Homepaper 2: Centrifugal Jewelry Casting Centrifugal casting is a cost-effective casting process that produces hollow cylindrical components, circular plates, and intricate parts 1 from practically any alloy. 2 As one might expect, centrifugal casting utilizes centrifugal force to drive molten metal into a mold or die cavity. As shown in Figure 1, molten metal is poured into a rotating mold and the centrifugal force from the mold rotation throws the liquid metal towards the outside of the mold. Rotational speeds range from 300-3000 rev/min, depending on the radius of the mold. This correlates to forces ranging from approximately 50-150 times the force of gravity experienced on the outside of the casting. 3 Common centrifugal castings include, gas and water pipes, engine-cylinder liners, large rolls, wheels, bushings, nozzles, gears, ship-shaft liners, bearing rings, and flanged shapes, as well as more intricately shaped castings like jewelry. 1 Centrifugal casting is classified into three more specific types of casting: true centrifugal casting, semicentrifugal castings, and centrifuging as seen in Figure 2. True centrifugal casting is the most common of the three types and is used for hollow symmetrical objects like pipes and steel tubing. This type of casting uses centrifugal forces to Figure 1: Horizontal Centrifugal Casting 3 Figure 2: Three types of centrifugal castings are: (a) centrifuging; (b) semicentrifugal; (c) true centrifugal. 1 Page 1 of 8
hold molten metal against the rotating walls of a hollow tubular mold. As no core is used, the volume of metal determines the thickness of the tube walls. This process is particularly effective because impurities and lighter dross tend to collect on the inside of the castings where they can be bored away. 1 Semicentrifugal casting is more or less true centrifugal casting with the addition of a core to the mold. This casting method is preferred for the formation of wheels, nozzles, and such. The liquid metal is poured into the mold via a gate placed on the central axis. Centrifugal forces then cause the metal to flow towards the extremities of the mold cavity. 1 Centrifuging involves several molds located radially around a central sprue or riser. For casting, the entire mold is rotated about the central sprue in which the molten metal is poured. Centrifugal forces push the liquid metal throughout the mold creating greater pouring pressure at all points in the mold cavity. This type of casting is very effective for casting small, intricate parts where the additional pressure from centrifugal force overcomes metal feeding problems that might otherwise occur. One major application for centrifuging is in the production of jewelry. 1 For centrifuge cast jewelry, investment or lost wax casting is typically used. This consists of creating a wax model of the desired piece, then placing the wax in a flask and surrounding it with investment. The investment is then cured and the wax burnt out of the mold. The mold is then placed into a centrifuge and cast before being removed and cooled. 4 More specifically, the first step of jewelry production involves creating a detailed wax model of the to-be-cast piece. Once the wax model has reached an acceptable level of refinement, wax wires called sprues are attached and the wax model mounted on a sprue former (a circular metal plate with a raised cone in the center). This setup can be seen in Figure 3. When the model is finally encased in the mold and the wax removed, the sprues will act as channels though which the molten metal will flow to reach the main cavity of the mold. As these channels are vital to the casting process, all sprues are very carefully formed and attached. 4 Figure 3: Wax Model Mounted on Sprue Former 4 Page 2 of 8
By design, sprues are always as short as possible. This reduces the distance that metal must flow during casting. Eight gauge wire is typically used for sprueing unless the model is too small to carry a wire of this size. For such small pieces, round wire as small as 14-gauge is used. This gauge size along with the minimized length of the sprues prevents the molten metal from solidifying in the sprues before the mold cavity has filled with metal. One end of the main sprue is carefully sealed to the heaviest portion of the wax model. A button 3/8 inch in diameter is build on to this main sprue ¼ inch below the point where the sprue connects to the wax model. The other end of the main sprue is then attached to the sprue former through the hole at the top of the sprue former s cone, and the cone filled in with wax. Auxiliary sprues are then attached to the main sprue and the model as needed to ensure the entire mold cavity gets filled adequately and to prevent shrinkage. These supplementary sprues are removed with a jewelers saw after casting. To promote a smooth flow of molten metal the surface of sprues are as smooth as possible, the sides parallel and slightly wider where they meet the model and at this junction corners are rounded. Anywhere from one to several models can be cast from the same sprue former providing they do not touch and the flask is of sufficient size. 4 An amount of metal is then determined either by a volume method such that in solid state the metal occupies a bit more volume than the wax model or a weight test using a conversion factor relating to the relative specific gravities of the wax and metal. After the amount of metal is determined and the sprues securely attached, the wax model is painted with a wetting or debubblizing solution. This solution relieves surface tension allowing the liquid investment to flow uniformly around the wax model and adhere to it well. This coating eliminates air bubbles between the mold and wax, creating a much smoother surface finish. As to not defeat the purpose of the coating, the wetting solution is applied very carefully to avoid air bubbles. This application either involves dunking the wax in a vat of solution several times or flowing the solution on with a soft brush. 4 The next step to centrifugal cast jewelry is flask selection. In order to reduce waste and cost, metal A cylindrical flasks are chosen such that the minimum amount of investment needed to resist the stresses of casting is used. For very small flasks (less than 1 inch diameter) this correlates to a minimum of ¼ inch of investment between the mold cavity and the flask boundaries. For larger flasks up to 8 inch in diameter, at least ½ inch of investment is designed A Flasks are typically made out of stainless steel for its favorable high temperature properties. Page 3 of 8
to be between the flask walls and the model and between ½ -1 inch of investment between model and the top and bottom of the mold. The larger the mold the longer the wax will take to burnout, which results in more energy and time consumption. 4 Once the proper size flask is selected, the following step is to create the casting mold out of investment. Investment is similar to plaster of Paris in the manner in which it mixes and sets, but is also able to withstand temperatures of 1350 ºF and has a favorable compressive strength of 1,500 psi, enough to resist the high g-forces of centrifuge casting. Investment is chiefly composed of plaster acting as a binder, silica for high refractory properties, boric acid for uniform thermal change during the wax burnout, and graphite to prevent oxidation. This combination creates a strong mold, which sets in smooth, firm contact with the wax model, yet is also porous enough to allow gases to escape ahead of molten metal. 4 Investment is mixed with room temperature water with a ratio of 1 part water to 2.2 parts cristobalite investment. The mixture is stirred in such a way as to not introduce air into the mix. After mixing, any air accidentally introduced is then removed either by vibrations or, if available, by a vacuum pump. The later is the superior method of air removal, resulting in consistently higher quality investment. When a vacuum pump is utilized the investment is subject to the vacuum twice: first in the mixing bowl and then in the flask. The reduced air pressure created by the vacuum causes the water in the investment mix to boil, eliminating all the air. If a vacuum pump is unavailable then it is necessary to coat the wetting layer on the wax model with a layer of investment about 1/8 inch thick, taking care to avoid air bubbles between the investment layer and the model. 4 After the investment has been prepared and the model sufficiently prepped, the empty flask is centered on and sealed to the sprue former, forming a container for the investment as seen in Figure 4. The investment is then poured into said container until it Figure 4: Coated Wax Model Centered in Flask 4 Page 4 of 8
overflows slightly as seen in Figure 5. The investment is then allowed to sit for roughly an hour, at which point the sprue former is removed with a gentle twisting motion and the spilled investment scraped away. The mold is then prepared for burnout. 4 Burnout works both to eliminate the wax, creating a negative of the wax model and to mature the investment to withstand the forces of casting. To remove the wax from the mold the flask is placed in a kiln with the sprue down as to allow the melted wax Figure 5: Flask Filled with Liquid Investment 4 to flow out. The temperature in the kiln is then raised gradually from room temperature to 1300 ºF for several hours. At 800 ºF the wax burns, but it leaves behind a carbon residue as seen in Figure 6, clogging the pores of the investment. Additional heating is required to eliminate this residue, allowing the air to pass through the pores to ahead of the molten metal. 4 Figure 6: Flask and Mold After Wax Removed 4 Once the residue has been removed from the mold, the flask is cooled to the desired temperature for casting. This temperature is dependant on the melting point of the metal tobe-cast. Ideally, the flask is at a high enough temperature such that the metal stays molten while traveling through the sprues, but low enough to solidify in the internal cavity. For sterling silver the ideal flask temperature is 900 ºF and gold is cast in a flask at around 700 ºF. Such a temperature is achieved by keeping the flask at the desired temperature in the kiln until just before it is placed into the casting machine. 4 As the flask is cooling to the desired temperature, the preweighed metal is cleaned of grease and oxides and painted with flux to prevent oxidation and aid in molten flow. The crucible is likewise cleaned and lined to prevent the metal from picking up impurities and the casting machine prepared for casting. The metal in the crucible is then melted to just about its flow point either with a torch or Page 5 of 8
machine furnace. Care is taken to avoid overheating the metal as this increases the likelihood of shrinkage and porosity. 4 When the metal has melted, the flask is removed from the kiln and placed in the casting machine and the crucible opening is carefully aligned with the main sprue. The metal is then reheated briefly to assure it is in the proper molten state. The machine is then started and the arm spins at about 300 revolutions per minute for 2-5 minutes forcing the molten metal into the mold with centrifugal force. Once the machine has come to a stop the flask is removed with tongs and then quenched in water. The quenching breaks up the investment and the casting is removed. Next, the casting is cleaned with a brush, annealed to a dull red glow, and then treated with an acid solution specific to the type of metal cast. This treatment removes any remaining investment and oxidation. The sprues are then cut away and the piece is washed, filed, and polished to its finished state. 4 This finish product is of consistently higher quality than one cast statically due to the many advantages of centrifugal casting. Beyond the major advantage of facilitating metal flow through small spaces, centrifugal casting has many other favorable attributes that lend themselves to jewelry production. One such factor is the concentration of impurities on the interior of the casting. 5 As the liquid metal is poured into the rotating mold and spread over the mold cavity, solidification begins immediately. Because most of the heat is dissipated through the mold, the solidification is progressive. As this solidification is occurring the remaining liquid metal feeds the solid, combining with centrifugal pressure to result in a dense, sound structure on the outside of the casting, with impurities concentrated towards the center where they do not effect the appearance or aging of the jewelry. 2 The mechanism of molten metal feeding the forming solid also acts to prevent shrinkage porosity. When most metals and metallic alloys cool from a liquid state to a Figure 7: Typical Columnar Grain Growth 5 solid state, they experience a solidification Page 6 of 8
shrinkage in which the metals volume decreases. If unaccounted for, this shrinkage can cause cavities inside the solidified castings. The elimination of such shrinkage gives centrifugal casting a distinct advantage over static casting. 5 Additionally, the favorable thermal conductivity of the mold walls result in cooling progressing from the outside in. This causes a favorable directional solidification and columnar grain growth B as shown in Figure 7. 5 From this figure one can also see that, unlike statically cast metals, centrifugally cast metals have a practically uniform microstructure at all depths. 5 The exceptions to this are the extreme interior, which contains impurities and the outside surface of the metal, which has finer crystal grains. This grain size and even the grain size of the columnar grain are smaller than that of metals produced by static casting. 5 These grain features give centrifugal castings superior mechanical properties to static castings 2 such as higher yield strength, toughness, and hardness due to greater total grain boundary area, which impedes dislocation motions. 6 As a result of these favorable qualities, centrifugal casting has proven itself to be an ideal choice to produce high quality jewelry. B Because of the complex shapes typically cast by centrifuging, columnar grain growth is not as perfect and consistent as it is for true centrifugal casting. Page 7 of 8
Works Cited 1 - Bralla, James G. Design for Manufacturability Handbook.New York, NY, USA: McGraw- Hill, 1998. http://site.ebrary.com/lib/swarthmore/doc?id=10180086&ppg=772 2 - Geng, Hwaiyu. Manufacturing Engineering Handbook. New York, NY, USA: McGraw-Hill, 2004. p 407. http://site.ebrary.com/lib/swarthmore/doc?id=10180040&ppg=407 3 - Waters. Fundamentals of Manufacturing for Engineers.Boca Raton, FL, USA: CRC Press LLC, 1996. p 31. http://site.ebrary.com/lib/swarthmore/doc?id=10099113&ppg=42 4 Story, Mickey. Centrifugal Casting as a Jewelry Process. Scranton, PA: International Textbook Company, 1963. 5 Janco, Nathan. Centrifugal Casting. Des Plaines, Ill: Amer Foundrymens Society, 1988. 6- Callister, William D.. Materials Science and Engineering: An Introduction. New York, NY: Wiley, 2006. Page 8 of 8