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Coating Errors Can Lead to Rough Results M


etalcasters often look toward problems in the sand system as the cause of

casting surface defects, but some surface roughness issues may have their root source in other areas. “Tear drop” and “orange peel” defects are the common names given to two surface roughness problems that are created by improperly mixing or applying refractory coatings on the mold or core prior to casting. Mold refractory coatings are part

of the complex mechanisms that take place inside the mold when the hot, molten metal interfaces with the mold or core surface. A better understand- ing of how the coating behaves in the mold-metal reaction can help maximize casting quality and minimize rework. Both of these defects are related to how the coating is applied to the sand mold or core and the coating viscosity.

TEAR DROPS “Tear drops” appear as droplet-

shaped protrusions of excess coating on the surface of a core or mold. In extreme cases, the droplets may be elongated. Typically they are found on the underside of cores or molds and result in a negative depression in the casting surface. Causes:

• Poor draining profile during coating application.

• Insufficient coating rheology (flow). • Overly thick coating viscosity. • Variations in core or mold temperature.

Understanding how the coating

behaves in mold-metal reactions can help maximize casting quality and minimize casting rework.

• Improper use of coating blow offs. Solutions:

• Adjust drain profile of core or mold to allow excess coating to drain off.

• Contact coating supplier to adjust coating rheology.

• Reduce coating viscosity or density to allow better coating leveling.

• Check core or mold temperature and allow to cool before application.

• Adjust coating blow offs to remove any excess coating during drain cycle (before drying begins).

ORANGE PEEL “Orange peel” is a core or mold

surface defect typically seen in spray coating applications that results in an irregular surface pattern resembling the pitted surface of an orange or an elephant’s skin. Te core or mold then reproduces a comparable surface roughness on the casting’s surface. Causes:

• Poor coating spray application tech- nique or equipment.

• Too high coating density. • Over application of coating.

• Poor coating leveling properties. Solutions:

• Practice proper coating spray tech- niques. Retrain coating personnel.

• Check spray equipment. Check and adjust spray fluid and air pressure.

• Reduce coating density or viscos- ity to improve coating leveling properties.

• Follow coating supplier guidelines for coating application properties. Proper mold coating material and

application should never be a crutch for overcoming poor foundry practice, such as improper sand control, core and mold production, venting, pour- ing and filling, and shot blasting. But by optimizing the coating material, and paying careful attention to coating storage, handling and application, mold coatings can help reduce casting de- fects and improve surface finish.

Tis information was adapted from the AFS wall chart, “Coating-Related Cast- ing Defects,” created by the AFS Mold- Metal Interface Reactions Technology Committee, 4F.

Excess coating on the surface of a core or mold (left) can lead to tear drops on the final casting (right).

Orange peel defects can occur from a variety of coating misapplications.

April 2014 MODERN CASTING | 41

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