Back in March of 2005, we witnessed foundry workers breaking the first Memorial Unit prototype out of its immense mold at MetalTek International’s facility in Pevely, Missouri. It took nearly half a day of working with jackhammers, bobcats, and shovels to free the piece, which then began its journey through heat treatment, testing, welding, finishing, and polishing. As is expected in prototyping, each step brought with it significant opportunities for learning and improving both the production processes and ultimately the design of the Memorial Units.
Although six feet in length and eighteen inches high, the “walls” of the Memorial Unit casting were optimized to be much thinner than a typical industrial cast, challenging the age-old foundry techniques employed in its manufacture. The victory of engineering the mold such that metal rose to fill the highest edges of the casting balanced against other complications, such as learning that cross bracing the cantilever caused the seating surface to bend out of tolerance as it cooled. The definition of “tolerance” in this case would be “the permissible deviation from a specified value of a structural dimension”, and quantifying acceptable tolerances for the Memorial Units soon became the topic of regular dialogue between the designers and MetalTek’s metallurgists, enginners, and foundry experts.
In a typical industrial casting process, a foundry would need to hold tight tolerances in two dimensions, but have the flexibility to “trick” the metal into expressing its natural tendency for movement in the third dimension where tolerances were not as significant. Because the criteria for the Memorial Units is visual and aesthetic, the design team had to understand- and convey to MetalTek- the point at which variance could possibly visually distinguish one piece from another, especially in a field of 184 Memorial Units organized proximately and in a linear fashion.
While considering tolerance requirements, the team launched a parallel effort to optimize the efficiency and accuracy of the casting and post-casting processes in preparation for production. The size of the Memorial Unit as a single cast remained an obstacle until the design team explored alternatives to the stainless steel fountain basin. Submerged the metal in this application had made corrosion a critical consideration, and drove the decision to cast out of a super-duplex stainless steel. This material, Ferralium, carries a significant cost premium and complicated the manufacturing process by its very strength, which made it difficult to produce the etched nameplates, in addition to requiring that all connections be analyzed to eliminate the concern over galvanic action.
The idea of exploring a pre-cast concrete basin initially came from the project’s fountain consultants, CMS Collaborative, based on their past experience. KBAS plugged the proposed new approach into their three dimensional computer model and developed a two piece Memorial Unit with a cast metal seating surface (still inlaid with synthetic terrazzo) bolted onto a pre-cast concrete fountain basin finished with state of the art waterproofing and sealing techniques perfected for high-end architectural concrete tubs. The foundry began “rapid prototyping” to determine how tight tolerances could be on a cast essentially half the size of the original.
This spring, designers once again visited the MetalTek foundry, this time to review six versions of the “half”-cast, and compare them against a perfect full-size template made out of high-density foam. The new approach provided the foundry greater control over many aspects of the casting and post-casting processes, and gave them an opportunity to “trick” the metal into actually moving into tolerance during cooling. An intricate heat treatment fixture was also engineered to control movement when the cast was rapid quenched after the final heating necessary to promote corrosion resistance.
The design team approved the final two prototypes in the series, giving the green light for these to move forward to Crown Polymers, where terrazzo applicators will install the epoxy polymer concrete that has been under testing and development with guidance from the Army Research Laboratory since last year. MetalTek was instructed to move forward in constructing a new pattern and tool for final Memorial Unit production. The first photographs of prototypes incorporating the anticipated casting, finishing, and EPC surface will be available this fall, as the first prototype pre-cast concrete basins are produced. If everything goes as anticipated, full and final production for the Memorial Units will begin in early 2007.
Jill Dowling
Design Manager
Centex Lee