Casting Complexity in Conformable Tank

Michigan’s REL Inc. aims to improve natural gas storage with an innovative aluminum casting produced via a modified low pressure permanent mold process.

Nicholas Leider, Associate Editor

(Click here to see the story as it appears in the February issue of Modern Casting.)

Metalcasters are well aware of the premium placed on energy efficiency across all industrial sectors. Whether the motive is commercial, environmental or regulatory, metalcasters know sacred cows cannot get in the way of progress. Casting buyers and end users want the highest quality performance at minimal cost, so suppliers continually seek ways to deliver improved mechanical and physical properties.

One challenge faced by REL Inc., Calumet, Michigan, a material process development company with casting capabilities, was repackaging fuel storage. Compressed natural gas (CNG) is commonly used to supplement traditional gasoline-powered engines. But improving the storage of CNG—most commonly in long cylindrical tubes—may make the cheaper, cleaner-burning fuel a commercial competitor to standard gasoline.

REL Inc., through a grant from a U.S. Department of Energy program, cast a technically viable conformable tank in an aluminum 206 alloy. The so-called conformable tank, cast in aluminum via a low pressure permanent mold process, better uses allowable packaging space to store as much as 35% more CNG. The shape of the tank also improves flexibility in the placement of the fuel, allowing it to be more commercially viable than the torpedo-shaped tanks currently in use. REL is now partnering with a major CNG supplier and a local university in a three-year, $2.1 million project to prove the technology is commercially viable.

Public Investment Kick-Starts Development

REL Inc., with much of its experience in squeeze casting, was capable of infiltrating metal into ceramic preform structures, with gaps as small as 10 microns. Adam Loukus, vice president, REL, saw potential in casting a tank with a thin internal structure to minimize the amount of tank material while still having the ability to hold high-pressure gas.

“Conventionally, you use cylindrical geometry when storing gas at high pressure,” said Josh Loukus, president, REL Inc. “That’s nature’s shape. Put those cylindrical tanks in the trunk, and as soon as you do that, you can’t put your groceries in there. With a conformable design, with a box, you can pack fuel more efficiently inside a vehicle.”

With the idea of a conformable CNG tank, REL applied for funding under the U.S. Department of Energy’s Advanced Research Projects Agency-Energy (ARPA-E) program. In July 2012, REL Inc. was awarded a $3 million grant to pursue its research.

The final goal being a design improvement from the cylindrical tanks, REL Inc.’s design team started with a squared structure. The corners were rounded to minimize high stress points. Through numerous iterations—including designs with pillar and bubble shaped internal structures—the REL Inc. team eventually pursued a path that already existed in nature.

The so-called Schwarz P surface features two intertwined cubic lattices. This design, which consists of two fully independent and intertwined cavities, features a high surface-to-volume ratio. Such a design is found in the skeletal plate of sea urchins, a fact that reinforced the REL Inc. team’s belief it was a naturally efficient design. The constant curvature, though, meant the Schwarz P design was especially difficult in simulation.

“Because of the constant curvature of the internal structure, it is extremely difficult to model efficiently.  The structure is such that we need to develop simplifying techniques to model and analyze the final tanks.  It almost cripples the CAD and FE programs we use,” Adam Loukas said. “While the structure is efficient, it’s computationally taxing.”

The final casting would be a single aluminum component, but it would require a number of sand cores, which could be stacked atop one another to produce the complex cavity. REL Inc. produced the majority of these cores in house, though some were 3-D printed for prototype models.

Castings to Meet Demanding Parameters

The tank needed to operate at 3,600 psi, though it had to withstand a burst pressure of 8,100 psi. This demand placed a premium on uniformity in casting. REL Inc. had some experience with the low pressure permanent mold process, which was appealing because it could reduce turbulence in filling the mold, but the company partnered with Eck Industries Inc., Manitowoc, Wisconsin, to produce the first version in 7075 aluminum, also produced in a low pressure permanent mold.

Those first articles proved promising enough for REL Inc. to invest in a low pressure permanent molding line at its facility in Calumet. Engineers then opted to cast the tanks in A206, one of the highest strength structural alloys.

“It wouldn’t do us much good to have great properties at the top of the tank if we have trouble near the gates,” Adam Loukus said. “It required a lot of process improvements and changes to the low pressure process. We needed to optimize the gating. We needed to introduce the metal in the most uniform manner possible.”

Once REL Inc. was able to produce acceptable castings, processing after solidification was relatively minor. The complex cores needed to be burned out, the gates removed and the tanks heat treated.  

Pursuing Commercialization

Proving the technology works is one thing; establishing a path to commercial feasibility is another. REL Inc., with the success of its ARPA-E project, sees the first half of the equation solved; the cast A206 aluminum tank can improve CNG storage. Now, the goal is to produce the tanks in a commercially viable way.

“It’s important to find resources to make that transition,” Adam Loukus said. “Who’s going to help you go from proving out the technology to getting prototypes out on the road?”

Now that REL Inc. is approaching the end of its participation in ARPA-E, the company has reached a partnership with Houston-based Southwestern Energy (SWN), the third largest producer of natural gas in the continental U.S. Improving CNG storage and use in automobiles and trucks holds obvious commercial appeal for SWN, which led to the company’s $2.1-million, three-year investment in December 2014.

“We want to get the tank under the bed,” said John Gargani, vice president, SWN. “Tanks today sit in the bed and take up a lot of the space, which is a drawback to the consumer.”

Michigan Technology Univ., Houghton, Mich., also will participate in the project by helping to optimize material used for the tank. MTU will also conduct testing of the tanks after they are cast by REL Inc.

“There will be mechanical engineering and materials science students and faculty working together at Tech,” said Greg Odegard, professor of computational mechanics, MTU. “Once the tank is fabricated, REL will give it back to us and we’ll test it—put it on a pickup truck, mount it, drive it around. REL and Michigan Tech are positioned ideally to develop this tank.”

With its own fleet of CNG vehicles, SWN has the potential to be REL Inc.’s first customer. A more environmentally friendly fuel than gasoline, CNG is also more affordable, costing drivers between $1-$1.50 per gallon. While recent decreases in gas prices limit cost savings in the near future, the CNG’s cost savings compared to gasoline are expected to be a major advantage of CNG-fueled vehicles in the long-term.

“We have proven the technology is viable,” Adam Loukus said. “The next step is a matter of making it competitive. I think this project has shown a lot of potential. Now it’s about improving our processes and materials so these castings are the best we can produce.”   

ncountering a scenario in which you are forced to suddenly and immediately suspend melting operations for an extended period can be a death sentence for many metalcasting facilities. Small to mid-size businesses are the backbone of the industry, but many do not survive when forced into extended downtime. One disaster-stricken metalcaster, however, found resilience through its own perseverance and a circle of support from peers, friends, suppliers, teams from installation and repair providers, an original equipment manufacturer and even competitors.
Tonkawa Foundry, a third-generation, family-owned operation in Tonkawa, Okla., was entering its 65th year of operation this year when a significant technical failure ravaged the power supply and melting furnaces on January 17. Thanks to the textbook evacuation directed by Operations Manager Carrie Haley, no one was physically harmed during the incident, but the extent of emotional and financial damage, and just how long the event would take Tonkawa offline, was unclear.
Tonkawa’s power supply and two steel-shell furnaces would have to be rebuilt. No part of the reconstruction process could begin until the insurance company approved removal of the equipment from the site. The potential loss of Tonkawa’s employees and customers to competing metalcasters seemed inevitable.
Within two days of the incident, repair, installation and equipment representatives were on site at Tonkawa to survey the damage. Once the insurance company issued approval to begin work, the installation team mobilized within 24 hours to remove the equipment and disassemble the melt deck.
Since the damaged equipment was installed in the 1980s and 1990s, Tonkawa and an equipment services and repair company quickly strategized a plan and identified ways to enhance the safety, efficiency and overall productivity of Tonkawa’s melt deck.
“The most critical issue was for our team to organize a response plan,” said Steve Otto, executive vice president for EMSCO’s New Jersey Installation Division. “We needed to arrive at Tonkawa ready to work as soon as possible and deliver quickly and thoroughly so they could get back to the business of melting and producing castings, and minimize their risk of closing.”
Several years after Tonkawa’s melt deck was originally installed, an elevation change was required to accommodate the use of a larger capacity ladle under the spout of the furnaces. Rather than raising the entire melt deck, only the area supporting the furnaces was elevated. As a result, the power supply and workstation were two steps down from the furnaces, creating a number of inconveniences and challenges that impacted overall work flow in the melt area. Additionally, the proximity of the power supply to the furnaces not only contributed to the limited workspace, but also increased the odds of the power supply facing damage.
The damage to the melt deck required it to be reconstructed. It was determined to be the ideal opportunity to raise the entire deck to the same elevation and arrange the power supply, workstation and furnaces onto one level. The furnace installation company provided the layout concepts, and with the aid of Rajesh Krishnamurthy, applications engineer, Oklahoma State Univ., Tonkawa used the concepts to generate blueprints for the new deck construction. The results yielded a modernized melt system with an even elevation, strategically placed power supply, enhanced worker safety and increased operator productivity.
“Eliminating the steps and relocating the power supply farther from the furnaces was a significant improvement to our melt deck,” Tonkawa Co-Owner Jim Salisbury said.
Within four days of insurance company approval, all damaged equipment had been removed and shipped for repair.
The insurance company required an autopsy on the damaged furnace before any repair work could begin. The forensic analysis was hosted by EMSCO in Anniston, Ala., in the presence of insurance company personnel, as well as an assembly of industry representatives from the companies who had received notices of potential subrogation from the insurance company.
Tonkawa’s furnace was completely disassembled while the insurance company’s forensic inspector directed, photographed, cataloged and analyzed every turn of every bolt on the furnace over a nine-hour workday. The coil was dissected, and lining samples were retained for future reference.
While the furnace sustained extensive damage, it did not have to be replaced entirely.
Structural reconstruction was performed to address run-out damage in the bottom of the furnace, a new coil was fabricated and the hydraulic cylinders were repacked and resealed. Fortunately, the major components were salvageable, and ultimately, the furnace was rebuilt for half the cost of a new furnace.
“The furnace experienced a significant technical failure,” said Jimmy Horton, vice president and general manager of southern operations, EMSCO. “However, not only was the unit rebuilt, it was rebuilt using minimal replacement parts.”
Though work was underway on the furnaces, Tonkawa was challenged with a projected lead time of 14 weeks on the power supply.
When accounting for the three weeks lost to insurance company holds and the time required for installation, Tonkawa was looking at a total production loss of 18-20 weeks. From the perspective of sibling co-owners Sandy Salisbury Linton and Jim Salisbury, Tonkawa could not survive such a long period of lost productivity. After putting their heads together with their furnace supplier, it was determined the reason for the long turnaround on the power supply could be traced to the manufacturer of the steel cabinet that housed the power supply.
The solution? The existing cabinet would be completely refurbished and Tonkawa would do the work rather than the initial manufacturer. This reduced the 14-week lead time to just five weeks.
Tonkawa is the single source for a number of its customers. Although lead-time had been significantly reduced, the Tonkawa team still needed a strategy to keep the single source customers in business as well as a plan to retain their larger customers.
Tonkawa pours many wear-resistant, high-chrome alloys for the agriculture and shot blast industries. Kansas Castings, Belle Plaine, Kan., which is a friendly competitor, is located 50 miles north of Tonkawa. Kansas Castings offered Tonkawa two to three heats every Friday for as long as it needed.
“We made molds, put them on a flatbed trailer, prayed it wasn’t going to rain in Oklahoma, and drove the molds to Kansas Castings. We were molding, shot blasting, cleaning, grinding and shipping every Friday,” Salisbury Linton said.
Others joined the circle of support that was quickly surrounding the Tonkawa Foundry family.
Modern Investment Casting Corporation (MICC) is located 12 miles east of Tonkawa in Ponca City, Okla. Though MICC is an investment shop and Tonkawa is a sand casting facility, MICC’s relationship with Tonkawa dates back years to when Sandy and Jim’s father, Gene Salisbury, was at the helm.
“Gene was always willing to help you out,” said MICC owner, Dave Cashon. “His advice was invaluable for us over the years, so when the opportunity arose to support Sandy and Jim, we volunteered our help.”
 MICC offered to pour anything Tonkawa needed every Friday in its furnace. Tonkawa brought its alloy, furnace hand and molds, while MICC provided its furnace and a furnace hand for three heats. Many of the specialty parts Tonkawa produces were completed with MICC’s support.
When Salisbury Linton approached Cashon and asked him to issue her an invoice to cover the overhead Tonkawa was consuming, Cashon told her if she brought in six-dozen donuts every Friday morning they’d call it even.
“We’re all kind of like family,” Cashon said. “We’re all part of the same industry and though we may be friendly competitors at times, you don’t want to see anybody go through what they’ve gone through and it could have just as easily been our furnace that failed. While we all take the appropriate measures and perform maintenance to prevent these scenarios from occurring, they unfortunately still occur from time to time in our industry.”
Tonkawa had recently added steel work to its menu of services and Central Machine & Tool, Enid, Okla., was able to take Tonkawa’s patterns and fulfill its steel orders so it would not fall behind with those customers, while CFM Corporation, Blackwell, Okla., took three of Tonkawa’s employees on a temporary basis and kept them working during the downtime. Additionally, a couple of Tonkawa’s major suppliers extended their payables terms.
Thanks to Tonkawa’s suppliers, friends and its personnel’s own passion, persistence and dedication, the business is up, running and recovering—placing it among the few shops of its size to overcome the odds and remain in business after facing calamity.
 Nearly eight months after that devastating Saturday evening in January, Salisbury Linton reflected on the people and events that helped Tonkawa rise from the ashes. “We certainly would not have the opportunity to see what the future holds for Tonkawa if it weren’t for all the kind-hearted people who cared about what happened to us. Everyone still checks in on us.”