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Sand treatment dominant topic.

Buying, treating, using and disposing of foundry sand dominated the technical sessions covering molding methods and materials.

The interest among foundrymen to know more about the age-old science of making sand, clay and water into more accurate molds was amply evident in the crowds attracted to the molding sessions. This year's presentations produced an excellent cross section of research and practical applications from the molders perspective on mold production in relation to casting quality.

Good molding practices simply make good business sense as the drive for casting quality mounts and the circle of environmental mandates regulating foundry operations grows ever tighter.

D. Leidel, Tanoak Enterprises, Inc., related the successful installation of a thermal sand reclaimer system by Wolverine Bronze Co., a nonferrous jobbing foundry whose main products are aluminum castings ranging from 1000 to over 20,000 lb using a nobake sand molding process. The foundry installed its reclamation system in 1984, but because sand amounts fluctuated widely, mounting energy and storage costs caused it to re-evaluate the system.

Two options were investigated: use its existing particulator (mechanical reclaimer) or use a more intensive form of reclamation resulting in lower new sand addition. Economics dictated the use of five minutes of intensive attrition without any thermal treatment (cold reclamation) instead of 10 minutes of intensive attrition accompanied by "flash calcining," which would cost more than twice that of the cold system.

The original system had heat-resistant components built in should calcining be required in the future. The reclaimer is already fitted with burner walls and needs only the addition of burners. Wolverine approximated the cost of thermal reclamation of a ton of sand at $42.23. This compares to the cost of operating the "cold" system at $9.72/ton.

The problem of the friability of hot sand was addressed by S. Neltner, Hill and Griffith Co., in his presentation on the defects resulting from high-temperature sand on casting quality. He said the best solution is a cooling process and a reduction in open time in the molding operation. Hot sand's effect on two types of bentonite clays in a preblend is not significant, provided the sand temperature is no higher than 130F.

Among the factors Neltner cited as contributing to the increase in sand temperature were initial metal pouring temperature, casting dimensions, sand-to-metal ratios, length of time from pouring to shakeout and base sand types. According to his observations of sand tested at different temperatures, as time increases in a 2:1 western-to-southern bentonite mixture (or the reverse), the percent of friability increases.

He stressed that the temperature-versus-time effect and the mold variations resulting from clay additions to new and/or reclaimed sand should be fully understood by the foundryman for his particular operation.

The foundryman must also understand that friability is not constant in that the percent of friability at the muller discharge may not automatically be acceptable at the pouring station. Compensation for moisture evaporation is an important consideration as it relates to the interval between mold preparation and pouring time.

In presenting the paper on the computerized evaluation and control of green sand processing systems, R. Green of AIMCOR said the program can monitor system performance. It also can serve as a sand diagnostic and process control tool to track causes for changes in the percentages of methylene blue, moisture compactibility and green strength of fully processed new sand mixtures.

He said changes in the methylene blue (MB) level are a major factor in producing uniform green sand properties in a sand system. When the test compactibility (TC) percentage is specified, the clay level in a particular system sand will dictate the total moisture (TM) percentage needed to obtain that specified compatibility. The combination of the percentages of TC, MB and TM establish the sand type and also the percentages of compactability efficiency (CE) and the strength processing efficiency (SPE) the system is able to produce.

A major problem in controlling the percentage of MB clay can come from core sand dilution; if it increases, new sand additions should be reduced to help maintain sand system consistency. Computer programs for monitoring and analyzing process performance and material balance will become available from AFS/CMI.

The results of a study of the effects of sand and sand additives on internal veining defects in 316 stainless steel castings averaging about 200 lb each were reported in a paper presented by M. Geoffrey, Acme Resin Corp./Borden, Inc. Core location, pouring time and pouring order were also part of the study.

Ground glass, black iron oxide and a silica/alumina/iron oxide blend were the additives chosen for the study, which found glass and the silica/alumina/iron oxide blend produced the fewest veins. An optimized model showed that only glass and the silica/alumina/iron oxide blend for eliminating the veining. The effects of changes in binder and reclaimed sand levels were small compared to the effect of changes in additive levels.

Cores using the blend showed less expansion at 1850F than those produced with the iron oxide additives. There was no indication of experimental bias based on core location in the mold and only a slight decrease in the number of veins as the order of pour increased. The number of veins was unaffected by changes in pouring time.

The results were then applied to production castings to verify the conclusions of the experiments used in the study. Reductions in grind times on castings made with cores containing glass were 50-100% compared to the times incurred on castings made with typical production cores.

Understanding the accumulation of nitrogen in green sand molding was the topic of a paper by V. LaFay, Hill and Griffith Co. Nitrogen pinhole porosity defects stem from green sand additives that become nitrogen "collectors." These collectors do not generate the nitrogen but act as vehicles to deliver the nitrogen to future mold making processes. If the quantity of nitrogen exceeds the recommended limits, the casting can have pinhole porosity.

"Available nitrogens" in the form of ammonia are generated by the decomposition of molding materials used in the various core and green sand processes during the casting process and collected and absorbed in the bulk of the molding sand. Because the green sand recirculates in the foundry, the ammonia can build up in the sand and finally be evident as pinholes in the castings.

Fireclay and seacoal with their volatiles removed were found not to be significant ammonia collectors, but western and southern bentonites tended to increase ammonia buildup significantly.

A project, partially funded by a hazardous waste reduction technology research, development and demonstration grant from the California EPA, was summarized by T. Corbett of GVT, Inc. The project investigated an alternative technology to replace the shell sand or Croning process for making cores. The process is widely used to make complex sand cores and has remained virtually unchanged since World War II.

The characteristics of the existing process include an intense ammonia-like odor during curing and poses possible exposure to phenol and formaldehyde emissions. After casting, Corbett explained, shell sand may contain significant amounts of residual phenol, making it difficult to reuse without costly thermal processing.

He said the alternative technology is based on a heat-curable epoxy resin bonded sand. It would allow foundries to use shell, hotbox or warmbox equipment and tooling to produce cores that will perform similarly to each of these processes.

The new process was evaluated in the laboratory to determine binder addition, sand mixing and core strength parameters. Subsequent foundry tests with new sand and waste sand were run to prove casting performance with both sands to characterize the products of combustion.

Waste sand used in the new process promises to be less toxic than sand from traditional shell cores and would allow sand normally considered as waste to be used to produce a damp, liquid resin warmbox sand or a dry resin-coated shell sand for core production. This would eliminate or reduce the waste sand. It would eliminate the need for waste sand disposal, the purchase of new raw sand and new resin-coated shell sand.

Though not yet endorsed by the California EPA, the process holds considerable promise for reducing raw materials and disposal costs, Corbett said.

Predicting the bentonite consumption in foundry recirculating sand system was reported in a review of an evaluation study by P. Bastien and F. Chiesa, Centre de Metallurgie du Quebec, Trois-Rivieres. Heat transfer modeling was used to study the overheating of layers of sand for different casting dimensions and aspect ratios for steel and aluminum to estimate the amount of bentonite destroyed.

In the normal operation of a foundry green sand system, small amounts of bentonite, new sand, water and organic materials must be added to replace the material destroyed or degraded by heat. Water additions, Bastien said, are easily controlled by compactibility testing; the others are based on foundry experience using quantities that maintain constant sand properties, mainly green strength, moisture and LOI.

It was logically inferred that the amount of destroyed bentonite will depend on the thermal properties of the sand and the cast metal (conductivity, specific heat and latent heat of fusion) and the initial concentration of bentonite in the sand. The refractoriness of bentonite in sand, the sand/metal ratio and the thickness (modulus) and shape (aspect ratio) of the castings are paramount factors affecting clay consumption.
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Title Annotation:CastExpo '93: 97th AFS Castings Congress, Chicago
Publication:Modern Casting
Date:Jun 1, 1993
Words:1560
Previous Article:Striving for a better melt.
Next Article:Furnace design, research & material control head sessions.
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