M S RAMAIAH INSTITUTE OF TECHNOLOGY

DEPARTMENT OF INDUSTRIAL ENGINEERING

AND MANAGEMENT

“FOUNDRY TECHNOLOGY”“BLIND RISERS”

Presented by,

Bharath M - 1MS09IM401Harisha N - 1MS09IM402Prathap S - 1MS09IM405Balaji A G - 1MS09IM400Nayaz Pasha- 1MS09IM404

Improving Blind Riser Efficiency :

Providing blind risers with adequate passage to atmospheric air and a hotter top metal front will increase your casting yield and reduce your energy costs.

Confronted with escalating raw material and energy costs, ferrous metal caster AG Anderson, a division of AmeriCast Technologies, London, Ontario, Canada, went back to the basics by re-evaluating sound and simple techniques.

Blind risers, or closed risers, are used to feed various casting sections. Although they can be indispensable when feeding certain complex configurations, they are less efficient than open risers. AG Anderson designed a new blind riser system that was able to improve casting 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 below the top risers, and like open risers, they act as reservoirs that supply liquid metal into a casting as it contracts on solidification. Open risers break through the cope to the top of the mold and are entirely exposed to the outside atmosphere. Blind risers' top metal fronts are not exposed to the atmosphere, so a vacuum pocket sometimes occurs at the top of the riser. In order to prevent the formation of the pressurized zone, the liquid contained by the blind riser must have a free passage to the outside atmosphere.

AG Anderson's venting technique offers a practical solution to avoid the vacuum pocket in blind risers.

Chamber Made :

Depending on its position in relation to the top or edge of the mold, the blind riser is subject to uneven and limited exposure to atmospheric pressure. Mold, core, mold wash and even sleeve permeability must be considered when assessing the exposure of the liquid metal inside the riser sleeve to the atmospheric air. A blind riser covered by 4 or 5 ft. of sand does not have the same exposure to atmospheric pressure as a riser placed close to the top of the mold and covered by 2 or 3 in. of sand.

Commonly, blind risers are provided with one or two vents drilled or molded from the closed end of the riser to the top of the mold. These vents, or pop-offs, only allow gases to escape the mold cavity as the metal rises into the mold. The metal entering the pop-off passageway solidifies almost instantly, blocking the connection of the liquid metal in the riser to 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 venting chamber on top of its riser sleeves (Fig. 1-2). The chambers can be made in no-bake or isocure 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 riser sleeve. 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 the cavity of the chamber and the top of the mold (Fig. 3). With the chamber-vent system in place, the only remaining obstruction between the liquid metal inside the blind riser and the atmosphere is the actual riser sleeve, but the high permeability of the sleeve, together with the hot zone at the top of the feeder, make this obstruction negligible. Practical results show that the downward movement of the liquid metal from the feeder head is significantly improved when the venting chamber is used.

Blind Look :

An extensive comparison study showed significant improvement when venting chambers were used on blind risers. One riser unassisted by a venting chamber failed to feed properly due to a skin of metal that formed prematurely on the surface. The riser moulded with a venting chamber, however, remained open at the top, allowing the atmospheric pressure to act through the chamber-vent system against the liquid metal front at the top of the riser.

The same two risers were radiograph tested and sectioned longitudinally to better understand the behaviour of the liquid metal and the extent of the feeding improvement. The riser unassisted by a venting chamber showed a skin of metal that formed at the top, along with an insignificant amount of metal loss. The riser assisted by a venting chamber was open at the top and showed significant metal loss.

More for Less :

The major metal loss in the properly vented riser demonstrated the effectiveness of the venting chamber in improving the efficiency of a blind riser when properly provided with an unobstructed passage to the atmospheric air. Today, the chamber vent system is a common practice at AG Anderson. The firm reviewed blind-riser jobs and reduced the size of many of the 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) steel diffuser had a pouring weight of 15,500 lbs. (7,031 kg). After the addition of a venting chamber to each blind riser, the riser method was reconfigured, reducing some riser sizes and eliminating other risers altogether. The same diffuser is now poured using 12,000 lbs. (5,443 kg) of metal, and casting yield improved from 51% to 66.7%. AG Anderson makes two of these 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 used to 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 the blind riser and the atmospheric air at the top of the mold, it cannot always prevent premature freezing at the surface of the feeder head. The solid skin formed on the surface can lead to the formation of secondary cavities that often penetrate into the casting.

The venting chamber method can be modified to increase the temperature gradient of the 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 the chamber collapses under the superheat, which allows the exothermic material to flow into the riser and float over the top of the liquid metal. The hot topping initiates an additional exothermic reaction that encourages the metal at the top of the riser to remain in a liquid phase even longer.

This reaction contributes to the caloric exchange between the molten metal and the riser sleeve, so the blind riser acts as an open one, well covered with hot topping and exposed to atmospheric pressure while still retaining the practical convenience of the blind riser. The hot chamber method can be adopted when casting yield is of significant importance or when casting configuration imposes the use of smaller risers.

Venting chambers also can be used for core venting. When molded on top of core prints, the chambers capture high amounts of core-generated gases, allowing them to escape unobstructed. The ends of the nylon vents often used inside cores for core venting can be captured under the venting chamber placed on the core print. Core venting through a venting chamber is recommended in situations were large cores (mainly split and glued) are surrounded by heavy walls of metal.

Under Pressure :

Venting a blind riser with a chamber is meant to increase casting yield and quality by improving its functionality and performance. The feeding distance, which is controlled by the casting wall thickness, is not affected by the use of the venting chamber. Previous studies have 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 pressure improves the ability of the metal to flow through the partially-solidified casting and reach areas away from the riser.

The principle of increasing the feeding distance by the use of pressurized risers has been experimentally proven but never adopted due to its limited practicality. In order to use pressurized risers, a continuous and controlled pressure must be applied from a source through 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 more practical approach to riser pressurization. The venting chamber could be connected to a source of argon or compressed air by using copper or flexible plastic tubing. In this case, the venting chamber becomes a pressure chamber that receives the gas from the line and transfers it into the riser. Future work at AG Anderson will focus on developing the schematic for a pressurized blind riser.

References :

Vasile Ionescu, AG Anderson, a div. of AmeriCast Technologies Inc., London, Ontario, Canada

Vasile Ionescu is senior metallurgist for AG Anderson, a division of AmeriCast Technologies Inc., 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 Learning

Vasile Ionescu "Improving blind riser efficiency: providing blind risers with adequate passage to atmospheric air and a hotter top metal front will increase your casting yield and reduce your energy costs". Modern Casting. FindArticles.com. 23 Apr, 2011. http://findarticles.com/p/articles/mi_hb6616/is_6_98/ai_n29446011/

COPYRIGHT 2008 American Foundry Society, Inc. COPYRIGHT 2008 Gale, Cengage Learning