as a tie bar between the legs of a U-shaped core to prevent distortion. Modify the core binder system and change the resin composition of the shell core as needed to improve strength. Work with the sand pro- vider on new formulations. Even after implementing these
suggestions, large shell cores can remain difficult to use in permanent mold applications. For example, at Wisconsin Aluminum Foundry Co. (WAFCO), Manitowoc, Wis., the large cylindrical core in Figure 3a failed at a 75% rate due to a number of problems. Engineers filled the 46-lb. (20.87-kg) shell core with air- set sand to produce castings, but this
Large shell cores present a special set of challenges, including the potential for core breakage,
cracking, distortion and gas-related porosity.
process increased both core weight and knockout costs. Similarly, the core in Figure 4, at nearly 26 x 6 in. (66.7 x 15.2 cm), featured a large flat section that developed cracks regu- larly. While not leading to outright
failure, as in the cylindrical core, it required additional inspection to verify the surface finish in the cored casting cavity and extra cleaning room labor to remove flashing caused by broken and cracked cores. WAFCO continues to minimize
variation in the core and molding process by working with suppliers to modify sand recipes to improve core integrity, discussing necessary core support prints with clients and
avoiding new high risk jobs based on experience. Tough networking with other metalcasting facilities is helpful in learning techniques to overcome challenges, large shell cores remain a complex endeavor.
CASE STUDY: CORE BINDER SELECTION AND CASTING QUALITY
Eck Industries, Manitowoc, Wis., inherited tooling for a semi- permanent aluminum wing casting (Figs. A-B) from a previous manu- facturer. The tooling was aluminum coreboxes, and the casting was designated for an A357-T6 alloy. The casting surface finish was a critical concern for the customer, and three separate bench operations were implemented to achieve the desired result. The casting also required X-ray soundness and aggressive mechanical property requirements. SO2
was chosen for the three
cores because it had produced sound castings for Eck in the past. How- ever, in this case, a residue developed on the die faces after outgassing that required frequent blasting and re- coating. Additionally, some core sections eroded, leading engineers to add a refractory coating to the affected areas. Still unable to achieve satisfac- tory results, Eck engineers changed to a phenolic urethane nobake
Figs. A-B: For the aluminum wing casting (top), Eck Industries tested different binders to produce a 21 x 22-in. (53.3 x 55.9-cm) mold with three sand cores (below).
An example of sand cores adding difficulty to the permanent mold process, this case shows that core selection may cause problems where the solutions are not intuitive. Because so many variables affect success or failure, semi- permanent molding remains a hit-or-miss proposition based largely on trial and error. Additionally, with current computer modeling techniques incapable of being reliable guides to core selection and use, metalcasting engineers must rely on past experience.
(PUNB) core material, which elimi- nated the die residue, improved pro- ductivity and decreased excessive cleaning room operations. However, scrap castings and the rework rate increased significantly due to shrink- age defects in the area between the cores and adjacent to the gating. The PUNB core also produced a vola- tile condensation zone in a region unaffected by the casting heat. Eck Industries has since gone back to the SO2
core. September 2013 MODERN CASTING | 39
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