Centrifugal Casting

Sufei Wei, The Centrifugal Casting Machine Company, and Steve Lampman, ASM International

CENTRIFUGAL CASTING is one of the density and freedom from oxides, gases, and the rotating mold. For example, with a vertical largest casting branches in the casting industry, other nonmetallic inclusions. When casting mold axis, the resultant force on the liquid is accounting for 15% of the total casting output solid parts, the pressure from rotation allows constant. This is not the case in a horizontal of the world in terms of tonnage. Centrifugal thinner details to be cast, making surface details mold. The other difference between horizontal casting was invented in 1918 by the Brazilian of the metal-cast components more prominent. and vertical mold orientation is the speed Dimitri Sensaud deLavaud, after whom the pro- Another advantage of centrifugal casting is the obtained by the molten metal as it spins around cess was named. DeLavaud’s invention elimi- elimination or minimization of gates and risers. the mold. When metal is poured into the hori- nated the need for a central core in the pipe zontally rotating mold, considerable slip occurs mold, and the mold was water cooled, allowing between the metal and the mold such that the for a high rate of repeated use. The technique Centrifugal Casting Methods metal does not move as fast as the rotating uses the centrifugal force generated by a rotat- mold. To overcome this inertia, the metal must ing cylindrical mold to throw molten metal Centrifugal casting machines are categorized be accelerated to reach the mold rotation speed. against a mold wall to form the desired shape. into three basic types based on the direction of This is not a problem in the vertical centrifugal Therefore, a centrifugal casting machine must the spinning axis: horizontal, vertical, or process, where the molten metal reaches the be able to spin a mold, receive molten metal, inclined. Centrifugal casting processes also have speed of the mold soon after pouring. However, and let the metal solidify and cool in the mold three types (Fig. 1): with a vertical mold axis, there is a tendency in a carefully controlled manner. for the molten metal to form a parabolic shape All metals that can be cast by static casting True centrifugal casting (horizontal, vertical, due to the competing gravitational and centrifugal can be cast by the centrifugal casting process, or inclined) forces. including carbon and alloy steels, high-alloy Semicentrifugal (centrifugal mold) casting corrosion- and heat-resistant steels, gray iron, Centrifuge mold (centrifugal die) casting ductile and nodular iron, high-alloy irons, True Centrifugal Casting stainless steels, nickel steels, aluminum alloys, The latter two methods are only done with copper alloys, magnesium alloys, nickel- and vertical spinning. This method, also referred to as just centrifu- cobalt-base alloys, and titanium alloys. Non- Horizontal centrifugal casting is mainly used gal casting, is characterized by an outer cylin- metals can also be cast by centrifugal casting, to cast pieces with a high length-to-diameter drical mold with no cores. The process can be including ceramics, glasses, plastics, and virtu- ratio or with a uniform internal diameter. Prod- vertical, horizontal, or inclined (Fig. 1). The ally any material that can be made into liquid ucts include pipe, tubes, bushings, cylinder permanent mold is rotated about its axis at high or pourable slurries. Centrifugal castings can sleeves (liners), and cylindrical or tubular speeds (300 to 3000 rpm), so that the molten be best described as isotropic, that is, having castings that are simple in shape. On the other metal is forced to the inside mold wall, where equal properties in all directions. This is not hand, vertical centrifugal casting is mainly for it solidifies. The casting is usually very fine true of a forging, rolling, or extrusion. castings with a low length-to-diameter ratio grained on the outer diameter, while the inside The centrifugal technique is used primarily (except vertically cast extralong rolls) or with diameter has more impurities and inclusions for the production of hollow components, but a conical diameter. The product range for that can be machined away. centrifugal casting is used to create solid parts. vertical centrifugal casting machines is wider, Centrifugal casting is used to produce cylin- The centrifugal casting process is generally pre- because noncylindrical (or even nonsymmetri- drical, tubular, or ring-shaped castings. The ferred for producing a superior-quality tubular cal) parts can be made using vertical centrifugal need for a center core is completely eliminated. or cylindrical casting, because the process is casting. All vertical centrifugal castings have Castings produced by this method will always economical with regard to casting yield, clean- more or less taper on their inside diameters, have a true cylindrical bore or inside diameter, ing room cost, and mold cost. The centrifugal depending on the gravitational (g) force applied regardless of shape or configuration. The bore force causes high pressures to develop in the to the mold and the casting size. The inclined of the casting will be straight or tapered, metal, and it contributes to the feeding of the centrifugal casting machine bears advantages depending on the horizontal or vertical spinning metal, with separation from nonmetallic inclu- and disadvantages of both horizontal and verti- axis used. Castings produced in metal molds by sions and evolved gases. In centrifugal casting cal castings and can be very useful in certain this method have true directional heat flow, of hollow sections, nonmetallic inclusions and applications. facilitating a planar solidification front move evolved gases tend toward the inner surface of Although both vertical and horizontal from the outside of the casting toward the axis the hollow casting. By using the outstanding methods employ centrifugal force, there are of rotation. This method results in the produc- advantage created by the centrifugal force of some differences in how the force is applied tion of high-quality, defect-free castings with- rotating molds, castings of high quality and with respect to the axis of the mold rotation out shrinkage, which is the largest single integrity can be produced because of their high and the speed of the molten metal relative to cause of defective sand castings. 668 / Centrifugal Casting

Horizontal true Vertical true Dimensional flexibility: Horizontal centrifu- centrifugal casting centrifugal casting gal casting allows the manufacture of pipes with maximum outside diameters close to 1.6 m (63 in.) and wall thicknesses to Molten Gate metal 200 mm (8 in.). Tolerances depend on part Mold size and the type of mold used. Quality: The centrifugal action removes Mold unwanted inclusions, dross, cleaner casting, and material that contains shrinkage, which Wheel can be machined away. Class I castings can Drive be produced without the need for upgrading and costly weld repairs. Vertical semi-centrifugal Properties: Mechanical properties are often Inclined true (centritugal-mold) casting superior to those of static castings due to the centrifugal casting finer grains resulting from the process, which Mold Integral tube are of constant size in circumferential and axial directions. Due to cleanliness and finer Flasks Cope grain size, good weldability is achieved.

Drag Semicentrifugal (Centrifugal Mold) Casting

In semicentrifugal casting (Fig. 1), a mold is rotated around its axis of symmetry. Cast con- figurations may be complex, determined by the shape of the mold. Molds for semicentrifu- Centrifuge gal casting often contain cores for production casting of internal surfaces. Directional solidification is obtained only by proper gating, as in static Molten casting. Castings that are difficult to produce metal statically can often be economically produced by this method, because centrifugal force feeds Mold Casting the molten metal under pressure many times higher than that in static casting. This improves casting yield significantly (85 to 95%), completely fills mold cavities, and results in a high-quality casting free of voids and porosity. Thinner casting sections can be produced with this method than with static casting. Typi- cal castings of this type include gear blanks, pulley sheaves, wheels, impellers, cogwheels, and electric motor rotors. The centrifugal force is used for slag separation and refilling of melt. Fig. 1 Methods of casting with rotating molds. True centrifugal casting (top row) can be horizontal, vertical, or at an incline. Semicentrifugal and centrifuge die casting are vertical methods with molds that can be designed to produce solid parts. Centrifuge (Centrifugal Die) Casting Centrifugal mold or die casting, also referred to as centrifuge casting, has the widest field of Centrifugal casting is also the preferred Advantages of centrifugal casting include: application. This casting method is typically method for Babbitting medium- and thick-wall, used to produce valve bodies and bonnets, half-shell or full-round (nonsplit) journal bear- Flexibility in casting composition: Centrifu- plugs, yokes, brackets, and a wide variety of ings, because it virtually eliminates porosity gal casting is applicable to nearly all compo- various industrial castings. and allows close control of the cooling process sitions, with the exception of high-carbon In this method (Fig. 1), the casting cavities to promote a strong bond. A disadvantage is the steels (0.40 to 0.85% C). Carbon segregation are arranged about the center axis of rotation need for more extensive equipment and tooling can be a problem in this composition range. like the spokes of a wheel, thus permitting the than for static casting. Minor segregation of the Wide range of available product character- production of multiple castings. Centrifugal intermetallics can occur across the thickness of istics: The metallurgical characteristics of force provides the necessary pressure on the the Babbitt, but segregation along the axial a tubular product are mainly characterized molten metal in the same manner as in semi- length of a statically cast bearing can be more by its soundness, texture, structure, and centrifugal casting. The centrifugal force con- serious and is more difficult to detect. The spin- mechanical properties. Centrifugal castings tributes to metal feed for casting thinner ning axis is usually horizontal, but vertical ori- can be manufactured with a wide range of sections and making surface details. entation is sometimes employed for unusual microstructures tailored to meet the demands Centrifuge casting is often used in conjunc- sizes (e.g., large diameters or short lengths). of specific applications. tion with investment casting. It is also used with Centrifugal Casting / 669 rubber molds made of silicone rubber with suffi- continue to use computers to collect, store, Vertical machines with a long mold need trun- ciently higher temperature resistance for and process the key production, equipment, nion wheels to support the mold around the cir- repeated use in casting without mold deteriora- and product information for high-tech products, cumference at the upper section. Small spinners tion. Centrifuge casting with rubber molds is also so that the production parameters can be opti- and molds can be clustered to form a turntable, referred to as spin casting. Suitable materials for mized and automatically set up in production. an over-and-under, or a Ferris wheel-type casting spin casting include zinc-base alloys, lead-base Centrifugal casting will continue to be prosper- machine, which shares the same pouring, heat- alloys, tin-base alloys, aluminum, and plastics. ous in the casting industry. ing, coating, and extracting systems, forming Spinners. Spinning of the mold is realized by a very efficient production cell. Equipment spinners. The achievable spinning speed and, Pour equipment and pour rate can be essen- sometimes, the ability to change speeds during tial pieces of the process for successful production of certain types of castings. Pouring is A centrifugal casting machine must be able the pour and solidification process is an important consideration for determining whether a critical factor in the following applications, to perform six operations accurately and with to name a few. The centrifugal ductile iron pipe repeatability: a casting geometry or a casting microstructure can be achieved. Spinning speeds are chosen casting process requires a long, moving trough The machine must be able to accelerate based on the centrifugal force requirement, and quadrant pour ladle to deliver the molten the mold to a predetermined speed, maintain which is measured by the multiple of the grav- iron along the mold to make 6 to 8 m (20 to smooth spinning, and decelerate to a stop in itational force (called g force). Horizontal cast- 26 ft) long pipe. The centrifugal soil pipe cast- a reasonable time frame. The machine also ing machines are typically spun at 45 to 60 g ing process needs a proper chute that can be should be able to change speeds during force on the casting inside diameter, and verti- cleaned and coated in a few seconds after each pour and solidification for some special cal casting machines are typically spun at 75 g pour and a consistent pour rate to dash the mol- applications. force. For some special applications, the spin- ten metal to the far end of the mold to form 3 m There must be a way to heat and coat the ning speed can be as high as 100 to 200 g force (10 ft) long pipes with even thickness. Centrif- mold before pouring the molten metal (e.g., horizontal roll castings and heat-resistant ugal heat-resistant steel tube castings rely on (except the cold mold method for ductile steel tube castings); in other cases, it can be as a fast pour rate and sufficient pour height to iron pipe). low as a fraction of a g to a few g’s (e.g., when help form the long thin-wall tube before the There must be a means to pour the molten vertically pouring a roll core, vertically pouring molten metal gets too cold. Vertical roll casting metal safely into the rotating mold at a con- solid bars, or horizontal two-speed pouring). uses a special-shaped down sprue to prevent the trolled rate, position, and orientation. Figure 6 in the article “Vertical Centrifugal metal flow from whirling. Once the metal is poured, a proper solidifica- Casting” in this Volume provides a convenient Casting Cooling. Centrifugal castings tion and cooling rate must be established in chart to check the centrifugal force against cast- should be cooled unidirectionally from outside the mold to obtain a desired casting micro- ing diameters and spinning speeds. to inside. Any two-way solidification will structure, protect the mold, and achieve In order to achieve the correct g force, the increase the chance of shrinkage porosity and required productivity. proper motor size is required. Too small a motor machining allowance, which should be avoided There must be a means of adding inoculants cannot meet process requirements for the spin- or minimized in thick-wall castings. Cooling or fluxes for some special applications. ning speed, and too big a motor can have an rate can affect the microstructure, casting hard- There must be a means of extracting the excessive acceleration rate that may cause ness, circumferential and axial cracks, machine solidified casting quickly from the mold at machine vibration. For large horizontal productivity, as well as mold life. In most elevated temperatures without deforming machines, hydraulic motors are preferred over cases, the early cooling rate of the castings is the casting. electric motors, because the former offer mainly controlled by the coating thickness, smoother spinning. Vibration monitors are coating texture, coating materials, as well as Centrifugal casting machines have realized installed for some large spinners to ensure mold thickness and mold materials; however, mechanization and automation basically in all smooth spinning. The spinners also need to be the later cooling rate is mainly controlled by of the aforementioned six tooling areas in many stopped at a predetermined time, which can be water cooling (unless the mold is not cooled by applications. An example of an automated hor- realized by the proper electric, hydraulic, or water). Water-cooling methods include water izontal machine and extractor is shown in mechanical means. Most vertical machines use submerge, waterjet spray, and water sleeve. Fig. 2. New casting and machine products are a single spinner, and the mold is spun on top of Ductile iron pipe production uses all three still evolving, such as metal-matrix composite the spinner. These vertical machines will have water-cooling methods mentioned previously, castings and magnetic stirring solidification. less stability than horizontal machines when the but other centrifugal castings mainly require Centrifugal casting and equipment engineers mold length-to-diameter ratio is greater than 1. waterjet spray, because of its simple setup and easy control of mold temperature. Roll production usually does not require water cooling, because the chill wall thickness is sufficient to absorb heat from the molten metal. Water cooling needs to be consistent, but sometimes, it needs intensity variation along the casting length. For example, the middle section of a long tube mold usually needs more water for cooling. As far as the grain structure of centrifugal castings (e.g., columnar versus equiaxed) is concerned, the pour temperature or the variable spinning speed plays a much more important role in obtaining the equiaxed grains than the water-cooling rate or mold temperature. Water cooling can be critical in certain products, such as high leaded bronze bushings and Babbitt bearings, where intensive water cooling assures Fig. 2 Automated horizontal casting machine. Courtesy of the Centrifugal Casting Machine Company that the lead segregation is suppressed. 670 / Centrifugal Casting

Some applications require continuous moni- horizontal machine can have a mold body with- wash, which is consistently replacing the other toring of casting and mold temperatures by out other mold parts. coating materials. Sand and resin sand linings a thermal couple probe inserted into the mold Mold materials include metallic permanent are quickly fading out, and graphite coating is cavity or infrared sensors aimed at the casting molds, refractory-lined metal molds, sand-lined mainly used for small nonferrous castings. The inside diameter and mold outside diameter, so metal molds, and other materials such as graph- dominance of water-based mold wash is estab- that the current process can be stopped and ite and rubbers. The metallic permanent molds lished on the fact that it offers significant the next process can be started. This is critical, are most widely used because of their reusabil- advantages over the other types. Some of these for example, to obtain a good bimetal bonding ity, accurate casting geometry, and high pro- advantages are: for roll production. ductivity. Mold inside diameters are subject to Casting Inoculation and Fluxing. Some thermal fatigue no matter what mold material Water-based mold wash is insulating enough castings necessitate the use of inoculants to is used. The low-carbon and low-alloyed forged for almost all applications. obtain the desired microstructure (such as duc- steel molds have much longer fatigue lives than It allows immediate metal pouring as soon tile iron and gray iron) or grain sizes (such as cast steel or cast iron molds, and they are also as the coating application is finished, which thick-wall chilled iron or steel tubes). In some safer to use. However, forged steel molds are greatly increases the casting productivity. applications, fluxes are needed to protect the more expensive to make than molds made of It offers a better surface finish to the casting inside diameter in order to obtain cast steel and iron. Copper alloys, tool steels, castings. a sound metallurgical bimetal bonding (such and superalloys are sometimes used for small Coating thickness can be easily used to con- as in the production of rolls and brake drums) castings that require high pour temperatures or trol the microstructure. or to prevent premature solidification or oxida- high cooling rates. Hardened rubber molds are It greatly reduces the friction between the tion (such as in thick-wall steel castings) of used for some metal jewelry with low melting casting and the mold. the casting inside diameter. Still others may points. It can be easily cleaned off the casting and use fluxes to purify the molten metal during Both horizontal and vertical molds must mold. pouring and solidification. All of these would meet certain geometrical precisions (straight- Its consumption is very small compared to require some means to deliver the inoculants ness, smoothness, roundness, concentricity, sand linings. or fluxes into the metal stream during pouring freedom from internal pores) to minimize The mold has a long life under its protection. or into the mold cavity before or during machine vibration, which can be critical to solidification. safety, bearing life, machine life, and product The mold can be heated from the outside or Casting Extraction. Castings need to be quality. The mold should also be maintained inside diameter on the machine with the mold extracted from the permanent mold. Small and or repaired during the life of its service. For spinning slowly by a row, or rows, of gas burners or off the machine in an oven at 180 low-production machines use manual extractors example, the inside diameter of a ductile iron and manpower to extract. Large castings, how- pipe mold is periodically peened to eliminate to 320 C (355 to 610 F). The mold tempera- ever, rely on mechanized extractors to do the tensile stress caused by thermal fatigue. Large ture can be critical for the coating adhesion to job. In extracting from horizontal machines, grooves or cracks on the mold inside diameter the mold, coating strength, as well as casting the extractor must overcome the friction acting can be repaired by subarc welding and grinding. surface quality. Once the mold is heated suffi- on the whole interface between the casting The mold geometric accuracy can be main- ciently, mold wash is applied on the mold and the mold. For vertical machines, the extrac- tained through remachining and grinding. inside diameter by spraying or flooding. The tor must overcome friction plus casting weight. Mold endplates should be properly designed mold wash thickness is usually between 0.5 For some applications, the castings are kept to prevent molten metal leakage. It cannot be and 3 mm (0.02 and 0.12 in.), depending on rotating during and/or after extraction to overemphasized that the mold endplate locks the application. The spraying method is pre- prevent the casting from warping. and mold fasteners are designed with a safety ferred for most processes because it gives more factor greater than 10 to prevent the endplates uniform coating thickness, smoother coating from coming off the mold or the molds from finish, and more consistent coating quality. Molds coming off the machine, which can cause cata- Spraying can also reach the areas that flooding strophic molten metal spilling. The push force cannot. The spraying method works for both Sand molds, semipermanent molds, and per- (F) on the endplates generated by the spinning horizontal and vertical molds, but the flooding manent molds can be used for the centrifugal metal is expressed by the following equation: method works only for horizontal molds. Figure 3 shows a patented automatic spray lance sys- casting process. Centrifugal action can also be 2ÀÁ N 2 combined with other molding methods such as F ¼ 5:375 d2 d2 tem that offers automatic control of the forward 100 o i investment casting. Selection of the type of and backward movement, moving velocity, and mold is determined by the shape of the casting, where r is the molten metal density (kg/m3), N spray on and off operations of the spray lance. the degree of quality needed, material to be is the mold spinning speed (rpm), d is the cast- It can also adjust height and tilt angle to align o the lance with all customer mold sizes. cast, and the production (number of castings) ing outside diameter (m), and di is the casting required. In addition to the processing parame- inside diameter (m). ters, proper mold design and selection is vital This equation illustrates that the pushing to producing quality castings. More details force increases rapidly with the spinning speed Defects in Centrifugal Castings on mold design are provided in the articles (squared), casting diameter (squared), casting “Horizontal Centrifugal Casting” and Vertical thickness (squared), as well as the metal den- The three most common defects observed in Centrifugal Casting” in this Volume. sity. Therefore, whenever a large or thick cast- centrifugal castings are segregation banding, Molds consist basically of four parts: the ing is to be poured, the pushing force on the raining, and vibration defects. mold body, track grooves or roller tracks on endplates must be calculated so that the stresses Segregation banding occurs only in true the body (these can be absent if the machine of the endplate locks, bolts, pins, and fasteners centrifugal casting, generally where the casting has thrust wheels to hold the mold axially), can be calculated and properly designed. wall thickness exceeds 50 to 75 mm (2 to 3 in.). endplates attached to the mold body, and end- Mold Heating and Coating Techniques. It rarely occurs in thinner-wall castings. Band- plate swing locks or taper pins/wedges. A verti- The centrifugal casting mold must be heated ing can occur in both horizontal and vertical cal mold needs fixtures to fasten it onto the and coated with ceramic mold wash, mold pow- centrifugal castings. Banding is more prevalent adapter table, and the table is bolted onto the der, sand, resin sand, or graphite. The most in alloys with a wide solidification range and spinner shaft. A mold used on a dual-faceplate widely used mold coating is water-based mold greater solidification shrinkage. Centrifugal Casting / 671

Lance Clamp spring Reverse Surface smoothness and dimensional accu- switch Spray stop stop racy of aluminum alloy centrifugal castings were much better than the gravity investment castings, but they were much more Spray wash in affected by the ﬁneness of investment material than centrifugal force. The soundness and mechanical properties Spray lance were improved with increased centrifugal Spray tip force.

Tilt adjustment The liquidus-solidus interval is important in Compressed air centrifugally cast investment mold castings, and final casting soundness depends on the gat- Vertical clamps Traverse ing system for castings of alloys with narrow drive and wide freezing ranges (Ref 3). Factors that influence soundness were identified to be Spray tank (Ref 3):

Hydraulic Ingate diameter Anchor bolts height Sprue diameter adjustment Metal pour temperature Mold temperature prior to pour Fig. 3 Patented automatic mold wash spray assembly. Courtesy of the Centrifugal Casting Machine Company Mold speed Spinning period Casting weight (expressed as a casting Bands are annular segregated zones of low- alloy and superalloy seal rings and valve seats length, l) melting constituents, such as eutectic phases for the auto industry; heat-resistant steel crack- Number of castings in each ring on the sprue and oxide or sulfide inclusions. They are char- ing tubes for petroleum refinery and radiant Distance (L) from the axis of rotation to the acterized by a hard demarcation line at the out- heating tubes and furnace rolls for heat treat- castings on the sprue (which characterizes side edge of the band that usually merges into ment furnaces; and copper alloys and Babbitt the sprue length) the base metal of the casting. alloys for bimetal bearings and bushings. See Liquidus-solidus interval (freezing range) Most alloys are susceptible to banding, but the article “Horizontal Centrifugal Casting” in the wider the solidification range and the this Volume for more information. For a wide freezing range (e.g., 100 K in greater the solidification shrinkage, the more Vertical machines are mainly used for shorter bronze), the casting soundness was improved pronounced the effects may be. Banding has parts, such as bushings, gear blanks, rings, short by increasing mold speed, ingate diameter, been found when some critical level of rota- rolls, wheels, aluminum and copper electric sprue diameter, and the product (l L) of spec- tional speed is attained, and it has been associ- motor rotors, jewelries, and vacuum titanium imen size and sprue length. Mold speed, n, was ated with very low speeds, which can produce alloy parts, and many different small round found to be the most significant variable. sporadic surging of molten metal. Therefore, parts, such as balls, valve bodies, and rings for For wide-interval alloys, optimal conditions both mechanisms may be involved. Minor the machinery industry. With proper support, (Ref 3) are “those under which each portion of adjustments to casting operation variables, such vertical casting machines can also produce molten metal newly arriving in the mold cavity as rotational speed, pouring rate, and metal extralong rolls. Slightly inclined machines are should spread out at the appropriate level and and mold temperatures, will usually reduce or used for producing ductile iron pipe for water freeze onto the solidification front without build- eliminate banding. and gas mains, and other inclined machines ing up a substantial surplus of molten metal. Raining is a phenomenon that occurs in hor- are used for rolls and conical bushings. See Thus, the optimum conditions for pouring izontal centrifugal castings. If the mold is the article “Vertical Centrifugal Casting” in this intricate castings in wide-interval alloys are rotated at too low a speed or if the metal is Volume for more information. those which produce layer-by-layer freezing.” poured into the mold too fast, the metal actually Centrifugal casting is also used to ensure good “Casting soundness decreased as the number rains or falls from the top of the mold to the filling in investment casting, as described subse- of castings at the same level on the sprue bottom. Proper process control can eliminate quently. Another related technique is spin cast- increased. Thus, as fewer castings are made, raining. ing, which uses rubber molds for casting of their density will increase. Increasing the num- Vibration Defects. Vibration can cause zinc, tin, and other alloys with low melting ber of ingates surrounding the sprue at the same a laminated casting. It can be held to a minimum points. It is also used in conjunction with nonme- level probably leads to the development of by proper mounting, careful balancing of the tallic materials such as glass and plastics. A more a hot spot, heat storage around the spot, and molds, and frequent inspection of rollers, bear- advanced materials application is the combus- departure from conditions required for layer- ings, and other equipment. tion synthesis of functionally graded materials. wise solidification. It is therefore best to assem- Investment Casting with Centrifugal Force ble the patterns in a helical array on the sprue (Ref 1). When the action of centrifugal force is for intricate castings in wide-interval alloys.” Applications combined with investment casting, the main bene- “Density comparisons between the centrifu- fit is an improvement in casting soundness with gal castings at the two extreme levels on the Typical products produced by horizontal cen- subsequent improvements in mechanical proper- sprue have shown that the sprue length has no trifugal casting machines include pipe and ties. The influence of centrifugal force in alumi- significant influence on density.” For narrow- tubes of all different metals; alloyed iron, steel, num investment castings (Ref 2) found that: interval alloys, there is a sprue-length effect. and high-speed steel rolls for steel mills and the Narrow-interval alloys require much greater food-processing, papermaking, and printing A down-tapered sprue was more effective in ingate and sprue cross sections than wide-inter- industries; iron and steel cylinder liners, gray eliminating turbulence of the metal stream val alloys, and narrow-interval alloy “castings iron piston rings, gray iron brake drums, and and nonmetallic incisions. should always be made on short sprues.” 672 / Centrifugal Casting

“Narrow-interval alloys require heat accumu- Spin casting typically refers to a special type into intricate areas of the mold. Depending on lation around the sprue and ingates. Sound cast- of centrifuge casting with rubber molds. In the the size of the mold and the material being cast, ings can be made by accumulating surplus early 1970s, heat-cured silicone rubber (Gen- spinning speeds range from 100 to 900 rpm. molten metal ahead of the solidification front. eral Electric Silcast) was introduced with a suffi- Metal products can range from less than 100 g In this case, the optimum conditions correspond ciently higher temperature resistance for molds to 1.2 kg (3.5 oz to 2.6 lb). Maximum product to wide-section ingates and sprues, mold pre- in casting plastics and metals with low melting size depends on mold diameter and thickness. heating, the use of short sprues, and some points, such as zinc alloys, tin alloys, and alu- Mold diameters can range up to approximately reduction in the relative rate of molten metal minum. Heat-cured silicone rubber molds can 60 cm (24 in.), with thicknesses of 10 cm supply to the mold cavities.” withstand temperatures up to approximately (4 in.) or more. In mold making, metal patterns “Intricate castings in wide-interval alloys 550 C (1020 F) at a rate of 50 to 60 cycles are laid on uncured silicone rubber sheets, must be made under conditions which will pre- per hour for hundreds of cycles. However, spin which are packed into a circular metal frame. vent excessive heat accumulation around the casting of metals with rubber molds is generally The circular metal frame is placed into gating system and mold cavities, and minimize limited to the low-melting-point alloys (below a hydraulic vulcanizer press and electrically the volume of molten metal ahead of the solid- 400 C, or 750 F), due to the degradation of heated for curing. ification front. Sound metal can be ensured by the silicone rubber. Mold life is on the order The curing temperature and duration depends directional solidification. In this case, mold pre- of 200 to 250 shots at 400 C (750 F). on the type of rubber and the mold thickness. heating, commensurate metal weights and sprue During pouring, the mold is placed on a cen- Pattern materials, such as pewter, can be used lengths, helical mold assemblies around the trifuge wheel to achieve adequate metal filling as long as they can withstand a temperature of sprue, and normal ingate and sprue cross sections should lead to a certain’freezing’ action, while rapid mold cavity filling with metal in this condition should lead to layerwise solidification” (Ref 3). Centrifugal Casting with Combustion 5 Synthesis. Combustion synthesis (CS) is an attractive technique for synthesizing a wide variety of advanced materials, including pow- ders and near-net-shape products of ceramics, 4 intermetallics, composites, and functionally graded materials. One type of CS is self-propagating high-temperature synthesis (SHS), as described in Ref 4. Combustion synthesis of highly exothermic reactions (typically reduction type) often results in completely molten products, which may be processed using common metallurgical methods. Casting of CS products under inert gas pressure or centrifugal casting has been used to synthesize 3 2 cermet ingots, corrosion- and wear-resistant coat- 1 ings, and ceramic-lined pipes. Casting under gas pressure is similar to the conventional SHS pro- (a) duction, while centrifugal casting allows for greater control of the distribution of phases by controlling the time of separation. For radial cen- 5 2 trifuges (Fig. 4a), the sample is placed at a fixed radial position from the axis of rotation, and the applied centrifugal force is parallel to the direction of propagation. The influence of centrifugal acceleration characterizes the phase distribution in the final product (Fig. 5). 4 The second type of centrifugal casting appa- ratus, called an axial centrifuge (Fig. 4b), is used for production of ceramic, cermet, or ceramic-lined pipes. The axial centrifuge casting method was developed further for production of long pipes with multilayer ceramic inner coatings. In this process, a thermite mixture (e.g., Fe2O3/2Al) is placed inside a rotating pipe and ignited locally. A reduction-type combustion reaction propagates through the mixture, and the centrifugal force results in sep- arate layers of metal and ceramic oxide, with the latter forming the innermost layer (Fig. 6). 1 The process is carried out in air under normal (b) 3 pressure, and pipes with diameters up to 30 cm (12 in.) and 5.5 m (18 ft) long have been Fig. 4 Schematic of self-propagating high-temperature synthesis plus centrifugal casting. (a) Radial centrifuge. obtained. (b) Axial centrifuge. 1, sample container; 2, reactant mixture; 3, ignitor; 4, axle; 5, reactor Centrifugal Casting / 673

mold is separated from the metal, and the pat- Plastic Products. Spin casting of thermoplas- terns are removed. Special-shaped tools are tics and thermoset plastics includes: used to cut gates and runners on the rubber. Air vents are also cut for air to escape from Plumbing fittings, valves, and couplings the mold cavity. Modifications to gates and Electrical switch plates, condensers, and vents are usually required after a few shots connector plugs to obtain the best results. Making of the mold Toy cars, wheels, and gears takes approximately 1 to 3 days, depending on Pen holders, clips, and fasteners the complexity of the product shape. Electronic computer parts While the mold is spinning, the liquid metal Calculator casing keys or plastic is melted in a suitable electric or gas- fired crucible furnace and then poured into the Wax patterns for investment casting are also center sprue of the mold. After the metal solid- produced by spin casting. ifies (the plastic parts set up), the parts are quickly removed from the mold. With metal, 50 to 60 cycles per hour can readily be made; REFERENCES Fig. 5 Degree of phase separation as a function of with plastic, 10 to 15 cycles are typical. Parts centrifugal acceleration, a, where g is with undercuts can be demolded from the rubber 1. E. Roberston, LA-12370-MS, Los Alamos acceleration due to gravity. Source: Ref 5 mold with no difficulty. Thin-walled parts can be National Laboratory, Sept 1993 molded comparable to die casting. The rubber 2. J.-N. Lee and G.-C. Yun, Effect of Centrif- 3MeO + 2Al Al2O3 + 3Me + Q (1) mold is not as precise as the metal mold, but ugal Force Applied During Investment the surface finish is generally good. However, Casting on the Soundness of Cast Al Ј 3MeO + 2Al + 3C Al2O3 + 3MeC + Q (2) blisters (very small pinpoint holes) were often Alloys, J. Korean Inst. Met., Vol 15 found on the surface of the thick section due to (No. 2), 1977, p 155–163 the shrinkage of metals. The blisters are the 3. V.I. Osinskii, M.V. Solov’ev, and A.V. defects for products with a plated glossy surface. Morozov, Soundness of Intricate Centrifu- Metal Products. Spin casting has the capabil- gally Cast Investment-Mold Castings ity to produce intricate designs for low- or high- (Translation), Sov. Cast. Technol., Vol 7, volume quantities. Metal products include: 1987, p 50–54 4. A. Varma and A.S. Mukasyan, Combustion Cams and levers in automotive control panels Synthesis of Advanced Materials, Powder Bushings Metal Technologies and Applications, Fig. 6 Concept of centrifugal process for production of Hub, clamps, and screws Vol 7, ASM Handbook, ASM International, ceramic-lined steel pipes. Source: Ref 6 Triggers, firing mechanisms, and cartridges 1998 Medals, pins, figurines, key chains, name 5. A.G. Merzhanov and V.I. Yukhvid, in 190 C(375F) under several thousands of plates, and costume jewelry Proc. First U.S.-Japanese Workshop or pounds pressure without distorting, melting, Door hardware, locks, hooks, and rings Combustion Synthesis, Vol 1, National breaking, or outgassing. The heat melts the rubber, Handles Research Institute for Metals, Japan, 1990 which is pressed against the surface of the pattern Belt buckles, buttons, and watch cases 6. O. Odawara and J. Ikeuchi, J. Am. Ceram. before the rubber sets. After vulcanization, the Gears, motor housings, and pump impellers Soc., Vol 69, 1986, p C80