 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... |
Open Die and Rolled Ring
Forged Metal Parts Deliver... |
| Castings |
- 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.
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