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Evaluating refractory coatings: a practical approach.

Providing a quality casting surface finish is only part of the necessary criteria when evaluating refractory coatings. It also is critical for foundries to understand what it expects from the coating in terms of performance, and define those requirements for suppliers.

When evaluating coatings for an operation, provide a list of coating requirements to suppliers. These suppliers then can test the coatings and provide data to the foundry before the metalcaster proceeds with test trials to select the coating.

Refractory coatings have a number of possible requirements. They can:

* be as permeable as possible to promote evacuation of gas;

* minimize air entrapment to promote ease of Baume control;

* anchor well to the core to prevent spalling;

* be fast drying;

* minimize core strength degradation;

* be environmentally friendly (water base, low silica);

* provide adequate protection against metal penetration;

* perform in dripping and flow coating applications;

* have an adequate suspension package to preclude refractory fallout;

* have an adequate biocide package to prevent biological growth under normal use conditions;

* level well and minimize runs and tear drops;

* be cost effective.

Once the suppliers provide data on a coating that meets the requirements, several tests exist for a foundry to verify the results.

The following information examines the foundry tests to evaluate refractory coatings. The testing methods described focus on core strength issues and coating monitoring and control. Most of the tests require the making of a standard test core (dog bone, common in most foundries) that will be used as the substrate to apply the coating. Test cores should simulate the foundry's process in type and amount of resin, sand and expansion control agents. The results from these tests determine which coating is correct for a particular operation.

Core Degradation

To evaluate the effect of refractory coatings on core strength, dip one set of test cores and leave the other set undipped. Place both sets in the drying oven until dry and allow them to cool to ambient temperature (approximately one hour). Then, when cool, evaluate both sets of cores for core strength. The comparative loss in strength of the coated cores will most likely be substantial.

Core degradation varies from coating to coating. The longer a core stays wet, the more core degradation will take place, so it is key to put cores into an oven heat zone as quickly as possible after the core is dipped. Most coatings use surfactants as wetting agents to allow the coating to penetrate the proper depth. These surfactants change the surface tension of the water, making it worse for core degradation.

The Baume Test

The Baume test is the most common test used in foundries to control coating because it is quick and easy. The test is performed with a hydrometer, which is a sealed glass tube that contains a calibrated scale in degrees Baume.

The hydrometers should be clean and dry with a resolution of at least [0.5.sup.[omega]]. The gages reference the material they are to be used for and the temperature the material should be to achieve measurement accuracy. The Baume scale of numbers relates to the specific gravity and body of a coating.

After mixing the coating sample thoroughly, immediately float the hydrometer in the coating slurry. When it stops sinking, read the degrees Baume directly from the hydrometer scale.

The testing of refractory coatings requires some special considerations. The coating should be homogeneous, at the correct temperature, have no air bubbles present and be completely still.

Weight Per Gallon

While not as convenient as the Baume test, the weight per gallon measurement is used in some foundries for coating control. This measurement requires coating material to be poured into a volumetric vessel or cup. A lid with a hole in it (allowing some coating to ooze through the hole) is placed on top, assuring that the cup is full.

When the excess coating is wiped away, the cup can be placed on an electronic balance to determine the weight of the coating per known volume, As with the Baume test, the coating must be homogeneous and with no air bubbles.

Viscosity

Viscosity, a measurement of material flow properties, is the best test for evaluating coatings because of its high correlation with the dried deposit on the core. There are several different viscosity methods:

Electric Spindle--While an electric spindle viscometer is the most accurate, it is a laboratory grade instrument and not suitable to the production floor. Depending on the material to be tested, various spindle sizes and viscometer speeds can be used.

Flow Cup--The flow cup measure of viscosity requires the use of a cup with a specific size hole in the bottom to match the material being used. A stopwatch is used as the cup is lowered into the coating and then taken from the surface of the coating after it has filled. The time it takes the coating to drain though the hole is the viscosity in number of seconds.

Run and Drip--Run and drip viscosity is measured by removing a hydrometer from the coating, holding it at a [45.sup.[omega]] angle and observing the way the coatings run off (stream, drip or a combination of both).

Solids Content

The solids in the coating must be measured because they are refractory materials that provide protection to the core. The higher the percent solids, the more protection the core is provided.

The solids content of a coating is determined by placing 20 ml of coating in a tare weighed 50 ml (or larger) beaker. The coating is weighed and then the beaker with coating is placed in a drying oven at 225F for two hours. After the sample cools down to ambient temperature, the material is reweighed. The solids percent can be determined by dividing the weight of the dried coating by the original weight and multiplying by 100.

Coating Deposit

Once the optimum Baume level of coating is defined to achieve a good surface finish to the casting, the amount of coating thickness can be determined.

Coating is removed from a flat surface on a core and the difference in the cored surface and the coated surface is measured. The amount of surface deposit can be used as a reference for future comparisons and making decisions about coating allowance in tooling design.

The dried coating deposit, because of its importance, is the dependent variable while other independent variables can be evaluated as to their correlation by regression analysis.

As an example, several different test procedure results can be evaluated and compared to coating thickness to see what might be the best method of control. In the case of Baume, weight per gallon, percent solids and electric spindle viscosity, viscosity shows the strongest correlation. This analysis also can be done using a metal substrate instead of a core because the substrate surface will remain consistent throughout the evaluation.

Coating Penetration

The distance the coating penetrates the core is an important feature to a coating's success. A coating that lies entirely on the surface of the core is not anchored well and will most likely spall away. A coating that penetrates too much will over degrade the core strength, resulting in core scabs or broken cores.

Coating penetration also is a function of core density. A core that is blown too tightly resists coating penetration, while one blown softly acts like a sponge and absorbs too much water. Therefore, any evaluation of coating penetration should be done on a core that is of normal production quality. Gore release agents may waterproof the core and affect coating penetration.

Penetration is evaluated by simply breaking or cutting a coated dried core and observing how far the coating penetrates the core. The usual reference is sand grains of penetration. A normal level of coating penetration is 2-4 sand grains. The coating can be adjusted without major reformulation by changing the concentration or type of surfactant.

Coating Permeability

Coating permeability is the amount of gas that can pass through the coating. The level of permeability is dictated by both the type and amount of refractory materials that are used in the coating formula and the dry film thickness deposit on the core. The permeability of the coating on the core is measured using a laboratory permmeter.

A coating with low permeability is desirable when directing evolved gasses to vent through specific areas of the core. A high permeability coating is best when the goal is the evacuation of core gasses through the coating.

The permeability of the coating at the coating-metal interface may be different than that measured on the core. Some constituents of the coating may quickly thermally decompose, leaving voids that result in higher permeability. Some may soften and flux resulting in lower permeability.

Pyrometric Cone Equivalent

Pyrometric cone equivalent testing determines the refractory value of a coating. This test, which must be performed by an accredited lab, is of value in quantifying the temperature that the coating can withstand.

The coating is dehydrated leaving the solids, which are then compressed into a standard cone shape. The coating cone is then subjected to high temperature along with a standard cone to determine the temperature at which the coating fails.

Biological Growth

Biological growth occurs in some coatings when biocides are left out of the coating. If this happens, the rheology of the coating can change when the biological growth adversely affects binders and suspension agents in the coating. If this occurs, less refractory ends up on the cores and casting quality can suffer.

A simple test can be performed to detect biological growth in coatings. Pour coating from a dip station into a glass graduated cylinder or a narrow jar 6-10 in. tall. Fill the container about 55-75% full. Using a felt tip marker, mark a line on the container at the coating level. Place a lid or stopper in the top, but make sure some air can get in. Leave the container for 24 hr and observe the coating level in reference to the line. If the coating has grown (moved above the line), there is biological activity.

pH

The pH test indicates of coating changes. Since coatings contain chemicals, a range of pHs exists that identifies a normal coating's alkalinity or acidity. Monitoring this feature may give some indication when there is a problem. Changes in pH can indicate the wrong materials used in manufacturing, spoilage of the material or contamination of the product.

The electrodes of the pH meter are immersed into the coating slurry. The pH should be read while gently moving either the coating sample around the electrodes or the electrodes themselves. This testing method also can be done using pH paper.

Monitoring Coating Performance

Coating performance can have a major impact on casting quality. For that reason, it makes sense to do some type of coating monitoring in the foundry. A suggested test program would be to pull a sample from a production dip station daily, record the lot number and perform the previously mentioned tests. Gores are not exactly reproducible, therefore using cores as a substrate to monitor coatings introduces some variability. One suggestion is to use a metal substrate when performing dry film measurements.

Goatings are an important part of a foundry's efforts to produce quality castings. The coating is the only thing standing between the molten metal and the core. When things go wrong, they can be catastrophic. By understanding the basics of coatings and how to evaluate them, a foundry is better prepared to recognize potential failure before it happens.

RELATED ARTICLE: Inside This Story:

* Practical knowledge in evaluating refractory coatings is provided, focusing on testing methods that pinpoint core strength and coating monitoring and control.

* This article gives an understanding of the basics of coatings and how to evaluate them, making a foundry better prepared to recognize potential failure before it happens.

About the Author

Stephen G. Baker is the senior materials engineer for Indianapolis Casting Corp., Indianapolis, Indiana. He received a bachelor of science degree from Purdue Univ. School of Engineering and Technology and has 28 yr of foundry experience in various management roles.
COPYRIGHT 2002 American Foundry Society, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 2002, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

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Author:Baker, Stephen G.
Publication:Modern Casting
Geographic Code:1USA
Date:Oct 1, 2002
Words:2016
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