Early in 2022, four seniors at Western Michigan University came up with a way to dovetail a personal hobby with their required senior project—the group approached their instructor, Robert Tuttle, professor of engineering, design, manufacturing, and management systems, about developing a casting that would lighten endurance bikes and make them more affordable. 

Thanks to a professional friendship between Tuttle and a former student, Phillip Messing, who is now an investment casting engineer at AFS Corporate Member Aristo-Cast, the project resulted in a 3.5-lb. magnesium prototype of a bike frame designed to hold wheels, pedals, seat, and handlebar in position and has a load bearing capacity of 200 lbs. 

With a weight reduction of 5.5 lbs. compared to traditional construction of endurance bicycles like those in the Tour de France, the casting didn’t sacrifice stress or strain characteristics. The final prototype could eliminate several days in constructing welding fixtures if the casting is commercialized in the future—the need for welding, machining, and assembly are removed. The casting measures in at 34.4 x 5.6 x 21.2 in.—but the lessons in metalcasting were immeasurable.

When the students started talking about magnesium, Tuttle had one thing to say: Aristo-Cast.

“If it’s complicated, a one-off, investment casting, and then magnesium too, I always think of them,” he said. 

The foundry collaboration with the team began almost immediately, said Messing, who made it a priority to ensure clear and frequent transfer of information between foundry and school. Using Inspire Cast software simulation, the academic “customer” began its design journey with basic assessments that included how much weight the frame would need to support and how long it should be expected to last, Tuttle said.   

“You have to look at things like, are the section sizes thick enough?” Tuttle said. “Also, you have to think about fatigue. While we were doing the classical finite element analysis, we were concurrently looking at the manufacturing issues and doing the casting simulation alongside the design. We often talk about concurrent engineering these days—trying to get your manufacturing issues in at the very early start of the design phase, and we very much did that. That’s how they knew they wanted a general mag alloy. Looking at the mechanical properties, we finally settled on AZ91E.”

In its poster presentation last spring, the team—comprised of Leah Fernandez, Tanner Schember, Connor Schuitema, and Ean Widmayer—summarized its design development:

“The frame is based on typical endurance road bike geometry,” the poster reads. “Instead of tubes, the frame is completely solid. The team analyzed three different cross-sectional configurations for strength, weight, castability, and appearance. The three cross-sections were inverted teardrop, rectangle, and I-beam configurations. Compared to I-beam, the teardrop and rectangle had 6% and 17% greater volume, respectively. Based on the criteria, the I-beam was the best option.”

As the design took shape, it didn’t take long for the young team to realize that cost was going to be prohibitive since their plan lacked a corporate sponsor. Recognizing the budget constraints of four undergrads, but also the value of fostering casting education that leads to well-equipped metalcasters and future customers alike, Aristo-Cast management made the call to donate all its labor and the materials. 

“The foundry shared the cost projection with them, and they were like, ‘Oh my gosh, is this accurate?!’ Tuttle recalled. “They didn’t realize what first articles cost and just because you’re making one casting, the cost doesn’t come down. 3D printing is a great enabler of what can still be a very expensive venture. So, there was a lot of learning! And we truly appreciate that partnership with Aristo-Cast.”

From the start, the complimentary services included a process called additively-assisted investment casting in which the wax pattern is printed—in this case, on a Voxeljet printer that prints PMMA material (polymethyl methacrylate), an acrylic plastic with excellent burnout behavior. Messing said the printer is capable of printing roughly 20 x 20 x 40 in. patterns, which was perfect for their process.

Hot Flashes

As the lightest structural metal, magnesium, in theory, had a lot to offer in the bike frame application. For example, it offers great strength-to-weight ratio compared to aluminum, it’s more cost-effective than carbon fiber, and has sustainability appeal through recycling and reuse. But a major lesson learned was that this element and investment casting do not play well together. Known as a fire-starter, magnesium’s flashpoint is very close to its melting point, Tuttle explained. But the team was in good hands at Aristo-Cast, which is one of only seven companies in the world that pours magnesium using the investment casting process. 

“If you try to pour magnesium into your ordinary ceramic shell, the magnesium will react to the silica and the oxygen,” Messing explained. “We developed a special process years ago that actually inhibits that reaction. We treat the shells with this proprietary material, and then we apply cover gas over it, which is nonreactive and will flush the oxygen out of the shell. And that’s how we’re able to pour magnesium.”

Could the project have been simplified as well as made more cheaply with another casting process? Green sand certainly offers a cost advantage and would probably be looked at for production volumes should the casting be further developed and commercialized in the future. However, the decision to use investment casting was based on its capability to handle a high level of complexity in “onesie or twosie” quantities, as Tuttle put it. 

“Also, a lot of the off-the-shelf sand 3D printers don’t have all the specialty additives necessary for magnesium,” Tuttle continued. “I’m aware of I think one facility that might, but my understanding is that it’s not standard fare.”

Steering Around Bumps 

Aristo-Cast took some preventive steps to ensure a smooth ride on the casting’s path. Once the 3D printed pattern was in hand, the foundry cleaned it and infused it with wax, which prevents ceramic slurry from seeping into the actual pattern, Messing explained. The pattern becomes like a sponge without wax around it, he said. He had also determined to use a mechanical manipulator to dip the pattern with a level of quality they wanted. 

“Then, once we started studying the shell, we were having questions about whether to pour vertically or horizontally,” Messing added. “Most investment casting shells are vertical instead of horizontal like sand castings, but we do have the capability in-house to dissect and patch up shell molds to be able to reconfigure and pour horizontally.” To play it safe on this large and complex casting, that is, in fact, what they did to avoid any issues with the shell cracking. 
With the initial casting produced, they discovered the need for some gating changes, Messing said. 

Tuttle added, “There were some misruns in that first casting—sometimes there are things not captured by the simulation. We knew it was castable, but it was just getting those fine details. But that’s another reason I picked Aristo-Cast. They do the very difficult jobs.”

The misruns were attributable to both the gating issues and the geometry transitions, Tuttle said. “You’re getting into some pretty thin walls where the bike seat fits—a little thinner than we thought.”

In response, the foundry tweaked the gating, repositioning and “adding some more meat” to the 12 gate entrances in some areas, Messing said.

Hands-On

The teaching mindset within WMU’s metalcasting program takes a very hands-on approach, Tuttle said. He parlays that into an experience that simulates a real design process at school, going from problem definition all the way through concept selections, detailed design and manufacture, prototype, and testing. In other words, they like their students to get their hands dirty, he added—in the case of this project, their hands needed some good oven mitts, too. 

In that DIY spirit, Tuttle had the team perform some machining themselves at the campus lab, a process which, when magnesium is involved, “can be more exciting than pouring it,” Tuttle laughed.

“If you’ve ever been out camping, it’s a fire starter,” he said, “which is why you have to have all these cover gases Phillip was talking about—otherwise, it’ll just burn on you.” 
“While you’re machining,” he added, “there are massive safety issues, which is why I don’t believe it whenever the automotive industry talks about moving to magnesium—nobody wants to deal with the fire hazard!” 

The students experienced the opportunity to build some fixturing for their machining, as well, and, said Tuttle, had a first-hand encounter with rationalizing the theory of casting design with the actual dynamics of casting application.

“There was an area on the back of the bike frame—the derailer that helps shift the chain—where I told them it was a little thin,” he said. “I kept saying, ‘I’m not sure that’s right.’ And one of the things they learned is that while, on the one hand, you think of a bike frame as always straight up and down—but when you get on it, it’s at an angle. 
“So, it was getting them to realize that you have an angle and you have to create a bending moment in the overall frame in the stress level. They kind of broke that little area—but they learned their lesson,” he added.

Entrusted with the full and practical learning experience of his students, Tuttle said he emphasized the importance of verifying the manufacturing results in a casting and understanding the production part approval process (PPAP.) The students performed some tensile testing and impact testing to get the full picture of all necessary manufacturing steps. 

Their project complete as well as their degrees, the four students have since gone out to make their way in the world. It remains to be seen whether one, who took a job with Trek Bikes, may one day resurrect the design work accomplished with the Aristo-Cast-produced magnesium bike frame of 2022. 
For Tuttle and Messing, the gratification of sending young professionals out with a solid casting education is not diminished by the fact that they don’t always go into casting careers. 

“It’s still a good thing,” Tuttle said, “because even if foundries don’t get an employee, there’s a good chance they’ll be getting a future customer who isn’t a rookie when it comes to casting.”  CS

Click here to view the article in the digital edition of September/October 2023 Casting Source.