Carbon sand: a nonsilica, round-grain carbon.

This unique molding medium provides a combination of properties for improved casting quality.

Carbon sand is another option in foundry technology for preventing casting defects due to sand while reducing cleaning costs. Used by itself or in mixtures with regular silica sands for cores and molds, carbon sand produces high-quality castings at near net shape, provides an exceptionally smooth surface finish and helps eliminate burn-on, expansion defects, penetration and veining.

Although carbon sand has been around since the early 1960s, only recently has it been available to foundries on a nationwide basis. Today's carbon sand was introduced to metalcasters in 1960 by Jack Gentry, who is now a consultant in Palm Springs, California. The product was produced in California in 1984 and distributed to foundries in that state.

A competitive product consisting of ground electrodes produced an irregular aggregate with an extreme surface area, which required excessive amounts of binder to coat and bond. Flowability was poor and the aggregate was difficult to ram or blow.

The carbon sand Gentry introduced was round-grain and had a screen distribution of four. Derived from calcined petroleum fluid coke, carbon sand can be used in a variety of applications. Because it is carbon, it brings to metalcasters a combination of properties not available in other aggregates.

Benefits of Carbon Sand

Carbon sand offers a number of enhanced properties, including low thermal expansion, shock resistance, low bulk density, high heat conductivity and no sintering point.

Because of its round grain, carbon sand won't smudge, resists crushing and promotes good flowability and better permeability. It produces lightweight cores and molds with less tendency to sag, which lightens labor and reduces power requirements on mixers and conveyors. In the case of core blowing, the round grains will blow easier and, consequently, improve density.

At 70 lb per cubic foot, carbon sand is lighter than silica, which reduces the workload on personnel and equipment. The lighter weight also reduces sagging in molds and cores.

Low thermal expansion is its most beneficial characteristic. Avoiding the sharp inversion expansion that silica sand undergoes, carbon sand can eliminate veining and scabbing defects, and permit hard ramming and high-pressure molding to further promote smoother finishes and dimensional accuracy.

Carbon sand is heat conductive. Because of its density, it doesn't provide heat sink capabilities, but will conduct heat for promoting directional solidification. Penetration is minimized with good heat conductivity.

In core applications, carbon sand is usually blended with silica sand. When blended at 50% by weight, silica sand core washing often can be eliminated.

Carbon sand is adaptable with all conventional binders, methods and equipment for use in iron or nonferrous castings, regardless of dimensions. In addition, it can be blended with any sand.

Carbon sand provides substantial cost reductions when replacing zircon, chromite, olivine and other nonsilica molding materials. In fact, zircon weighs 240% more than an equal volume of carbon sand. To make an equal number of cores, 2-1/3 tons of zircon are needed to just one ton of carbon sand.

Clay-Bonded Carbon Sand

The performance of carbon sand with various ratios of southern and western bentonites was measured through a series of tests. Twelve batches were prepared for testing--six with 100% carbon sand and six blended 50/50 by weight with silica sand having a rounded 59.9 AFS grain fineness. The mixes were prepared in a 24-in. vertical wheeled mull or with a batch size of 8000 grams.

Analysis of the basic green sand tests revealed a logical pattern of results, except for the 1800F (982C) hot compressive strength. Results were low when compared to a 100% silica sand mixture, which exhibits a peculiar pattern not found with silica sand compositions.

It can be seen that hot strengths decrease as the western bentonite ratio is increased. This pattern also continues for the 50/50 blend. This implies that having a western bentonite ratio greater than 25% does not increase hot compressive strengths. Hot compressive strengths are influenced more by adjusting the methylene blue clay percentage in carbon sand mixtures than by increasing the western bentonite in the clay ratio.

Shop Experience

In actual practice, casting defects attributable to lower hot strengths haven't been encountered. What is recognizable from production test data is the durability of carbon sand grains. An isolated production molding floor was established with 5000 lb of carbon sand. Additives to the sand were an 80% southern/20% western bentonite ratio and city water.

The mixture was mulled in a vertical wheel batch muller until a compactibility of 40 was achieved. Molding was accomplished by shoveling into the flask and compaction by jolt-squeeze/matchplate machines. Cores were not used in casting production so core dilution was not a consideration.

Three metal types--iron, bronze and aluminum--were poured and sand was monitored over an eight-month period. Pouring temperatures for the three metals ranged from 1375-2650F (746-1454C).

Sand was turned two to three times daily during the test period, and shakeout was done by hand to allow continued isolation. Except from additions of bentonite and water, and retempering after each turn, the pile was maintained as originally established during the eight months.

Under the above conditions, grain size remained close to its original size. This result was obviously influenced by the resistance to fracturing caused by thermal shock. The mix of metals had no known adverse effect on the sand properties.

Surface finish on the castings is comparable between the three metals cast. Since no carbonaceous compounds were added to the mixture, peel and casting finish were determined largely by the characteristics of the carbon sand.

The test revealed that:

* the low bulk density of carbon sand, as compared to other foundry aggregates, must be allowed for when considering binder requirements;

* hot compressive strengths in carbon sand mixtures are influenced largely by methylene blue clay percentage;

* carbon sand can serve as a common aggregate for both ferrous and non-ferrous metals. Casting finish, resistance to burn-on and peel are superior without the use of carbonaceous additives;

* carbon sand's durability is exceptional. Its inherent ability to avoid thermal shock prevents the fracturing down of sand from liquid metal shock.

"Silica sand is not the ideal molding material," Gentry said. "Carbon sand is a manufactured product and we can make as much of it as the market requires. With it, castings can be improved with less effort, less craft and less time, and I feel it's the sand of the future."

References

E.G. Gentry and C. Lear, "Calcined Fluid Coke as New Molding Medium-Preliminary Evaluation," AFS Transactions, vol 69 pp 320-327 (1961).

E.G. Gentry, "Thermal Stability of Carbon Sand and Other Non-Silica Molding Materials," AFS Transactions, vol 74, pp 142-149 (1966).

Carbon Sand Produces Quality Cores, Helps Save Production

Officials at R&D Pattern & Foundry, Tulsa, Oklahoma, say carbon sand works especially well for their pump impeller cores. Located in the heart of the oil industry, the foundry manufactures many castings for pumps, an integral part of the petroleum industry.

The petroleum industry depends, to a large extent, on the movement of liquid products. It's easy to see why efficiency is so vital. Maintenance-free operation also is essential because down-time of a single pump may interrupt widespread operations. Immersion pumps with tiny ferrous impellers may operate at the bottom of a well for years without attention. All this reliability can ride on the performance of a wafer-thin impeller core in somebody's foundry.

"Two years ago, we began using carbon sand to replace zircon in our most difficult impeller cores," said Scott Ramsey, R&D superintendent. "Now we use it extensively because our customers require high-efficiency impellers. That means no veining, burn-on or penetration in the vanes of impellers of red brass, high-leaded bronze, cupro-nickel and other non-ferrous alloys."

For most impeller cores, R&D uses a 50/50 blend by weight of carbon sand 75 and an Ottawa #72 silica sand with 1.25-1.5% Isocure|TM~. Cores are coated with an alcohol-base graphite wash. R&D uses carbon sand in some nobake jobs and have had good results with resin-coated carbon sand/silica blended shell sand.

Although on a much more limited basis, Waupaca Foundry, Marinette, Wisconsin, has used carbon sand for producing cores for two years. After experiencing veining problems on a special job with their phenolic urethane system, foundry officials needed to come up with a solution to keep from shutting down production for a certain customer.

The foundry has used carbon sand both with 50/50 and 60/40 silica blends for certain short-term jobs. Waupaca officials are investigating a carbon shell bonded sand to replace other additives.

"Carbon sand has gotten us out of a number of binds," said Gary Gigante, plant manager. "We were up against the wall, and there's no doubt it worked for us as a quick fix."