benefit of surface smoothness realized through the AFS-GFN of the sand.
CONCLUSION
It was seen that currently available molding aggregates have the ability to achieve surface roughness values of less than 200 RMS micro inches. Tese values are slightly within the values associated with investment castings. For the materials tested, each exhibited a decrease in casting roughness with increasing aggregate AFS grain fineness. Tis was true with all materials up to a thresh- old value, at which time no further decrease in casting roughness was seen with increasing AFS-GFN. Tis was supported by previously con- ducted research. Within all material groups, the effect of AFS-GFN was secondary to both calculated surface area and aggregate permeability. While permeability can be thought to describe the open areas of the com- pacted sand, the surface area better
describes the screen distribution of the sand and corresponding amount of fine particles. Both permeability and surface area were directly related to casting surface smoothness. Tis was true for aggregates within a shape group. Although angular and sub- angular aggregates had high surface areas, their permeability was high and indicated an open surface. Spherical and rounded aggregates exhibited the smoothest surfaces combining low permeability with high surface area. Originally, it was believed surface
wettability as measured by contact angle between liquid metal and the bonded aggregate was a critical factor in the resulting casting surface fin- ish. While it was shown that contact angle on various materials at similar AFS-GFN was not proportional to the casting roughness, grain shape was confirmed as a major factor. Te absence of a relationship
between contact angle and casting surface roughness might be explained
by the fact that grain shape was seen as a major influence in surface roughness. Te contact angle of various materi- als possibly was affected more by the grain shape and resulting surface smoothness than that of the wettabil- ity of the material alone. As with all measuring instruments, artifacts of the test method may influence the results to some degree. Te increase in casting roughness may be due to the shape of the peaks and valleys created with the coatings. By definition and measurement, the refractory coatings only increased the surface roughness over non-coated samples. All of the refractory coatings were successful in improving the surface roughness of the 3-D printed sands. Te surface finish of the test castings from coated samples appeared to be somewhat independent of the starting substrate sand. Te coatings had a major effect on the surface finish but further work is required to revise the coatings to improve casting finishes. ■
May/Jun 2017 | METAL CASTING DESIGN & PURCHASING | 45
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