CTI Improves Cyclotron Ring Production with Forging Process

The growing diversity of medical diagnostic systems and related equipment has led to an increasing demand for forged parts in the medical industry, such as large rings and hubs for MRI scanners, PET scanners, cyclotrons, and other equipment. Application requirements call for these parts to be of the highest quality, while budgets continue to call for cost-efficient production. CTI Molecular Imaging (Knoxville, TN), in a joint effort with Scot Forge recently met both of these requirements by converting a cast component to a forging in the production of steel rings and hubs for its Eclipse Cyclotrons.

CTI is the leading provider of positron emission tomography (PET) scanners and cyclotrons. Established in 1983, the company has won numerous awards for its advances in molecular imaging. CTI provides total solutions for the PET industry with its integrated line of scanners, cyclotrons, molecular probes, detector materials, and support services-all aimed at helping physicians diagnose diseases earlier and more accurately than with traditional imaging technologies. A cyclotron is essentially a particle accelerator for the production of F-18, the radioactive biomarker that is injected into patients undergoing PET scans for early cancer detection.

A main component of a cyclotron is a steel ring, providing a circular arena wherein proton particles are accelerated at high speed to generate an 11 meV proton beam essential for the production of F-18. Each 57" OD x 47" ID, 12" thick ring is plated with nickel and copper to convey RF energy as well as to optimize the high vacuum environment needed for proper system operation. Two steel hubs, 57" OD, 17" long, also help provide a vacuum sealed environment. The rings and hubs comprise the flux path for the electromagnet and must have a tightly controlled carbon content.

Until recently, the metal rings and hubs in CTI cyclotrons were cast, but problems with porosity in the castings were causing expensive rework during the machining and plating. Andy Williamson, Mechanical Design Manager for CTI, elaborates. "The surface finish was often too porous for the plating to seal properly without special rework," he says. Also, pinholes in the surface invited unwanted contaminants.

Bringing the cast rings up to specification involved "excessive filling of holes and welding," Williamson says. "Rework was necessary during both the machining and plating operations, causing cost issues and delays. We needed another solution."

Looking to replace the casting with an alternative metalworking process, CTI did extensive research into using forgings. Forging is becoming an increasingly desirable alternative to casting in the production of large medical components. The forging process, regardless of section size, guarantees internal soundness, whereas the internal structures of cast parts weaken in larger-sized parts, resulting in porosity. And with advancements in forging technologies and methods, forged parts are now available in a variety of shapes-rivaling cast part shapes-at less cost than in the past. CTI quoted the parts from several forging companies, which reinforced these findings. Williamson says, "we concluded that the overall cost of a forged and machined part with plating would be 10 percent less expensive than what we were paying for a cast, machined and plated part."

Comparative Analysis

When compared to castings open die and rolled ring forged metal parts deliver:

• Directional grain flow and superior final part strength
• Structural integrity and product reliability
• Reduced process control and inspection requirements
• More predictable response to heat treating

CTI chose Scot Forge, of Spring Grove, Illinois, to supply the forged parts after Williamson and Purchasing Manger Juel Hensley visited Scot Forge's plant. "Scot Forge had by far the best capability to produce the rings and hubs," Williamson says. The 100-year-old ISO 9001:2000 certified forging company uses thousands of tools, torch cutting, sawing, machining, and presses that are custom-designed by their own engineers to produce uniquely shaped, repeatable parts at competitive prices.

Tom Schwingbeck, Jr., Director of Technical Sales and Services at Scot Forge, states that "our forging expertise, extensive capabilities, and proactive approach with CTI to understand their requirements and expectations, helped to cost effectively convert castings to forgings while supplying a superior product". Scot Forge produced each Eclipse Cyclotron ring to near net shape by forging a wrought billet into a rough donut on a 1,250 ton hydraulic press, then rolling the part on a Wagner ring mill to create a seamless rolled ring with highly desirable mechanical properties and metallurgical soundness.

Porosity is prevented through this forging process because, in the forging of a heated cast ingot, the ingot is consolidated, providing a sound center component. The coarse grain cast structure is broken up and replaced by a fine grain wrought structure providing a sound center product with excellent structural integrity.

Following the forging and rolling process, each ring is rough machined at Scot Forge to 57-1/2" OD, 46-3/4" ID and 12-1/4" thick. The hubs are forged to near net shape on a 3,000 ton hydraulic press, then rough machined to 59" OD x 10-3/4" long, stepped down to 40-1/2" OD x 9-7/8" long. Finally, the rings and hubs are normalized to ensure a uniform microstructure with high dimensional stability for finish machining.

Another important specification met by Scot Forge was the low carbon material required for the ring and hubs. "In order to keep the cyclotron magnet power low, the steel needs good magnetic permeability, so it must have a very low carbon chemistry," notes Williamson. The CTI specification called for a steel with a carbon content not above .10%. "Scot Forge's metallurgical engineers worked with us to meet this specification," Williamson says. "In fact, they exceeded the spec, giving us a specially formulated 1008 (.08-.10 carbon) material."

CTI has now tested the forged ring and hub prototypes with excellent results, and the parts have been qualified. Plating has been applied to the finish machined surface with no sealing problems. Based on these results, CTI recently placed eight production orders with Scot Forge.

"Just six years ago, we didn't consider forgings because castings were less expensive and could accommodate more configuration features," says Williamson. Now, companies like Scot Forge are offering more complex forged parts than ever before available, at lower overall production cost. Consequently, CTI will be using forged rings and hubs in all of its future cyclotrons.