or dimensional accuracy tendencies of the test casting. Te test cast- ings can be poured from a variety of metals including gray iron, steel and copper-based alloys. Pouring times for the molds are 10-12 seconds. Once the castings cool to room temperature, they are removed and the gates are sectioned off with loose sand. Te castings are wire-brushed and sand blasted to remove any loose sand on the surface and then are tested for dimensional accuracy. Fol- lowing this, they are sectioned and evaluated for veining and penetra- tion defects. A coordinate measuring machine (CMM) was used to measure the dimensions of cores and castings. Step cone cores were first measured 24 hours after being made. Te cores were then coated with a high solid zircon refractory coating, as per industry practice, and dried at 212F (100C). in an oven before being measured again to determine coating thickness. Once the cast- ings were poured, they were sand blasted to remove adhering sand and measured along the six differ- ent steps. From the core and casting data, dimensional deviation of the casting from the core was calculated and plotted.
Fig. 4. Shown are core temperatures at 5% solid fraction, which illustrates the changes in core temperatures on a simple test casting.
3
Results and Conclusions Te difference between
the actual casting dimension and the theoretical casting dimension calculated from
the shrink rule is apparent. Te actual casting dimension depends on the section thickness of the casting. At the thinner casting sections, that is at the 4 in. (10.16 cm) step to the 3 in. (7.62 cm) step on the core, the cast- ing is smaller than the original core size. However, at the thicker casting
sections, the casting is larger than the original section size. Te thicker metal sections take longer to solidify and the temperature of the core is higher than at the other sections. Silica with 10% zircon displays a
similar trend as silica in the thinner metal sections. At the 2 (5.08 cm) and 2.5 inch (6.32 cm) steps, a marked difference between the two samples appears. At the 1-in. (2.54 cm) step, however, both silica and silica with 10% zircon display a dimensional change of plus-0.03 in. (0.076 cm) in the casting.
Fig. 5. The expansion curves for silica sand and various blends of zircon and silica sand are shown.
38 | MODERN CASTING July 2016
In the thinner sections, the metal solidifies at a faster rate and is solid enough not to move with the core expansion. Due to similar alpha- beta peak expansion between the two samples, the thinner sections are similar to each other. However, a steeper contraction is seen in the zir- con blend sample after the alpha-beta transition when compared to baseline silica. Tis will affect the thicker sec- tions more as the metal solidifies at a slower rate and is mobile to move with the core which reaches higher temperatures when compared to the thinner sections. Tis will lead to a low expansion or contraction of the core at that particular step, leading to better dimensional accuracy. At the thickest section of the cast-
ing, the metal will remain in a liquid state longer and is mobile when the
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