The oil alternative for sand molding.
The U.S. foundry industry has up to 150 foundries utilizing oil tempered molding sand systems, producing primarily aluminum and other nonferrous castings for demanding markets such as aerospace, automotive and art casting. A few iron foundries also utilize oil tempered molding sand as an alternative to water tempered green sand when castings demand high quality surface finish and definition.
What is Oil Tempered Molding Sand?
Oil tempered molding sand was initially developed in the 1960s, by upgrading oil sand core technology for the purpose of molding. Intended primarily for use as facing sand for green sand molding, it was discovered that the material could be utilized for the entire mold media. Oil tempered sand incorporates a clay binder similar to water tempered green sand, but the clay binder has been processed and converted to an organophilic clay, which will accept oil as the tempering agent instead of water.
The oil tempered bond was developed as a method of producing high quality precision castings using the existing equipment and skills found in the green sand foundry. This alternative molding material provides significant operational differences and measurable increases in casting quality.
Following is a look at the materials used in an oil tempered sand system.
Sand--The majority of foundries use a silica or olivine sand, but all commercially available sands may be used with oil tempered bond. It is more important to select high quality sand with known characteristics regarding grade, fineness and distribution.
The sand selected for oil tempered bond will have a significant effect on the molding properties of the prepared sand and the surface finish of the casting. If optimum results are expected, certain limits should be placed on the sand selected. All sands must be washed and dried with a clay content less than 1% and moisture content below 0.5%. The selection process should focus on 3-4 screen sand with an AFS grain fineness number ranging from 100-190. Coarser sands are better for heavier castings. Round grain sands tend to work better than angular or sub angular.
Oil--Most oil companies have several petroleum oils available, however, a 30 weight non-detergent oil is recommended. Any oils containing inhibitors should not be used for they may interfere with the bonding action. Table 1 shows the recommended oil property levels.
Catalyst--Bonding materials that require a catalyst may use methyl (wood) alcohol, ethyl alcohol or proprietary catalysts recommended by the manufacturer of the bonding agent. Catalysts should be handled and stored with precautions normally taken with volatile, explosive liquids.
Additives--Oil tempered molding sand does not typically require additions of any additives such as cellulose, cereals and starches. This is due to the fact that a majority of foundries using oil tempered sand pour nonferrous metals.
Principles and Properties
The molding and physical properties of oil tempered sands are very similar to water activated green sand--both systems activate a binder with a liquid to form a molding material capable of withstanding the stresses of the metalcasting process. However, the utilization of oil as a tempering agent capitalizes on a few key differences in the physical properties of water and oil.
The single greatest difference between oil and water is the significantly lower gas evolution of oil during the metalcasting process. Upon contact with molten metal, water expands 1600 times its original volume during its phase change from liquid to gas. Typical oils used in oil tempered sands only expand 400 times their original volume.
This reduces the quantity of gas by approximately 85-90% within the mold, reducing the necessary venting function of the prepared sand mold. The lower permeability requirement allows for the use of a finer grain sand while avoiding the potential for gas related casting defects.
Oil also has a considerably higher boiling point in comparison to water. Typically, the oil utilized in oil tempered sands are selected based on the boiling point, with a minimum of 400F the generally accepted standard. In comparison, water boils at 212F and begins producing an extreme quantity of gas at the mold metal interface. This scenario creates a poor mold environment that is highly dependent on proper mold venting to avoid gas related casting defects.
Oil does not evaporate like water, allowing for sand preparation and molding to be completed independent of melt and core production. The prepared molding sand can wait to be molded and the prepared molds stored until production is available. The reduced effects of evaporation on an oil tempered system allow the user to maintain a lower temper point that will produce less gas when contacted by the molten metal.
The use of oil as the tempering agent within a sand mixture has a noticeable impact on the physical nature of the molding material as well as its physical properties. Oil tempered sands are known for their extreme toughness, high deformation and high plasticity. Since most oil tempered sand foundries do not have the proper sand system controls in place, an over tempered sand with a high level of compactability--over 50%--typically occurs.
Oil tempered sand systems should be controlled similar to water-based systems, targeting a compactability range of 35-45%. Oil tempered sand possesses a much wider temper window and tolerance to changes in compactability levels. If compactability drops below 35%, oil tempered sand systems are more resistant to friable mold surface.
Operational Differences & Attributes
It is a mistake to compare these two sand systems in terms of advantages or disadvantages because the systems are substantially different. It is better to describe the differences and attributes of oil tempered sand systems in regard to its application within the metalcasting process.
Oil tempered sand systems also have lacked the intense research efforts that have benefited water-based systems, which leaves terms like "advantage" untested or unproven. The operational differences associated with the use of oil tempered system sands are numerous and notably an extension of the base physical properties of the material.
Surface Finish--Oil tempered sand systems were designed to take advantage of the lower gas evolution characteristics of oil, which allows for the use of a finer grain sand resulting in an improved surface finish. Sand molding has a generally accepted surface finish range of 300-150 RMS, and by following the recommended sand selection criteria, it is possible to achieve surface finishes at the lower portion of the accepted range using oil tempered bond.
The reducing atmosphere produced at the mold-metal interface by the burning oil results in the high-quality finish. The ignition of the oil produces pyrolitic and lustrous carbon deposits on the mold surfaces, which improves the sand peel from the casting while fewer oxidizing defects occur in the mold cavity.
The reducing atmosphere, which helps produce a smoother casting surface, consequently reduces cleaning costs. The oil tempered sand also causes fewer casting defects by not allowing hydrogen to come in contact with the metal, thereby greatly reducing the chance of porosity.
Shelf Life -- The main attribute and dominate reason foundries utilize oil tempered sand is the long shelf life of the material. Oil tempered sand can be stored exposed to the atmosphere for 7-10 days, with no discernible impact on the physical properties. Prepared molds are therefore not affected by production downtime and can remain unpoured until production resumes.
The organophilic processing, which allows the platelets to accept oil also causes the platelets to remain open for extended periods of time. In contrast, when the cation is not tied up in a water tempered sand, the moisture will evaporate causing the platelets to collapse, requiring more mulling energy and moisture to reopen them.
Water tempered sands are highly susceptible to evaporation, which has a dramatic impact on the friability of the mold surface. The surface integrity of a water tempered sand system deteriorates exponentially if the mold remains unpoured.
Recovery/Reuse--Oil tempered sand systems are exceptionally resilient to the metalcasting process. Satisfactory molds can be made at elevated temperatures and shakeout sand will recover "good feel" upon cooling. The high durability characteristics of these systems reduce the re-bonding requirements for additional oil and binder. Best results are obtained by running the system like a water-based system, at low temper point targets that require minimal additions and re-mulling after each use.
When the green strength reaches a minimum level, the sand can be revitalized by remulling with 15-25% of the original amount of oil added during mulling. To rejuvenate the sand completely and regain the original bond strength, a small percentage of each of the original ingredients also should be added. Additions of 1% bonding agent, 0.5% oil and 0.25% catalyst are recommended as a starting point. The order of addition and the mulling times are the same as those used to make the original mix.
Some foundries have gone as long as 12-18 months without remulling or adding new bond to the system. In order for a western bentonite to function properly in a water tempered system, it must inherently contain 7-8% moisture before addition and mulling. When water tempered bentonite is exposed to the high temperature of molten metal, the heat drives the inherent moisture below that 7-8% level, permanently collapsing the platelets. This is what is known as dead burnt clay. The oil tempered material prolongs this process because it requires exposure to higher temperatures for extended periods of time to completely drive off the oil.
In spite of its advantages, oil tempered systems were deemed impractical for widespread use due to low strengths and handling difficulties in comparison to water-bonded systems. During the last several years, the strength issue has been addressed with the addition of new and improved materials used in the organophillic process to bolster strengths, while the handling difficulties can be corrected by controlling the oil additions/content and proper mulling techniques.
Although the current economic downturn in the foundry industry has forced the closing of several small foundries using oil tempered bond, there continues to be more foundries trying, using and becoming more competitive with oil tempered bond by reducing cleaning costs. In addition to the small nonferrous shops using this material, at least three foundries in North America currently use oil tempered bond on automatic molding machines.
Several iron foundries also have begun using this material as facing sand on fine detailed work with outstanding results. The ability to produce superior quality precision castings using conventional green sand equipment will draw more foundries back to this forgotten molding medium in search of a competitive advantage.
Table 1 Recommended Property Levels for Oil Viscosity at 100F approx. 100 SSU Aromatic content 10-20% Naphthenic content 35-45% Paraffinic content 40-50% Viscosity index Less than 52 SSU
For More Information
"Foundry Sands," Aluminum Casting Technology, AFS, 1986, p.151-157.
"Foundry Sand," Basic Metalcasting, J. P. LaRue, AFS, 1989, p.113-116.
RELATED ARTICLE: Inside This Story:
* Oil tempered bond molding was developed in the 1950s as an alternative method to green sand molding for producing high quality precision castings with improved surface finish.
* Existing green sand system equipment and skills can be used for oil tempered systems with an adjustment of the system settings.
* Oil tempered sand systems take advantage of the lower gas evolution characteristics of oil, which allows for the use of a finer grain sand, lending to better surface finishes.
About the Author:
Tim Butler is the owner of Indian Ridge Mineral Co., Antioch,, Illinois. He has over 17 years experience in foundry consumable product sales and development. Jon N. Lund is product manager at Waukesha Foundry, Waukesha, Wisconsin. He has over 10 years of quality assurance management and metalcasting related sales experience.
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|Author:||Lund, Jon N.|
|Date:||Feb 1, 2003|
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