resin so that it can be heated by the pattern and hardened into a shell around the pattern. Because of the resin and finer sand, it gives a much finer surface finish. The process is easily automated and more precise than sand casting. Common metals that are cast include cast iron, aluminium, magnesium, and copper alloys. This process is ideal for complex items that are small to medium-sized.

Investment casting

Investment casting (known as lost-wax casting in art) is a process that has been practiced for thousands of years, with the lost-wax process being one of the oldest known metal forming techniques. From 5000 years ago, when beeswax formed the pattern, to today's high technology waxes, refractory materials, and specialist alloys, the castings ensure high-quality components are produced with the key benefits of accuracy, repeatability, versatility, and integrity.

Investment casting derives its name from the fact that the pattern is invested, or surrounded, with a refractory material. The wax patterns require extreme care for they are not strong enough to withstand forces encountered during the mold making. One advantage of investment casting is that the wax can be reused.

The process is suitable for repeatable production of net shape components from a variety of different metals and high performance alloys. Although generally used for small castings, this process has been used to produce complete aircraft door frames, with steel castings of up to 300 kg and aluminium castings of up to 30 kg. Compared to other casting processes such as die casting or sand casting, it can be an expensive process. However, the components that can be produced using investment casting can incorporate intricate contours, and in most cases the components are cast near net shape, so require little or no rework once cast.

Waste molding of plaster

A durable plaster intermediate is often used as a stage toward the production of a bronze sculpture or as a pointing guide for the creation of a carved stone. With the completion of a plaster, the work is more durable (if stored indoors) than a clay original which must be kept moist to avoid cracking. With the low cost plaster at hand, the expensive work of bronze casting or stone carving may be deferred until a patron is found, and as such work is considered to be a technical, rather than artistic process, it may even be deferred beyond the lifetime of the artist.

In waste molding a simple and thin plaster mold, reinforced by sisal or burlap, is cast over the original clay mixture. When cured, it is then removed from the damp clay, incidentally destroying the fine details in undercuts present in the clay, but which are now captured in the mold. The mold may then at any later time (but only once) be used to cast a plaster positive image, identical to the original clay. The surface of this plaster may be further refined and may be painted and waxed to resemble a finished bronze casting.

Evaporative-pattern casting

This is a class of casting processes that use pattern materials that evaporate during the pour, which means there is no need to remove the pattern material from the mold before casting. The two main processes are lost-foam casting and full-mold casting.

Lost-foam casting

Lost-foam casting is a type of evaporative-pattern casting process that is similar to investment casting except foam is used for the pattern instead of wax. This process takes advantage of the low boiling point of foam to simplify the investment casting process by removing the need to melt the wax out of the mold.

Full-mold casting

Full-mold casting is an evaporative-pattern casting process which is a combination of sand casting and lost-foam casting. It uses an expanded polystyrene foam pattern which is then surrounded by sand, much like sand casting. The metal is then poured directly into the mold, which vaporizes the foam upon contact.

Non-expendable mold casting

Non-expendable mold casting differs from expendable processes in that the mold need not be reformed after each production cycle. This technique includes at least four different methods: permanent, die, centrifugal, and continuous casting. This form of casting also results in improved repeatability in parts produced and delivers near net shape results.

Permanent mold casting

Permanent mold casting is a metal casting process that employs reusable molds, usually made from metal. The most common process uses gravity to fill the mold. However, gas pressure or a vacuum are also used. A variation on the typical gravity casting process, called slush casting, produces hollow castings. Common casting metals are aluminum, magnesium, and copper alloys. Other materials include tin, zinc, and lead alloys and iron and steel are also cast in graphite molds. Permanent molds, while lasting more than one casting still have a limited life before wearing out.

Die casting

The die casting process forces molten metal under high pressure into mold cavities (which are machined into dies). Most die castings are made from nonferrous metals, specifically zinc, copper, and aluminium-based alloys, but ferrous metal die castings are possible. The die casting method is especially suited for applications where many small to medium-sized parts are needed with good detail, a fine surface quality and dimensional consistency.

Semi-solid metal casting

Semi-solid metal (SSM) casting is a modified die casting process that reduces or eliminates the residual porosity present in most die castings. Rather than using liquid metal as the feed material, SSM casting uses a higher viscosity feed material that is partially solid and partially liquid. A modified die casting machine is used to inject the semi-solid slurry into reusable hardened steel dies. The high viscosity of the semi-solid metal, along with the use of controlled die filling conditions, ensures that the semi-solid metal fills the die in a non-turbulent manner so that harmful porosity can be essentially eliminated.

Used commercially mainly for aluminium and magnesium alloys, SSM castings can be heat treated to the T4, T5 or T6 tempers. The combination of heat treatment, fast cooling rates (from using uncoated steel dies) and minimal porosity provides excellent combinations of strength and ductility. Other advantages of SSM casting include the ability to produce complex shaped parts net shape, pressure tightness, tight dimensional tolerances and the ability to cast thin walls.

Centrifugal casting

In this process molten metal is poured in the mold and allowed to solidify while the mold is rotating. Metal is poured into the center of the mold at its axis of rotation. Due to inertial force, the liquid metal is thrown out toward the periphery.

Centrifugal casting is both gravity and pressure independent since it creates its own force feed using a temporary sand mold held in a spinning chamber. Lead time varies with the application. Semi- and true-centrifugal processing permit 30–50 pieces/hr-mold to be produced, with a practical limit for batch processing of approximately 9000 kg total mass with a typical per-item limit of 2.3–4.5 kg.

Industrially, the centrifugal casting of railway wheels was an early application of the method developed by the German industrial company Krupp and this capability enabled the rapid growth of the enterprise.

Small art pieces such as jewelry are often cast by this method using the lost wax process, as the forces enable the rather viscous liquid metals to flow through very small passages and into fine details such as leaves and petals. This effect is similar to the benefits from vacuum casting, also applied to jewelry casting.

Continuous casting

Continuous casting is a refinement of the casting process for the continuous, high-volume production of metal sections with a constant cross-section. It's primarily used to produce a semi-finished products for further processing.  Molten metal is poured into an open-ended, water-cooled mold, which allows a 'skin' of solid metal to form over the still-liquid center, gradually solidifying the metal from the outside in. After solidification, the strand, as it is sometimes called, is continuously withdrawn from the mold. Predetermined lengths of the strand can be cut off by either mechanical shears or traveling oxyacetylene torches and transferred to further forming processes, or to a stockpile. Cast sizes can range from strip (a few millimeters thick by about five meters wide) to billets (90 to 160 mm square) to slabs (1.25 m wide by 230 mm thick). Sometimes, the strand may undergo an initial hot rolling process before being cut.

Continuous casting is used due to the lower costs associated with continuous production of a standard product, and also increased quality of the final product. Metals such as steel, copper, aluminum and lead are continuously cast, with steel being the metal with the greatest tonnages cast using this method.

Upcasting

The upcasting (up-casting, upstream, or upward casting) is a method of either vertical or horizontal continuous casting of rods and pipes of various profiles (cylindrical, square, hexagonal, slabs etc.) of 8-30mm in diameter. Copper (Cu), bronze (Cu•Sn alloy), nickel alloys are usually used because of greater casting speed (in case of vertical upcasting) and because of better physical features obtained. The advantage of this method is that metals are almost oxygen-free and that the rate of product crystallization (solidification) may be adjusted in a crystallizer - a high-temperature resistant device that cools a growing metal rod or pipe by using water.

The method is comparable to Czochralski method of growing silicon (Si) crystals, which is a metalloid.

Cooling curves

Cooling curves are important in controlling the quality of a casting. The most important part of the cooling curve is the cooling rate which affects the microstructure and properties. Generally speaking, an area of the casting which is cooled quickly will have a fine grain structure and an area which cools slowly will have a coarse grain structure. Below is an example cooling curve of a pure metal or eutectic alloy, with defining terminology.

Note that before the thermal arrest the material is a liquid and after it the material is a solid; during the thermal arrest the material is converting from a liquid to a solid. Also, note that the greater the superheat the more time there is for the liquid material to flow into intricate details.

Chvorinov's rule

The local solidification time can be calculated using Chvorinov's rule, which is:

Where t is the solidification time, V is the volume of the casting, A is the surface area of the casting that contacts the mold, n is a constant, and B is the mold constant. It is most useful in determining if a riser will solidify before the casting, because if the riser does solidify first then it is worthless.

The gating system

The gating system serves many purposes, the most important being conveying the liquid material to the mold, but also controlling shrinkage, the speed of the liquid, turbulence, and trapping dross. The gates are usually attached to the thickest part of the casting to assist in controlling shrinkage. In especially large castings multiple gates or runners may be required to introduce metal to more than one point in the mold cavity. The speed of the material is important because if the material is traveling too slowly it can cool before completely filling, leading to misruns and cold shuts. If the material is moving too fast then the liquid material can erode the mold and contaminate the final casting. The shape and length of the gating system can also control how quickly the material cools; short round or square channels minimize heat loss.

The gating system may be designed to minimize turbulence, depending on the material being cast. For example, steel, cast iron, and most copper alloys are turbulent insensitive, but aluminium and magnesium alloys are turbulent sensitive. The turbulent insensitive materials usually have a short and open gating system to fill the mold as quickly as possible. However, for turbulent sensitive materials short sprues are used to minimize the distance the material must fall when entering the mold. Rectangular pouring cups and tapered sprues are used to prevent the formation of a vortex as the material flows into the mold; these vortices tend to suck gas and oxides into the mold. A large sprue well is used to dissipate the kinetic energy of the liquid material as it falls down the sprue, decreasing turbulence. The choke, which is the smallest cross-sectional area in the gating system used to control flow, can be placed near the sprue well to slow down and smooth out the flow. Note that on some molds the choke is still placed on the gates to make separation of the part easier, but induces extreme turbulence. The gates are usually attached to the bottom of the casting to minimize turbulence and splashing.

The gating system may also be designed to trap dross. One method is to take advantage of the fact that some dross has a lower density than the base material so it floats to the top of the gating system. Therefore, long flat runners with gates that exit from the bottom of the runners can trap dross in the runners; note that long flat runners will cool the material more rapidly than round or square runners. For materials where the dross is a similar density to the base material, such as aluminium, runner extensions and runner wells can be advantageous. These take advantage of the fact that the dross is usually located at the beginning of the pour, therefore the runner is extended past the last gate(s) and the contaminates are contained in the wells. Screens or filters may also be used to trap contaminates.

It is important to keep the size of the gating system small, because it all must be cut from the casting and remelted to be reused. The efficiency, or yield, of a casting system can be calculated by dividing the weight of the casting by the weight of the metal poured. Therefore, the higher the number the more efficient the gating system/risers.

Shrinkage

There are three types of shrinkage: shrinkage of the liquid, solidification shrinkage and patternmaker's shrinkage. The shrinkage of the liquid is rarely a problem because more material is flowing into the mold behind it. Solidification shrinkage occurs because metals are less dense as a liquid than a solid, so during solidification the metal density dramatically increases. Patternmaker's shrinkage refers to the shrinkage that occurs when the material is cooled from the solidification temperature to room temperature, which occurs due to thermal contraction.

Solidification shrinkage

Most materials shrink as they solidify, but, as the adjacent table shows, a few materials do not, such as gray cast iron. For the materials that do shrink upon solidification the type of shrinkage depends on how wide the freezing range is for the material. For materials with a narrow freezing range, less than 50 °C (122 °F), a cavity, known as a pipe, forms in the center of the casting, because the outer shell freezes first and progressively solidifies to the center. Pure and eutectic metals usually have narrow solidification ranges. These materials tend to form a skin in open air molds, therefore they are known as skin forming alloys. For materials with a wide freezing range, greater than 110 °C (230 °F), much more of the casting occupies the mushy or slushy zone (the temperature range between the solidus and the liquidus), which leads to small pockets of liquid trapped throughout and ultimately porosity. These castings tend to have poor ductility, toughness, and fatigue resistance. Moreover, for these types of materials to be fluid-tight, a secondary operation is required to impregnate the casting with a lower melting point metal or resin.

For the materials that have narrow solidification ranges, pipes can be overcome by designing the casting to promote directional solidification, which means the casting freezes first at the point farthest from the gate, then progressively solidifies toward the gate. This allows a continuous feed of liquid material to be present at the point of solidification to compensate for the shrinkage. Note that there is still a shrinkage void where the final material solidifies, but if designed properly, this will be in the gating system or riser.

Solidification shrinkage of various metals

Metal	Percentage
Aluminium	6.6
Copper	4.9
Magnesium	4.0 or 4.2
Zinc	3.7 or 6.5
Low carbon steel	2.5–3.0
High carbon steel	4.0
White cast iron	4.0–5.5
Gray cast iron	−2.5–1.6
Ductile cast iron	−4.5–2.7

Risers and riser aids

Risers, also known as feeders, are the most common way of providing directional solidification. It supplies liquid metal to the solidifying casting to compensate for solidification shrinkage. For a riser to work properly the riser must solidify after the casting, otherwise it cannot supply liquid metal to shrinkage within the casting. Risers add cost to the casting because it lowers the yield of each casting; i.e. more metal is lost as scrap for each casting. Another way to promote directional solidification is by adding chills to the mold. A chill is any material which will conduct heat away from the casting more rapidly than the material used for molding.

Risers are classified by three criteria. The first is if the riser is open to the atmosphere, if it is then it is called an open riser, otherwise it is known as a blind type. The second criterion is where the riser is located; if it is located on the casting then it is known as a top riser and if it is located next to the casting it is known as a side riser. Finally, if the riser is located on the gating system so that it fills after the molding cavity, it is known as a live riser or hot riser, but if the riser fills with materials that have already flowed through the molding cavity it is known as a dead riser or cold riser.

Riser aids are items used to assist risers in creating directional solidification or reducing the number of risers required. One of these items are chills which accelerate cooling in a certain part of the mold. There are two types: external and internal chills. External chills are masses of high-heat-capacity and high-thermal-conductivity material that are placed on an edge of the molding cavity. Internal chills are pieces of the same metal that is being poured, which are placed inside the mold cavity and become part of the casting. Insulating sleeves and toppings may also be installed around the riser cavity to slow the solidification of the riser. Heater coils may also be installed around or above the riser cavity to slow solidification.

Patternmaker's shrink

Shrinkage after solidification can be dealt with by using an oversized pattern designed specifically for the alloy used. Contraction rules, or shrink rules, are used to make the patterns oversized to compensate for this type of shrinkage. These rulers are up to 2.5% oversize, depending on the material being cast. These rulers are mainly referred to by their percentage change. A pattern made to match an existing part would be made as follows: First, the existing part would be measured using a standard ruler, then when constructing the pattern, the pattern maker would use a contraction rule, ensuring that the casting would contract to the correct size.

Note that patternmaker's shrinkage does not take phase change transformations into account. For example, eutectic reactions, martensitic reactions, and graphitization can cause expansions or contractions.

Typical patternmaker's shrinkage of various metals

Metal	Percentage	in/ft
Aluminium	1.0–1.3	1⁄8–5⁄32
Brass	1.5	3⁄16
Magnesium	1.0–1.3	1⁄8–5⁄32
Cast iron	0.8–1.0	1⁄10–1⁄8
Steel	1.5–2.0	3⁄16–1⁄4

Mold cavity

The mold cavity of a casting does not reflect the exact dimensions of the finished part due to a number of reasons. These modifications to the mold cavity are known as allowances and account for patternmaker's shrinkage, draft, machining, and distortion. In non-expendable processes, these allowances are imparted directly into the permanent mold, but in expendable mold processes they are imparted into the patterns, which later form the mold cavity. Note that for non-expendable molds an allowance is required for the dimensional change of the mold due to heating to operating temperatures.

For surfaces of the casting that are perpendicular to the parting line of the mold a draft must be included. This is so that the casting can be released in non-expendable processes or the pattern can be released from the mold without destroying the mold in expendable processes. The required draft angle depends on the size and shape of the feature, the depth of the mold cavity, how the part or pattern is being removed from the mold, the pattern or part material, the mold material, and the process type. Usually the draft is not less than 1%.

The machining allowance varies drastically from one process to another. Sand castings generally have a rough surface finish, therefore need a greater machining allowance, whereas die casting has a very fine surface finish, which may not need any machining tolerance. Also, the draft may provide enough of a machining allowance to begin with.

The distortion allowance is only necessary for certain geometries. For instance, U-shaped castings will tend to distort with the legs splaying outward, because the base of the shape can contract while the legs are constrained by the mold. This can be overcome by designing the mold cavity to slope the leg inward to begin with. Also, long horizontal sections tend to sag in the middle if ribs are not incorporated, so a distortion allowance may be required.

Cores may be used in expendable mold processes to produce internal features. The core can be of metal but it is usually done in sand.

Filling

There are a few common methods for filling the mold cavity: gravity, low-pressure, high-pressure, and vacuum.

Vacuum filling, also known as counter-gravity filling, is more metal efficient than gravity 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 vacuum 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. The first patented vacuum casting machine and process dates to 1879.

Low-pressure filling uses 5 to 15 psig (35 to 100 kPag) of air pressure to force liquid metal up a feed tube into the mold cavity. This eliminates turbulence found in gravity casting and increases density, repeatability, tolerances, and grain uniformity. After the casting has solidified the pressure is released and any remaining liquid returns to the crucible, which increases yield.

Tilt filling

Tilt filling, also known as tilt casting, is an uncommon filling technique where the crucible is attached to the gating system and both are slowly rotated so that the metal enters the mold cavity with little turbulence. The goal is to reduce porosity and inclusions by limiting turbulence. For most uses tilt filling is not feasible because the following inherent problem: if the system is rotated slow enough to not induce turbulence, the front of the metal stream begins to solidify, which results in mis-runs. If the system is rotated faster it induces turbulence, which defeats the purpose. Durville of France was the first to try tilt casting, in the 1800s. He tried to use it to reduce surface defects when casting coinage from aluminium bronze.

Macrostructure

The grain macrostructure in ingots and most castings have three distinct regions or zones: the chill zone, columnar zone, and equiaxed zone. The image below depicts these zones.

The chill zone is named so because it occurs at the walls of the mold where the wall chills the material. Here is where the nucleation phase of the solidification process takes place. As more heat is removed the grains grow towards the center of the casting. These are thin, long columns that are perpendicular to the casting surface, which are undesirable because they have anisotropic properties. Finally, in the center the equiaxed zone contains spherical, randomly oriented crystals. These are desirable because they have isotropic properties. The creation of this zone can be promoted by using a low pouring temperature, alloy inclusions, or inoculants.

Inspection

Common inspection methods for steel castings are magnetic particle testing and liquid penetrant testing. Common inspection methods for aluminum castings are radiography, ultrasonic testing, and liquid penetrant testing.

Defects

There are a number of problems that can be encountered during the casting process. The main types are: gas porosity, shrinkage defects, mold material defects, pouring metal defects, and metallurgical defects.

Casting process

There are numerous different casting processes that can be classified according to several criteria.

Casting in molds that closely match the shape of the finished part is called form casting, which is the most commonly used method. Other methods include casting into bars or slabs, ingot casting, and casting continuous, theoretically endless strands, known as continuous casting.

Depending on the type of mold filling, a distinction is made between gravity casting, the standard method in which the melt falls into the mold due to the effect of gravity, as well as centrifugal casting with centrifugal forces (for rotationally symmetrical parts) and pressure casting, in which the melt is pressed into the mold by piston pressure.

A particularly important classification distinguishes between processes with molds that are used only once and are destroyed when the castings are removed (lost mold) and permanent molds that are used multiple times:

Lost-mold casting: Models are used to create the molds. A further distinction is made between models that can be used once (lost model) or multiple times (permanent model).

Casting with lost molds and permanent patterns: This is also called sand casting because the molds are made of sand. Depending on the type of mold production, it is suitable for individual parts and series production, as well as any workpiece mass, and is therefore of great economic importance. Numerous vehicle components are manufactured this way. Because sand is very temperature-resistant, it is primarily used for casting cast iron and cast steel.

Casting with lost molds and lost patterns: These include the lost -wax process (investment casting) and full-mold casting. In investment casting, the patterns are made from wax and encased in clay or ceramic. The wax is then melted out and the mold filled with molten material. This process is only suitable for small batches and small workpiece masses, but achieves high quality. In full-mold casting, the molds are made from Styrofoam, surrounded by any molding material, and then, without removing the patterns, molten material is poured over them, which burns the patterns. It is suitable for small batches and very large castings.

Casting with permanent molds: These are usually made of steel. They wear due to contact with the molten metal, resulting in poorer surface quality and dimensional accuracy than with lost-molten metals. For low-melting materials such as aluminum, they can be cast very frequently; for high-melting materials such as copper, they can be cast less frequently. Because permanent molds are very expensive, the number of pieces has a significant impact on the unit price. 

Gravity casting: gravity casting with permanent molds

Variants of centrifugal casting

Die casting: Here, the molten metal is pressed into the mold using a piston. The time required to fill the mold is reduced, making it ideal for large-scale production of small workpieces. It is particularly commonly used for aluminum casting, known as aluminum die casting.

centrifugal casting

Continuous casting

Low pressure casting: Here the air pressure around the melt is increased to force it into a riser pipe that leads to the mold.

There are also a number of special processes such as thixocasting, vacuum casting and squeeze casting.

Process chain during casting

The casting process chain consists of

the preparation with the production of the molds and the melting of the materials,

the casting, which includes filling the mold and solidifying the melt, as well as

post-treatment: demoulding, fettling and heat treatment.

Preparation

In preparation for the actual casting process, the casting, the molds are built, for which models are previously made. To create cavities in the workpieces, cores are manufactured and placed in the molds. Parallel to these tasks, the melt is prepared. 

Model making

The wax models for investment casting are either manually carved from a wax model or cast from a master model. In full-mold casting, the Styrofoam models are cut from blocks and sometimes assembled and glued from individual parts. Permanent models for sand casting can be made of wood, ceramic, or metal, which can be used for varying numbers of times. Cheap wooden models can sometimes only be used five times, while metal models can be used much more often. 

Mold making and mold material preparation

Permanent molds are forged or milled from steel and can cost several hundred thousand euros. Lost molds consist of molding material, usually sand held together with small amounts of clay and water. The molds can be milled from solidified sand (direct molding material milling). They are sometimes manufactured using 3D printing. Usually, however, models are used for mold construction, over which the molding material is placed. The still loose molding material must then be solidified, for which numerous different processes can be used. This includes simple shaking and pressing, which is used in series production because it can be automated (machine molding). In shell molding, a relatively thin layer of molding material is placed over the model and soaked with resins. These resins harden in the oven.

The molds must not only contain the shape of the workpieces to be manufactured, but also additional openings for feeding the melt. The cavity into which the melt is poured is called the gate. After the mold is filled, the volume of the liquid, cooling melt decreases, which is why more material must flow into the mold. A single gate is often not sufficient for this, which is why so-called risers are added, which are removed again with the gate after solidification. In the simplest case, the gate consists of an opening directly above the cavity for the workpiece. Better workpiece quality can be achieved if the melt falls to the floor in a separate shaft and flows laterally into the workpiece. There are numerous variations for the shape, size and number of risers and gates, as they have a major influence on component quality. 

Core production and core molding material preparation

Cores are necessary to produce castings with cavities. The cores are placed in the molds and removed after solidification. For permanent molds, the cores are usually also made of metal, while for sand molds, they are made of sand. They are also destroyed after casting. Cores are also needed for undercuts. 

Melt preparation

Preparation of the melt includes the selection of raw materials, melting in furnaces and melt treatment.

Metals can be used directly from the steelworks as raw materials, but large quantities of scrap are processed in the foundry. The industry is characterized by a high recycling rate. Some of the scrap is generated in the foundry itself; this includes defective castings as well as removed feeders and gating systems, which serve as recycled material. Old scrap from used and crushed components is also processed.

To obtain an alloy with the desired composition, various starting materials must be mixed. This mixture is called the charge. Using special software, it is possible to calculate which quantities and in which ratios are needed to achieve the most optimal charge.

Various industrial furnaces are used to melt metals. The cupola furnace, the arc furnace, and the induction furnace are particularly important. They are suitable to varying degrees for different materials. Cupola furnaces are used for ferrous materials, while arc furnaces are used for steel and non-ferrous metals, but both are only used for melting. Induction furnaces and resistance furnaces are also suitable for keeping the melt hot. About 60 to 70% of a foundry's energy consumption is attributable to melting. 

The melt is then treated. Various substances are added to the melt to prevent it from reacting with the oxygen in the air and from undergoing other unwanted changes. Sulfur hexafluoride is used for magnesium melts; its high density prevents contact between the melt and the oxygen in the air. During inoculation, foreign substances are added to the melts to influence solidification and thus the hardness and strength of the finished casting. 

Most of the time, the metals are melted in the foundry, sometimes in the steel or metallurgical works and then transported to the foundry by liquid metal transport.

Cast

The pouring of the melt into the mold is called casting. Afterward, and sometimes in parallel, the melt solidifies. These processes have a decisive influence on the quality of the castings. The melt can be poured directly from the furnace into the mold, but it can also be poured into ladles or ladles and only then into the molds. 

Mold filling

The melt can be poured directly into the molds, which is particularly common with molds that are open at the top. Pouring into a special sprue system is common. The melt can fall into the mold from above or flow into it from the side or from below. Falling melts lead to turbulence and turbulent flows. The melt mixes with air and absorbs unwanted gases, which later remain as pores in the casting. The molds are filled relatively quickly, however. As the mold is filled, there is little or no turbulence, which leads to laminar flows. The castings then contain very few pores. Tilt casting combines both methods.

The melt cools upon contact with the mold. Solidification should not begin until the mold is completely filled, as otherwise voids may occur. Some melts become viscous upon cooling, which promotes voids. The corresponding casting property is the mold filling capacity. Some molds are heated to keep the temperature difference as small as possible, or molds with low thermal conductivity are used. However, this prolongs the subsequent solidification. Other molds, especially permanent molds, are cooled to accelerate the process and reduce the thermal stress. 

Cooling and solidification of the melt

At the latest after the mold is completely filled, the melt cools and reduces its volume, which is referred to as (liquid) shrinkage. Melt must flow from the gate and the risers until it solidifies. During solidification, shrinkage also causes volume changes. These can no longer be compensated for by risers. After that, the volume decreases further until the casting reaches room temperature (solid-state shrinkage).

The exact process of solidification and subsequent cooling has a decisive influence on the microstructure and thus on the hardness and strength of the castings. Various substances are dissolved in the melt. As the solubility decreases during cooling, these substances are precipitated. In cast iron, for example, graphite (carbon) is precipitated. Cast iron grades are classified according to the (microscopic) shape of the graphite into cast iron with lamellar graphite, cast iron with vermicular graphite (worm-shaped graphite) and cast iron with spheroidal graphite, which differ in their hardness and strength. Dissolved gases can also be precipitated from the melt. If they cannot escape from the mold, they remain as pores or shrinkage cavities. The cooling rate also influences the hardness and strength of the castings. Slow cooling, for example, produces gray cast iron, which is easy to machine, while rapid cooling produces chilled cast iron. 

Aftercare

Post-processing includes demolding, in which the castings are removed from the molds. This can occur after they have cooled to room temperature or immediately after solidification. Especially in series production, the castings are removed from the molds as early as possible. This prevents further shrinkage from being hindered by the mold and also allows the molds to be quickly available for re-casting. In permanent molds, the castings are removed with ejectors; in lost molds, the mold is destroyed.  

Further treatment mainly involves cleaning and sometimes heat treatment.

Cleaning

Cleaning includes separating the gate and risers, removing cores, descaling (burn marks), desanding (removing molding material residue), repairing casting defects, and cleaning the surface. In some cases, machining allowances are also removed. Cleaning accounts for a large portion of the total foundry costs, as it can only be partially automated. A cleaning-friendly design of the casting is therefore crucial for unit costs. 

Heat treatment

Heat treatment is intended to improve the mechanical properties of the casting. For malleable cast iron (a type of cast iron), it is an integral part (tempering is a form of heat treatment). Cast steel is also usually annealed because the cast structure is very coarse-grained. For other materials, heat treatment may be omitted. 

Casting process simulation

Casting processes simulation uses numerical methods to calculate cast component quality considering mold filling, solidification and cooling, and provides a quantitative prediction of casting mechanical properties, thermal stresses and distortion. Simulation accurately describes a cast component's quality up-front before production starts. The casting rigging can be designed with respect to the required component properties. This has benefits beyond a reduction in pre-production sampling, as the precise layout of the complete casting system also leads to energy, material, and tooling savings.

The software supports the user in component design, the determination of melting practice and casting methoding through to pattern and mold making, heat treatment, and finishing. This saves costs along the entire casting manufacturing route.

Casting process simulation was initially developed at universities starting from the early 1970s, mainly in Europe and in the U.S., and is regarded as the most important innovation in casting technology over the last 50 years. Since the late 1980s, commercial programs are available which make it possible for foundries to gain new insight into what is happening inside the mold or die during the casting process.

Economic importance

Casting complex workpieces has the advantage over other production methods in that it involves relatively few process steps and reduces material consumption, which occurs, for example, in milling. Casting is also becoming increasingly important compared to machining for weight-optimized component geometries, as required in aircraft construction or titanium casting in medical technology. Although foundry production accounts for only about one percent of total manufacturing output in Germany, there are numerous sectors that rely on foundries as suppliers. The main customers, at over 50%, are the automotive industry (with a sharp increase in recent decades) and mechanical engineering. In contrast, the coal and steel industry's demand for castings has declined sharply.

In 2011, there were 78,000 employees in Germany, working in approximately 500 foundries. Production volume is expressed as the total mass of the workpieces produced in the foundry. In 2011, it was 5.8 million tons for Germany. The global annual production of castings was over 100 million tons in 2013. In 2013, China was the most important producer with 42.5 million tons, followed by the USA (12.8 million tons) and India (9.3 million tons). This was followed by Japan, Germany, and Russia with 5.3 to 4.3 million tons. 

Development prospects

German vehicle manufacturers alone source approximately 3 million tons from the production of German foundries. This shows that the industry is likely to be severely affected by the rise of electromobility, which will lead to the elimination of heavy mechanical components (engines, transmissions, etc.). The focus of automobile production is also shifting to Asia. Another important trend is the development of lightweight castings. Hand-molding of large individual pieces and small series has largely been discontinued in Germany for efficiency reasons, which means that large, hand-cast individual pieces often have to be sourced from abroad (e.g., Brazil).

Competing procedures

Many manufacturing processes can be used alternatively. Casting competes primarily with forming (forging) and machining (turning, drilling, milling, grinding). However, these require raw material in solid form, which is usually produced by casting. Casting can also produce very complex shapes and is suitable for large series production. Small and medium-sized workpieces are more commonly forged or machined.

Materials with a very high melting point are often produced using powder metallurgy. Instead of a melt, metal powder is used. For individual pieces, 3D printing is an alternative, either producing the part directly  or using an investment casting mold.

Sourced from Wikipedia