casting 101 C
Coatings Can Cause Surface Finish Problems
Members of the AFS Mold Metal Interface Committee, 4-F W
hen working with foundry person- nel, casting buy- ers may assume that all casting
surface defects are related problems in the sand system or mold making process. But some roughness issues may have their root source in other ar- eassuch as the mold refractory coating. Many foundries ap- ply refractory coat- ings on sand molds and cores. Mold coatings can be used to help protect the sand from molten metal penetration and minimize other potential casting surface defects. But if coatings are not mixed, stored and ap- plied correctly, the coatings themselves may become the source of casting surface defects. Refractory coatings are part of the
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complex mechanisms that take place inside the mold when the hot molten metal interfaces with the mold or core surface. With a better understanding of how the part coating behaves in the reaction between the mold and metal, your supplier can maximize casting
quality and minimize rework related to surface defects. One common problem related to coatings is known as “tear drops.” T ese droplet-shaped protrusions of excess coating on the surface of the core or mold can result in unacceptable surface fi nish. Typically, the droplets are found on the underside of cores or molds and
• Improper use of coating blow off s. Tear-drop-related defects might
Understanding how the part coating behaves in mold-metal reactions can help maximize casting quality and minimize casting rework.”
result in a negative depression in the casting surface. In extreme cases, the droplet can be elongated, appearing as though it was running down the core or mold’s side when the metal was poured. T e most likely causes of this defect
include: • Poor draining profi le during coating application.
• Insuffi cient coating fl ow. • Excessive thickness in coating. • Variations in core or mold temperature.
result from any of these issues. T e solution for the metalcaster depends on proper process control. Your supplier should ensure the coating can properly drain from the core or mold or adjust coating blow off to remove any excess. T e metalcaster may have to contact the coating sup- plier concerning the coatings fl ow and viscosity or density. Proper mold coating material and application should never be an excuse for a metalcasting fa-
cility’s poor practices, such as improper sand control, core and mold production, venting, pouring and fi lling, and shot blasting. By optimizing the coating material, and paying careful attention to storage, handling and application, a metalcaster can use mold coatings to minimize defects, improve surface fi nish and deliver higher quality castings. This information was adapted from the AFS wall chart, “Coating-Related Casting Defects,” created by the AFS 4F Mold-Metal Interface Reactions Technology Committee.
Excess coating on the surface of a core or mold (left) can lead to an unacceptable surface fi nish on the casting (above).
May/Jun 2014 | METAL CASTING DESIGN & PURCHASING | 47
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