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Casting Answers & Advice.

Q: Our foundry uses core wash coatings for cores and molds but we are experiencing inconsistent casting finish. Is there a way to ensure that the coating produces a consistent surface finish? Is there a basic coating control test program we can institute?

Background: The foundry regularly runs Baume tests on the coatings at each work station as a coating control program. The coating supplier provides a Certificate of Analysis indicating that every shipment of material is within product specifications.

Refractory coatings have two interactive parameters that can affect casting finish--refractory components and the rheological characteristics of the coating. Coating performance depends on a consistent deposit of the refractory components on the core or mold and the rheological characteristics determine the consistency of the coating applied. Due to the interaction of these two factors, Baume testing is not always sufficient and a multi-test in-house coating control program is necessary.

The AFS Mold and Core Coatings Manual recommends several mold and core coating test procedures. The refractory components can be measured by using a combination of the following AES coating test methods:

* AFS 409-87-TS, Specific Gravity of Refractory Coatings: Hydrometer Method (i.e. Baume);

* AFS 410-87-TS, Specific Gravity of Refractory Coatings: Gravimetric Method (i.e. density or weight per gallon).

Since the refractory component of a coating is heavier than water, coatings have a lower specific gravity when they are diluted with water from concentrate to application level. The "Determination of Total Solids Content" directly measures the refractory content of a coating but it is a relatively slow test (approximately 2 hr) and does not identify the type of refractory components present. "Gravimetric Method Specific Gravity" is a fairly quick test (approximately 5 mm) that allows inferences to be drawn about the total solids and the refractory components present in the coating independent of the rheological characteristics. If the dilution factor of the coating is constant (same amount of water added each time) then the principle variable in the Gravimetric Method Specific Gravity is associated with the amount and type of refractory present.

The rheological characteristics of a coating are best measured with a viscosity test using one of the two viscosity test methods:

* AFS 407-87-TS, Brookfield Viscosity Test (rotational viscosity);

* AFS 408-87-TS, Zahn Cup Viscosity Test (efflux or flow viscosity).

Viscosity provides information on how a diluted coating will behave when it is applied to a core or mold surface by measuring how the solids (refractory component) and liquid interact in the coating. More specifically, viscosity describes the ability of a coating system to flow during application.

Either of these AES viscosity tests will yield the information necessary to control the refractory deposit left on the core. Both methods are relatively fast tests (approximately 2-5 mm) and either test is a good indicator of coating performance during application.

Recommendations: Since this foundry is already using Baume as a control test, it is reasonable to continue this testing. However, there is a potential for high operator variability with hydrometer testing and test parameters must be controlled. Operator consistency and length of test time are critical. When Baume is used as a control test, operators should time the test then select a single test time so that all operators use the same length during testing (30 sec to 2 mm).

However, Baume and Hydrometers are strongly influenced by both refractory concentration and viscosity, and the Baume test alone does not provide sufficient information to fully control coating parameters.

Baume was originally designed to measure the density of salt-water solutions with much lower viscosity than coatings. Because Baume is greatly affected by coating viscosity, the refractory contribution to specific gravity can not be segregated from the viscosity contribution when Baume is the only test used. This can result in undetected changes to the coating deposit on the core or mold that can cause inconsistent coatings and random casting surface finish defects.

When Baume is used in combination with a Gravimetric Method Specific Gravity and one of the viscosity tests, the combined results can become a useful diagnostic tool. Results from interactive test methods may be graphed against each other and the relationship evaluated (Fig 1).

Results from three sets of test methods can be plotted together to define a three dimensional domain that can define coating parameters and optimize process requirements.

The information in this column was supplied by AFS Mold-Metal Interface Committee (4-F)

Recommendations are the opinion of the AFS Technical Dept. Based on referenced literature and experience. If you need assistance with at technical issue, fax or email your question to: Casting Answers & Advice. C/o MODERN CASTIMG, at 847/824-7848 or moderncasting

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Publication:Modern Casting
Date:Apr 1, 2003
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