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Ex or


based on observation or supplier product information. Traditional logic holds that exothermic sleeves should be used on smaller diameter risers and insulating sleeves on larger risers. However, another line of rationale suggests exothermic sleeves could be used on larger risers as a way of compensating for restrictions like limited furnace capacity or flask size. Some believe the insulating proper- ties of a burned-out exothermic riser sleeve are the same as those manu- factured to be insulating, suggesting that the exothermic sleeve will always lead to greater yield. In other circles, the suggestion that both sleeves have the same insulating properties is not thought to be valid. Bearing in mind the difference in cost of exothermic sleeves (which are more expensive) versus insulating sleeves, how can steel casters deter- mine the most economical practice? A 2010 survey of steel casters posed the question, “How do you deter- mine whether to use an insulating or exothermic sleeve or sand for a riser?” Te survey also polled the casters on hot-topping practices and methods


S


teel casters have devel- oped their own rules for using insulating or exothermic riser sleeves


Insulating othermic?


Steel Casters State Their Cases


Metalcasters share their practices and preferences for insulating and exothermic riser sleeves in a recent survey. MALCOLM BLAIR AND RAYMOND MONROE, STEEL FOUNDERS’ SOCIETY OF AMERICA, CRYSTAL LAKE, ILLINOIS


for determining riser size. Of the 21 respondents, nine (43%) used both insulating and exothermic sleeves (Fig. 1). A total of 76% used exo- thermic sleeves. Only 24% used insulat- ing hot topping material, while 71% used exothermic hot topping material. Seventy-one percent of respondents


used simulation to determine riser size while 31% used modulus calculation. A few used both. Two-thirds of respondents col- lected data to support their riser sleeve practice, suggesting there is not a developed technical approach to determine the optimal manufac- turing procedures.


Using Both Of the nine steel casters using


various insulating and exothermic media sleeves, six have established changeover points to determine their sleeve choice. Two conflicting ratio- nales were given for using a particular sleeve type. Tree casters use insulat- ing sleeves at smaller diameters, while three others use exothermic sleeves at the smaller diameters, changing to insulating sleeves as the diameter increases. One company used exo- thermic sleeves up to 24 in. Small diameter risers have higher surface to volume ratio than larger


sleeves, suggesting that exothermic risers would be more effective at the smaller sizes. At larger diameters, the surface to volume ratio drops, so the transfer to the use of an insulating sleeve might seem appropriate either based on performance or cost effectiveness. A number of respondents indicated


they used exothermic sleeves at larger diameters to overcome a metal short- age, which would otherwise prevent them from using an insulating sleeve. One respondent commented that sometimes it uses exothermic sleeves when space in the mold is restricted. An additional deciding factor for a few was a concern over the pickup of aluminum from exothermic sleeves, which may produce aluminum nitride under-riser cracking.


Simulation Results Te Steel Founders’ Society


of America has conducted recent simulations of the effect of sand and insulating and exothermic sleeves for a 2-in. diameter, 4-in. tall riser on a 2 x 2-in. bar and a 10-in. diameter, 20-in. tall riser on a 10 x 10-in. bar. Te exothermic sleeve on the 2 x 2-in. bar appeared to be slightly more efficient. On the 10 x 10-in. bar, the results for the sleeves were similar. In both cases, the sand riser was clearly


May 2011 MODERN CASTING | 39


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