Benchlife: the core quality issue.

AFS Molding Div. Cured Sand Committee (4-I)

Inside This Story:

* Foundries using phenolic urethane coldbox cores and molds can experience quality problems if the prepared sand mixture is not properly stored and handled prior to curing.

* Foundries must monitor the four variables--moisture, temperature, process parameters and sand characteristics--most likely to affect coldbox sand benchlife and the resultant cores.

The most commonly addressed issues relating to coldbox core quality are the need to properly store chemicals, maintain and calibrate mixing equipment, and control the blowing and curing process. These issues, however, don't account for a lesser known phenomenon in the storage and handling of mixed sand--benchlife.

All mixed sand--before it is hardened with the reactive gas--has a determined benchlife or amount of time before it degrades and loses the strength necessary to withstand the molten metal and ensure casting quality. To ensure quality cores, foundries must understand the variables affecting benchlife and monitor them.

Reducing Benchlife

Benchlife of any sand mixture can be determined by sand testing. To measure benchlife, tensile specimens (made from a single batch of sand) are produced at regular time intervals and the immediate tensile strength of the specimens are measured and recorded. As the sand mix ages, there will be a point in time when the strength of the specimen degrades to the extent that it precludes the use of cores in production.

Chemically speaking, the phenomenon of benchlife occurs when part 1 resin and part 2 coreactant of a phenolic urethane binder system undergo one or more chemical reactions. The binders may react with one another, water and other contaminants. If extensive reactions occur before the part 3 amine is introduced to catalyze part 1 and 2, problems can occur.

When part 1 and 2 react by themselves, the sand becomes less flowable. This decrease in flowability results in decreased core density and reduced tensile strength. Side reactions that can take place (such as the reaction of the isocynate in the part 2 with moisture) also result in poor core quality because less polymerization occurs, resulting in weaker cores. Under normal conditions, the chemical reactions that take place between part 1 and 2 ate slow (until catalyzed by part 3 amine) as are the side reactions. However, when four foundry variables--moisture, temperature, process parameters and sand characteristics--aren't held in check, a reduction in benchlife can occur.

Moisture

Isocyanate is the major component of the part 2 coreactant. This isocyanate polymerizes, of crosslinks, with the part 1, but also reacts with water. Any source of moisture inhibits the polymerization and decreases the number of possible resin bridges. Less polymerization results in lower ultimate tensile strengths of the cured cores.

Quantitatively speaking, one molecule of water can react with two molecules of isocyanate. As a result, even low levels of moisture can have a significant impact on the benchlife of the sand mixture. The longer the isocyanate is exposed to moisture prior to use, either in mixed sand of during storage, the greater the chance for the isocyanate to react with water. Proper storage and handling of the part 2 coreactant is necessary to prevent it from reacting with moisture in the bulk or day tanks. The use of desiccant dryers is the best way to ensure that the isocyanate is not exposed to moisture prior to use.

Moisture can come from many sources, but the most common sources ate the sand and sand additives, and humidity in the ambient, blow and purge air. The moisture content of foundry sand and other aggregates must be constantly monitored and minimized to ensure it remains below 0.1%. The relative humidity of air used for sand transport and blowing and purging must also be controlled. As the relative humidity increases (especially in the summer months), the plant air dryers mist take more moisture out of the incoming air source. If the air dryers are not maintained properly, moisture can be introduced, shortening the benchlife of mixed sand.

If possible, it is ideal to have the p[ant air at a dew point of -40F. Condensation of water on cool sand can also be a contributing factor if the dew point is not maintained at a sufficiently low level. Similarly, if a blower is used to deliver sand from the supplier, the ambient relative humidity can be condensed, introducing moisture into the sand.

Hot sand also can cause moisture to condense in the silo. It is good practice to have moisture and temperature specifications with the sand supplier and to measure these properties on incoming loads to verify conformance to the specifications.

Temperature

As with many chemical reactions, an in crease in the temperature of the components increases the rate of the reaction--this especially holds true for the sand in your system. A rule of thumb for this type of reaction is that every 18F (10C) increase in the temperature of the components doubles the reaction rate. Any increase in reaction rate decreases the sand benchlife.

Compounding the adverse affect of elevated temperature, the binders contain solvents that provide handling and performance properties. As the temperature rises, the solvents evaporate, which allows the part 1 and part 2 to react more quickly, further decreasing the benchlife of the sand mix.

Because sand generally constitutes more than 98% of the final mold/core mixture, sand temperature is one of the largest factors affecting benchlife of mixed sand. Elevated resin and sand additive temperatures can theoretically contribute to a decrease in the benchlife characteristics of mixed sand. while their temperature is only a minor contributor to the temperature of the final sand mixture, controlling and monitoring the temperature of the components is recommended

Process Parameters

Process parameters--the way the foundry runs their core operation--are the elements in the operation over which personnel have the greatest amount of control. To maximize the benchlife of mixed sand, several factors such as mixing time, batch size and 'fugitive amine' must be optimized and controlled.

Mixing--During mixing of the sand, resin and additives, several factors can result in decreased benchlife. The process of mixing promotes solvent evaporation due to constant renewal of the surface at the mixed sand and air interface. Additionally, heat generated during the mixing pro cess can compound the solvent evaporation and lead to further benchlife degradation. To minimize these adverse effects, the mixing process should be optimized so that the mixing time is just long enough to ensure that the sand grains and sand additive particles ate evenly coated.

There are numerous variables that must be considered when establishing the mixing time, including mixer el]3ciency, batch size, resin viscosity and cycle time requirements. Optimization can be accomplished by varying the mixing time for a specific mixer, resin level and batch size combination and comparing the tensile strength of the resulting cores. The batch size and level probe position don't affect the actual benchlife properties of the sand mixture, but they can contribute to occurrence of benchlife problems if they are not optimized.

For example, during summer, a smaller batch size would minimize the amount of time mixed sand is exposured to ambient humidity, and the sand would be used up prior to a significant decrease in core tensile strength, If possible, minimizing the surface area at the mixed sand and air interface will decrease solvent evaporation and the reaction of the isocyanate and moisture.

Amine--Fugitive amine is another condition that can and should be controlled to increase the time the mixed sand is usable. As happens in the corebox, any binder exposed to the amine catalyst will react and harden. If seals, piping joints, connections and valves are not properly maintained, they will leak. The resulting fugitive amine in the core room can react with the sand mix in the hopper, resulting in problems identical to those brought about by decreases in benchlife.

Sand Characteristics--In the phenolic urethane coldbox process, the reaction rate between the part 1 and part 2 is dramatically increased, of catalyzed, by the part 3 amine (which is a base). It follows that components or contaminants in the sand or sand additive that are alkalinic (have a pH greater than 7) may also increase the reaction rate and thus decrease the benchlife of the sand mixture. The pH of the sand should be monitored to ensure that it is constant and that there has not been any contamination that may affect the properties of the mixed sand.

The acid demand value (ADV) of sand is related to pH and is a measure of the number of reactive basic groups in the aggregate sample. Much like pH. this property also should be monitored to ensure consistency. If the ADV increases, it indicates the presence of alkalinic constituents that can lead to a decrease in benchlife.

The grain fineness number (GFN) and particle size distribution of the sand and additives also affect benchlife characteristics of mixed sand. High GFN sand has an exponentially greater surface area. When the binder is properly mixed with the san& the resin coats each individual grain. The higher the GFN number for a given resin level, the thinner the resin layer. In turn as the resin layer thickness diminishes, the solvents evaporate more quickly, allowing the reaction rate to increase.

The particle size distribution is also a key characteristic to sand surface area and benchlife. When the percentage of fines in the sand is elevated, the surface area is significantly increased even though the GEN may be within normal ranges. Thus, particle size distribution must also be closely monitored because dusty sand dramatically lowers the benchlife.

Keep Monitoring

Benchlife of mixed sand in the phenolic urethane coldbox process is an often overlooked yet critical factor in

core quality. By controlling and monitoring factors such as temperature, moisture, process parameters and sand properties, the benchlife of mixed sand can be maximized to ensure the consistent and efficient production of quality cores.

For More Information

"What Variables Affect Core Erosion Resistance," D. Kahles, M. Nikolai, C.M. Henry and R.E, Showman, MODERN CASTING, May 2003, p. 37.

"In Search of a Cure: Optimizing Coldbox Core Systems," M. Adamovits, B. Thomas and W. Tinker, MODERN CASTING, May 2002, p. 30.

"Core Binders: A Look Into the Future," G. Tackes, MODERN CASTING, October 2001, p. 24.