Coating chemistry key to FPC consistency.

Coating Chemistry Key to FPC Consistency

For some foundry applications, foam pattern casting (FPC) is gaining support, but effective pattern coating remains a stern master requiring much sophisticated research and a lot of trial and error.

According to the experts, evaporative foam pattern casting, when compared to conventional bonded sand casting techniques, offers significant across-the-board savings by: * requiring fewer skilled people; * simplifying casting equipment investment; * eliminating costly coring; * allowing use of inexpensive flasks; * minimizing casting cleaning; * reusing sand and eliminating need for binders and other sand additives.

In brief, foam pattern casting (FPC) offers lower operating costs and improves product quality in a cleaner, quieter environment. So why isn't every foundry more committed to the process? Well, for one thing, foundrymen are slow to change, and casting processes that have taken 5000 years to perfect can't be all bad.

But a paramount reason was discussed at the recent AFS conference, Evaporative Foam Pattern Casting, by three experts in the field: the U.K.'s Nigel K. Graham, Foseco International, Ltd; Robert L. Snook, Ceramco/Ashland Chemical Co; and Raymond W. Monroe, Steel Founders' Society of America. Their collective opinion: coatings are an essential part of the process, affecting defect formation and metal flow and controlling finished product quality. The ceramic pattern coatings in the FPC process must meet demanding requirements, but coating formulations and effectiveness vary widely.

As seen by Snook and Monroe, the pattern coating acts as a barrier between the sand and the metal, allowing the vaporized foam pattern to escape while becoming a control point for the mass and heat flow during pouring.

Coating Technology for FPC

Simply stated, the coating is the mold from which the casting is formed, permitting the escape of decomposition products from the pattern while retaining the metal and excluding the sand. The coating is only one part of the FPC system comprised of coating, sand, compaction and often a vacuum. Changing one component can result in the need for change in another.

The role of the coating is critical, however, since it alone is responsible for the casting's surface finish and final dimensions. Coatings that can assure these conditions already are in use in hundreds of foundries here and abroad, but much more needs to be explored to fully exploit this relatively new production technology.

Typically, FPC coatings consist of the following ingredients: * carrier; * refractory materials; * suspending agents; * binder; * rheological agents; * preservatives.

These must be combined in a manner that ensures a consistent result. Consistency is the key; once having found the right combination of each to produce the desired result, repeatability becomes an important concern. The operating limits within which a given casting can be made successfully are small; the narrower these limits, the more difficult the process.

The function of the carrier, usually water, is solely to convey the refractory mineral, the heart of the coating, onto the full geometry of the pattern. The suspended mineral largely determines a coating's permeability, heat transfer or insulating properties, as well as providing the aforementioned molten metal/sand barrier.

The suspending agent provides a stable slurry of water and binders in a ratio sufficient to adequately coat the pattern and hold the mixture in place after drying. The dried coating also provides some dimensional stability to the pattern, or cluster, and resists sand abrasion. Rheological agents in the coating slurry promote the even flow of the coating over the pattern, and the preservatives and bactericides retard deterioration of organic ingredients from biological degradation.

While the basic composition of coatings is relatively simple, achieving the desired process properties is difficult. As of this writing, short of trial and error, there is no hard and fast rule for specific coating formulations for specific metals. Snook and Monroe revealed that casting experience has given them some basic criteria on permeability and heat transfer, but added that there is still a real need for a simple, repeatable test to determine coating permeability before and after the casting process.

Coating Testing

Before testing a coating, it must be homogenized so that all ingredients are evenly dispersed throughout the slurry. Gravity begins to act on its components when the slurry mixer stops, and, although it may look like no material separation is occurring in the coating mixture, sedimentation and stratification start almost at once.

These subtle, short-term changes in the suspension will lead to inconsistent or poor casting results. A variable-speed mixer of sufficient capacity to create and maintain an even dispersion of coating ingredients is critical to coating uniformity.

A number of procedures are useful to mark coating quality control. They include Baume, density, percent of solids, viscosity and dry coating weight tests. Baume is the poorest test since it is intended to measure the specific gravity of liquids, and coatings are not liquids, but suspensions of solids in a liquid. They are slurries, and determining their true density is simple.

Using a weight cup or a volumetric container, measure and weigh a sample; its weight in grams divided by the volume in millimeters yields specific gravity or density.

A second method to determine the percent of solids in a coating is to dry a known sample weight of the coating, and divide that number by the weight of the original wet sample. Since it is the combination of viscosity and percent of solids that determines the weight of coating applied to a pattern, a viscometer should be part of the laboratory testing equipment.

Dry Coating Weight

Dry coating weight is the most important indicator of a successful casting result. It is the proper amount of dry coating required to produce a specific casting as well as a measure of coating properties. Repeated experimentation will reveal a range of coating weights that will yield good iron castings free of carbon defect and burned on sand.

The weight of dry coating should be recorded and correlated with casting results, but the range of coating weights within which good castings can be produced will vary with cluster size and surface area. A good coating should produce an operating range of from 10-20% of the total dry coating weight. For example, a cluster with a total dry coating weight of 500 grams should have a range of 50-100 grams per cluster and should yield reproducible results between 475 and 525 grams of dry coating.

The dry coating weight can also be useful as a control procedure. The coating, drying and weighing of a standard pattern can be correlated with the proper weight of coating on a larger cluster, serving as an empirical method of controlling and adjusting the slurry to ensure the proper weight of coating on the cluster. This dry coating range then becomes a constant for other castings.

Rheological properties can be affected greatly by temperature, tank size, mixing equipment and methods of application. Often, it has been necessary to alter the rheological properties of the coating to meet changing conditions as the FPC process moves from a pilot operation to full production. If the desired coating weight is known, this can be a relatively easy procedure; without it, much time and effort will be lost.

Future of Coatings in FPC

Coatings are still evolving, but currently there are areas where they could be made more effective and where they could contribute to improved processing or casting quality. Better control of casting dimensions using chemically-altered coatings for FPC technology is a natural future development.

A primary area of improvement exists in the removal of plastic residue from the casting cavity after molding, a major cause of casting defects and a complex chemical and metallurgical problem. Since the plastic foam is not eliminated uniformly, its heat-destroyed residue builds up on the leading edge of the advancing metal stream. Foams that decompose at higher gas fractions are easier to eliminate, hence the reason for coating additives to catalyze the polymer breakdown and aid cleaner ash disposal.

New chemical combinations, as yet unused in coatings formulations, may improve casting quality, extend casting sizes made with FPC as well as prevent sand from entering the metal. Altering coatings could also bridge the beady-textured surface of the foam pattern, preventing it from reproducing itself on the casting surface.

The coating should also be a greater factor in achieving dimensional stability, acting as a pattern stiffener. This would help casting clusters resist distortion during handling and compaction and reduce pattern aging. Greater flowablity, innovations in pattern application systems and devising quick-dry coatings that change color when they are ready for use would be useful advances.

In summary, the most fruitful areas of investigation into progress in FPC technology encompass the physical and chemical roles played in formulating and applying FPC coatings. Sand and coating interact strongly to control many aspects of casting quality, but coating application and formulation must still be the areas that receive the most intense investigation.

Current tests of density or permeability are still limited in their ability to predict casting results. The conclusion of the presenters is that a better understanding of the roles played by coatings in the FPC process will lead to improved quality control tests as well as better castings.

PHOTO : A specially racked FPC mold cluster is automatically dipped in a coating bath prior to

PHOTO : racking and drying.