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Control the quality of your refractory mold coating.

Scrutinize the consistency of your coating practice to be sure that you're realizing the full benefit of your mold and core refractories.

According to conventional wisdom, a refractory coating at the mold-metal interface serves as "cheap insurance" against burn in, other surface imperfections and misruns. Coatings can be used to prevent molten metal penetration and to encourage a smooth, as-cast finish. However, this "insurance" rarely solves problems resulting from molds or cores that have "soft spots" due to poor sand compaction or engineering, and without the proper care and storage, your coating could actually be more trouble than it is worth.

Mold and core coatings take many forms, but they are essentially composed of refractory particles (minus 325 mesh) in a liquid suspension, such as alcohol or water. Alcohol-based zircon coatings are typical of what is used in steel foundries, but ever-tightening environmental regulations seeking to reduce volatile organic compound (VOC) emissions have pushed the industry toward adopting water-based coatings as the norm. Even though this article uses alcohol-based zircon coatings as a primary example, the control principles discussed are equally valid for all refractory coatings.

The manufacturing specifications of coatings are more complex than the formula for house paint, and coatings may contain additives such as surfactants, binders and biocides. Many foundries do not properly store, stir or test their refractories, and, as a result, they wind up with inconsistent castings. The formulation of the coating is the vendor's responsibility, but its homogeneity is the responsibility of the foundry.

Vendor Certification

The first step in assuring the quality of your refractory coatings is to make sure that they are "certified" by the manufacturer. The vendor should certify the chemical purity of the raw materials and should be contractually bound to inform you of any change in formulation or in the primary ingredients.

Each "lot" of refractory coating should have a certification accompanying it that lists:

* specific gravity (SG)/density of the coating;

* SG/density of solids and liquids;

* solids content;

* manufacture date;

* volume and weight of components.

One of the easiest incoming quality assurance checks is to weigh the container (or check the manufacturer's recorded weight on the shipping invoice). If the wrong mineral was used, there usually will be a major discrepancy.


Storage of a coating allows time for its suspended refractory to settle out, and the vibrations during shipment accelerate this settling of minerals. If the storage time has surpassed the manufacturer's recommendation, the supplier must remix the material just prior to delivery. This is called "reconstitution" and requires a heavy-duty mixer (3-5 HP driving a 7-8-in., three-blade propeller). With the right equipment, it can be done in a few minutes, and the reconstitution date should then be clearly marked on the container.

Coatings should not be stored more than 30-90 days, depending on the formulation, without vigorous, complete remixing. If the supplier logistics are awkward, then consider installing a coating mixer - which is more powerful than a "stirrer." You can tell when a coating needs to be remixed by attempting to stick a 1/4-in. rod down the side to the bottom of the container. If you encounter any resistance along the sides or if you cannot touch the bottom of the container with the rod, the solids in the coating may have settled, in which case the coating needs to be reconstituted.

Shop Stirring

A "cake" of sediment at the bottom of a dram can mean that you're not stirring your coating properly, and this could be an expensive mistake. The binders and suspension agents dissolved in the liquid may become more dilute in the coating used, and this could mean that your molds and cores might not receive the full benefit of the refractory coating.

If the container has been in a shop for longer than a week, it must be stirred for a minimum of 30 min at full speed (approximately 1725 rpm). The stirrer does not need to be as robust as the mixer used to reconstitute the coating, but it should have a stainless steel three,blade propeller on a shaft that is at least 3/4-in. around. A larger diameter shaft prevents "whipping," which causes air entrainment and splashing. The propellers, which should extend to within 2-3 in. of the bottom, will wear away, so check them with a gauge each time the container is changed. The mixer should force the coating downward. Also, keep in mind that multiple blades along the length of the shaft may splash coating onto the container's sides as the liquid is drawn down. Be sure to properly ground your mixing and delivery systems to prevent an accident.

Reduce the motor speed of the stirrer as the container is drawn down. Coating that is splashed onto the walls of the container dries and forms a cake, which in turn peels off and falls back into the container, where it can clog the spray system. This also can cause the solids ratio and SG to vary. Check the mixer's speed once per shift and manually adjust it as necessary. Ideally, when the stirrer is operating, the level of the liquid will rise only 1-2 in. You should see a slow movement of material, not violent splashing.

Operating a mixer continuously will cause a coating to lose solvent, since the internal friction actually heats the liquid. Keep your mixer on a timer so that the coating is stirred for about 5 min every hour. Once a container is started, stir it routinely until it is empty to keep the refractory in suspension. The mixer can be shut off overnight or on weekends, but the next time a drum is used, stir the contents continuously for 15-30 min. In addition, keep the mixer tank covered to minimize evaporation and contamination.

Consistent Application

Modern, high-solids coatings will change in viscosity and gel with prolonged overmixing. For this reason, it's better to make sure that the stirrer is always turned off during spraying. Be consistent, and program this consistency into your mixer.

If you use a double-diaphragm pump to spray a coating onto your cores or molds, extend the intake to within 1-2 in. of the bottom of the container or the pump will suck air and the spray will cause a "stucco" effect. The same is true for the intake plumbing of the pump itself. If it is not hermetically sealed, it will suck air, which becomes mixed with the coating, causing the sprayer to sputter.

The intake should be 2-3 in. from the propeller tip, and the pump should be securely attached to the 1/4-in. plate on the top of the stirrer so that the relative positions cannot change. The intake or discharge should have a screen on it, and this screen should be inspected and cleaned at each container change. If chunks of coating are observed on the screen, the coating is being thrown up onto the side of the container and drying. Turn down the speed of the mixer to prevent this.

As the solvent evaporates, a coating becomes stiffer and more likely to drag sand when brushed or swabbed onto a core or mold surface. Mold finishers should empty their coating pails into the stirring tank every couple of hours and refill them with "fresh" coating to get the best mold or core finishes.

If coating is applied by spray, a three-way valve will allow you to draw your sample from the user's end of the hose. This valve also will make flushing the system with solvent easier.

Measuring SG

It's also important to regularly test a coating to assure that the formulation is consistent. Traditionally, mold coatings have been tested with a SG hydrometer "float" or stick meant to ensure that they were properly diluted or mixed. Anyone who has done this test can relate to the degree of imprecision in making the reading. Errors can be made if:

* the hydrometer is scratched or broken;

* the float is dirty;

* the coating is being stirred;

* the sample container is not deep enough;

* the float sinks and then bobs back to its equilibrium position.

There are drastic differences between the SG of the liquids and the displacement of the solids. Further, the additives and binders used to promote the suspension of the solids in the coating may produce other rheological effects, or effects related to the surface energies of the liquid.

The net effect is that the defining equation varies with each supplier and each coating formula and, therefore,. must be adjusted for each mixture.

SG = M/(M+D) where M = 145 and

D = Baume reading [+ or -] rheological properties

The proper control strategy is to measure the SG of the refractory coating and occasionally estimate the rheological effects with careful Baume and/or viscosity measurements. Baume is a composite number, influenced by the SG of the liquid and the suspended solids, the shape of the gauge, the liquid's viscosity and plasticity, the gelling characteristics, and other liquid characteristics including pseudoplasticity, rheopexy, thixotrophy, dilatency, Bingham plasticity and zeta potential. Many foundries use Baume to control their coatings, but this test alone is not enough to maintain a consistent refractory. The SG can be used to determine when the coating must be diluted or thickened.

SG measurement is most easily accomplished using a "mud balance" [ILLUSTRATION FOR FIGURE 1 OMITTED] to weigh a known volume of the coating, but a precision container and an analytical balance will accomplish the same job. A mud balance measures weight of the sample per unit volume or density. SG is generally defined as the ratio of the density of a material to that of water (when referring to the SG of liquids). To obtain the SG of a coating, take the density of the coating indicated by the mud balance and divide it by the density of water (approximately 8.33 lb/gal).

Most of the potential error arises from sampling techniques. The coating that is being tested must be representative of the coating that is used. If your coating is drawn from the bottom of the container, then the sample should be drawn from the same place. Be sure to flush the material that may be sitting or settling in a valve or piping by filling a container and dumping it back into the tank before drawing a sample. In addition, be sure to keep the sample cup clean.

Precontrol Chart

Plot your results on a precontrol chart, and adjust your coating when out of range [ILLUSTRATION FOR FIGURE 2 OMITTED]. Other data such as new batches, lot numbers and motor speeds can be recorded on these charts to provide a comprehensive record of coating control. Checklists are sometimes used as part of quality control procedures, so that key tasks, like probing the bottom of a container to check for sediment or cleaning intake filters, are done periodically.

Have your supplier provide a graphic representation of the relationship between SG and additions for coating formulation [ILLUSTRATION FOR FIGURE 3 OMITTED]. Be aware of the amount of refractory coating to be adjusted. Make any additions, stir for 15 min and retest, making sure to remark the precontrol chart.


The solids content of a coating should be checked occasionally, depending on the usage. If a new drum is opened every day, then it only needs to be tested once. If the use is one drum per week, it might be best to check the drum twice.

This is a simple test where a clean watch glass is weighed, the coating sample is put on the glass, and it is then reweighed. The carrier (solvent) is driven off by drying at 215F (102c). The heating must be done slowly to avoid spillage. The watch glass is reweighed after the coating has dried, and the remaining percent solids is calculated.

Some foundries use a "settling" or sediment test, in which a graduated cylinder is filled with coating and the solids content is estimated by the volume that settles to the bottom, usually overnight. This test does not yield a true volume or weight percent solids, however, the results should be reasonably reproducible, if compared at the same settling times.


The other important property of mold or core coating is viscosity. Viscosity is a measure of a liquid's internal friction, or resistance to flow, and there are more than 120 different procedures for measuring this. The best measure for our purpose is a Zahn cup or equivalent. These cups hold a given volume and have carefully constructed drain holes in the bottom. The cup is dipped in the liquid and withdrawn. The drain time is measured, and a chart is used to convert this time to centipoise. The drain hole can wear, so check it with the gauge supplied by the manufacturer.

The viscosity will change (usually it decreases) while being stirred. For this reason, the mixer should always be turned off, prior to testing the coating. Also, temperature is a critical factor in determining viscosity.

Coating viscosity probably does not need to be measured routinely, but, if there are suspected problems, it is one of the parameters to check. The results can indicate variations in the nature of the solvent due to aging, temperature, overmixing or some reaction with the container. It also can point toward a major problem with the binders or the suspending agent.


When changing containers, there should be enough room in the new container to add and stir in the residual material from the old one. One foundry fabricated a simple rack, out of angle iron, that allowed the barrel to be tipped to drain completely into a 5-gal pail. With some coatings costing over $0.50/lb, this can be worthwhile.

Store refractory coatings inside and under moderate conditions. Coatings allowed to freeze may lose binder, and exposure to direct sunlight may cause the coatings to cake.

Instead of buying "paste" you can use the thick residue from the bottom of the last container for the solids addition when your coating is too thin. Scrape the residue into an airtight sealable pail for storage until needed, because once it has dried it cannot be reused. The other correction strategy for a thin coating is to stir the drum (with the lid taken off) until the percentage solids is corrected via evaporation.

In addition, save the solvent you use for flushing and cleaning your system. This solvent can be used to dilute the coating to the proper consistency.

RELATED ARTICLE: Use SG to Identify "Ingredients"

If there is a need to identify the materials in your coating, the weight in grams of 100 cc of the supernatent liquid (that liquid which is floating on the top of the container before stirring or reconstitution) will be equal to the SG. However, the volume must be exact, and graduated cyliners are normally used. If this number equals 1.0, then the solvent is water. Organic solvents have densities less than one, as shown in the table below.


If you know the specific gravity of a coating as well as the percent solids and the solvent, then, by back calculating, you can "guess" the refractory constituent of the coating. The table contains SG statistics for the minerals commonly used in refractory coatings. Mineral "purity" can be an issue, so there actually may be a "range" for the SG.

Solvent Weight+Refractory Weight/Solvent Volume+Refractory Volume = SG
COPYRIGHT 1998 American Foundry Society, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1998, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

Article Details
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Author:Vingas, George J.
Publication:Modern Casting
Date:May 1, 1998
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