Читать книгу Metal Shaping Processes - Vukota Boljanovic - Страница 11
Оглавление2
METAL CASTING PROCESSES
2.3 Other Expendable Mold Casting Processes
2.4 Permanent Mold Casting Processes
2.5 Melting Practices and Furnaces
Metal casting processes are among the oldest methods for manufacturing metal goods. In most early casting processes the mold or form used had to be destroyed in order to remove the product after solidification. This type of mold is called an expendable mold. Since a new mold is required for each new casting, production rates in expendable mold processes are often limited by the time required to make the mold, rather than by the time needed to make the casting itself. The second type of mold is the permanent mold; permanent molds are used to produce components in endless quantities.
Casting has significant advantages compared with other methods of component manufacture. Castings are generally cheaper than components made in other ways. The casting process in one or another of its forms provides the designer with an unrestricted choice of shape that can be made in a single stage. A casting can usually be made much closer to the chosen design, which provides savings in both material and finishing processes compared with other methods of manufacture. In addition, the cast structure has the highest resistance to deformation at elevated temperatures, so that castings have higher creep strengths than wrought and fabricated components. Cast metal may also have superior wear resistance than the equivalent forged metal. These advantages combine to ensure that casting has become the most important process for the manufacture of components in metals (and in some other materials). In 2004 the total worldwide casting production was 75 million tons/year. Major applications of casting include the following:
•Transport: automobile, airspace, railways, shipping
•Heavy equipment: machining
•Plant machinery: chemical, petroleum, paper, sugar, textile, steel and thermoplastic
•Defense: vehicles, artillery, munitions, storage and supporting equipment
•Electrical machines: motors, generators, pumps, compressors
•Household: appliances, kitchen and gardening equipment, furniture, and fitting
•Art objects: sculptures, idols, furniture, lamp stands, and decorative items.
The advantages of casting have, in the past, been offset by significant disadvantages compared with wrought products. Castings are considered to be less ductile than the equivalent wrought product, and they have a less consistent performance in fatigue; they also have inferior integrity. The difference in ductility may be more apparent than real, however. A forged or rolled component may have a higher ductility than a casting in the direction of forging or rolling but a significantly lower transverse ductility. This is a distinct advantage if the longitudinal direction has to resist the principal stress, but it is not necessarily a sign of inferiority of the casting process.
Metal casting processes may be classified in several different ways:
•According to the mold type: (1) expendable mold (destroyed after each casting) and (2) permanent mold (reused many times);
•According to the type of pattern used for making a sand mold: (1) expendable pattern (melted for each mold), the pattern material being wax; and (2) permanent pattern (reused for many molds), the pattern material being wood or metal.
•According to the type of core used for producing a hole in casting: (1) expendable core (used in both sand and metal molds), the core material being sand; and (2) a permanent core (used with a permanent mold only), the core material being metal.
•According to the method by which the mold is filled: (1) gravity (sand casting, gravity die casting); (2) pressure (low and high pressure die casting); and (3) vacuum (vacuum investment casting).
Sand casting is a metal-forming process in which a molten metal compound is poured into a sand mold to produce a workpiece’s desired shape.
Sand casting has historically been the most popular casting method, producing by far the greatest tonnage of castings used in any country. Today, however, with the widespread conversion of automotive components from ferrous metals to aluminum, sand casting’s position as the dominant molding method is threatened. It is usually the least expensive way of making a component; its inherent cost advantage over other methods continues to make it an attractive molding method.
Basically, sand casting consists of six production steps:
Pattern. Preparing and placing a pattern having the shape of the desired workpiece.
Molding. Making a mold and incorporating a gating system using a molding machine.
Pouring. Pouring the molten compound metal into the mold.
Cooling. Cooling and solidifying the metal in the mold to form a desired shape.
Sand removal. Removing sand and scales from the surface of a separated workpiece, and removal of risers and gates.
Inspection. Performing in-process and preshipment inspection in accordance with standards.
Silica sand (SO2) is the molding aggregate most widely used by the foundry industry. Silica’s high fusion point, 1760°C (3200°F) and low rate of thermal expansion produce stable cores and molds compatible with all pouring temperatures and alloy systems. Its chemical purity also helps prevent it from interacting with catalysts or with the curing rate of chemical binders. There are two basic types of silica sand that are commercially available as a mold material. The first type is a round-grain silica sand containing roughly 99% or higher silica with minimal amounts of trace materials. The second type is lake sand. This sand contains approximately 94% silica, with the balance containing iron oxide, lime, magnesia, and alumina. Since impurities are removed, round-grain sands possess higher refractoriness than the lake sands. However, because of this much higher refractoriness, round-grain sands may have a higher propensity for veining and metal penetration defects. Lake sand has lower refractoriness but also has a lower tendency for casting defects. For proper functioning, molding sand must be able to withstand the high temperatures of molten metals, hold the shape of the mold when moist (usually with the aid of a bonding agent such as clay), be permeable enough to release gases, have sufficient strength to support the weight of the metal, and be of a fine enough texture to result in a smooth casting.
A pattern is used to make a cavity in the sand mold into which molten metal is poured. The pattern is a full-size model of the part, enlarged to account for shrinking and machining allowances in the final casting. The selection of the material used to make the pattern depends on the size and shape of the casting, the dimensional accutance and the quantity of castings required, and the molding process. Generally, material used to make patterns include wood, plastics, and metals. Patterns may be made of a combination of materials to reduce wear in critical regions, and they usually are coated with a patting agent – a liquid used over a patterns that leaves a slick film–to facilitate the removal of the casting from the molds.
There are four types of patterns: solid patterns, split patterns, match-plate patterns, and cope-and-drag patterns.
a) Solid Patterns
Figure 2.1 shows a solid pattern, also called loose pattern, made of one piece, used for simple shape and low-quantity production; its geometry is the same as the casting, adjusted in dimensions for shrinking and machining. Generally, it is made from wood and is inexpensive. However, determining the location of the parting line between two halves of the mold and positioning the gate system can be a problem.
Fig. 2.1 Solid pattern.
b) Split Patterns
A more complex pattern (Fig. 2.2) for round or irregular-shaped workpieces made in two or more parts is called split pattern. The two pattern halves usually predetermine the parting line of the mold. Split patterns are used for complex shape of the workpieces and moderate production quantities.
Fig. 2.2 Split pattern.
c) Match Plate Patterns
Match plate patterns are split patterns assembled on opposite sides of wooden or metal plates known as match plates (Fig. 2.3). Holes in the plate allow the top (core) and bottom (drag) sections of the mold to be aligned accurately. For long runs and top quality, match plate patterns, or cope-and-drag plate, are used. The major advantage to this is that a single machine can make both cope and drag molds from one pattern.
Fig. 2.3 Match plate pattern.
Early match plate molding required operators to assemble a pair of removable snap-flasks together with the pattern, and then fill each side of the mold with sand, followed by a simple machine squeeze cycle to make the mold. Those early mold machines were generically called squeezer machines. After stripping the mold, the snap flasks remained at the machine for reuse, and flockless molds were delivered onto a mold handling system for pour-off and cooling prior to shakeout.
d) Cope-and-Drag Patterns
Cope-and-drag patterns (Fig. 2.4) are similar to match plate patterns except that each half of the split pattern is assembled to a separate plate so separate patterns, and possibly separate machines, are used to make the mold halves. Cope-and-drag patterns include a gating and riser system.
Fig. 2.4 Cope-and-drag pattern.
The first step in match-plate design for both match plate patterns and cope-and-drag patterns is to modify the cast part’s geometry for the sand casting process. The part is scaled to accommodate metal shrinkage during the casting and machining in finishing operations. Shrink rate varies with alloy and part geometry. After the parting line is defined, draft is applied to the part. Typically two degrees, the draft allows the pattern to be removed from the cope and the drag.
As mentioned above, match plates use split patterns. The part file is separated along the parting line and the two halves are attached to the match plate base. The half that forms the cope side of the mold is assembled to the top face of the match plate, and drag side is placed on the bottom face. Next, runners, gates, risers, and wells are added. To align the cope and drag, locations are also added to the match plate. For storage purpose this type of patterns uses a removable sprue; a mounting pad is placed where the sprue will be attached.
To define the internal shape of the casting, a core is required. A core is a full-scale model of the internal surface of the casting part, which is placed in the mold cavity to form the internal surface and removed from the finished part during shakeout and further processing. The actual size of the core must include allowance for shrinking and machining. Sand cores are made of special sands, which are mixed with binders and rammed into a core box that has been made to produce cores of the proper dimensions. Cores must be baked at carefully controlled temperatures to make them hard enough to withstand the pressure exerted by the molten material. Cores of complex shapes may be made in several sections and cemented together. Depending on the geometry of the casting, the cores may require structural supports to hold them in the proper position in the mold cavity during the pouring of the molten metal. These supports are called chaplets. On pouring and solidification, the chaplets are integrated into the casting. The portions of the chaplets protruding from the castings are cut off. Figure 2.5 schematically illustrates how a core is held in the mold cavity with and without chaplets.
Fig. 2.5 Core held in place in the mold cavity: a) core held in with chaplets; b) core held without chaplets.
Sand molds are characterized by the types of sand that compose them and by the methods used to make the molds. They are often classified as greensand, dry sand, skin-dried, and no-bake molds.
Greensand molds. Clay-bonded sands have provided the principal medium from which molds for castings have been produced for centuries. In essence, the mold material consists of sand, usually silica in a quartz form, clay, and water. The water develops the bonding characteristics of the clay, which binds the sand grains together. Under the application of pressure, the mold material can be compacted around a pattern to produce a mold having sufficient rigidity to enable the metal to be poured into it to produce a casting. When the mold is used in its moist condition, it is referred to as green and the method of producing the molds is referred to as the greensand molding process. The term greensand does not refer to color but to the fact that the raw sand and binder mixture in the mold is moist or damp while the metal is being poured into it. Greensand molding is the least expensive method of making a mold, and the sand is easily recycled for subsequent use.
The sand used for greensand molding must fulfill a number of requirements:
1.It must pack tightly around the pattern, which means that it must have flowability.
2.It should be capable of being deformed slightly without cracking, so that the pattern can be withdrawn. In other words, it must exhibit plastic deformation.
3.It must have sufficient strength to strip from the pattern and support its own weight without deforming, and to withstand the pressure of the molten metal when the mold is cast. It must therefore have green strength.
4.It must be permeable, so that gases and steam can escape from the mold during casting.
5.It must have dry strength, to prevent erosion of the mold surface by liquid metal during pouring as the surface of the mold cavity dries out.
6.It must have refractoriness, to withstand the high temperature involved in pouring without melting or fusing to the casting.
7.With the exception of refractoriness, all of these requirements are dependent on the amount of active clay present and on the water content of the mixture.
Dry sand molds. If the mold is made using oily or plastic binder and dried at a temperature just above 180°C (356°F) the majority of the free moisture will be removed. This is the principal of the dry-sand molding process. Removal of the free moisture is accompanied by a significant increase in the strength and rigidity of the mold. This enables the mold to withstand much greater pressures and so, traditionally; the dry-sand process has been used in the manufacture of large, heavy castings. A dry sand molds provides better dimensional control in the cast product compared to greensand mold.
Skin-dried molds. Skin-dried mold is made in the same way as a greensand mold, but after it is made, the inside cavity surfaces need to be sprayed with a mixture of 10% water to one part molasses or lignin sulphite. The sprayed areas are dried using torches, heating lamps, or other means to a depth of about 10 to 15 mm, (0.4 to 0.6 in.), leaving a smooth hard skin.
No-bake molds. In the no-bake mold process, a synthetic liquid resin is mixed with the sand to form a filled mold that hardens at room temperature. This type of mold has a good dimensional control in high-production applications.
Molding material is a mixture of sand and other components that serve as binders. In industry the ingredients are blended together in mulling machines. To form the mold cavity, the traditional method is to pack the molding sand in a box called a flask, around a pattern, and with a gate system (pouring basin, sprue, sprue base—wall, runner, riser, and gate). Hand ramming of sand around a pattern is rarely used today except under special circumstances for simple casting.
To allow for easy removal of the pattern, the flask is made to separate horizontally at a parting line. When the pattern is drawn from the mold, if holes and cavities are required inside, they are made by inserts of sand (cores) before the two mold halves (the cope and drag) are reassembled; a cavity remains in the sand.
To increase production rate and improve quality of casting, a sand mixture is compacted around the pattern by a molding machine.
There are a number of techniques for doing this.
Squeeze molding machines. Squeeze-molding machines automatically insert and compact sand in a mold. The processes used are designed to produce a uniform compaction. Jolting is sometimes used to help settle the sand in a mold. These molds are made in flasks.
Sandslingers. High-speed streams of sand fill the flask uniformly and tend to pack the material effectively. Sandslingers are used to fill large flasks and are typically operated by machine.
Impact molding. A controlled explosive impulse is used to compact the sand. The mold quality with this technique is quite good.
An alternative to the traditional flask for each sand mold is flaskless molding, which refers to the use of one master flask in a mechanized system of mold production. Each sand mold is produced using the same master flask. The most frequently used include the following:
Vertical flaskless molding. In flaskless molding, the master flask is contained as an integral unit of the totally mechanized mold-producing system. Once the mold has been stripped from the integral mold-producing unit, it is held against the other half of the mold with enough pressure to allow the metal to be poured.
In the vertical flaskless systems the completely contained molding unit blows and squeezes sand against a pattern (or multiple patterns), which has been designed for a vertical gating system. Molds of this type can be produced in very high quantities per hour, and they are of high density with excellent dimensional reproducibility.
Among the disadvantage of flaskless molding are those restrictions that apply to the size of casting, the use of complicated cores and core assemblies, and the number of castings per mold. Mold handling may be more difficult.
2.2.6 The Sand Casting Operation
In order to produce a sand casting, a typical outline of the manufacturing steps that need to be followed in the sand casting operation is shown in Fig. 2.6.
1.A mechanical drawing of the part is used to generate a design of the pattern and core (if necessary). Decisions about such issues as part shrinkage, material to be used for the pattern, and draft must be built into the drawing. Core drawings need to define how to hold the core in place.
2.Patterns are made and mounted on plates equipped with pins for alignment. Core boxes produce core halves, which are pasted together.
Fig. 2.6 Outline of production steps in sand casting operation.
3.The cope half of the mold is assembled by securing the cope pattern plate to the flask with aligning pins and attaching inserts to form the gate system (sprue and risers). The flask is rammed with sand. Sand is packed about the pattern and gate systems. The half pattern and other inserts are removed. The drag half is made in the same manner with the pattern inserted. A bottom board is placed below the drag and aligned with pins. The pattern, flask, and bottom board are inverted; and the pattern is withdrawn leaving the appropriate cavity. The core is set in place with drag cavity to make concave or internal features for the cast part.
4.The cope is placed on top of the drag and the assembly is secured with pins mating the mold halves. The flasks are then subjected to pressure to counteract the force of buoyancy. (Buoyancy results from the weight of the liquid metal being displaced by the core, according to Archimedes’ law.) The force tending to lift the cope is equal to the weight of the displaced liquid less the weight of the core. A mathematical expression of this situation is
| Fb = Wm − Wc | (2.1) |
where
| F b | = | force of buoyancy N, (lb) |
| W m | = | weight of molten metal displaced N, (lb) |
| W c | = | weight of the core N, (lb) |
5.Molten metal is preheated in a furnace or crucible to pouring temperature. The exact temperature may be closely controlled depending upon application. Degassing and other treatment procedures, such as removal of impurities (i.e., slag) may be done at this time.
6.The molten metal is poured slowly but continuously into the mold until the mold is full. As the molten metal solidifies and cools, the metal will shrink. As the molten metal cools, the volume will decrease. During this time, molten metal may backflow from the risers to feed the casting cavity and maintain the same shape.
7.After the metal solidifies below the eutectic point, the casting is removed from the mold with no concern for final metal properties. At this point, the sand mold is broken up and the casting removed. The bulk of the remaining sand and cores can be removed by using a vibrating table, a sand/shot blaster, hand labor, etc.
8.The sprue and risers are cut off and recycled. The casting is cleaned, finished, and heat treated (when necessary).
9.The casting is inspected using nondestructive testing (NDT) and destructive methods in accordance with standards.
The rammed graphite mold is typically used for large industrial casting for reactive metals such as titanium and zirconium. It uses graphite instead of sand in a process similar to sand casting. Traditionally, a mixture of properly size-fractioned graphite powder, pitch, corn syrup, and water is rammed against a wooden or fiberglass pattern to form a mold section. The mold sections are air-dried, baked at 175°C (350°F) and then fired in furnace for 24 hours at 1025°C (1877°F). This causes the mold to carbonize and harden. Mold ramming is a labor-intensive process that cannot be easily mechanized. The graphite mold is so hard that it must be chiseled off the cast parts. The castings are generally cleaned in an acid bath, followed if necessary by chemical milling, to remove any reaction zone, and weld-repaired, then sand-blasted for a good surface appearance.
Rammed-graphite molds need to be stored under controlled humidity and temperature.
2.3 OTHER EXPENDABLE MOLD CASTING PROCESSES
After sand-casting, the oldest expendable mold casting technology, was developed, several other expendable mold casting processes were invented to meet special needs. The differences among these methods are in the composition of the mold material, the methods by which the mold is made, or in the way the pattern is made.
An expandable mold is formed of refractory materials. The thermal conductivity of the mold is a property of the material selected. It is also a function of particle size and distribution. The thermal conductivity influences the rate of transfer through the mold and therefore the rate of solidification which, in turn, influences the metallurgical integrity of the casting.
When choosing the mold material, one should be chosen that is sufficiently refractory to withstand the pouring temperature of the particular metal being cast without melting or softening. As long as the material is pure, the melting point value is a good guide to refractoriness. However, the melting point can be reduced dramatically by adding very small amounts of alkali metal salts or iron oxide.
All of these involve the use of temporary and not reusable molds, and they need gravity to help force the molten fluid into the casting cavities. In this process the mold is used only once.
Shell molding is a foundry process in which the molds are made in the form of thin shells. This technique is also called the “C” process or Croning; the Croning process was developed in Germany after World War II and patented by Johannes C. A. Croning.
Shell molds are made in the following sequence of operations:
1.Initially preparing a match-plate pattern or cope-and-drag pattern. In this process, the pattern is made of ferrous metal or aluminum.
2.Mixing fine silica sand with 3 to 6 % thermosetting resin binder. Common shell molding binders include phenol formaldehyde resins, furan, or phenolic resins and baking oils similar to those used in cores.
3.Heating the pattern, usually to between 230 and 280°C (446 to 536°F) and placing it over a dump box containing sand mixed with binder.
4.Inverting the damp box (the sand is at one end of a box and the pattern at the other) so that sand and resin binder fall onto the hot pattern and form a shell of the mixture to partially cure on the surface to form a hard shell. The box is inverted for a time determined by the desired thickness of the shell. In this way, the shell mold can be formed with the required strength and rigidity to hold the weight of the molten metal. The shells are light and thin, usually 6 to 10 mm (0.2 to 0.4 in.) in thickness.
5.Repositioning the drag box and pattern so that loose uncured particles drop away.
6.Heating the shell with the pattern in an oven for several minutes to complete curing.
7.Removing the shell mold from the pattern.
8.Repeating for the other half of the shell.
9.Joining the two mold halves together and supporting shell mold by sand or metal shot in the flask, and pouring the molten metal.
10.Removing the casting, cleaning, and trimming.
The steps in shell molding are illustrated in Fig. 2.7.
There are many advantages to the shell-mold process:
•Rigidly bonded sand provides great reproducibility and produces castings nearer to net shape with intricate detail and high dimensional accuracy of ±0.25 mm (±0.010 in).
Fig. 2.7 Steps in shell molding: a) pattern heated and clamped over a box; b) inverted box; c) repositioned box; d) shell with pattern is heated in oven; e) half shell is stripped from the pattern; f) shell mold supported by metal shot in flask and poured melted metal; g) finished casting.
•Castings can range from 30 g to 12 kg (1 oz to 25 lb).
•There is a virtual absence of moisture, resulting in a lack of moisture-related defects. In fact, the burning resin provides a favorable anti-oxidizing atmosphere in the casting surface.
•Because mold shells are thin, permeability for gas escape is excellent, allowing the use of finer sands. Finer sand and excellent flowability produce dense mold surfaces and contribute to producing complex casting with high-quality surface finish 1.25 µm (50 µin.) RMS (root mean squere).
•Heat from burning slows the casting-cooling rate, yielding a more machinable structure.
•Resin-bond strength allows smaller draft angles, deep draws, and built-in mold locators that prevent mold shift mismatch.
Disadvantages to the shell-mold process are:
•Since the tooling requires heat to cure the mold, pattern costs and pattern wear can be higher.
•Energy costs are higher than for other processes.
•Material costs are higher than those for greensand molding.
Tooling for shell molds is generally more expensive than for other processes because it is more precise and must resist heat and abrasion. Also, heat and resin binders are necessary. For these reasons, shell molding is most suitable for medium-to high-volume parts, where the manufacturer utilizes added value.
Tooling for shell molds is generally more expensive than for other processes because it is more precise and must resist heat and abrasion. Also, heat and resin binders are necessary. For these reasons, shell molding is most suitable for medium-to high-volume parts where the manufacturer utilizes added value.
The V-process, or vacuum molding, is one of the newest casting processes; in it, unbonded sand is held in place in the mold by a vacuum. In this process, a thin plastic film of 0.7 to 2.0 mm (0.03 to 0.08 in.) is heated and placed over a pattern. The softened film drops over the pattern with 26 to 52 kPa, (3.8 to 7.6 psi) and the vacuum tightly draws the film around the pattern. The flask is placed over the plastic-coated pattern and is filled with dry, unbonded, extremely fine sand and vibrated so that the sand tightly packs the pattern. The flask walls also create a vacuum chamber with the outlet shown in Fig. 2.8 at the right side of each illustration.
Another unheated sheet of plastic film is placed over the top of the sand in the flask, and the vacuum is applied to the flask. The vacuum hardens the sand so the pattern can be withdrawn. The other half of the mold is made the same way. After cores are put in place (if needed), the mold is closed. During pouring the mold is still under vacuum but the casting cavity is not. When the metal has solidified, the vacuum is turned off and the unbonded sand runs out freely, releasing a clean casting with zero draft, high-dimensional accuracy and with a 125 to 150 RMS surface finish.
Fig. 2.8 Steps in V-process: a) the pattern is placed on a hollow carrier plate; b) a heater softens the plastic film; c) the vacuum draws the plastic film tightly around pattern; d) the flask is placed on the film-coated pattern; e) the flask is filled with dry sand; f) the back of the mold is covered with unheated plastic film; g) the vacuum is released and the mold is stripped; h) the cope and drag assembly form a plastic-lined cavity; during pouring, molds are kept under vacuum; i) the vacuum is released and as the sand flows freely, the casting is freed; j) finished part.
This process is economical, environmentally and ecologically acceptable, energy thrifty, versatile, and clean. A disadvantage of the V-process is the necessity of plated pattern equipment. The steps of the process are shown in Fig. 2.8.
2.3.3 Evaporative Pattern Casting Process
This process is also referred to as any of the following: expanded polystyrene (EPS) process, lostfoam process, expendable pattern casting, and lost pattern process. It is unique in that a mold and pattern must be produced for every casting. Evaporative-pattern casting is a manufacturing technique in which an expanded pattern is used during the casting process. Preforms of the parts to be cast are molded in expanded polystyrene; an aluminum master mold is used to create the pattern. Pre-expanded polystyrene beads are injected into the pattern mold. Steam expands the bead to bond the beads together and fill most of the space. As expandable polymers are steamed, the beads continue to bond and create a solid pattern. The mold is then cooled and opened, and the polystyrene pattern is removed. Bonding various individual segments of the mold together, using hot-melt adhesive, allows a complex shape of the pattern to be formed.
The individual patterns are assembled into a cluster around a sprue and then coated with a refractory compound. After the coating has dried, the foam pattern assembly is positioned on several inches of loose dry sand in a vented flask. Additional sand is then added while the flask is vibrated until the pattern assembly is completely compacted and embedded in sand. Finally, molten metal is poured into the pattern, the pattern vaporizes upon contact with the molten metal, which replaces it to form the casting. Gas formed from the vaporized pattern permeates through the coating on the pattern, the sand, and finally through the flask vents. The sequence in this casting process is illustrated in Fig. 2.9.
Fig. 2.9 Evaporative-pattern process: a) foam pattern of polystyrene is coated with refractory compound; b) coated foam pattern is placed in flask and sand is compacted around pattern; c) molten metal vaporizing the pattern and replacing it to form the casting.
A significant characteristic of the evaporative-pattern casting process is that the pattern need not be removed from the mold, no cores are needed, inexpensive flasks are satisfactory for the process, complex shapes can be cast, no binders or other additives are required for the sand, and machining can be eliminated. However, expensive tooling (i.e., a new pattern is needed for every casting) restricts the process to long-run casting.
Investment or lost-wax casting is primarily a precision method of casting metals to fabricate near-net-shaped metal parts from almost any alloy. Intricate shapes can be made with high accuracy. In addition, metals that are hard to machine or fabricate are good candidates for this process. This process is one of the oldest manufacturing processes and was developed by the ancient Egyptians some 4000 years ago.
Investment casting got its name from the fact that the pattern is invested (covered completely) with the refractory material. It can be used to make parts that cannot be easily produced by other manufacturing processes, such as turbine blades, and other components requiring complex, often thin-wall castings, for example aluminum structural parts having a wall dimension of less than 0.75 mm (0.03 in.).
The sequences involved in investment casting are shown in Fig. 2.10. A mechanical drawing of the part is the starting point of the process; the drawing illustrates an injection die in the desired shape. This die will be used to inject wax or a plastic such as polystyrene to create the pattern needed for investment casting. The patterns are attached to a central wax sprue, creating an assembly, or mold. The sprue contains the pouring cup from which the molten metal will be poured into the assembly.
Fig. 2.10 Steps in investment casting: a) wax patterns are produced by injection molding; b) the patterns are attached to sprue to form a pattern assembly (tree); c) pattern assembly is coated with a thin layer of refractory material; d) the mold is completed by covering coated tree with sufficient refractory material to make it rigid; e) the mold is held in an inverted position and dried in, the wax is melted out of the cavity; f) preheated mold is poured with molten metal; g) the mold is shaken out; h) casting is separated from the sprue.
The pattern assembly (tree) is then dipped into a slurry of very fine-grained silica and binders including water, ethyl silicate, or other refractory material, and allowed to dry. After this initial coating has dried, the final mold is achieved by repeatedly dipping the pattern tree into the refractory slurry until a shell of 6 to 8 mm (0.24 to 0.31 in.) has been applied.
The mold is allowed to air dry in an inverted position for about 8 to 10 hours to melt out the wax. The mold is then pre-heated to 800 to 1000°C (1474 to 1832°F) for 3 to 4 hours (depending on the metal to be cast) to drive off water, remove any residues of wax, and harden the binder.
Pouring into the preheated mold also ensures that the mold will fill completely. Pouring can be done using gravity or vacuum conditions. After the metal has solidified, the mold is broken up and the casting is removed. Most investment castings need some degree of post-casting machining to remove the sprue and runners and to improve the surface finish. The gate is ground off. Parts are also inspected to make sure they were cast properly, and if not, are either fixed or scrapped.
Investment casting produces exceedingly fine quality products made of all types of metals. It has special applications in fabricating very high-temperature metals such as superalloys for making gas turbine engine blades and nozzle guide vanes.
The variety of steels used and of parts cast has increased dramatically as designers and engineers have realized the potential of investment castings. The aerospace, armament, automotive, food, petrochemical, nuclear, textile, valve and pump, and other general engineering industries all use the technique.
Aluminium alloys are the most widely used nonferrous investment castings in the fields of electronics, avionics, aerospace, pump and valve applications, and military command equipment.
Titanium alloy investment castings are produced for static structural applications requiring metallurgical integrity with high fracture toughness.
Plaster mold casting is similar to sand molding except that plaster is substituted for sand.
Plaster of Paris, or simply “plaster,” is a type of building material based on calcium sulfate hemi-hydrate, nominally CaSO4 · 0.5H2O. It is created by heating gypsum to about 150°C (302°F). The reaction for the partial dehydration is
CaSO4 · 2H2O → CaSO4 · 0.5H2 + 1.5H2O (released as steam).
In plaster mold casting a plaster is mixed using talc, sand, sodium silicate, and water to form a slurry and to control contraction and setting time, reduce cracking, and increase strength. This slurry is poured over the polished surfaces of the pattern halves (usually plastic or metal) in a flask and allowed to set. The slurry sets in less than 15 minutes to form the mold. The mold halves are extracted carefully from the pattern and then dried in an oven at a temperature range of 120 to 260°C (248 to 500°F) to remove moisture.
The mold halves are carefully assembled to form the mold cavity and are preheated to about 120°C (248°F). Prior to mold preparation the pattern is sprayed with a thin film of patting compound to prevent the mold from sticking to the pattern. Molten metal is then poured into mold. After the metal is solidified and cooled, the plaster mold is broken away ftom the finished casting. This process is normally used for nonferrous metals such as aluminum, zinc, or copper-based alloys. It cannot be used to cast ferrous material because the sulfur in gypsum slowly reacts with iron.
The minimum wall thickness of aluminum plaster castings typically is 1.5 mm (0.06 in.). Plaster molds have high reproducibility, permitting castings to be made with fine details and close tolerances. Because plaster molds have very low permeability, any gas produced during the solidification of the metal cannot escape, because mechanical properties and casting quality depend on alloy composition and foundry technique. Slow cooling due to the highly insulating nature of plaster molds tends to magnify solidification-related problems, and thus solidification must be controlled carefully to obtain good mechanical properties.
Casting sizes may range in weight from less than 30 grams (1 oz) to 7 kg (15 lb). The draft allowance is 0.5 to 1.0 degree. Good surface finish and dimensional accuracy, as well as the capability to make thin cross-sections, are advantages of plastermold casting.
The ceramic mold casting process, also called cope-and drag investment casting, uses a permanent pattern made of plastic, wood, or metal. To make the slurries for molding, fine-grained zircon (ZrSiO4), aluminum oxide, and fused silica are mixed with bonding agents and poured over the pattern, which has been placed in a flask. These slurries are comparable in composition to those used in investment casting. Like investment molds, ceramic molds are expendable. However, unlike the single-part molds obtained in investment castings, ceramic molds consist of a cope and drag setup.
Making a ceramic mold is similar to making a plaster mold in that the ceramic slurry is poured over the pattern. It hardens rapidly to the consistency of rubber; after that, the halves of the mold are removed from the pattern and reassembled. The volatiles are removed using a flame torch or in a low-temperature oven. The mold is then baked in a furnace at about 1000°C (1832°F). The mold is now capable of high-temperature pours. This ceramic molding can be used to cast ferrous and other high-temperature alloys, stainless steel, tool steels, and titanium. Its advantages (good accuracy and surface finish) are similar to those of plaster mold casting. A draft allowance of 1° is recommended. Parts made as a ceramic mold casting can be very small or up to a ton.
The process is expensive but can produce casting with fine detail and eliminate secondary machining operations.
2.4 PERMANENT MOLD CASTING PROCESSES
Permanent mold casting refers to all casting technologies in which the mold cavity is reused many times and is made of a metallic material or graphite. This is in contrast to other casting technologies, such as sand casting, investment casting, and others in which the mold is made of nonmetallic materials. Metal mold casting is the predominant way to manufacture shape castings. Specifically, about 90% of all aluminum castings produced are in metal molds, including gravity fed, low-pressure, and high-pressure die castings.
Permanent mold casting technologies are classified as gravity, pressure, squeeze, and specialized processes.
2.4.1 Basic Permanent Mold Casting
The permanent mold casting process is the production of castings by the pouring of molten metal into permanent metal molds using gravity or tilt pouring. These molds are commonly made of steel or cast iron, and their cores are made from metal or sand. Metal molds are constructed of two or more sections that are designed for easy, precise opening and closing. The cavity designs for these molds do not follow the same rules for shrinkage as do sand casting molds, due to the fact that the metal molds heat up and expand during the pour, so the cavity does not need to be expanded as much as in the sand castings.
Typical parts made by permanent mold casting are automobile pistons, cylinder heads, gears, and kitchenware. Parts that can be made economically generally weigh less than 25 kg (55 lb), although special castings weighing a few hundred kilograms have been made using this process. The cavity, with gating system included, is machined into the halves to provide accurate dimensions and good surface finish. Steps in the basic permanent mold casting process are shown in Fig. 2.11.
Fig. 2.11 Steps in basic permanent-mold casting: a) mold is preheated and coated; b) mold is closed and molten metal poured; c) mold is opened and casting is ejected; d) finished part.
In preparation for casting, the mold is first preheated at 150 to 260°C (302 to 500°F); then a refractory washer mold coating is brushed or sprayed onto those surfaces that will be in direct contact with the molten metal alloy. The proper operating temperature for each casting is set. Cores, if applicable, are inserted, and the mold is closed manually or mechanically. The alloy is heated at the pouring temperature and is poured into the mold through the gating system. Unlike expendable molds, permanent molds do not collapse, so the mold must be opened before appreciable cooling contraction occurs in order to prevent cracks from developing in the casting.
It is desirable and generally more economical to use permanent steel cores to form cavities in a permanent mold casting. When the casting has re-entrant surfaces or cavities from which one-piece permanent metal cores cannot be withdrawn, destructive cores made of sand, shell, plaster, and other materials are used. This process then is called semipermanent mold casting. Sectional steel cores are used in some instances.
Advantages of permanent mold casting include the facts that cast surfaces are generally smoother than sand castings, and closer dimensional tolerances can be maintained. Permanent mold castings usually have better mechanical properties than sand castings because solidification is more rapid and fill is more laminar.
This process is used mostly for aluminum, magnesium, copper alloys, and gray iron because of their generally lower melting points. The process is not economical for small production runs and intricate shapes because of the difficulty in removing the casting from the mold.
2.4.2 Vacuum Permanent Mold Casting
Vacuum permanent mold casting (not to be confused with vacuum molding) is similar to low-pressure permanent mold casting, except for the step of filling the mold. In this case, the molten metal is sucked upward into the mold by vacuum pump. A schematic illustration of the vacuum casting is shown in Fig. 2.12.
Fig. 2.12 Schematic illustration of the vacuum casting process a) before flow up of molten metal, and b) after flow up of molten metal into cavity.
The permanent mold is enclosed in an airtight bell housing. The housing has two openings: the sprue at the bottom, through which molten metal enters the mold; and the vacuum outlet at the top. The sprue opening is submerged below the surface of the molten metal and the vacuum is drawn within the housing, creating a pressure differential between the mold cavity and the molten metal in the crucible. This pressure differential causes the molten metal to flow up the sprue and into the mold cavity, where it solidifies. The mold is removed from the housing, opened, and the casting ejected.
By controlling the vacuum, the pressure differential between the mold cavity and the molten metal can be varied, allowing for the differential fill rates that are necessitated by certain part designs and gating requirements. This results in tight control of the fill rate, which also directly influences the soundness of the casting. Through proper part design, mold design, and the use of the vacuum mold process, voids, shrinks, and gas pockets can be greatly reduced or eliminated in critical areas. Because the sprue opening is submerged beneath the surface of the molten metal, only pure alloy, free from oxides and dross, can enter the die cavity. This helps to produce clean, sound castings with minimal foreign materials that detract from strength, appearance, and machinability.
The mechanical properties of the vacuum permanent mold casting are 10 to 15% superior to those of the traditional permanent mold casting. Castings range in size from 200 g to 4.5 kg (6 oz to 10 lb).
Slush casting is a special type of permanent mold casting in which the molten metal is not allowed to completely solidify. In this process a metal mold in two or more sections is used. The mold is filled with molten metal. After partial solidification of the liquid metal on the surface in the desired thickness, the mold is inverted in order to drain out the still-liquid metal at the center, resulting in a hollow casting. The mold halves then are opened, and the casting removed.
Solidification begins at the walls because they are relatively cool; it then works inward, so the thickness of the shell is controlled by the amount of time allowed before the mold is drained. This is a relatively inexpensive process for small production runs and generally is used only for low-melting lead and zinc-based metals and to produce ornamental items that need not be strong, such as statues, lamp pedestals, and toys.
Pressure casting, also called low pressure casting, is the principle of pressing the molten metal through a refractory tube into the mold from below using an overpressure by gas of about 0.08 to 0.1 MPa (12 to 15 psi). The volume of the circulating material is widely reduced. Supported by the pressurization of the holding crucible, the molten metal is forced into the cavity from beneath so that the flow is upward, with low turbulence, a regulated casting pressure, and a controlled casting speed. The pressurization chamber with the crucible and the mold make one unit, connected by the refractory tube as illustrated in Fig. 2.13. Most crucibles are induction heated to keep the metal warm, but not to melt it.
Fig. 2.13 Schematic illustration of the low-casting process.
The advantages of low pressure casting are good fillability for thin-walled and large area parts and a close structure for pressurized parts. By maintaining several casting parameters, a very high constant quality in the serial production and process safety can be realized. The overpressure during the casting process and the following adjustable holding time acts in opposition to the mold shrinkage by the cooling down.
The low pressure casting process is a cost-effective production process (for medium-run series) that provides excellent mechanical properties of casting.
Die casting is one of the most important and versatile quantity production processes used in the metalworking industry. The traditional die casting process may be described as injection under high pressure, typically of 10 to 350 MPa (1450 to 50,000 psi) into a steel mold (otherwise known as a die) of a molten metal alloy. This solidifies rapidly to form a net-shaped part; then the die is opened and the casting is automatically ejected. After the casting has been removed, the die is closed, and the cycle is ready to be repeated. To run this cycle at a high rate of speed a die casting machine is used. As in other casting processes, after the casting is formed and removed from the die, the sprue and runners must be cut off.
Die casting is an efficient, economical process offering a broader range of shapes and components than any other manufacturing technique. The advantages of die-casting include the following:
High-speed production. Die casting provides complex shapes within closer tolerances than many other mass production processes. Little or no machining is required, and thousands of identical castings can be produced before additional tooling is required.
Dimensional accuracy and stability. Die casting produces parts that are durable and dimensionally stable, while maintaining close tolerances. They are also heat-resistant.
Strength and weight. Die cast parts are stronger than plastic injection moldings having the same dimensions. Thin-wall castings are stronger and lighter than those possible with other casting methods. In addition, because die castings do not consist of separate parts welded or fastened together, the strength is that of the alloy rather than the joining process.
Multiple finishing techniques. Die cast parts can be produced with smooth or textured surfaces, and they are easily plated or finished with a minimum of surface preparation.
Simplified assembly. Die castings provide integral fastening elements, such as bosses and studs. Holes can be cored and made to tap drill sizes, or external threads can be cast.
There are two basic die casting processes, differentiated only by their methods of metal injection: hot chamber and cold die chamber.
a) Hot Chamber Process
The hot chamber process is only used for zinc and other low melting point alloys that do not readily attack and erode metal pots, cylinders, and plungers. Development of this technology, through the use of advanced materials, allows this process to be used for some magnesium alloys. The basic components of a hot chamber die casting machine and die are illustrated in Fig. 2.14. In this machine there is a main pot or crucible in which is immersed a fixed cylinder with a spout firmly connected against the die. A plunger operates in the cylinder. Raising the plunger uncovers or opens a port or slot that is below the molten metal level, and molten metal fills the cylinder.
When the plunger is forced downward, the metal in the cylinder is forced out through the spout into the die. The plunger is withdrawn as soon as the metal solidifies in the die. The die is then opened and the casting ejected. After that the die is closed and securely locked into position, and the casting cycle is repeated.
b) Cold Chamber Process
Cold chamber die casting cycle, Fig. 2.15, differs from hot chamber in that the injection system is not submerged in molten metal. Molten metal is still forced into the die by a hydraulically activated plunger.
Fig. 2.14 Schematic illustration of the hot chamber die casting process: a) die is closed, plunger withdrawn, and molten metal flows into chamber; b) plunger forces metal in chamber to flow into die cavity, maintaining pressure during solidification; c) plunger is withdrawn, die is opened, and casting is ejected; d) finished part is shown.
Fig. 2.15 Schematic illustration of cold chamber die casting process: a) die is closed, plunger withdrawn, and molten metal is poured into chamber; b) plunger forces metal in chamber to flow into die cavity, maintaining pressure during solidification; c) plunger is withdrawn, die is opened, and casting is ejected; d) finished part is shown.
However, in this process, the metal is poured into a “cold chamber” through a port or pouring slot with a ladle that only holds enough metal for one die filling or casting cycle. Immediately after the ladle is emptied the plunger advances, seals the port, and forces the molten metal into the die.
As the molten metal does not remain in the cold chamber very long, higher melting point metals like the copper alloys can be cast in this type of machine. It operates at a much slower cycle than the other machines in hot chamber process.
Extra material is used to force additional metal into the die cavity to compensate for the shrinkage that takes place during solidification.
In this group of processes, the molten metal is forced to distribute into the mold cavity by centrifugal acceleration. The process of centrifugal casting is long established, coming originally from a patent taken out by A. G. Eckhardt of Soho, England, in 1809. Centrifugal casting processes have greater reliability than static casting. They are relatively free from gas and shrinking.
There are three types of centrifugal casting processes: true centrifugal casting, semi-centrifugal casting, and centrifuging casting.
a) True Centrifugal Casting
The following operations are included in true centrifugal casting. One possible setup of true centrifugal casting is illustrated in Fig. 2.16. A mold is set up and rotated at a known speed along a horizontal axis; the mold is coated with a refractory coating.
Fig. 2.16 Setup for true centrifugal casting.
While horizontal, mold-rotating molten metal is poured into mold at one end. The high-speed rotation results in centrifugal forces that cause the metal to take the shape of the mold cavity. After the part has solidified, it is removed and finished.
The axis of rotation is usually horizontal but can be vertical for short workpiecers. The outside shape of the casting can be round or of a simple symmetrical shape. However, the inside shape of the casting is always round. During cooling, lower density impurities will tend to rise toward the center of rotation. Consequently, the properties of the casting can vary throughout its thickness.
Typically, in this casting process three structure zones may occur: the first zone is a layer of fine equiaxed structure that forms almost instantaneously at the mold wall. The second zone consists of directionally oriented crystals approximately perpendicular to the mold surface, and the third zone is nearest to the center and is characterized by a large number of uniformly grown crystals. The true centrifugal casting process is suitable for the production of hollow parts with large dimensions, such as pipes for oil, chemical industries, water supply, etc. Cylindrical parts ranging from 15 mm to 3 m (0.6 in. to 10 ft) in diameter and 15.5 m (50 ft) long can be cast centrifugally with wall thicknesses from 6 to 125 mm (0.25 to 5 in.). Typical metals cast are steel, iron, nickel alloys, copper alloys, and aluminum alloys.
Let us consider how fast the mold must rotate in horizontal centrifugal casting for the process to work successfully. Centrifugal force acting on a rotating body is defined by the following equation:
| (2.2) |
where
| Fc | = | centrifugal force, N (lb) |
| m | = | mass, kg (lb) |
| v | = | velocity, m/s (ft/sec) |
| R | = | inside radius of the mold, m (ft). |
Gravitational force is its weight,
| Fg = mg | (2.3) |
where
| F g | = | gravitation force, N (lb) |
| m | = | mass, kg (lb) |
| g | = | acceleration of gravity, m/s2 (ft/sec2) = 9.81 m/s2(32.2 ft/sec2). |
Velocity v can be expressed as
| (2.4) |
where
| N | = | rotation speed, rev/min. |
G-factor is the ratio of centrifugal force divided by the gravitation force:
| (2.5) |
where
| GF | = | gravitation factor. |
Solving further, we get
| (2.6) |
where
| D | = | inside diameter of the mold, m (ft). |
If the G-factor is too low in centrifugal casting, the liquid metal will not remain forced against the mold wall during the upper half of the circular path, but will drop inside the cavity. Too high a speed results in excessive stresses and hot tears in the outside surface—on an empirical basis of GF = 50 to 100 for the metal mold and GF = 25 to 50 for a sand cast mold.
True centrifugal casting is characterized by better mechanical properties of the cast than is true in conventional static casting: nonmetallic impurities that segregate toward the bore can be machined off; the casting is relatively free from defects; there is less loss of metal compared to that in conventional sand casting; production rate is high; there are no parting lines; the scrap rate is law; and the process can be used to manufacture bimetallic tubes.
b) Semicentrifugal Casting
During semicentrifugal casting (Fig. 2.17), the mold is rotated around its axis of symmetry.
Fig. 2.17 Semicentrifugal casting
The molds used can be permanent or expendable and may contain cores. The detailed shape is given by the shape cavity of the rotating mold.
The centrifugal force is utilized for slag separation, refilling of melt metal, and increase of the filling power in order to cast parts with thin walls. In general, the rotational speed is lower than that used in true centrifugal casting and is usually set so that G-factors of around 15 are obtained. The mold is designed with risers in the center to supply the feed metal. The central zone of the parts (near the axis of rotation) has inclusion defects and thus is suitable only for parts where these can be machined away. Cogwheels are an example of parts that can be cast using this method.
c) Centrifuging Casting
The principle of centrifuging casting is illustrated in Fig. 2.18. The mold is designed with parts cavities that are symmetrically grouped in a ring located away from the axis of rotation. From a central inlet the molten metal is forced outwards into the mold cavity by centrifugal force. The method is extensively used for casting smaller parts. It is also used in the dental industry to cast gold crowns for teeth.
Fig. 2.18 Centrifuging casting: a) mold for eight castings, b) the casting.
Squeeze casting, also known as liquid metal forging, is a die casting method that is a combination of casting and a forging process. It is based on slower continuous die filling and high metal pressures.
This interesting process aims to improve product quality by solidifying the casting under a metallostatic pressure head sufficient to:
1.prevent the formation of shrinkage defects, and
2.retain dissolved gases in solution until freezing is complete.
This method was originally developed in Russia in the 1960s and has undergone considerable improvement in the U.S.
The sequence of operations in squeeze casting is shown in Fig. 2.19. Liquid metal is poured into an open die, just as in a closed die forging process. The dies are then closed and pressure is applied. The amount of pressure thus applied is significantly less than that used in forging, and parts of great detail can be produced. Coring can be used with this process to form holes and recesses. The porosity is low and the mechanical properties are improved.
During the final stages of closure, the liquid is displaced into the further parts of the die. No great fluidity requirements are demanded of the liquid, since the displacements are small. Both ferrous and nonferrous materials can be utilized with this method. This process also can cast forging alloys, which generally have poor fluidities that normally preclude one taking the casting route. The method produces heat-treatable components that can also be used in safety-relevant applications and are characterized by higher strength and ductility than conventional die castings. For example, in comparison with nonrein-forced aluminum alloy, aluminum alloy matrix composites manufactured by squeeze casting techniques can double fatigue strength at 300°C. Hence, such reinforcements are commonly used at the edges of the piston head of a diesel engine where requirements are particularly high. Because squeeze casting is relatively new, much work needs to be done to better understand the fundamentals of the process. By improving the process, the casting industry will discover new options for producing the complex, lightweight aluminum parts that are increasingly demanded in the automotive industry.
Fig. 2.19 Sequence of operations in the squeeze casting process: a) pouring molten metal into die, b) closing die and applying pressure, c) ejecting squeeze casting.
2.5 MELTING PRACTICE AND FURNACES
Melting the metal and handling liquid metal are two of the most critical components in the overall metal casting operation. The manner in which the metal is melted, the way the metal is transferred into the casting cavity, and the whole liquid metal-handling process have a significant impact on productivity, on the cost of operations, and certainly on the quality of the resultant cast components.
Processing the metal in its molten start is the activity wherein the most gains can be achieved. Molten metal processing is an opportunity for refining and for quality enhancement. For example, processes such as alloying, degassing, filtration, fluxing, and grain refinement and modification in aluminum are usually carried out in the liquid metal prior to casting. The mass transfer rates and the kinetics are such that these reactions are carried out much more effectively in the melt.
Today in the foundry industry there is extensive awareness and recognition of the importance of molten metal processing. It is well understood that the highest quality can be obtained by working metals in the molten state. The level of hydrogen can be reduced and liquid and solid inclusions can be removed and controlled, thus improving the quality of the overall casting. The practice of degassing the metal prior to casting is carried out as a normal step throughout the industry. Filtration and removal of inclusions is another molten metal processing technology that has evolved over the past decade or so, and today most foundries and metal casting operations filter the metal to remove inclusions prior to casting. In addition, there is active development of measurement devices and equipment to monitor the quality of molten metal.
Furnaces use insulated, heated vessels powered by an energy source to melt metal. Furnace design is a complex process, and the design can be optimized based on multiple factors. Furnaces in foundries can be any size, ranging from mere ounces to hundreds of tons, and they are designed according to the types of metals that are to be melted in them. Also, furnaces are bound by the fuel available that will produce the desired temperature. For low temperature melting point alloys, such as zinc or tin, melting furnaces may reach around 850°C (1262°F). From steel, nickel-based alloys, tungsten, and other elements with higher melting points, furnaces can reach to over 3850°C (6962°F). The fuel used to allow furnaces to reach these high temperatures can be electricity, natural gas or propane, charcoal, coke, fuel oil, or wood.
Melting furnaces used in the foundry industry are of many diverse configurations. The selection of the melting unit is one of the most important decisions foundries must make, with due consideration to several important factors including:
1.The temperature required to melt the alloy.
2.The melting rate and quantity of molten metal required.
3.The economy of installation and operation.
4.Environmental and waste disposal requirements.
Some of the more commonly used melting furnaces include cupolas, direct fuel-filled furnaces, crucible furnaces, induction furnaces, and electric arc furnaces.
a) Cupolas
Some ferrous foundries use cupola furnaces. For many years the cupola was the primary method of melting used in iron foundries. The cupola furnace has several unique characteristics that are responsible for its widespread use as a melting unit for cast iron:
•The cupola is one of the only methods of melting that is continuous in its operation.
•It has high melt rates.
•It has helatively low operating costs.
•It has ease of operation.
In more recent times the use of the cupola has declined in favor of electric induction melting, which offers more precise control of melt chemistry and temperature and much lower levels of emissions.
The construction of a conventional cupola (Fig. 2.20) consists of a vertical steel shell lined with a refractory type of brick.
The charge is introduced into the furnace body by means of an opening approximately halfway up the vertical shaft. The charge consists of alternate layers of the metal to be melted, coke fuel, and limestone flux. The fuel is burned in air that is introduced through tuyeres positioned above the hearth. The hot gases generated in the lower part of the shaft ascend and preheat the descending charge.
Most cupolas are of the drop-bottom type with hinged doors under the hearth, which allows the bottom to drop away at the end of melting to aid cleaning and repairs. At the bottom front is a taping spout for the molten iron at the rear, and positioned above the taping spouts is a slag spout. The top of the stack is capped with a spark/fume arrester hood.
Fig. 2.20 Cupola furnace used in foundries.
Typical internal diameters of cupolas are 450 to 2000 mm (17.7 to 78.8 in.) diameter. A typical operation cycle for a cupola would consist of closing and propping the bottom hinged doors and preparing a hearth bottom. The bottom is usually made from low-strength molding sand and slopes towards a tapping hole. A fire is started in the hearth using lightweight timber, and coke is loaded (charged) on top of the fire and burned by increasing the air draught from the tuyeres. Once the coke bed is ignited and of the required height, alternate layers of metal, flux, and coke are added until the level reaches the charged doors. The metal charge would typically consist of pig iron, scrap steel, and domestic returns.
An air blast is introduced through the wind box and tuyeres located near the bottom of the cupola. The air reacts chemically with the carbonaceous fuel, thus producing heat of combustion. Soon after the blast is turned on, molten metal collects on the hearth bottom where it is eventually tapped out into a waiting ladle or receiver. As the metal is melted and fuel consumed, additional charges are added to maintain a level at the charging door and provide a continuous supply of molten iron.
At the end of the melting campaign charging is stopped, but the air blast is maintained until all of the metal is melted and tapped off. The air is then turned off and the bottom doors opened, allowing the residual charge material to be dumped.
b) Crucible Furnaces
A crucible is a container in which metals are melted. Foundry crucibles are usually made of graphite, with clay as a binder, and they are very durable and resist temperatures to over 1600°C (2912°F). A crucible is placed into a furnace and, after the melting, the liquid metal is taken out of the furnace and poured into the mold.
Crucible furnaces are one of the oldest and simplest types of melting unit used in the foundry. The furnaces use a refractory crucible, which contains the metal charge. If scrap metal is used as charge it must be cleansed and heated before being introduced into the furnace, as any oil or moisture could cause an explosion. The charge is heated via conduction of heat through the walls of the crucible. The heating fuel is typically coke, oil, gas, or electricity. Crucible melting is commonly used where small batches of low melting point alloy are required, such as bronze, brass, and alloys of zinc and aluminum. The modest capital outlay required for these furnaces makes them attractive to small nonferrous foundries.
Crucible furnaces (Fig. 2.21) are typically classified according to the method of removing the metal from the crucible:
Fig. 2.21 Three types of crucible furnaces: a) lift-out furnace, b) bale-out furnace, c) tilting.
Lift-out furnace. In this type, the crucible and molten metal are removed from the furnace body for direct pouring into the mold.
Bale-out furnace. In a bale-out furnace the molten metal is ladled from the crucible to the mold.
Tilting furnace. In this type of furnace, the molten metal is transferred to the mold or ladle by mechanically tilting the crucible and furnace body.
c) Induction Furnaces
The principle of induction melting is that placing a metal body in an alternating magnetic field creates eddy currents, causing losses through which the metal is heated. Skin effects concentrate these currents in the outer layers. The inductor traversed by an alternating current creates a magnetic field, which should be optimally adapted to the metal.
Varying the alternating current frequency can influence the depth of heating, but it also depends on the concentration of flux capacity, on the length of treatment, and the material’s conduction properties. Medium frequency is usually used for melting metal.
There are two main types of induction furnace: coreless and channel.
Coreless induction furnace. The coreless induction furnace is composed of a refractory container, capable of holding the molten bath, which is surrounded by a water-cooled helical coil connected to a source of alternating current. In Fig. 2.22 is shown a simplified cross-section of a coreless induction furnace.
The alternating current applied to the coil produces a varying magnetic field concentrated within the helical coil. This magnetic field passing through the charge induces a secondary current in the charge piece. The current circulating in the charge produces electrical losses that heat the charge and eventually melt it.
Fig. 2.22 Simplified cross-section of a coreless induction furnace.
When the charge material is molten, the interaction of the magnetic field and the electrical currents flowing in the induction coil produce a stirring action within the molten metal. This stirring action forces the molten metal to rise upwards in the center, causing the characteristic upward on the surface of the metal. The degree of stirring action is influenced by the power and frequency applied, as well as the size and shape of the coil and the density and viscosity of the molten metal. The stirring action within the bath is important, as it helps with mixing of alloys and melting of turnings as well as homogenizing of temperature throughout the furnace. Excessive stirring can increase gas pickup, lining wear, and oxidation of alloys.
The coreless induction furnace has largely replaced the crucible furnace, especially for melting high melting point alloys. The coreless induction furnace is commonly used to melt all grades of steels and irons, as well as many nonferrous alloys. The furnace is ideal for remelting and alloying because of the high degree of control over temperature and chemistry, while the induction current provides good circulation of the melt.
Channel induction furnace. The channel type induction furnace derives its name from the fact that it is constructed with a small channel of molten metal passing through the magnetic core, which has a primary winding wound around it. This channel of molten metal acts like the secondary of a short-circuited transformer, causing current flow through the metal in the channel and causing heat loss to occur by the Joule effect.
The typical arrangement of a channel furnace is shown in Fig. 2.23. The liquid metal is kept in a crucible and flows through a channel that is connected with the bottom of the crucible.
The whole furnace is typically covered by metal plates that serve both as mechanical protection and electromagnetic shielding.
Channel induction furnaces were used initially as containers for molten metal, but are now commonly used for some melting applications as well.
d) Electric Arc Furnaces
This type of furnace draws an electric arc that rapidly heats and melts the charge metal. When the molten metal is ready to pour, the electrodes are raised and the furnace is tilted to pour the molten metal into a receiving ladle.
Fig. 2.23 Simplified cross-section of a typical channel induction furnace.
Electric arc furnaces produce tremendous quantities of metal fumes; however, the furnace is normally equipped with a fume-capture system to reduce both workplace and air pollution. These furnaces are common in large ferrous foundries. Temperatures inside an electric arc furnace can rise to approximately 1900°C (3450°F). Electric arc furnaces may be categorized as direct arc or indirect arc.
Direct arc furnaces. In a direct arc furnace (Fig. 2.24) scrap steel or direct reduced iron is melted by direct contact with an electric arc. The arc is produced by striking current from a charged electrode to the metal (the neutral point), through the metal, and drawn to an oppositely charged electrode.
Fig. 2.24 Schematic illustration of direct arc furnace.
At the start of the direct arc melting process (the electrodes and roof are raised and swung to one side), a charge of steel scrap is dropped into the furnace from a clamshell bucket. The roof is sealed, the electrodes lowered, and an arc strikes the electrode. The charge is heated both by the current’s passing through the charge and by the radiant energy evolving by the arc.
The electrodes are automatically raised and lowered by a positioning system, which may use either electric winch hoists or hydraulic cylinders. The regulating system maintains an approximately constant current and power input during the melting of the charge, even though scrap may move under the electrodes while it melts. The arc contacts the scrap charge and the metal is melted. Since direct arc furnaces are sometimes used to perform refining operations, the molten metal may remain in the furnace after melting. When the melting and refining operations have been completed, the molten metal is tapped into a ladle for casting. Some of the advantages of direct arc furnaces include high melt rates, high pouring temperatures, and excellent control of melt chemistry.
Indirect arc furnaces. Indirect arc furnaces (Fig. 2.25) generally consist of a horizontal barrel-shaped steel shell lined with refractory. The arcing between two horizontally opposed carbon electrodes effects the melting of the metal. Heating is via radiation from the arc to the charge. The barrel-shaped shell is designed to rotate and reverse through approximately 180° in order to avoid excessive heating of the refractory above the melt level and to increase the melting efficiency of the unit.
Fig. 2.25 Schematic illustration of indirect arc furnace.
Indirect arc furnaces are suitable for melting a wide range of alloys but are particularly popular for the production of copper-base alloys. The units operate on a single-phase power supply, and hence the size is usually limited to relatively small units.
REVIEW QUESTIONS
2.1What are the major applications of a casting?
2.2Name the two basic categories of casting processes.
2.3Name the two basic types of silica sand.
2.4What is a pattern and how many types are there?
2.5What is the difference between a split pattern and a match plate pattern?
2.6What is a chaplet?
2.7What properties determine the quality of sand for greensand molding?
2.8What is the difference between dry sand molds and shell molds?
2.9What is the EPS process?
2.10What is the difference between investment casting and plaster mold casting?
2.11What common metals processes use die casting?
2.12What is the difference between vacuum permanent mold casting and vacuum molding?
2.13How many types of centrifugal casting are there, and what are the differences among them?
2.14What is the difference between a cupola and a crucible furnace?
