Refractory coatings: making the right choice.

Proper use of these products will reduce scrap and labor costs and improve productivity to make higher quality castings.

With the never-ending quest for reduced scrap and higher productivity, making the right choice of a refractory coating and application system for foundry operations has never been more important. Refractory coatings have come a long way since the days when foundries produced simple products on-site to simply wash cores or molds.

Today's coatings are sophisticated performance products offering foundrymen an important tool for reducing scrap and labor costs, and improving productivity to make higher quality castings. Coating application methods also have improved substantially. Careful planning, however, is required to take full advantage of the benefits offered by refractory coatings.

Refractory Coatings

Foundry refractory coatings are "paints" specially developed for the metalcasting industry. Refractory coatings differ from paints in that they are designed to withstand high temperatures of molten metal while acting as a TABULAR DATA OMITTED barrier between molten metal and a core or mold surface.

In general, refractory coatings are suspensions of a high-melting point mineral, or refractory, in a liquid carrier. In addition to the major refractory, modern coatings are comprised of many other ingredients, including: a carrier (or solvent), a binder, a suspending agent, defoamers, wetting agents and biocides. These additives all serve a specific function and many times must be customized for a particular application.

After being applied to a core or mold surface, the liquid carrier is removed by evaporation or combustion, leaving a layer of refractory on the sand surface. This layer prevents or minimizes the penetration of molten metal into the sand, reduces or prevents "burn-on" and erosion of the sand, and generally improves the quality of a casting surface. The saying, "The coating is the mold," is a fair representation of the importance of the coating.

Coating Benefits

The decision to use a refractory coating must be based on overall casting economics. To justify using a coating, a reduction in costs must result, either from cleaning, repairs or scrapped castings. The cost savings must more than offset the cost of purchasing, transportation, mixing, applying and drying the coating, as well as disposing any waste coatings.

In general, refractory coatings:

* reduce or eliminate metal penetration;

* reduce or prevent burn-on;

* prevent erosion of sand by molten metal;

* provide a smoother surface finish;

* improve overall casting quality;

* reduce scrap;

* cut cleaning costs.

Total cost/benefit economics can be quantified with the help of suppliers. Coatings are only one part of the casting process. They do not operate independently of the metal, sand, binder, equipment and foundry personnel.

The benefits of a foundry coating are achieved only if the coating is properly selected, prepared, applied and dried. If done incorrectly, coatings can do more harm than good, resulting in poor casting finish and/or applied coating defects such as scabs, blows, gas holes and inclusions.

Refractory Coating Factors

Many factors must be considered in choosing the right coating. Some of the more important are:

* types of metal poured;

* pouring temperatures;

* cross-sectional areas of casting;

* resistance of refractory to metal penetration;

* "peel" characteristics;

* thermal conductivity of refractory;

* thermal expansion and contraction;

* application characteristics;

* toxicity of refractory;

* uniformity of refractory from shipment to shipment;

* permeability.

Today's coatings are often mixed-refractory systems, offering the advantages and synergy of more than one type of refractory.

Once the proper refractory is chosen, the carrier or solvent must be determined. There also are many factors to consider with carrier selection, including: compatibility of carrier with sand binder and/or refractory; method of drying; flammability and "burning" characteristics; toxicity and odor; application; labor; and floor space.

The most commonly used carriers are water, isopropyl alcohol and 1,1,1 trichloroethane. When selecting a carrier, health and safety considerations include toxicity, flammability, ventilation, and equipment such as respirators, goggles and protective glasses.

Choosing the Right Form

Choosing the delivery form of your coating is in many ways as important as selecting the coating type. Among the physical considerations are: handling; degree of in-plant control; coating performance and uniformity; type of container and cost of disposal; available mixing equipment and procedures; convenience; and costs such as shipping, labor and quality.

Perhaps the biggest trend of recent years has been the switch from pastes to premixes or slurries. The perceived high cost of "shipping solvent" associated with a slurry is often offset by substantial labor savings, less coating disposal and better quality control, leading to lower scrap costs and better castings.

Slurry--Slurry coatings are manufactured either as ready-to-use or as a heavy slurry needing the addition of more solvent to reach operating consistency. The distinction of slurry coatings is that they are more fluid than pastes, and the coating additives are more fully activated during manufacturing. Slurries are more homogenous and generally more consistent in product quality.

Unlike paste and powder coatings, slurries do not require intensive mixing or aging before use. They only require dilution and relatively short mixing cycles. More foundries are moving toward slurry products, eliminating the need for on-site mixing and improving overall quality control in foundry coating operation.

Paste--Paste coatings are not fluid and may not be as convenient to use as slurry coatings. The additives in paste coatings often are not fully activated during manufacturing and may require aging or tempering after dilution to allow the Baume and viscosity to stabilize. Pastes are also less homogenous and less consistent, as well as more difficult to handle and prepare than slurries.

Powder--Powder coatings are the least expensive to purchase. They require intensive mixing, however, and may require long mixing cycles (two hours or more) at a high Baume before dilution. Powder coatings also may not suspend as well as other forms, particularly in alcohol. The cost of mixing and the difficulty in controlling the finished coating often outweigh the lower purchase price.

If a foundry chooses to use a powder, moist powder coatings are recommended over dry powders to eliminate dust when the powder is added to the mixing container or otherwise handled.

Choosing a Supplier

Just as foundry coatings have changed in recent years, so have the expectations of coating suppliers. Suppliers can play a key role to help make coating operations successful and reduce the cost.

Suppliers should help in explaining the technical options available in choosing a coating and work with foundries to select the best product for optimum economics. Options in coating applications should be explained, including advice on all aspects of delivery, mixing, application and drying, and general factors, such as storage and proper use factors to minimize product spoilage.

On-site support is perhaps the most important area where suppliers can help on an ongoing basis. Suppliers should provide training, operations audits, technical updates and literature to assure coating success.

Quality control in refractory coatings is absolutely required. The very nature of refractories--minerals from the ground--makes your suppliers' quality control the key to your success. Your supplier should be willing to provide product specifications, SPC data and demonstrate expertise in making successful coatings. Quality audits of your suppliers should be an integral part of your purchasing strategy.

The ability to supply products on short notice is becoming even more important as coatings become more sophisticated and specialized. Additionally, flexibility in packaging type and the ability to offer any needed consultation on specialized applications equipment are important.

Finally, it is crucial to develop and maintain a healthy supplier/customer relationship so that if crises and problems develop, they can be handled effectively.

WATER

Flammability: not flammable Toxicity: nontoxic Cost: no charge Flash Point: none Evaporation Rate: very low

Comments:

The least expensive and safest of the carriers.

Labor and/or burden rate occupies mixer volume.

The use of water-based coatings is highly recommended if other factors make their use impractical. Not recommended for use with magnesite.

WATER-BASED COATING

Advantages:

1. Lowest cost to purchase. 2. Nontoxic. 3. Nonflammable.

Disadvantages:

1. Requires heat to dry coating. 2. Complete drying of deep pockets in a reasonable time can be difficult. 3. Greater tendency than "light-off" or air-dry coatings for tears or runs. 4. Reduces tensile strength of urethane nobakes, coldbox and silicate sands. 5. Increases potential for core breakage. 6. Possible core degradation during storage. 7. Loss of floor space because of ovens and extra conveyors. 8. Requires high initial investment for ovens. 9. Requires operating and maintenance costs for oven. 10. Coating can freeze. 11. Contamination can cause spoilage.

ISOPROPYL ALCOHOL

Flammability: high Toxicity: moderate Threshold Limit Value: 400 ppm Cost: moderate Flash Point 53-61 |degrees~F (91-99% purity) Evaporation Rate: moderate Burning Characteristics: clean flame (no smoke), does not burn excessively hot

Comments

Recommended if fast drying is required and if flammability is not a problem.

All containers containing either isopropyl alcohol or an alcohol-based coating must be kept covered when not in use; kept properly grounded; and kept away from heat, sparks and open flames.

Coating suppliers generally recommended the use of only 99% isopropyl alcohol.

The use of 91% alcohol can cause coatings to thicken, to loose suspension, or not to "light-off" or burn completely dry. Reducer compatibility should be checked before using.

LIGHT-OFF COATINGS

Advantages:

1. Dries fast. 2. Wets well. 3. Penetrates into sand. 4. Retards moisture absorption by stored cores. 5. Generates heat by flame that accelerates cure of nobake sands. 6. Reduces handling of cores. 7. Lowers labor costs. 8. Increases productivity. 9. Does not require ovens for drying. 10. Utilizes floor space.

Disadvantages:

1. Fire hazard. 2. Potential toxicity hazard. 3. Possible blistering of coating. 4. Possible degradation of cores and molds by hot burning solvents and by excessive solvent penetration. 5. Purchase costs are often higher than water-based.

1,1,1 TRICHLOROETHANE

Flammability: not flammable Toxicity: moderate Threshold Limit Value: 350 ppm Flash Point: none Evaporate Rate: high

Comments:

1,1,1 trichloroethane is the least toxic of the chlorinated solvents. However, good ventilation in work area must be provided. Also, 1,1,1 trichloroethane will be unavailable shortly, since it is an ozone depleter.

AIR DRY COATINGS USING 1,1,1 TRICHLOROETHANE

Advantages:

1. Dries rapidly. 2. Nonflammable. 3. Less detrimental than water and alcohol to the tensile strengths of urethane nobakes, coldbox and silicate sands. 4. Less core breakage. 5. Retards moisture absorption by stored cores. 6. Reduces handling of cores. 7. Lowers labor costs. 8. Increases productivity. 9. Utilizes floor space better (no ovens and fewer conveyors).

Disadvantages:

1. Toxicity hazard. 2. Decomposition of solvent to hydrogen chloride can begin at temperatures exceeding 300F or by exposure to an open flame. 3. Possible objection to odor by foundry personnel. 4. Highest cost to purchase. 5. Requires good ventilation (vapor heavier than air). 6. Use is being outlawed since 1,1,1 trichloroethane is an ozone depleter.