Outside the Sand Box

May 29, 2005
While trying to minimize casting thin-wall metal sections, Ashland Specialty Chemical discovers that the answer to a problem sometimes means changing the way people think.

Some call it an additive, others a replacement. Some even consider it an anti-chill material. Regardless of the title, Ashland Casting Solution’s Exactherm technology provides a new approach to the problem of how to cast thin-walled metal sections.

Now metalcasters have the ability to custom tailor the thermal properties of a mold or core to meet key casting sections needs. Developed in the late 1990s, Exactherm is a continued development of the company’s Exactcast riser sleeve technology. Ron Aufderheide, Ashland’s senior product manager, explains, “We had good surface results on the risers and wondered if we could transfer the technology to a different part of the process.”

Their curiosity paid off. The result of that inquiry is better control of the casting process, elimination of mis-run defects, blows, and hot tears, and overall improvement of casting surface finishes. Even more important, this product makes casting thin-wall metal sections achievable, whereas previously it could have created many challenges.

Exactherm is a low-density refractory aggregate that can be used at levels anywhere from 5% to 100% in sand molds and cores. Its density, thermal conductivity, and specific heat is about 25% of silica sand. “The more you use, the more insulating the core,” states Aufderheide.

The product is offered in two different grades. The G 220 grade is the standard and is suitable for most metalcasting applications. The STL 210 grade generally is used for applications where thicker-walled castings and high pouring temperatures are present. Since the thermal properties of the mixture can be custom blended to meet the needs of the specific casting section, the degree of insulating performance is directly proportional to the percentage of Exactherm in the mold or core. In addition to increased insulating properties, higher percentages in the mold or core also results in lower density of the composite blend. When blended in volumetric ratios, the change in these properties is linear as shown in the charts on this page.

The additive can be used either in the core or as a facing material in the mold or both. The total amount that is in contact with casting section determines how fast or slow the metal in that specific section will cool. Therefore, custom designs can be used to make thin sections within a given casting cool at a rate similar to thicker sections of the same casting. As a result, it is compatible with all traditional mold and coremaking processes including green sand, cold box, no-bake, and shell.

Shifting paradigm

Slowly, but surely, Exactherm is becoming increasingly popular in the metalcasting market. “In the beginning the problem was getting people to go outside traditional thinking,” Aufderheide details.

The low density and fine particle size of Exactherm sand replacement demands a change in thinking regarding the amount of binder that is needed to bond the final blend. Rather than thinking of the amount of binder as a percentage of the sand, as is typically done, the volume of Exactherm blend that is being coated with the resin must be considered. For example a core made with 100% will require from 8 to 10% resin by weight. Compared to sand — which weighs four times as much —this is equivalent, by volume, to 2 to 2.5% by weight of sand. The resin needed for blends between 5 and 100% can be calculated by separating the amount of sand and Exactherm and determining their resin percentages individually, and then adding the total together. The chart on this page shows how the total amount of binder (by weight) varies with the blend ratio of sand and Exactherm.

“Customers that have used it are pleased with the results,” asserts Aufderheide. “A steel caster in West Jefferson, OH, was having trouble casting closed-vein impeller ends. After using about a 60-40% blend of Exactherm and sand, the problem was resolved.” Currently, Ashland is coordinating various trial applications with a number of its customers.