sand when subjected to a high metal- lostatic height (approximately 18 in.) and severe heat flow. Te bauxite 40/50 product per-
formed better than silica sand and comparable to chromite sand for gray and ductile iron castings. Te uncoated cores exhibited less metal penetration than silica sand but were observed to have a very rough surface. For coated cores, the degree of metal penetration was reduced for the bauxite 40/50 aggregate but, again, a rough surface was observed. Both of these observa- tions for coated and uncoated bauxite 40/50 for gray iron is a strong indica- tion that the high permeability of the product is detrimental in surface quality. Tis was also confirmed with the ductile iron castings. In this case, silica sand did not exhibit any metal penetration, though substantial vein- ing, for graphite coated and uncoated cores. For ductile iron, metal penetra- tion is not commonly seen on coated or uncoated cores because of the higher surface energy between ductile iron and the molding aggregate. Bauxite 40/50 did have appreciable metal penetration but was observed to have significant rough surface features on the entire surface of the cores, confirming the screen distribution of Bauxite 40/50 is not appropriate for casting applications. Under extreme conditions such
as steel casting temperatures, bauxite 40/50 did not perform as well as silica and chromite sands. Te major observa- tion was the fused mass surrounded by steel penetrating the synthetic aggregate, making it extremely difficult to remove and rapidly degrading the cutting saw blade. Chromite and silica
Table 5. Screen Distribution Screen 20 30 40 50 70
100 140 200 270 Pan
Bauxite 40/50 0.02 0.02
58.89 34.28 6.00 0.75 0.01 0.00 0.00 0.00
32 | MODERN CASTING July 2015
sand was observed to have extensive metal penetration, though not as severe with chromite. However, depending on the depth of metal penetration, shallow penetration could be removed. Another series of Gertzman tests
were performed to investigate the synthetic aggregate bauxite 40/100 product. Te experimental binder sys- tem used for the test cores was 1.25% phenolic urethane nobake poured with a class 35 gray iron at 2,600F. As expected, the silica sand exhib- ited massive metal penetration in the uncoated condition and extensive veining defects for the zircon coated cores. Te bauxite 40/100 product did not exhibit any veining defects for the coated and uncoated condition. Slight metal penetration was observed for both the uncoated and zircon coated cores. However, when comparing to bauxite 40/50, significant reduction in metal penetration was observed, confirming aggregate distribution contributes significantly to penetra- tion resistance for the bauxite product. Te bauxite 40/100 product performed equally well when compared to chro- mite and zircon sand.
Industrial Casting Trial A casting trial at a steel casting
facility was performed to advance the product from the laboratory environment to actual part produc- tion, measuring the performance of the bauxite product to silica sand. A complex stainless steel valve body with two internal cores was selected as the test casting, because the solidification pattern around the two cores pro- duces heat saturation, giving a higher propensity to cause metal penetra-
Bauxite 40/100 0.00 0.03
18.66 28.16 18.61 24.06 8.09 1.60 0.34 0.22
Bauxite 40/70 0.00 0.05
32.14 44.45 19.08 3.74 0.46 0.02 0.00 0.00
tion, aggregate sintering and veining. Because of this casting geometry, the metalcaster coated the two cores with a zircon water-based solution. To increase the casting difficulty of the bauxite 40/100 product, the two cores were not coated. Te stainless steel casting was
poured at 2,850F and allowed to cool for approximately four hours before shakeout and blast cleaning. Te inter- nal features were observed for surface finish, comparing the zircon-coated silica sand cores to the uncoated bauxite 40/100 cores. For both castings, no metal penetration or veining defects was observed. Te bauxite 40/100 cored area showed a slightly rougher surface than the zircon coated silica sand cored area. However, the manager indicated the surface finish of the bauxite 40/100 product was accepted and would be included into the finish casting lot.
Conclusions Te bauxite aggregate’s per-
formance is comparable to other metalcasting aggregates. No signifi- cant physical properties issues were observed to affect the performance for three different sand binder systems. Te heat capacity property of bauxite is similar to silica sand, expansion characteristics comparable to zircon and chromite sand, and chemical properties analogous to the aggregate materials investigated. Trough some modification with the screen distribu- tion, the bauxite aggregate performed as well as silica sand, chromite sand and zircon sand. Te series of evalu- ation tests clearly illustrated bauxite can be used as synthetic sand for the metalcasting industry.
Bauxite 70/140 0.01 0.00 0.06 0.04
18.34 56.23 19.44 4.04 1.02 0.66
Chromite 40/100 0.74 5.51
15.33 31.00 34.47 11.75 0.88 0.18 0.04 0.02
Silica 50/140 0.00 0.02 4.16
21.72 23.38 26.85 16.90 5.10 1.06 0.68
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