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Sand handling systems affect castings produced in green sand.

Sand Handling Systems Affect Castings Produced in Green Sand

For some 200 years, green sand has been the most widely used metal molding material because it is plentiful, reusable, easily worked and produces reliable results. But, in order for the foundryman to maintain these desirable and consistent properties, he must devise a sand use system that uses the right compliment of equipment and follows testing procedures that promote casting consistency.

Contributing design factors, such as sand volumes, shakeout, raw sand handling processes, sand cooling, mixing capacity and dust control are critical. These variables, coupled with the effects of other process variables, like rate and amount of core sand influx, burnout levels of clay and carbons, sand temperature and sand-to-meal ratio, impede the stability that can be sustained in most high-production foundry sand systems. This forces sand controls to focus only on maintaining consistency, limiting them to actual sand parameters rather than optimizing sand properties and composition.

Simply testing sand does not define sand control, but the importance, and cost, of testing is unarguably of fundamental importance to good green sand castings. Unfortunately, most sand control programs are limited to collecting test data to plot wall charts, rather than using molding sand data to maintain sand uniformity and quality. Data collection should involve more than testing sand in the mixer, which really provides a record of the finished product rather than an effective sand control methodology.

System Control

There are actually three steps to system control:

* identification of the sand process variables that affect sand consistency and uniformity;

* getting control of as many variables as possible;

* optimizing process conditions.

The most controllable sand variables contributing to consistency are the additions of water, bond, carbonaceous material and new sand. Those less controllable include the rate and amount of core sand influx, sand temperature, cooling systems, mixing, the composition, homogeneity and processing of returned sand and the amount of fines removed by dust collection equipment. If these factors are erratic, the composition and molding quality of the sand will vary widely.

As with any system, the success of each component in the system depends on the functioning of the others. Adequate control is required of the whole system, including sand mixing, molding, pouring, shakeout, cooling, sand handling equipment (belt conveyors, hoppers, bins), dust collection equipment, screens, magnetic separators and molding floor practices.

Many conventional sand systems are designed for high-volume molding and casting production and often neglect the effect that the type of shakeout system, muller, sand cooler and other equipment has on the uniformity and molding properties of the sand. In these cases, sand testing becomes increasingly important.

Systems Analysis

The physical properties and composition of prepared sand are routinely monitored in most foundries by standard weekly tests that include: combustibles at 1800F (LOI); combustibles after was (LOI on washed AFS clay residue remaining after screen analysis); calculation of "adjusted clay" [AFS clay - (LOI - LOI after wash)]; and volatiles at 900F.

Ideally, optimum sand properties should be determined by casting results, but in practice are often adjusted to meet the requirements of the sand system and molding method. No ideal sand formulation exists that will produce satisfactory results in every system.

The prepared sand tests listed above provide a great deal of information with a minimum of testing and the data should be used to control mix uniformity. It also can be used to build files of information for later reference to scrap and defect data using statistical computer analysis.

System analysis data is critical to achieving structural or compositional properties at other stages in the system prior to the prepared sand condition. Table 1 lists examples of system analysis tests that can be adapted to suit specific systems.

The ideal sand preparation system design provides homogeneous sand and simplifies control by minimizing or eliminating many of the factors that cause variation.

Raw Sand Handling

Even if a supplier provides a sand with a reasonably consistent grain distribution, segregation in the raw sand handling system can result in varying grain distributions entering the system sand. These variations are significant because they greatly affect binder requirement, permeability and casting finish. Antisegregation equipment should be incorporated into all raw sand storage hoppers to counteract such variations.

Tight sieve analyses specifications for grain fineness and distribution are often placed on incoming sand, but sieve analysis should be done at various points in a foundry to determine if the raw sand handling system is transporting it without major changes in grain fineness and distribution due to segregation. The test data then can be used to determine where antisegregation devices are needed to improve in-plant process control.

Consistency of Additions: Water,

Bond, Carbons, Mixing

Poor moisture control is one of the leading causes for casting defects. The compactability of sand must be carefully controlled, and its value versus moisutre control is well-documented. Even if a moisture probe was developed for instant moisture readings, moisture content would nto serve as an effective control because as the sand composition changes, the amount of moisture required to maintain the desired compactability changes.

Maintaining constant compactability ensures that the sand during molding will compact consistently from one mold to the next. The compactability test simulates the behavior of the sand as it is compacted at the molding machine. Because of the speed and simplicity of the test, it can be performed manually at the mixing station. The successful use of this test allows automatic compactability control at the mixer, the best solution for on-line control of water addition.

In some molding processes, sand flowability requirements limit the compactability level during molding. It is desirable, however, to maintain a slightly higher compactability at the mixer than that required in production in order to develop and disperse the clay properly and to accommodate for the loss of some moisture that occurs between mixing, molding and pouring.

Optimum sand properties cannot be achieved when mixing at low compactability levels. Excessively high moisture during mixing has a tendency to create balls and clusters in the sand that affect compaction and surface finish. Prosimity of the modlign area to the muller may determine a decrease in compactability from the mixer and before molding should be tested and moisture levels adjusted to provide proper molding station compactability.

It is desirable for the compactability to be as high as possible during molding without compromising the flowability of the sand. The sand is at its minimum bulk density and compaction is optimized when the sand is at its temper point. Once the sand is compacted, moisture loss continues, especially at the compacted mold surface.

As it dries out, the friability of the sand increases, lowering its resistance to surface abrasion. Mold surfaces are easily scuffed in normal mold handling and core setting, and the sand becomes more prone to producing dirt, erosion and inclusion defects during the erosive flow of molten metal.

The friability test measures the resistance of the compacted sand surface to abrasion. In this test, two standard specimens are prepared and abraded in a rotary screen for one minute, and the weight percent of sand removed from the surface is expressed as percent friability. The test is performed using the instrument formerly used to measure moldability (see Fig. 1).

A friability curve, obtained by testing standard specimens after air-drying for increasing periods of time, provides important information about the length of time that can elapse before the friability of a particular sand becomes detrimental. Some molding sands are extremely moisture sensitive and the friability can double within a short period of time. The curve can be used to establish the maximum time that should be allowed to elapse before the molds are poured. Molds exceeding this time should not be poured.

The mixer sequence, from additions to discharge, should be regulated and timed, with provisions made to allow for variable molding rates and demand for sand. This dpends upon having adequate sand mixing capacity and provisions to feed off excess prepared sand.

One system design problem in conventional sand systems is dependence upon the mixer alone to maintain uniform sand. In conventional sand systems, the mixer is often depended upon to disperse not only the new clay and temper water and develop the bond, but it additionally must homogenize the new additionally must homogenize the new additions with the mixture of shakeout sand in a very short mulling cycle.

Moisture, compactability, green properties, structural properties and methylene blue clay tests are usually run routinely on one sample of the prepared sand. These tests also can be run, as a system analysis series of tests, on samples taken from different areas of the same batch of sand, or at different times in a continuous mixture system.

The data recovered can provide information on how homogenous the sand is when using the mulling cycle normally used in production. Uneven distribution of clay and additives in the sand can be a serious problem, especially for moisture distribution, where imbalances can cause wet spots in the mold resulting in pinholes or blows in castings.

The thermal gradient created in the mold after pouring results in varying degrees of thermal degradation of sand depending on factors such as pouring temperature and cooling time before shakeout. Shakeout sand exists as a variable mixture of sand in several states of thermal degradation with the possible iclusion of thermally degraded core sand and sand from unpoured molds, all contributing to a nonuniform sand mixture.

In short mulling cycles, the conventional mixer cannot corret the sand variations and adequately disperse clay and new additives into an homogeneous mixture for the casting process. The primary function of the mixer should be to disperse evenly clay and temper water to develop the bond.

This is accomplished much more effectively if mulling time is increased or control is exercised on the material going into the mixer. The more consistent the sand and additives going into the mixer, the less is required from the mixer.

Certain things can be done to improve consistency of additions. The use of slurry systmes for bond addition should be avoided as this usually complicates the problem of controlling bond and moisture levels, resulting in greater variation than should be necessary. Dry bond additions are recommended.

The additions of new sand, bond and carbons should be as consistent as possible. Weighing additions allows closer control than does volumetric measures. Carefully weighing returned sand is especially important to eliminate errors that result from variations in the bulk density of the sand due to differences in compactability, composition and temperature. The use of weighed quantities of preblends can help maintain control of the additions by reducing the possible error from multiple additions.

The weights of additions should be determined by anticipatory control, based on the expected core sand dilution and the amount of burnout of clay and carbons, and should be aimed at maintaining a constant sand composition.

Occasional large additions of new sand and bond, or large variations due to plugging of devices that add dry bond, will cause large fluctuations in sand composition and control. A system that minimizes these occurrences of out-of-specification additions is essential.

Changes in additions can be computer-controlled, based on pattern scheduling. The computer files can contain information on pattern number, poured weight per mold, bond additions required per mold, carbon and/or other additives required per mold, molding rate, core sand dilution per mold, metal pouring temperature, desired compactability, etc.

A computer-integrated sensing device could also be used to detect unpoured molds at shakeout, and reduce additions when the sand from the unpoured molds circulates to the point in the system where the additions are made.

It is important to ensure that all materials going into the sand system are consistent, and are not sources of variation in themselves. This includes not only sands, but bentonites, seacoal or other additives, preblends, water (if the water is recycled), and materials from dust collectors if they are returned to the sand. These materials should be tested to ensure consistency.

Soluble and leachable calcium and magnesium tests, the apparatus for which is shown in Fig. 2, are new tests that have been developed in cooperation with the Sand Committee of the Iron Castings Research Institute in answer to the need for tests that can be used to check consistency of raw incoming bentonites and preblends.

Just as incoming raw sands must be fairly consistent in grain fineness and distribution per sieve analysis monitoring, so must raw clays be checked for consistency. Soluble and leachable calcium and magnesium tests provide a tool for checking consistency of clays through measurement of the soluble salts and exchangeable ions present in the clay.

The physical properties of the clays are dependent to a large extent on the soluble salts and exchangeable ions. Minor differences should be expected, since clay is a natural material, but large variations can indicate that the incoming clay is not the same Western or Southern bentonite from shipment to shipment. Since Western and Southern bentonites have different physical properties, depending upon their soluble salts and ions, it is important to receive the same clay or blend of clays consistently in order to maintain system control.

Since all clays are not the same, they are not readily interchangeable without producing differences in the properties of the molding sand. The calcium and magnesium tests for checking clay blending consistency will become even more important as blends become more widely used.

When unexplainable sand changes occur, check the system for possible causes, such as changes in moisture or incorrect bond additions. Data should be available on the clays, sands, carbons and other raw materials to ensure that they have not changed. Checking incoming materials routinely reduces the chances of problems resulting from variations in raw stocks.

If separate additions of Western and Southern bentonites are made in the foundry, their ratio must be controlled. Wet tensile strength tests can be used as a rough control of the bond formulation in the molding sand (see Fig. 3 and 4).

The clay level of the sand determines the initial wet tensile strength level. As the ratio of Western and Southern bentonite changes, the wet tensile strength changes. The more Western bentonite, the higher the wet tensile strength; the more Southern, the lower it is.

Wet tensile strength is also affected by contaminants in the sand. Decomposition products, like seacoal and certain core binders, lower it, while others, like soda ash, activate the clay and raise wet tensile strength.

While the test serves to monitor the condition of the clay in the system as it recirculates, the live clay level, as determined by the methylene blue clay test, must be known when deciding what action to take when deciding what sand drops. If the sand strength drops, but the methylene blue total clay level doesn't, the solution is not to add more clay, but to check the mixer. The methylene blue test of the return sand is a monitor for the amount of clay burnout, and is useful as a guide for additions.

Carbon additions to molding sands are usually controlled using the loss-on-ignition test as a measure of the total amount of organics, although some of the loss is also due to water of crystalization in the clay, base sand carbonates and residual core binder. The volatiles test also is used to measure live or volatile organics driven off rapidly at low temperatures. Unlike LOI testing, the volatiles test allows for the important distinctions between coked and uncoked carbons in the sand, and between carbonaceous materials with varying volatile contents.

Gas pressure tests are normally applied to core sands, mold washes and core pastes, but they can be used to test molding sands to obtain the same information provided by both LOI and volatile tests. All provide a measure of the rate and volume of gas evolved. The gas test can be completed in 12 minutes, as opposed to the hours required for LOI and volatile tests.
COPYRIGHT 1990 American Foundry Society, Inc.
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Author:Pedicini, Louis J.
Publication:Modern Casting
Date:Feb 1, 1990
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