New Conditioning Practice Improves Sand Performance.

High-intensity vacuum mixing and cooling can help green sand metalcasters improve molding sand and casting quality while operating more efficiently and cost-effectively.

While foundry technology rapidly has been advancing, there have been few innovations in mixing and cooling, aside from the addition of more modern peripheral controls. Already employed successfully in Europe and Canada, new vacuum mixing and cooling technology for green sand casting operations is gaining attention in the U.S., as a Texas foundry has decided to implement the technology. This preparation practice can effectively cool molding sand while mixing and provide improved sand properties. Additionally, because the need for a separate sand cooler and peripheral equipment is eliminated, this process provides economic benefits.

In this process, a high-intensity mixer is fitted with a vacuum-rated shell. A mixing tool with blades rotates counter-clockwise, while a rotary pan rotates clockwise. This induces a counter-current material flow pattern, which provides maximum mixing intensity through application of intense shearing action on the sand mixture. The rotating pan on the mixer is inclined so that, as the sand mixture is moved upward by the rotating pan toward a stationary blade, the sand is folded over with the aid of both the stationary blade and gravity into the high-intensity mixing tool. These operations produce uniform sand and high green strengths, which are a requirement for most of today's high-production molding processes.

Cooling Efficiency

Drawing a vacuum during mixing provides effective sand cooling and improved development of sand properties. In this process, extra water is added for cooling. This extra water is vaporized in the cooling process, while the water needed to temper the sand mixture to the desired compactibility remains in the sand mixture. Following the water addition, when a deep vacuum is applied, the water is drawn into the vapor state. As the water goes from liquid to vapor, heat energy is released, causing fast and effective sand cooling.

Figure 1 shows the results of recent research in which a high-clay molding sand with a formulation typical of that for vertically parted molding was heated and introduced into a vacuum mixer/cooler. Boiling water then was added. After high-intensity mixing under a vacuum, the sand temperature dropped from 200F (93C) to 120F (49C) in 40sec. Boiling water was added to prove that the sand was not simply being quenched by the large volume of extra water. To further prove this point, Fig. 1 also shows the results of repeating the same procedure, but adding cool water in an attempt to quench and without applying vacuum. The results of this experiment yielded a hot, steamy mass of molding sand that was stiff and sticky, typical of the problematic hot sands that metalcasters have struggled with for decades.

Water Distribution

Applying vacuum during high-intensity mixing not only produces effective cooling, but also provides additional benefits. When a vacuum is applied, the water is vaporized and huge volumes of steam are generated in the mixer. This water vapor surrounds every exposed surface of the clay, bringing it into contact with the water. Under vacuum, water molecules lose their surface tension. Instead of beading like water on waxed paint at atmospheric pressure, water in a vacuum moves freely upon the surfaces it contacts, wetting every clay interlayer between the silicate sheets, providing almost instant clay activation. Also, under deep vacuum, all of the air molecules that must normally be displaced as the water molecules move are not present, so the air molecules do not hinder the movement of the water molecules. In contrast, with conventional mixing at atmospheric pressure, the water droplets with high surface tension must penetrate and be "worked into" the clay, and must displace air molecules as they move, all by mechanical action alone.

As the vacuum continues and the water wets all of the bentonite, the water molecules needed by the clay are oriented and rigidly held in the clay interlayer. When atmospheric pressure is restored in the mixer, the return of the air molecules and the pressure exerted also helps to force the water deeper into the clay, while the surface tension of the water returns, locking the water rigidly in place. Any excess water that is not held rigidly by the clay is drawn off by the deep vacuum. This produces clay particles that are hydrated well between the interlayers, causing bonding strength to develop, while the surface of the clay itself is not over-saturated with free water. Figure 2 illustrates how the vacuum process can avoid over-saturation of the surface clay while eliminating the moisture-deficient interlayers of the clay.

The difference in the distribution of the water between molding sand prepared by conventional mixing at atmospheric pressure and molding sand prepared by this process and cooling causes a difference in the final molding sand properties. With this newer mixing and cooling process, the molding sand is characterized by a drier feeling surface, which provides improved free flow characteristics of the molding sand in hoppers and in deep pockets or intricate areas on the pattern prior to compaction, yet the interlayers are more thoroughly saturated, which leads to improved bonding potential during compaction at molding. This process can prevent a wet surface layer, which causes stickiness and hinders flowability, and drier clay interlayers, lowering the efficiency of clay activation and bonding.

Sand Properties

Figures 3 and 4 show friability and moldability test results, which provide evidence of this difference in moisture distribution and its effect in the mechanical properties of the sand. Both of these tests use the same instrument, which is a device with a rotary screen that rotates over a catch tray. In the friability test, compacted cylindrical specimens are placed in the rotating screen and are abraded against one another and the screen to measure how well the compacted sand resists scuffing or abrasion, which on a sand mold leads to erosion and inclusions. The higher the friability, the lower the resistance of the sand to abrasion and the greater the tendency for erosion and inclusions. The moldability test uses a loose mass of sand, which is rotated in the screen, and it measures the stickiness or cohesiveness of the sand mass as a measure of how free flowing it is. The higher the moldability, the more free-flowing the sand, which is important in hoppers and on the pattern.

Improvements in bonding strength are evident from the results of the cone jolt toughness test (Fig. 5). In this test, a specimen is jolted on a platform while it is under the weight of a cone-shaped weight. The number of jolts until the specimen fails are counted. This test measures not only strength, but also bulk brittleness, in contrast with friability, which measures surface brittleness. The higher the cone jolt toughness, the tougher the compacted sand. Sands with low cone jolt toughness tend to produce broken molds and cause difficulties in pulling deep pockets in a pattern.

This increased bonding potential also may be a benefit, in combination with high-pressure molding, to provide strengths adequate for maintaining the ever more stringent dimensional tolerances that are required of today's metalcaster.

Sand brittleness, as measured by the friability and cone jolt toughness tests, is reduced, because, as a result of high core sand dilution, sand brittleness is one of the most common problems in U.S. molding sands. When core sand dilution is high, an excessive amount of new sand enters the molding sand system from cores. This over-volume of new material influx leads to the addition of new clay and carbons and dumping of excess good molding sand that could otherwise be recirculated. If the molding sand could remain in the system longer, with a minimum of new sand added to dilute fines and maintain refractoriness, the sand would be mulled cumulatively by repeated passes through the mixer, the benetonite's properties would be more thoroughly developed, and the sand would be less brittle, reducing the tendency for broken molds, erosion and other molding difficulties and brittle sand-related defects. While future foundry designs should consider minimization of core sand dilution for the sake of economy and effici ency of material reuse and waste disposal, this mixing process may provide a new option for dealing with brittle sand, even in systems with high core sand dilution, because it accomplishes the same thing that cumulative mulling does, and it overcomes sand surface and bulk brittleness through more effective hydration of the bentonite's interlayers.

Additional Benefits

Research in the U.S. and Europe confirms that this process raises green sand permeability (Fig. 6). This effect is possibly due to increased swelling of the clay caused by more thorough hydration of the interlayers. SEM photomicrographs of molding sands have revealed that bentonite in system sands is not present in smooth concentric layers around the sand grains as one might imagine, but is, in reality, present as small masses or cushions of clay smeared between and on sand grains.

Increased swelling of the cushions of clay may push the sand grains further apart, increasing permeability. This, however, would not increase the tendency for penetration the way a coarse sand does, as it creates larger empty voids between sand grains with increasing permeability. The clay, which is a refractory material, swells between the sand grains as they are pushed apart, and the masses of clay prevent mechanical penetration. This also lowers the bulk density of the final molding sand mixture, and lower bulk densities are a characteristic also confirmed by research in both the U.S. and Europe.

Differential scanning calorimetry data also suggests that molding sand prepared by vacuum mixing and cooling has a tendency to release its water more consistently than molding sand prepared by conventional mixing at atmospheric pressure, probably due to more consistent hydration of the clay interlayers. Molding sand mixed at atmospheric pressure showed variable temperatures for loss of water. During the metalcasting process, this would mean less inherent tendency in the molding sand for the formation of localized hot spots to develop on the mold metal interface, more even sand peel and more even heat extraction from the casting as it solidifies. More even heat extraction in turn may lead to finer grain structure in the cast metal, which may produce metallurgical advantages in some cases. Further research in these areas is in progress.

In addition to these technological benefits, the vacuum process requires less equipment and completely eliminates the major dirty air component in a return sand system. The vacuum-tight process also yields the following cost savings:

* the major dust collection and buffer storage silo used to route hot return sand through a separate sand cooler is not required. Condensation in the cooler's exhaust ductwork and in the dust collector itself resulting from this hot, humid sand cooler exhaust also does not exist for the vacuum mixer/cooler;

* the size of the dust collector required for the return sand system can be reduced by 50%. Additional materials handling equipment (bucket elevator and belt conveyors) and buffer storage silo are not required;

* the size of the building (or sand tower) required to house the return sand vacuum mixing/cooling system is typically 20% less than required to house a system with a separate sand cooler.