Foundry - Blind Risers


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Improving Blind Riser Efficiency

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Foundry - Blind Risers

  2. 2. Improving Blind Riser Efficiency :Providing blind risers with adequate passage to atmospheric airand a hotter top metal front will increase your casting yield andreduce your energy costs. Confronted with escalating raw material and energy costs, ferrous metal caster AGAnderson, a division of AmeriCast Technologies, London, Ontario, Canada, went back to thebasics by re-evaluating sound and simple techniques. Blind risers, or closed risers, are used to feed various casting sections. Although theycan be indispensable when feeding certain complex configurations, they are less efficientthan open risers. AG Anderson designed a new blind riser system that was able to improvecasting yield and reduce labour and energy costs for large carbon and low alloy steel castings. Blind risers are located in the lower section of castings, or just hidden at a level belowthe top risers, and like open risers, they act as reservoirs that supply liquid metal into acasting as it contracts on solidification. Open risers break through the cope to the top of themold and are entirely exposed to the outside atmosphere. Blind risers top metal fronts are notexposed to the atmosphere, so a vacuum pocket sometimes occurs at the top of the riser. Inorder to prevent the formation of the pressurized zone, the liquid contained by the blind risermust have a free passage to the outside atmosphere. AG Andersons venting technique offers a practical solution to avoid the vacuumpocket in blind risers.
  3. 3. Chamber Made :Depending on its position in relation to the top or edge of the mold, the blind riser is subjectto uneven and limited exposure to atmospheric pressure. Mold, core, mold wash and evensleeve permeability must be considered when assessing the exposure of the liquid metalinside the riser sleeve to the atmospheric air. A blind riser covered by 4 or 5 ft. of sand doesnot have the same exposure to atmospheric pressure as a riser placed close to the top of themold and covered by 2 or 3 in. of sand. Commonly, blind risers are provided with one or two vents drilled or molded from theclosed end of the riser to the top of the mold. These vents, or pop-offs, only allow gases toescape the mold cavity as the metal rises into the mold. The metal entering the pop-offpassageway solidifies almost instantly, blocking the connection of the liquid metal in the riserto the atmospheric air. Often, the pop-off metal freezes before reaching the top of the mold. In order to gain more efficiency out of its blind risers, AG Anderson utilizes a ventingchamber on top of its riser sleeves (Fig. 1-2). The chambers can be made in no-bake orisocure sand and are shaped like cups with 2-3 in. (5.08-7.2 cm) diameter and 0.375 in.(0.953 cm) wall thickness. Before the sand is packed in the mold, the chamber is placed on top of the blind risersleeve. A 0.375-in. diameter dowel penetrates the top of the chamber, but not the riser sleeve.After the sand is cured, the dowel is removed and leaves an unobstructed passage between thecavity of the chamber and the top of the mold (Fig. 3). With the chamber-vent system inplace, the only remaining obstruction between the liquid metal inside the blind riser and theatmosphere is the actual riser sleeve, but the high permeability of the sleeve, together with thehot zone at the top of the feeder, make this obstruction negligible. Practical results show thatthe downward movement of the liquid metal from the feeder head is significantly improvedwhen the venting chamber is used.Blind Look : An extensive comparison study showed significant improvement when ventingchambers were used on blind risers. One riser unassisted by a venting chamber failed to feedproperly due to a skin of metal that formed prematurely on the surface. The riser mouldedwith a venting chamber, however, remained open at the top, allowing the atmosphericpressure to act through the chamber-vent system against the liquid metal front at the top ofthe riser. The same two risers were radiograph tested and sectioned longitudinally to betterunderstand the behaviour of the liquid metal and the extent of the feeding improvement. Theriser unassisted by a venting chamber showed a skin of metal that formed at the top, alongwith an insignificant amount of metal loss. The riser assisted by a venting chamber was openat the top and showed significant metal loss.
  4. 4. More for Less : The major metal loss in the properly vented riser demonstrated the effectiveness ofthe venting chamber in improving the efficiency of a blind riser when properly provided withan unobstructed passage to the atmospheric air. Today, the chamber vent system is a commonpractice at AG Anderson. The firm reviewed blind-riser jobs and reduced the size of many ofthe risers. As a result, the overall casting yield was significantly improved. Before the implementation of the new venting system, an 8,000lb. (3,629-kg) steeldiffuser had a pouring weight of 15,500 lbs. (7,031 kg). After the addition of a ventingchamber to each blind riser, the riser method was reconfigured, reducing some riser sizes andeliminating other risers altogether. The same diffuser is now poured using 12,000 lbs. (5,443kg) of metal, and casting yield improved from 51% to 66.7%. AG Anderson makes two ofthese diffusers per week, saving 364,000 lbs. (165,108 kg) of metal over a one-year period--the weight equivalent of 30 diffusers.The new venting system also saves energy and reduces cost based on the decreased time usedto upgrade and remove risers, as well as the reduction in pour weight (Fig. 5).Hot to Top : While the venting chamber provides an open passage between the liquid metal in theblind riser and the atmospheric air at the top of the mold, it cannot always prevent prematurefreezing at the surface of the feeder head. The solid skin formed on the surface can lead to theformation of secondary cavities that often penetrate into the casting. The venting chamber method can be modified to increase the temperature gradient ofthe feeder toward the top. Filling the chamber with ordinary exothermic hot topping creates a"hot venting chamber." After the metal reaches the top of the riser, the sleeve below thechamber collapses under the superheat, which allows the exothermic material to flow into theriser and float over the top of the liquid metal. The hot topping initiates an additionalexothermic reaction that encourages the metal at the top of the riser to remain in a liquidphase even longer. This reaction contributes to the caloric exchange between the molten metal and theriser sleeve, so the blind riser acts as an open one, well covered with hot topping and exposedto atmospheric pressure while still retaining the practical convenience of the blind riser. Thehot chamber method can be adopted when casting yield is of significant importance or whencasting configuration imposes the use of smaller risers. Venting chambers also can be used for core venting. When molded on top of coreprints, the chambers capture high amounts of core-generated gases, allowing them to escapeunobstructed. The ends of the nylon vents often used inside cores for core venting can becaptured under the venting chamber placed on the core print. Core venting through a ventingchamber is recommended in situations were large cores (mainly split and glued) aresurrounded by heavy walls of metal.
  5. 5. Under Pressure : Venting a blind riser with a chamber is meant to increase casting yield and quality byimproving its functionality and performance. The feeding distance, which is controlled by thecasting wall thickness, is not affected by the use of the venting chamber. Previous studieshave shown that by applying pressure over the liquid front of the metal at the top of the riser,the amount of feed metal supplied through the solidifying wall will increase. The pressureimproves the ability of the metal to flow through the partially-solidified casting and reachareas away from the riser. The principle of increasing the feeding distance by the use of pressurized risers hasbeen experimentally proven but never adopted due to its limited practicality. In order to usepressurized risers, a continuous and controlled pressure must be applied from a sourcethrough the mold and inside a riser sleeve in the area where the metal is still liquid. The use of a venting chamber with a slightly modified venting system may be a morepractical approach to riser pressurization. The venting chamber could be connected to asource of argon or compressed air by using copper or flexible plastic tubing. In this case, theventing chamber becomes a pressure chamber that receives the gas from the line and transfersit into the riser. Future work at AG Anderson will focus on developing the schematic for apressurized blind riser.References :Vasile Ionescu, AG Anderson, a div. of AmeriCast Technologies Inc., London, Ontario,CanadaVasile Ionescu is senior metallurgist for AG Anderson, a division of AmeriCast TechnologiesInc., London, Ontario, Canada.For More Information"Improving Productivity in Ductile Iron Castings--Some Ideas to Improve Casting Yield," N.T. Rizzo Downes and S. Kannan, 2003 AFS Transactions (03-081), p. 825.COPYRIGHT 2008 American Foundry Society, Inc.COPYRIGHT 2008 Gale, Cengage LearningVasile Ionescu "Improving blind riser efficiency: providing blind risers with adequatepassage to atmospheric air and a hotter top metal front will increase your casting yield andreduce your energy costs". Modern Casting. 23 Apr, 2011. 2008 American Foundry Society, Inc.COPYRIGHT 2008 Gale, Cengage Learning