Manufacturing 13

Because of the precision and finish, most die castings require no finish machining except for the removal of excess metal fin, or flash, around the parting line and the possible drilling or tapping of holes. Production rates are high, and a set of dies can produce many thousands of castings without significant change in dimensions.While die casting is most economical for large production volumes, quantities as low as 2000 can be justified if extensive secondary machining or surface finishing can be eliminated. Thin-wall zinc die casting is now considered to be a significant competitor to plastic injection molding.The die castings are stronger, stiffer, more dimensionally stable, and more heat resistant. In addition, the metal parts are more resistant to ultraviolet radiation, weathering, and stress cracking when exposed to various reagents. Figure 13-7 presents a variety of aluminum and zinc die castings.Table 13-3 compares the key features of the four dominant families of die-casting alloys, and Table 13-4 compares the mechanical properties of various die-cast alloys with the properties of other engineering materials. ■ 13.4 SQUEEZE CASTING AND SEMISOLID CASTING Squeeze casting and semisolid casting are methods that enable the production of highquality, near-net-shape, thin-walled parts with good surface finish and dimensional precision as well as properties that approach those of forgings. Both processes can be viewed as derivatives of conventional high-pressure die casting, since they employ tool steel dies and apply high pressure during solidification. While the majority of applications involve alloys of aluminum, each of the processes has been successfully applied to magnesium, zinc, copper, and a limited number of ferrous alloys.

CLEANING AND FINISHING After solidification and removal from the mold, most castings require some additional cleaning and finishing. Specific operations may include all or several of the following: 1. Removing cores 2. Removing gates and risers 3. Removing fins, flash, and rough spots from the surface 4. Cleaning the surface 5. Repairing any defects Cleaning and finishing operations can be quite expensive, so consideration should be given to their minimization when designing the product and selecting the specific method of casting. In addition, consideration should also be directed toward the possibility of automating the cleaning and finishing. Sand cores can usually be removed by mechanical shaking.At times, however, they must be removed by chemically dissolving the core binder. On small castings, sprues, gates, and risers can sometimes be knocked off. For larger castings, a cutting operation is usually required. Most nonferrous metals and cast irons can be cut with an abrasive cutoff wheel, power hacksaw, or band saw. Steel castings frequently require an oxyacetylene torch. Plasma arc cutting can also be used. The specific method of cleaning often depends on the size and complexity of the casting. After the gates and risers have been removed, small castings are often tumbled in barrels to remove fins, flash, and sand that may have adhered to the surface.Tumbling may also be used to remove cores and, in some cases, gates and risers. Metal shot or abrasive material is often added to the barrel to aid in the cleaning. Conveyors can be used to pass larger castings through special cleaning chambers, where they are subjected to blasts of abrasive or cleaning material. Extremely large castings usually require manual finishing, using pneumatic chisels, portable grinders, and manually directed blast hoses. While defect-free castings are always desired, flaws such as cracks, voids, and laps are not uncommon. In some cases, especially when the part is large and the production quantity is small, it may be more attractive to repair the part rather than change the pattern, die, or process. If the material is weldable, repairs are often made by removing the defective region (usually by chipping or grinding) and filling the created void with deposited weld metal. Porosity that is at or connected to free surfaces can be filled with resinous material, such as polyester, by a process known as impregnation. If the pores are filled with a lower-melting-point metal, the process becomes infiltration. (See Chapter 16 for a further discussion of these processes.) HEAT TREATMENT AND INSPECTION OF CASTINGS Heat treatment is an attractive means of altering properties while retaining the shape of the product. Steel castings are frequently given a full anneal to reduce the hardness and brittleness of rapidly cooled, thin sections and to reduce the internal stresses that result from uneven cooling.Nonferrous castings are often heat treated to provide chemical homogenization or stress relief as well as to prepare them for subsequent machining. For final properties, virtually all of the treatments discussed in Chapter 5 can be applied. Ferrous-metal castings often undergo a quench-and-temper treatment,and many nonferrous castings are age hardened to impart additional strength.The variety of heat treatments is largely responsible for the wide range of properties and characteristics available in cast metal products. Virtually all of the nondestructive inspection techniques can be applied to cast metal products. X-ray radiography, liquid penetrant inspection, and magnetic particle inspection are extremely common. dega-c13_313-333-hr 1/9/07 3:53 PM Page 329 330 CHAPTER 13 Multiple-Use-Mold Casting Processes ■ 13.10 AUTOMATION IN FOUNDRY OPERATIONS Many of the operations that are performed in a foundry are ideally suited for robotic automation since they tend to be dirty, dangerous, or dull. Robots can dry molds, coat cores, vent molds, and clean or lubricate dies. They can tend stationary, cyclic equipment, such as die-casting machines, and if the machines are properly grouped, one robot can often service two or three machines. In the finishing room, robots can be equipped with plasma cutters or torches to remove sprues, gates, and runners.They can perform grinding and blasting operations, as well as various functions involved in the heat treatment of castings. In the investment-casting process, robots can be used to dip the wax patterns into refractory slurry and produce the desired molds. In a similar manner, robots have been used to dip the Styrofoam patterns of the full-mold and lost-foam processes in their refractory coating and hang them on conveyors to dry. In a fully automated lost-foam operation, robots could be used to position the pattern, fill the flask with sand, pour the metal, and use a torch to remove the sprue. ■ 13.11 PROCESS SELECTION As shown in the individual process summaries that have been included throughout Chapters 12 and 13, each of the casting processes has a characteristic set of capabilities, assets, and limitations.The requirements of a particular product (such as size, complexity, required dimensional precision, desired surface finish, total quantity to be made, and desired rate of production) often limit the number of processes that should be considered as production candidates. Further selection is usually based on cost. Some aspects of product cost, such as the cost of the material and the energy required to melt it, are somewhat independent of the specific process.The cost of other features, such as patterns, molds, dies, melting and pouring equipment, scrap material, cleaning, inspection, and all related labor, can vary markedly and be quite dependent on the process. For example, pattern and mold costs for sand casting are quite a bit less than the cost of die-casting dies. Die casting, on the other hand, offers high production rates and a high degree of automation.When a small quantity of parts is desired, the cost of the die or tooling must be distributed over the total number of parts, and unit cost (or cost per casting) is high. When the total quantity is large, the tooling cost is distributed over many parts, and the cost per piece decreases. Figure 13-20 shows the relationship between unit cost and production quantity for a product that can be made by both sand and die casting. Sand casting is an expendable mold process. Since an individual mold is required for each pour, increasing quantity does not lead to a significant drop in unit cost. Die casting involves a multiple-use mold, and the cost of the die can be distributed over the total number of parts. As shown in the figure, sand casting is often less expensive for small production runs, and processes such as die casting are preferred for large quantities. One should note that while the die-casting curve in Figure 13-20 is a smooth line, it is not uncommon for an actual curve to contain abrupt discontinuities. If the lifetime of a set of tooling is 50,000 casts, the

Centrifuging can also be used to drive pewter, zinc, or wax into spinning rubber molds to produce products with close tolerances, smooth surfaces, and excellent detail. These can be finished products or the low-melting-point patterns that are subsequently assembled to form the "trees" for investment casting. ■ 13.6 CONTINUOUS CASTING As discussed in Chapter 6 and depicted in Figure 6-5, continuous casting is usually employed in the solidification of basic shapes that become the feedstock for deformation processes such as rolling and forging. By producing a special mold, continuous casting can also be used to produce long lengths of complex cross-section product, such as the one depicted in Figure 13-13. Since each product is simply a cutoff section of the continuous strand, a single mold is all that is required to produce a large number of pieces. Quality is high as well, since the metal can be protected from contamination during melting and pouring, and only a minimum of handling is required. TABLE 13-5 Centrifugal Casting Process: Molten metal is introduced into a rotating sand, metal, or graphite mold and held against the mold wall by centrifugal force until it is solidified. Advantages: Can produce a wide range of cylindrical parts, including ones of large size; good dimensional accuracy, soundness, and cleanliness. Limitations: Shape is limited; spinning equipment can be expensive. Common metals: Iron; steel; stainless steel; and alloys of aluminum, copper, and nickel. Size limits: Up to 3 m (10 ft) in diameter and 15 m (50 ft) in length. Thickness limits: Wall thickness 2.5 to 125 mm (0.1-5 in.). Typical tolerances: O.D. to within 2.5 mm (0.1 in.); I.D. to about 4 mm (0.15 in.). Draft allowance: 10 mm/m ( in./ft). Surface finish: 2.5-12.5 µm (100-500 µin.) rms. 1 8 dega-c13_313-333-hr 1/9/07 3:53 PM Page 324 SECTION 13.7 Melting 325 FIGURE 13-13 Gear produced by continuous casting. (Left) As-cast material; (right) after machining. (Courtesy of ASARCO, Tucson, AZ.) ■ 13.7 MELTING All casting processes begin with molten metal. Ideally, the molten metal should be available in an adequate amount, at the desired temperature, with the desired chemistry and minimum contamination. The melting furnace should be capable of holding material for an extended period of time without deterioration of quality, be economical to operate, and be capable of being operated without contributing to the pollution of the environment. Except for experimental or very small operations, virtually all foundries use cupolas, air furnaces (also known as direct fuel-fired furnaces), electric-arc furnaces, electric resistance furnaces, or electric induction furnaces. In locations such as fully integrated steel mills, molten metal may be taken directly from a steelmaking furnace and poured into casting molds. This practice is usually reserved for exceptionally large castings. For small operations, gas-fired crucible furnaces are common, but these have rather limited capacities. Selection of the most appropriate melting method depends on such factors as (1) the temperature needed to melt and superheat the metal, (2) the alloy being melted and the form of available charge material, (3) the desired melting rate or the desired quantity of molten metal, (4) the desired quality of the metal, (5) the availability and cost of various fuels, (6) the variety of metals or alloys to be melted, (7) whether melting is to be batch or continuous, (8) the required level of emission control, and (9) the various capital and operating costs. The feedstock entering the melting furnace may take several forms. While prealloyed ingot may be purchased for remelt, it is not uncommon for the starting material to be a mix of commercially pure primary metal and commercial scrap, along with recycled gates, runners, sprues, and risers, as well as defective castings. The chemistry can be adjusted through alloy additions in the form of either pure materials or master alloys that are high in a particular element but are designed to have a lower melting point than the pure material and a density that allows for good mixing. Preheating the metal being charged is another common practice, and it can increase the melting rate of a furnace by as much as 30%. CUPOLAS A significant amount of gray, nodular, and white cast iron is still melted in cupolas, although many foundries have converted to electric induction furnaces. A cupola is a refractory-lined, vertical steel shell into which alternating layers of coke (carbon), iron (pig iron and/or scrap), limestone or other flux, and possible alloy additions are charged and melted under forced air draft. The operation is similar to that of a blast furnace, with the molten metal collecting at the bottom of the cupola to be tapped off either continuously or at periodic intervals. Cupolas are simple and economical, can be obtained in a wide range of capacities, and can produce cast iron of excellent quality if the proper raw materials are used and good control is practiced. Control of temperature and chemistry can be somewhat

Die temperatures are usually maintained at about 150° to 250°C (300° to 500°F) below the solidus temperature of the metal being cast in order to promote rapid freezing. Since cast iron cannot withstand the high casting pressures, die-casting dies are usually made from hardened hot-work tool steels and are typically quite expensive. As shown in Figure 13-4, the dies may be relatively simple, containing only one or two mold cavities, or they may be complex, containing multiple cavities of the same or different products, or even be an assembly of multiple subcomponents. The rigid dies must separate into at least two pieces to permit removal of the casting. It is not uncommon, however, for complex die castings to require multiple-segment dies that open and close in several different directions. Die complexity is further increased as the various sections incorporate water-cooling passages, retractable cores, and moving pins to knock out or eject the finished casting. Die life is usually limited by wear (or erosion), which is strongly dependent on the temperature of the molten metal. Surface cracking can also occur in response to the large number of heating and cooling cycles that are experienced by the die surfaces. If the rate of temperature change is the dominant feature, the problem is called heat checking. If the number of cycles is the primary cause, the problem is called thermal fatigue. In the basic die-casting process, water-cooled dies are first lubricated and clamped tightly together. Molten metal is then injected under high pressure. Since high injection pressures cause turbulence and air entrapment, the specified values of pressure

In all variations of the process, die-casting dies fill with metal so fast that there is little time for the air in the runner system and mold cavity to escape, and the metal molds offer no permeability.The air can become trapped and cause a variety of defects, including blowholes, porosity, and misruns. To minimize these defects, it is crucial that the dies be properly vented, usually by wide, thin (0.13-mm or 0.005-in.) vents positioned along the parting line. Proper positioning is a must, since all of the air must escape before the molten metal contacts the vents.The long thin slots allow the escape of gas but promote rapid freezing of the metal and a plugging of the hole. The metal that solidifies in the vents must be trimmed off after the casting has been ejected. This can be done with special trimming dies that also serve to remove the sprues and runners. Risers are not used in the die-casting process since the high injection pressures ensure the continuous feed of molten metal from the gating system into the casting.The porosity that is often found in die castings is not shrinkage porosity; it is more likely to be the result of either entrapped air or the turbulent mode of die filling. This porosity tends to be confined to the interior of castings, and its formation can often be minimized by smooth metal flow, good venting, and proper application of pressure. The rapidly solidified surface is usually harder and stronger than the slower-cooled interior and is usually sound and suitable for plating or decorative applications. Sand cores cannot be used in die casting because the high pressures and flow rates cause the cores to either disintegrate or have excessive metal penetration. As a result, metal cores are required, and provisions must be made for their retraction, usually before the die is opened for removal of the casting. As with all mating segments and moving components, a close fit must be maintained to prevent the pressurized metal from flowing into the gap. Loose core pieces (also metal) can also be positioned into the die at the beginning of each cycle and then removed from the casting after its ejection. This procedure permits more complex shapes to be cast, such as holes with internal threads, but production rate is slowed and costs increase. Cast-in inserts can also be incorporated in the die-casting process. Examples include prethreaded bosses, electrical heating elements, threaded studs, and high-strength bearing surfaces. These high-temperature components are positioned in the die before the lower-melting-temperature metal is injected. Suitable recesses must be provided in the die for positioning and support, and the casting cycle tends to be slowed by the additional operations. Table 13-2 summarizes the key features of the die-casting process. Attractive aspects include smooth surfaces and excellent dimensional accuracy. For aluminum-, magnesium-, zinc-, and copper-based alloys, linear tolerances of 3 mm/m (0.003 in./in.) are not uncommon. Thinner sections can be cast than with either sand or permanent-mold casting.The minimum section thickness and draft vary with the type of metal, with typical values as follows:

In each of the expendable-mold casting processes discussed in Chapter 12, a separate mold had to be created for each pour. Variations in mold consistency, mold strength, moisture content, pattern removal, and other factors contribute to dimensional and property variation from casting to casting. In addition, the need to create and then destroy a separate mold for each pour results in rather low production rates. The multiple-use-mold casting processes overcome many of these limitations, but they, in turn, have their own assets and liabilities. Since the molds are generally made from metal, many of the processes are restricted to casting the lower-melting-point nonferrous metals and alloys. Part size is often limited, and the dies or molds can be rather costly. ■ 13.2 PERMANENT-MOLD CASTING In the permanent-mold casting process, also called gravity die casting, a reusable mold is machined from gray cast iron, alloy cast iron, steel, bronze, graphite, or other material.The molds are usually made in segments, which are often hinged to permit rapid and accurate opening and closing. After preheating, a refractory or mold coating is applied to the preheated mold, and the mold is clamped shut. Molten metal is then poured into the pouring basin, and it flows through the feeding system into the mold cavity by simple gravity flow. After solidification, the mold is opened and the product is removed. Since the heat from the previous cast is usually sufficient to maintain mold temperature, the process can be immediately repeated, with a single refractory coating serving for several pouring cycles.Aluminum-, magnesium-, zinc-, lead-, and copper-based alloys are the metals most frequently cast, along with gray cast iron. If graphite is used as the mold material, iron and steel castings can also be produced. Numerous advantages can be cited for the permanent-mold process. Near-net shapes can be produced that require little finish machining.The mold is reusable, and a good surface finish is obtained if the mold is in good condition. Dimensions are consistent from part to part, and dimensional accuracy can often be held to within 0.25 mm (0.010 in.). Directional solidification can be achieved through good design or can be promoted by selectively heating or chilling various portions of the mold or by varying the thickness of the mold wall. The result is usually a sound, defect-free casting with good mechanical properties. The faster cooling rates of the metal mold produce a finer grain structure, reduced porosity, and higher-strength products than would result from dega-c13_313-333-hr 1/9/07 3:53 PM Page 313 314 CHAPTER 13 Multiple-Use-Mold Casting Processes a sand casting process. Cores, both expendable sand or plaster or retractable metal, can be used to increase the complexity of the casting, and multiple cavities can often be included in a single mold. When sand cores are used, the process is often called semipermanent mold casting. On the negative side, the process is generally limited to the lower-melting-point alloys, and high mold costs can make low production runs prohibitively expensive. The useful life of a mold is generally set by molten metal erosion or thermal fatigue. When making products of steel or cast iron, mold life can be extremely short. For the lowertemperature metals, one can usually expect somewhere between 10,000 and 120,000 cycles. The actual mold life will depend upon the following: 1. Alloy being cast. The higher the melting point, the shorter the mold life. 2. Mold material. Gray cast iron has about the best resistance to thermal fatigue and machines easily. Thus it is used most frequently for permanent molds. 3. Pouring temperature. Higher pouring temperatures reduce mold life, increase shrinkage problems, and induce longer cycle times. 4. Mold temperature. If the temperature is too low, one can expect misruns and large temperature differences in the mold. If the temperature is too high, excessive cycle times result and mold erosion is aggravated. 5. Mold configuration. Differences in section sizes of either the mold or the casting can produce temperature differences within the mold and reduce its life. The permanent molds contain the mold cavity, pouring basin, sprue, runners, risers, gates, possible core supports, alignment pins, and some form of ejection system. The molds are usually heated at the beginning of a run, and continuous operation then maintains the mold at a fairly uniform elevated temperature.This minimizes the degree of thermal fatigue, facilitates metal flow, and controls the cooling rate of the metal being cast. Since the mold temperature rises when a casting is produced, it may be necessary to provide a mold-cooling delay before the cycle is repeated. Refractory washes or graphite coatings can be applied to the mold walls to control or direct the cooling, prevent the casting from sticking, and prolong the mold life by minimizing thermal shock and fatigue. When pouring cast iron, an acetylene torch is often used to apply a coating of carbon black to the mold. Since the molds are not permeable, special provision must be made for venting.This is usually accomplished through the slight cracks between mold halves or by very small vent holes that permit the escape of trapped air but not the passage of molten metal. Since gravity is the only means of inducing metal flow, risers must still be employed to compensate for solidification shrinkage, and with the necessary sprues and runners, yields are generally less than 60%. Mold complexity is often restricted because the rigid cavity offers no collapsibility to compensate for the solid-state shrinkage of the casting. As a best alternative, it is common practice to open the mold and remove the casting immediately after solidification.This prevents the formation of hot tears that may form if the product is restrained during the shrinkage that occurs during cooldown to room temperature. For permanent-mold casting, high-volume production is usually required to justify the high cost of the metal molds. Automated machines can be used to coat the mold, pour the metal, and remove the casting. Figure 13-1 shows a variety of automobile and truck pistons that were manufactured by the permanent-mold process, which is summarized in Table 13-1. SLUSH CASTING Hollow castings can be produced by a variant of permanent-mold casting known as slush casting. Hot metal is poured into the metal mold and is allowed to cool until a shell of desired thickness has formed. The mold is then inverted and the remaining liquid is poured out. The resulting casting is a hollow shape with good surface detail but variable wall thickness. Common applications include the casting of ornamental objects such

In the squeeze casting process, molten metal is introduced into the die cavity of a metal mold, using large gate areas and slow metal velocities to avoid turbulence. When the cavity has filled, high pressure (20 to 175 MPa, or 3000 to 25,000 psi) is then applied and maintained during the subsequent solidification. Parts must be designed to directionally solidify toward the gates, and the gates must be sufficiently large that they freeze after solidification in the cavity, thereby allowing the pressurized runner to feed additional metal to compensate for shrinkage. Intricate shapes can be produced at lower pressures than would normally be required for hot or cold forging. Both retractable and disposable cores can be used to create holes and internal passages. Gas and shrinkage porosity are substantially reduced, and mechanical properties are enhanced.While the squeeze casting process is most commonly applied to aluminum and magnesium castings, it has also been adapted to the production of metal-matrix composites where the pressurized metal is forced around or through foamed or fiber reinforcements that have been positioned in the mold. dega-c13_313-333-hr 1/9/07 3:53 PM Page 321 322 CHAPTER 13 Multiple-Use-Mold Casting Processes For most alloy compositions, there is a range of temperatures where liquid and solid coexist, and several techniques have been developed to produce shapes from this semisolid material. In the rheocasting process, molten metal is cooled to the semisolid state with constant stirring.The stirring or shearing action breaks up the dendrites, producing a slurry of rounded particles of solid in a liquid melt.This slurry, with about a 30% solid content, can be readily shaped by high-pressure injection into metal dies. Because the slurry contains no superheat and is already partially solidified, it freezes quickly. In the thixocasting variation, there is no handling of molten metal.The material is first subjected to special processing (stirring during solidification as in rheocasting) to produce solid blocks or bars with a nondendritic structure. When reheated to the semisolid condition, the thixotropic material can be handled like a solid but flows like a liquid when agitated or squeezed. The solid material is then cut to prescribed length, reheated to a semisolid state where the material is about 40% liquid and 60% solid, mechanically transferred to the shot chamber of a cold-chamber die-casting machine, and injected under pressure. In a variation of the process, solid metal granules or pellets are fed into a barrel chamber, where a rotating screw shears and advances the material through heating zones that raise the temperature to the semisolid region.When a sufficient volume of thixotropic material has accumulated at the end of the barrel, a shot system drives it into the die or mold at velocities of 1 to 2.5 m/sec (40-100 in./sec). The injection system of this process is a combination of the screw feed used in plastic injection molding and the plunger used in conventional die casting. In all of the semisolid casting processes, the absence of turbulent flow during the casting operation minimizes gas pickup and entrapment. Because the material is already partially solid, the lower injection temperatures and reduced solidification time act to extend tool life.The prior solidification coupled with further solidification under pressure results in a significant reduction in solidification shrinkage and related porosity. The minimization of porosity enables the use of high-temperature heat treatments, such as the T6 solution treatment and artificial aging of aluminum, to further enhance strength. Since the thixocasting process does not use molten metal, both wrought and cast alloys have been successfully shaped. Walls have been produced with thickness as low as 0.2 mm (0.01 in.). ■ 13.5 CENTRIFUGAL CASTING The inertial forces of rotation or spinning are used to distribute the molten metal into the mold cavity or cavities in the centrifugal casting processes, a category that includes true centrifugal casting, semicentrifugal casting, and centrifuging. In true centrifugal casting, a dry-sand, graphite, or metal mold is rotated about either a horizontal or vertical axis at speeds of 300 to 3000 rpm. As the molten metal is introduced, it is flung to the surface of the mold, where it solidifies into some form of hollow product. The exterior profile is usually round (as with gun barrels, pipes, and tubes), but hexagons and other symmetrical shapes are also possible. No core or mold surface is needed to shape the interior, which will always have a round profile because the molten metal is uniformly distributed by the centrifugal forces. When rotation is about the horizontal axis, as illustrated in Figure 13-8, the inner surface is always cylindrical. If the mold is oriented vertically, as in Figure 13-9, gravitational forces cause the inner surface to become parabolic, with the exact shape being a function of the speed of rotation.Wall thickness can be controlled by varying the amount of metal that is introduced into the mold. During the rotation, the metal is forced against the outer walls of the mold with considerable force, and solidification begins at the outer surface. Centrifugal force continues to feed molten metal as solidification progresses inward. Since the process compensates for shrinkage, no risers are required. The final product has a strong, dense exterior with all of the lighter impurities (including dross and pieces of the refractory mold coating) collecting on the inner surface of the casting. This surface is often left in the final casting, but for some products, it may be removed by a light boring operation. Products can have outside diameters ranging from 7.5 cm to 1.4 m (3 to 55 in.) and wall thickness up to 25 cm (10 in.). Pipe (up to 12 m, or 40 ft, in length),

LOW-PRESSURE AND VACUUM PERMANENT-MOLD CASTING Gravity pouring is the oldest, simplest, and most traditional form of permanent-mold casting. In a variation known as tilt-pour permanent-mold casting, the molten metal is placed in the pouring basin and the mold then rotates to induce flow into the mold cavity. In this way, turbulence is minimized as the metal flows through the gating system and into the mold. In low-pressure and vacuum permanent-mold casting, the mold is turned upside down and positioned above a sealed, airtight chamber that contains a crucible of molten metal.A small pressure difference then causes the molten metal to flow upward into the die cavity. In the low-pressure permanent-mold (LPPM) process, illustrated in Figure 13-2, a low-pressure gas (3 to 15 psi) is introduced into a sealed chamber, driving molten metal up through a refractory fill tube and into the gating system or cavity of a metal mold. This metal is exceptionally clean, since it flows from the center of the melt and is fed directly into the mold (a distance of about 10 cm, or 3 to 4 in.), never passing through the atmosphere. Product quality is further enhanced by the nonturbulent mold filling, which helps to minimize gas porosity and dross formation. Through design and cooling, the products directionally solidify from the top down. The molten metal in the pressurized fill tube acts as a riser to continually feed the casting

Some type of pouring device, or ladle, is usually required to transfer the metal from the melting furnace to the molds. The primary considerations for this operation are (1) to maintain the metal at the proper temperature for pouring and (2) to ensure that only high-quality metal is introduced into the molds.The specific type of pouring ladle is determined largely by the size and number of castings to be poured. In small foundries, a handheld, shank-type ladle is used for manual pouring. In larger foundries, either bottom-pour or teapot-type ladles are used, like the ones illustrated in Figure 11-6. These are often used in conjunction with a conveyor line that moves the molds past the pouring station. Because metal is extracted from beneath the surface, slag and other impurities that float on top of the melt are not permitted to enter the mold. High-volume, mass-production foundries often use automatic pouring systems, like the one shown in Figure 13-19. Molten metal is transferred from a main melting furnace to a holding furnace. A programmed amount of molten metal is further transferred into individual pouring ladles and is then poured into the corresponding molds

and the time and duration of application vary considerably. The pressure need not be constant, and there has been a trend toward the use of larger gates and lower injection pressures, followed by the application of higher pressure after the mold has completely filled and the metal has started to solidify. By reducing turbulence and solidifying under high pressure, this cycle reduces both the porosity and inclusion content of the finished casting.After solidification is complete, the pressure is released, the dies separate, and ejector pins extract the finished casting along with its attached runners and sprues. There are two basic types of die-casting machines. Figure 13-5 schematically illustrates the hot-chamber, or gooseneck, variety.A gooseneck chamber is partially submerged in a reservoir of molten metal. With the plunger raised, molten metal flows through an open port and fills the chamber.A mechanical plunger then forces the metal up through the gooseneck, through the runners and gates, and into the die, where it rapidly solidifies. Retraction of the plunger then allows the gooseneck to refill as the casting is being ejected, and the cycle repeats at speeds up to 100 shots per minute. Hot-chamber die-casting machines offer fast cycling times (set by the ability of the water-cooled dies to cool and solidify the metal) and the added advantage that the molten metal is injected from the same chamber in which it is melted (i.e., there is no handling or transfer of molten metal). Unfortunately, the hot-chamber design cannot be used for the higher-melting-point metals, and it is unattractive for aluminum since the molten aluminum tends to pick up some iron during the extended time of contact with the casting equipment. Hot-chamber machines, therefore, see primary use with zinc-, tin-, and lead-based alloys. Zinc die castings can also be made by a process known as heated-manifold directinjection die casting (also known as direct-injection die casting or runnerless die casting). The molten zinc is forced through a heated manifold and then through heated mininozzles directly into the die cavity.This approach totally eliminates the need for sprues, gates, and runners. Scrap is reduced, energy is conserved (less molten metal per shot and no need to provide excess heat to compensate for cooling in the gating system), and product quality is increased. Existing die-casting machines can be converted through the addition of a heated manifold and modification of the various dies. Cold-chamber machines are usually employed for the die casting of materials that are not suitable for the hot-chamber design. These include alloys of aluminum, magnesium, and copper as well as high-aluminum zinc.As illustrated in Figure 13-6, metal that has been melted in a separate furnace is transported to the die-casting machine, where a measured quantity is fed into an unheated shot chamber (or injection cylinder) and subsequently driven into the die by a hydraulic or mechanical plunger.The pressure is then maintained or increased until solidification is complete. Since molten metal must be transferred to the chamber for each shot, the cold-chamber process has a longer operating cycle compared to hot-chamber machines. Nevertheless, productivity is still high

cost per part for 45,000 pieces, using one set of tooling, would actually be less than for 60,000 pieces, since the latter would require a second set of dies. In most cases, multiple processes are reasonable candidates for production, and the curves for all of the options should be included. The final selection is often based on a combination of economic, technical, and management considerations. Table 13-6 presents a comparison of casting processes, including green-sand casting, chemically bonded sand molds (shell, sodium silicate, and air-set), ceramic mold and investment casting, permanent-mold casting, and die casting. The processes are compared on the basis of cost for both small and large quantities, thinnest section, dimensional precision, surface finish, ease of casting a complex shape, ease of changing the design while in production, and range of castable materials

difficult, however.The nature of the charged materials and the reactions that occur within the cupola can all affect the product chemistry. Moreover, by the time the final chemistry is determined through analysis of the tapped product, a substantial charge of material is already working its way through the furnace. Final chemistry adjustments, therefore, are often performed in the ladle, using the various techniques of ladle metallurgy discussed in Chapter 6. Various methods can be used to increase the melting rate and improve the economy of a cupola operation. In a hot-blast cupola, the stack gases are put through a heat exchanger to preheat the incoming air. Oxygen-enriched blasts can also be used to increase the temperature and accelerate the rate of melting. Plasma torches can be employed to melt the iron scrap. With typical enhancements, the melting rate of a continuously operating cupola can be quite high, such that production of 120 tons of hot metal per hour is not uncommon. INDIRECT FUEL-FIRED FURNACES (OR CRUCIBLE FURNACES) Small batches of nonferrous metal are often melted in indirect fuel-fired furnaces that are essentially crucibles or holding pots whose outer surface is heated by an external flame.The containment crucibles are generally made from clay and graphite, silicon carbide, cast iron, or steel. Stirring action, temperature control, and chemistry control are often poor, and furnace size and melting rate are limited. Nevertheless, these furnaces do offer low capital and operating cost. Better control of temperature and chemistry can be obtained, however, if the crucible furnaces are heated by electrical resistance heating. DIRECT FUEL-FIRED FURNACES OR REVERBERATORY FURNACES Direct fuel-fired furnaces, also known as reverberatory furnaces, are similar to small open-hearth furnaces but are less sophisticated. As illustrated in Figure 13-14, a fuelfired flame passes directly over the pool of molten metal, with heat being transferred to the metal through both radiant heating from the refractory roof and walls and convective heating from the hot gases. Capacity is significantly greater than that of the crucible furnace, but the operation is still limited to the batch melting of nonferrous metals and the holding of cast iron that has been previously melted in a cupola.The rate of heating and melting and the temperature and composition of the molten metal are all easily controlled. ARC FURNACES Arc furnaces are the preferred method of melting in many foundries because of the (1) rapid melting rates, (2) ability to hold the molten metal for any desired period of time, and (3) greater ease of incorporating pollution control equipment.The basic features and operating cycle of a direct-arc furnace can be described with the aid of Figure 13-15.The top of the wide, shallow unit is first lifted or swung aside to permit the introduction of charge material. The top is then repositioned, and the electrodes are lowered to create an arc between the electrodes and the metal charge. The path of the heating current is usually through one electrode, across an arc to the metal charge, through the metal charge, and back through another arc to another electrode.

during solidification.When solidification is complete, the pressure is released and the unused metal in the feed tube simply drops back into the crucible. The reuse of this metal, coupled with the absence of additional risers, leads to yields that are often greater than 85%. Nearly all low-pressure permanent-mold castings are made from aluminum or magnesium, but some copper-based alloys can also be used. Mechanical properties are typically about 5% better than those of conventional permanent-mold castings. Cycle times are somewhat longer, however, than those of conventional permanent molding. Figure 13-3 depicts a similar variation of permanent-mold casting, where a vacuum is drawn on the die assembly and atmospheric pressure in the chamber forces the metal upward. All of the benefits and features of the low-pressure process are retained, including the subsurface extraction of molten metal from the melt, the bottom feed to the mold, the minimal metal disturbance during pouring, the self-risering action, and the downward directional solidification. Thin-walled castings can be produced with high metal yield and excellent surface quality. Because of the vacuum, the cleanliness of the metal and the dissolved gas content are superior to that of the low-pressure process. Final castings typically range from 0.2 to 5 kg (0.4 to 10 lb) and have mechanical properties that are even better than those of the low-pressure permanent-mold products. ■ 13.3 DIE CASTING In the die-casting process, or more specifically pressure die casting, molten metal is forced into metal molds under pressures of several thousand pounds per square inch (tens of MPa) and held under high pressure during solidification. Because of the combination of metal molds or dies and high pressure, fine sections and excellent detail can be achieved, together with long mold life. Most die castings are made from nonferrous metals and alloys, with special zinc-, copper-, magnesium-, and aluminum-based alloys having been designed to produce excellent properties when die cast. Ferrous-metal die castings are possible but are generally considered to be uncommon. Production rates are high, the products exhibit good strength, shapes can be quite intricate, and dimensional precision and surface qualities are excellent. There is almost a complete elimination of subsequent machining. Most die castings can be classified as small- to medium-sized parts, but the size and weight of die castings are continually increasing. Parts can now be made with weights up to 10 kg (20 lb) and dimensions as large as 600 mm (24 in.). dega-c13_313-333-hr 1/9/07 3:53 PM Page 316 SECTION 13.3 D

pressure vessels, cylinder liners, brake drums, the starting material for bearing rings, and all of the parts illustrated in Figure 13-10 can be manufactured by centrifugal casting. The equipment is rather specialized and can be quite expensive for large castings.The permanent molds can also be expensive, but they offer a long service life, especially when coated with some form of refractory dust or wash. Since no sprues, gates, or risers are required, yields can be greater than 90%. Composite products can also be made by centrifugal casting of a second material on the inside surface of an already-cast product. Table 13-5 summarizes the features of the centrifugal casting process. In semicentrifugal casting (Figure 13-11) the centrifugal force assists the flow of metal from a central reservoir to the extremities of a rotating symmetrical mold.The rotational speeds are usually lower than for true centrifugal casting, and the molds may be either expendable or multiple-use. Several molds may also be stacked on top of one another, so they can be fed by a common pouring basin and sprue. In general, the mold shape is more complex than for true centrifugal casting, and cores can be placed in the mold to further increase the complexity of the product. The central reservoir acts as a riser and must be large enough to ensure that it will be the last material to freeze. Since the lighter impurities concentrate in the center, however, the process is best used for castings where the central region will ultimately be hollow. Common products include gear blanks, pulley sheaves, wheels, impellers, and electric motor rotors. Centrifuging, or centrifuge centrifugal casting (Figure 13-12), uses centrifugal action to force metal from a central pouring reservoir or sprue, through spoke-type runners, into separate mold cavities that are offset from the axis of rotation. Relatively low rotational speeds are required to produce sound castings with thin walls and intricate shapes. Centrifuging is often used to assist in the pouring of multiple-product investment casting trees.

Fluxing materials are usually added to create a protective slag over the pool of molten metal. Reactions between the slag and the metal serve to further remove impurities and are efficient because of the large interface area and the fact that the slag is as hot as the metal. Because the metal is covered and can be maintained at a given temperature for long periods of time, arc furnaces can be used to produce high-quality metal of almost any desired composition. They are available in sizes up to about 200 tons (but capacities of 25 tons or less are most common), and up to 50 tons per hour can be melted conveniently in batch operations. Arc furnaces are generally used with ferrous alloys, especially steel, and provide good mixing and homogeneity to the molten bath. Unfortunately, the noise and level of particle emissions can be rather high, and the consumption of electrodes, refractories, and power results in high operating costs. Figure 13-16 shows the pouring of an electric-arc furnace. Note the still-glowing electrodes at the top of the furnace. INDUCTION FURNACES Because of their very rapid melting rates and the relative ease of controlling pollution, electric induction furnaces have become another popular means of melting metal.There are two basic types of induction furnaces. The high-frequency, or coreless units, shown schematically in Figure 13-17, consist of a crucible surrounded by a water-cooled coil of copper tubing. A high-frequency electrical current passes through the coil, creating an alternating magnetic field.The varying magnetic field induces secondary electrical currents in the metal being melted, which bring about a rapid rate of heating. Coreless induction furnaces are used for virtually all common alloys, with the maximum temperature being limited only by the refractory and the ability to insulate against heat loss.They provide good control of temperature and composition and are available in a range of capacities up to about 65 tons. Because there is no contamination from the heat source, they produce very pure metal. Operation is generally on a batch basis. Low-frequency or channel-type induction furnaces are also seeing increased use.As shown in Figure 13-18,only a small channel is surrounded by the primary (current-carrying or heating) coil.A secondary coil is formed by a loop, or channel, of molten metal, and all the liquid metal is free to circulate through the loop and gain heat.To start,enough molten metal must be placed into the furnace to fill the secondary coil, with the remainder of the charge taking a variety of forms.The heating rate is high, and the temperature can be accurately controlled.As a result, channel-type furnaces are often preferred as holding furnaces, where the molten metal is maintained at a constant temperature for an extended period of time. Capacities can be quite large, up to about 250 ton

vFluxing materials are usually added to create a protective slag over the pool of molten metal. Reactions between the slag and the metal serve to further remove impurities and are efficient because of the large interface area and the fact that the slag is as hot as the metal. Because the metal is covered and can be maintained at a given temperature for long periods of time, arc furnaces can be used to produce high-quality metal of almost any desired composition. They are available in sizes up to about 200 tons (but capacities of 25 tons or less are most common), and up to 50 tons per hour can be melted conveniently in batch operations. Arc furnaces are generally used with ferrous alloys, especially steel, and provide good mixing and homogeneity to the molten bath. Unfortunately, the noise and level of particle emissions can be rather high, and the consumption of electrodes, refractories, and power results in high operating costs. Figure 13-16 shows the pouring of an electric-arc furnace. Note the still-glowing electrodes at the top of the furnace. INDUCTION FURNACES Because of their very rapid melting rates and the relative ease of controlling pollution, electric induction furnaces have become another popular means of melting metal.There are two basic types of induction furnaces. The high-frequency, or coreless units, shown schematically in Figure 13-17, consist of a crucible surrounded by a water-cooled coil of copper tubing. A high-frequency electrical current passes through the coil, creating an alternating magnetic field.The varying magnetic field induces secondary electrical currents in the metal being melted, which bring about a rapid rate of heating. Coreless induction furnaces are used for virtually all common alloys, with the maximum temperature being limited only by the refractory and the ability to insulate against heat loss.They provide good control of temperature and composition and are available in a range of capacities up to about 65 tons. Because there is no contamination from the heat source, they produce very pure metal. Operation is generally on a batch basis. Low-frequency or channel-type induction furnaces are also seeing increased use.As shown in Figure 13-18,only a small channel is surrounded by the primary (current-carrying or heating) coil.A secondary coil is formed by a loop, or channel, of molten metal, and all the liquid metal is free to circulate through the loop and gain heat.To start,enough molten metal must be placed into the furnace to fill the secondary coil, with the remainder of the charge taking a variety of forms.The heating rate is high, and the temperature can be accurately controlled.As a result, channel-type furnaces are often preferred as holding furnaces, where the molten metal is maintained at a constant temperature for an extended period of time. Capacities can be quite large, up to about 250 ton

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