Working in a Vacuum
Less is more. Over 70 years ago, Ludwig Mies van der Rohe popularized that phrase in the context of modern architecture, but it’s an equally relevant truth today when you contrast a complex weldment or fabricated part to a streamlined casting. When AFS Corporate Member TPI Arcade (Arcade, New York) converted a 14-piece welded frame for a medical system to a single, all-encompassing aluminum casting, the customer, Hologic, got more than they’d hoped for––and, at the same time, much less. Less cost, less material to procure, and nothing to weld.
TPI Arcade Sales Manager Erik Depczynski said the relationship between his company and Hologic dates back to the late 1990s when the foundry served as a supplier through a third party, usually a machine shop—that changed when they hired Casting Development Associates with Charlie Simmons, who cultivated a more active book of business with Hologic. The foundry has done about a half-million in sales with Hologic over the past four years alone compared to $30,000 in previous legacy business. “It’s been a good partnership,” Depczynski said, referring to both Hologic and Simmons.
This year, TPI Arcade was recognized with the Outstanding Achievement award in AFS’s Casting Competition for the formerly fabricated part that acts as the framework for Hologic’s Fluent fluid management system used in operating rooms for hysteroscopic procedures. During an annual Kaizen improvement exercise at the manufacturer, the frame was identified for redesign in 2019 because it’s been pricey to produce since the device’s inception seven years ago. Procurement and assembly of its many bits of hardware contributed to inefficiency, and the entire process of acquiring aluminum tubing, cut at various angles, was both expensive and complicated. After the pieces of tubing were cut, the supplier manually welded them together and installed threaded inserts. Once delivered back to the manufacturer, Hologic attached four separate brackets to the frame.
“As we looked at everything and thought about our [redesign] options, machining this part out of a solid piece of aluminum would have created a lot of material waste, so we didn’t pursue that idea,” said Jim Woodman, Hologic CAD specialist, manufacturing operations. “Then we were talking with Charlie, and he said, ‘Why don’t we try to incorporate as many things as we can into one casting?’ So that’s where it went, and we removed costs associated with buying the cylinders plus having to assemble them afterwards.
“The guys in production were happy we made those changes, because they had a lot of headaches,” he added.
TPI Arcade poured the casting in A356, which Depczynski said is the most comparable sand cast alloy to a 6061 wrought aluminum, which the weldment had been made from.
Vacuuming Out Cost
Having recently manufactured its 3,000th Fluent unit, the company has ordered 1,200 frame castings from TPI Arcade for 2022, and Woodman estimated the company has saved a quarter of a million dollars annually since the redesign, which represents about half the original per-part cost.
Money saved was just the beginning of benefits the customer was to enjoy from its partnership with the foundry. TPI Arcade adheres to an aerospace quality system (AS9100 Rev. D) for all its parts, and Woodman asserted they’ve experienced no problems whatsoever with their delivered castings for the three-year duration of the order; only a minor initial slag issue occurred in the first-batch machining, which was immediately corrected.
But it’s the V-process used at the foundry—its only process—that is generating more advantages than Woodman originally anticipated.
For starters, once a PO is released, TPI turns around castings in about five weeks, with three weeks added for outsourced machining plus a week for finishing, which includes heat treating and chromating.
“We’re delivering ready-to-assemble components in the time that they’d still be in tooling construction with permanent mold, die casting or investment casting,” said Depczynski.
And the part density is much better, Woodman said, despite the fact that the casting weighs in lighter than its welded predecessor—4.9 lbs. today versus its previous 5.4-lbs.
A parts designer with considerable casting expertise, Woodman noted the absence of drafts was a significant surprise that emerged from this first experience with vacuum casting.
“Normally you have to worry about where the drafts are going to be and are things still going to fit the same way with the draft,” he said. “Being able to have a part without any drafts made it a lot easier. We didn’t have to worry about changing the mounting of any of the components that are attached to the frame.” Depczynski said, “Our casting process yields tighter dimensional castings that are more cosmetically appealing. But one of the big advantages of this particular design is we don’t need draft angles. So, you’ll see a lot of the geometry has vertical walls, and Hologic leveraged a lot of those attributes into their equipment, which is going into hospital settings where the parts have to look nice and be dimensionally accurate.
“For this particular application, the vertical walls without draft angles is why they partnered with us to convert this into a casting—if it had been a traditional casting, we would need eight draft angles on the vertical walls, adding weight, which the customer was trying to get out of their design.”
He added that because the V-process results in a finer surface finish than a regular sand casting, additional savings were realized from having less finishing work to do.
Tooling costs shrink with the V-process, too, because patterns are covered and protected from wear over the course of thousands of pours.
“We build a pattern, just like a traditional sand foundry would, but we honeycomb the pattern and drill small holes through it, which we call vacuum vents,” said Depczynski. “Then we place the pattern on the line, hook a vacuum pump up underneath the pattern, and suck a vacuum pull through the tool itself—through the vents that we’ve drilled through the pattern. Then we take a piece of Surlyn film, which is a heavy-duty Saran-wrap like material, and drape it over the pattern. As that film gets close to the pattern, the vacuum pull sucks the film down tight up against the tool. That’s the first step in the process—it provides a barrier between the pattern and the sand media we’re going to pour into. And since the sand doesn’t touch the tool, the plastic film can slide off the tool, thus forming a vertical wall.”
Because the vacuum vacates air from the sand, holding it in place, sand binding chemicals aren’t required, so the foundry can use very fine grain sand, which produces good surface finish.
The sand is compacted and a second piece of Surlyn film is placed on top of the flask. In other words, the sand is captured between two pieces of Surlyn film.
“That prevents wear on the tool—the tool is good for life,” Depczynski said.
No Problem That Collaboration Can’t Solve
Overall, the Fluent frame casting enjoyed about as smooth a design and execution process as any aluminum casting could hope for. The two companies collaborated seamlessly to arrive at an optimum castable design, but a few challenges had to be addressed.
“Most of our sticking points were how to actually make this moldable,” Depczynski said. “If you look at the original fabrication, you’ll see a lot of the features are basically square or rectangular stock, and the customer wanted to save weight where they could. If you compare the original design to the casting, you’ll see a lot of walls that are thinned out and just thick around mounting locations—we did a lot of back and forth on thinning walls out here and adding thickness there. It was a little challenging arriving at what the customer could live with and where was it possible to reduce weight.”
Then, another puzzle cropped up: Wanting to incorporate as many features as possible into the casting, the engineering teams were challenged to find the best way to add a top bracket that mounts the device’s display to the unit.
“Because it had such an overhang, we had to think about it carefully,” Woodman said. “The foundry’s recommendation was to put some gussets in to give it additional strength. That’s what we ended up doing and it actually gave us the strength we needed to make sure there wasn’t going to be any deflection.
“They were a great team to work with,” Woodman added, “and the machine shop they work with, too. I had a lot of conversations with that fellow, as well. We had some holes we were going to add at an angle [for putting in a cable clamp], but because of the way they machine it, they preferred to go straight in. Working with the machine shop, he made a design change suggestion, modified the model, and actually it was a great idea—it worked very well.”
Thanks to the success of the Fluent frame casting conversion, Woodman is confident his team’s annual Kaizen review of products eligible for cost and performance improvement will spotlight new opportunities to replace other weldment parts with castings.
“I’m working on one now, in fact,” Woodman said. “It’s got like 25 helicoils involved, but I’ll bring in Charlie and Erik to talk about it and get some quotes, and I’ll meet with our engineering group—I just have to show them that we now have a good example with this frame, and we’ve had no issues. They shouldn’t hesitate to let us do more.” CS