Drifting away from seacoal.
* Seacoal has been a popular green sand additive because of the positive effects it can have on casting quality. But because it is the main contributor to emission characteristics, many metalcasters are searching for replacements.
* Detailed within is an evaluation of replacement materials that can provide the same benefits as seacoal without the environmental concerns.
Seacoal has long been a part of green sand casting--and with good reason. The bituminous coal is used by metalcasters to form a layer of lustrous carbon that works to keep molten metal from penetrating the sand. By keeping the molten metal at bay, mold wall movement can be reduced, limiting the chances for defects to occur. However, seacoal does have its downside.
The additive is responsible for the bulk of the emission characteristics during green sand casting. Because of the negative environmental consequences, investigations have been conducted to determine the effects of reducing or eliminating seacoal and seacoal supplements from green sand casting.
The goal is to find suitable replacements that reduce the emission characteristics of the carbonaceous additives generated during metalcasting without sacrificing the quality of castings.
Application of Seacoal
In order to eliminate or reduce seacoal from green sand, it is important to first understand its use in the metalcasting industry Seacoal is utilized in green sand molding because:
* it develops lustrous carbon, which is used to develop a layer of carbonaceous materials at the mold-metal interface;
* during the decomposition of seacoal in the metalcasting process, its "fixed carbon" will develop into coking material that reduces mold wall movement and develops a "cushion" between the sand grains;
* it contributes to the development of an oolitic layer on the surface of sand grains that improves the flowability of molding sand and reduces the roughness of the casting surface;
* it contributes to a reducing atmosphere in the mold during the metalcasting process, thus creating an oxygen scavenger.
Application of Replacement Materials
When seacoal is eliminated from the green sand casting equation, so is the layer of lustrous carbon, which means another material must be substituted in its place to ensure the quality of the casting will not suffer. A popular material used as a replacement for producing lustrous carbon is graphite, which can be added to green sand as a preblend or a separate addition. Graphite also can be added as a release agent that is sprayed on the molding machine pattern. The beneficial effect of adding graphite into or on the surface of the molding sand has the same effect as the decomposition characteristic of seacoal to develop lustrous carbon.
When seacoal is eliminated, a process change is required to accommodate the reduction of mold wall movement. Because it works to develop the cushion material, seacoal is one of the most effective materials to reduce mold wall movement, which is the result of the expansion characteristics of the base sand used in molding sand.
Figure 1 shows the expansion characteristics of various sands used in the metalcasting industry. The desired method used to reduce or eliminate expansion defects is the addition of cellulose into the green sand. The quantity added is based upon the sand type, casting design and other conditions in the metalcasting facility.
[FIGURE 1 OMITTED]
Oolitic layers can be developed without seacoal but at a much slower rate. The process can be sped up through the addition of cellulose, return core sand, decomposition of resin (from core processes) and other sources that support the development of oolitic layers. How fast the process occurs is dependant upon the addition of the materials and conditions in the facility.
Another way to enhance the density of molding sand is with the addition of western bentonite and/or a modified western bentonite. Maintaining an improved density of molding sand through the formation of oolitics or with the addition of western bentonite/ modified western bentonite is critical to casting quality when eliminating or reducing seacoal.
The reducing atmosphere theory also is important to consider. This theory has been debated on various occasions. The Univ. of Alabama has pointed out in previously published data that, "Since cast iron naturally provides a reducing atmosphere at the mold-metal interface, the effect of seacoal in decreasing the propensity of metal penetration is not due to formation of a reducing atmosphere."
For those who suggest the requirement of a reducing atmosphere through the addition of carbonaceous additives will enhance the quality of castings, the need to have organic materials (such as seacoal) in the molding sand is important. Without the addition of seacoal, cores can serve as an available source of carbonaceous materials. The cores are formulated with organic materials that have similar properties to seacoal that can develop lustrous carbons. However, the addition of materials, such as graphite, cellulose and western bentonite do not generate sufficient reducing atmospheres to replace seacoal.
Casting weight and design (surface area) should be the first area reviewed when removing seacoal from green sand. Smaller iron castings (having a pouring weight of 120 lbs. or less) with varying designs are usually an ideal application for molding sand without seacoal. The least desirable applications are large "chunky" iron castings (having a pouring weight of 120 lbs. or greater).
The reason that smaller iron castings with varying designs are ideal is because they can accommodate the replacement materials. The addition of cellulose into the molding sand allows a sufficient cushion to maintain the integrity of the casting. The addition of graphite into the molding sand or through the application of a release agent sway allows smaller castings to take advantage of the properties found in the graphite to replace the need for the generation of lustrous carbon.
A western bentonite-bonded system also can enhance the integrity of the casting, and the return core sand and the core itself will contribute to the reducing atmosphere due to the carbonaceous material formed during the core processes. If there are no cores in the process, a higher level of graphite may be required.
Casting design also has an important impact on successfully using green sand without seacoal. For years, facilities have discussed sand-to-metal ratio, not surface area to the science of metalcasting. But surface area has recently been considered as a stronger influence in the understanding of the metalcasting process over the traditional application of sand-to-metal ratio. An example of the application of casting design over sand-to-metal ratio is a study by the Casting Emission Reduction Program (CERP) concerning emission characteristics. A graphical representation of this data can be found in Fig. 2. This information was developed as a comparison of casting surface area to emission characteristics of green sand while maintaining constant sand-to-metal ratio. The graph demonstrates that cast surface area can have a dramatic effect on the characteristics of molding sand. This information correlates directly with the required quantity of additives into green sand that is necessary to produce a desired casting. The need for alternative materials into the molding sand without seacoal will be affected by cast surface area.
[FIGURE 2 OMITTED]
Metalcasting facilities that produce large "chunky" iron castings require the addition of seacoal supplements in addition to the other changes made with the smaller castings. These compounds have been added to molding sand for many years and have the advantage of developing desirable properties in green sand without many of the negative properties associated with seacoal.
Popular replacements in this application are anthracite coal and causticized lignite. The desirable properties that are enhanced when larger castings are produced apply the enhanced "cushion theory," reducing atmosphere and the development of oolitic layers on the molding sand rapidly. Both materials effectively develop these properties.
Testing Without Seacoal
To further understand the performance of sand systems with and without seacoal, a series of test castings was produced. Figure 3 shows a casting that was produced with a seacoal-based sand system that will be compared to other additives.
[FIGURE 3 OMITTED]
The primary drawback to the removal of seacoal is the lack of integrity to the casting (primarily casting peel and sand "sticking" to the casting prior to shakeout). Figure 4 shows castings produced without modifications to the molding sand (additives include predominately western bentonite and no carbonaceous additives). With the addition of cellulose and a predominately western bentonite molding sand, an improvement was noticed in the casting surface and a reduction in the sand adhering to the casting prior to shakeout was achieved.
[FIGURE 4 OMITTED]
Figure 5 presents a casting produced without seacoal in the molding sand with 2% cellulose. To enhance the casting surface, the addition of graphite or a graphite-containing release agent can be applied. Figure 6 shows a casting that was produced without seacoal but had the addition of an anthracite coal, and Fig. 7 shows a casting produced without seacoal but with causticized lignite.
[FIGURES 5-7 OMITTED]
Life Without Seacoal
Seacoal has proven its worth to metalcasters by helping to reduce mold wall movement and stopping sand grains from sticking to the finished casting. But other materials are available that can achieve the same results without the environmental concerns. The decision for metalcasters to make is if the positive characteristics of seacoal outweigh the negatives. Green sand casters contemplating removing seacoal from their operation should consider:
* the addition of a graphitic material into the green sand system can act as a replacement for the lustrous carbon generated by seacoal;
* the addition of varying amounts of cellulose into the molding sand can act as a replacement for the "cushion" material required in the green sand molding process;
* the bonding mechanism of the molding sand will prefer western bentonite or modified western bentonite in order to enhance the integrity of the molding sand.
For More Information
Visit www.moderncasting.com to view "Evaluating Emission Characteristics of Seacoal and Seacoal Supplements Using Advanced Analytical Techniques," V.S. Lafay, S.L. Neltner, D.N. Taulbee and R.J. Wellbrock, AFS Transactions (paper 00-046), 2000.
Victor S. LaFay and Stephen L. Neltner have more than 40 years of combined experience at The Hill & Griffith Co., Cincinnati, providing metalcasting facilities information and analysis related to green sand molding and its additives and defects.
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|Date:||Dec 1, 2004|
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