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Foundry sand testing focus of molding division.

Sand. To the layman, it conjers up beaches or deserts, but to the foundryman sand is the substance of his livelihood, at once his promise and his problem. Molding sand characteristics were very much in the forefront of the technical sessions.

Combining clays to enhance sand binding characteristics (90-24) was the topic of D. Hentz, E. Olson, American colloid Co. They reported that there are several kinds of bentonite clays, few of which are used as foundry sand binders, though rarely will all these clays satisfy all foundry sand system requirements. Controlled mixing of defined bentonite characteristics, however, can lead to more effective binder applications. This was demonstrated by the reaction of various mixes of three sodium bentonites, showing the synergistic effects alone and in combination with a flowability enhancer.

The relatively few clays used today have significant differences. It would be expected that a mix of two bentonites in a sand system would yield results at or near the average of the measured parameter of each clay, but this not always the case. Synergistic reactions occur. A commercially available flowability enhancer was added to mulled clay to gage flowability, and to test for other properties evident due to the addition. Mixtures were mulled for five different time periods and tested for green and dry compressive strength, compactability, flowability, density and wet tensile strength.

All three test bentonites were distinct and reacted quite differently when subjected to different tests. It was found that the flowability enhancer acted as a gel breaker, the gel being formed when water and bentonite are mixed and allowed to stand for up to a minute. Without the flow enhancer, the bentonite-water gel was much stiffer and resisted compaction. Forthcoming tests under actual production conditions will investigate the effectiveness of the use of a flow enhancer to improve sand preparation and moldability.

Thermal sand reclamation to minimize waste foundry sands (90-150) was the subject of G.J. Reier, GMD Engineered Systems, Inc. Sand reclamation, he said, may be the only way foundries have to meet the requirements of the Resource Conservation and Recovery Act (RCRA).

It makes sense to eliminate or minimize foundry sand waste rather than approach the problem through costly waste site or treatment processes that take time and money. One reclamation possibility is using a thermal sand reclaimer with a post-pneumatic scrubber. This method will handle the three main forms of waste materials generated by sand systems: degraded and undegraded organic hydrocarbons, calcined and uncalcined inorganic compounds and elemental metals and their oxides.

Waste minimization and the resultant cost reductions are the obvious driving forces behind the utilization of thermal sand reclamation. Other key benefits of the operation of a thermal sand reclaimer are recovering a natural resource, reducing waste problems and related costs, reducing new sand costs and confining landfill disposal to contaminated baghouse dust and fines.

Effective sand cooling and its relationship to lower sand-related scrap (90-35) was addressed by M.J. Granlund, Foundry Systems Control and M. J. Aklinski, National Engineering Co. Hot sand, according to the authors, is any sand that has a temperature so high that sand preparation, molding or casting is difficult. Generally, sands above 12OF are considered hot. Mulling produces sands that are inconsistent and difficult to control. With more restrictive cast metal quality requirements, increased foundry competition, the need for higher molding line utilization and less storage space, interest in sand condition has become important.

Hotsand affects every aspect of green sand molding. Depending on a foundry's ability to control sand temperature, casting results can range from high scrap (sand inclusions, washes or erosion scabs, surface roughness, pinholes, crushes and broken molds) to complete loss of system control. increased moisture in the sand also causes more turbulence in the metal flow.

Because system shakeout sand varies widely in moisture content, residual binder and temperature, some means of compensation for these variations must be developed. The use of a sand cooler to control the temperature of the return sand is one compensations path. In one foundry, the installation of a sand cooler system decreased scrap and improved casting appearance sufficient for a less than 15 months return on investment.

The effects of sand and carbon fiber additions on plaster mold and casting properties (90-95) was explained by J.W. Davis and J.R. Brevick, Pennsylvania State Univ.

Plaster mold casting is an old technology now receiving attention as a means of making precision castings requiring little final finishing. Castings made by this technique exhibit less distortion, facilitate making thin sections and in some cases improve the cast metal's properties. It also reduces the need for some machining, and can be adapted for production quantities as well as for very short runs.

Plaster has problems with mold strength, permeability and its vulnerability to high temperatures, but, used in nonferrous castings, it produces the surface finish, detail and integrity required in diecasting prototypes. Another drawback is its insulating nature which tends to retard casting solidification, producing castings with physical properties inferior to diecast parts. Limited published data suggests that the addition of sand increases the chilling tendency of the mold media, producing stronger castings.

Experiments show that the solidification time of castings made in green sand is shorter than for castings made in plaster. The addition of 25, 50 and 75 volume percent sand to the plaster molds decreased solidification time compared to the pure plaster molds, though the upper two volumes did not significantly outperform to lower figure. The addition of carbon fibers had no effect on casting solidification time. The Brinell hardness of castings made in green sand is greater than that of those made in any of the plaster molds.

Castings made in green sand had a higher tensile strength than castings made in plaster molds, but the addition of 25 percent volume sand to the plaster mold significantly increased the tensile strength of castings made in pure plaster molds. The tensile strength of plaster molded castings was increased by adding sand to the plaster, the improvement attributable to reducing solidification time. Fracture bar surfaces showed that the grain size of the castings became increasingly finer as solidification time decreased. The addition of carbon fibers to the plaster/sand mixture increased the strength of the plaster molds without affecting he physical properties of the castings.

Understanding green strength and compactability, and how they aid green sand systems (90-31), was presented by R.A. Green, Applied industrial Materials Corp, R.W. Heine, Univ of Wisconsin/Madison and T. S. Shih, National Central Univ, Taiwan.

The relationship of green compressive strength and compactability to percent methylene blue clay and percent moisture of fully processed bentonite bonded green sands with seacoal in a 3:1 clay:seacoal ratio can be classified as clay rich or clay poor. In clay poor NaB sands, green strength at a given compactability increases with increasing clay content up to about 7% clay. Clay rich sands show little or no increase in strength beyond 7% clay. More moisture is required as clay content is increased.

Clay and moisture are the ingredients that determine the properties affecting compactability and green compressive strength, the author reported. The relationship of compactability, clay content and moisture reveals a clear distinction between clay poor and clay rich green sands. Clay rich sands appear to start at 7% methylene blue clay where they appear to approach a constant level at a given compactability. Green strength of clay poor sands, compared with clay rich sands, is lower, generally containing 6% or less methylene blue.

Packing of sand grains together with clay coatings reaches its maximum green strength effect at less than 30% equilibrium compaction in the 6% methylene blue range. Green sand strength depends on the strength of the clay mass as the clay content exceeds 8% methylene blue. Bentonite clays do not affect the physic al principles operating in a mixture, but do change the clay:moisture ratio level needed to produce a specific compactability and green strength.

The formation and causes of chromite double skin defect were reviewed by J. Howden, Volclay, Ltd (90-28), and solutions, such as raw material quality controls and technical controls relating to melting procedures, pouring speeds, etc, were offered. According to Howden, "the parameters for elimination of the defect have developed and been in place for approximately nine months. During this period, the double skin defect has become an extremely rare occurrence, and now can usually be traced back to a deviation from the outlined procedures."

"Control of sand variables is the key to casting success," said D. Hoyt, Wedron Silica Co, who discussed the design of a sand storage silo discharge system to minimize segregation by reblending the sand (90-29). It was found that proper reblending of foundry sands can be accomplished using several base guidelines and modeling techniques, and it is easy to design a sand storage model to any foundry's requirements for delivery of sand to a mixing device.

A panel on the three main "Chemical Tests in Green Sand" included presentations on Ammonical Nitrogen, by J. Ward, Navistar; Methylene Blue, by L. Soderling, Hill & Griffith Co; and Loss on Ignition, by R. Praski, Carpenter Bros, Inc. Another paper described Gage R&R studies on various MB test procedures (90-07) to improve an understanding of the value of the numbers generated given by V. LaFay, Hill & Griffith Co.
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Publication:Modern Casting
Date:Jun 1, 1990
Words:1558
Previous Article:A356: alloy of the nineties.
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