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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. T ese values are slightly within the values as- sociated with investment castings. For the materials tested, each exhibited a decrease in casting roughness with in- creasing aggregate AFS grain fi neness. T is was true with all materials up to a threshold value, at which time no fur- ther decrease in casting roughness was seen with increasing AFS-GFN. T is was supported by previously conducted research. Within all material groups, the eff ect of AFS-GFN was second- ary to both calculated surface area and aggregate permeability. While perme- ability can be thought to describe the open areas of the compacted sand, the surface area better describes the


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screen distribution of the sand and corresponding amount of fi ne particles. Both permeability and surface area were directly related to casting surface smoothness. T is was true for aggre- gates within a shape group. Although angular and sub-angular aggregates had high surface areas, their perme- ability 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 fi n- 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 confi rmed as a major factor. T e absence of a relationship between contact angle and casting surface rough-


ness might be explained by the fact that grain shape was seen as a major infl uence in surface roughness. T e contact angle of various materials possibly was aff ected more by the grain shape and resulting surface smoothness than that of the wet- tability of the material alone. As with all measuring instruments,


artifacts of the test method may infl uence the results to some degree. T e increase in casting roughness may be due to the shape of the peaks and valleys created with the coatings. By defi nition 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. T e surface fi nish of the test cast- ings from coated samples appeared to be somewhat independent of the starting substrate sand. T e coatings had a major eff ect on the surface fi nish but further work is required to revise the coatings to improve casting fi nishes.


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