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Keywords of this article:  injection molding 
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Frog Legs' second generation caster wheels make use of two different long carbon fiber reinforced composite materials from PlastiComp.
Frog Legs' second generation caster wheels make use of two different long carbon fiber reinforced composite materials from PlastiComp.
Photo: 12

PlastiComp Inc. and Frog Legs Inc. have jointly developed a new generation of wheelchair caster wheels made of carbon fiber composite.

Back in 1997, Frog Legs' Founder Mark Chelgren created an alternative to the traditional rigid casters used for the front wheels on wheelchairs that allow them to make tight turns.

Instead of fixed forks that require the wheel to move up and over obstructions – bouncing the rider in the process – Chelgren designed his forks with a patented pivot point and wedge shaped shock absorber that allows them to move over impediments in an arc path, functioning much like airplane landing gear, smoothing out the ride.

"An arc is a much more efficient movement," said Chelgren. "But its compression zones are not linear, so instead of a traditional cylinder shaped shock absorber, Frog Legs uses a wedge shape to better handle the differential loadings from front to rear."

Testing performed by the Center for Industrial Research and Service at Iowa State University showed that 80% of the vibrations wheel chair riders experienced were created by the front caster wheels and that Chelgren's design improvements reduced those vibrations from being transmitted to the wheelchair frame by 76%.

The challenge this time is to lighten the products, because people with disabilities are greatly affected by any additional weight, he said.

"This time we looked at a different type of manufacturing process than machining aluminum. Injection molding allowed us to have a much more complex shape and carbon fiber composites really gave us advantages in what our design parameters could be," Chelgren explained.

"If the idea is to take an aluminum part and make it using carbon fiber, I think people are missing the boat," he continued. "A carbon fiber part and an aluminum part shouldn't look identical, instead you need to design for the material and process combined. It would be almost impossible to machine or forge the bottom part of our new fork design."

Frog Legs’ second generation caster wheels make use of two different long carbon fiber reinforced composite materials from PlastiComp. Machined aluminum has been replaced by a long carbon fiber reinforced polyamide (PA) 6/6 composite in the wheel forks and a long carbon fiber reinforced thermoplastic polyurethane (TPU) in the wheel hub.

PlastiComp custom developed the reinforced polyurethane composite to bond with the urethane used for the outer rolling surface of the wheel. "Using thermoplastic polyurethane for the hub allows us to obtain a chemical bond," said Chelgren.  "With an aluminum hub there are dissimilar materials that never fully bond and can slip."

Changing from aluminum to long carbon fiber reinforced composites reduced the weight of a pair of Frog Legs caster wheels by 33% – over 280g lighter.

The new design and materials passed RESNA (Rehabilitation Engineering Society of North America) industry standard testing that put them up against million cycle impact and drop forces. Frog Legs went on to test them to failure to have data to compare with traditional metal caster wheels.

PlastiComp's application design and performance analysis services assisted the product development cycle. "Overall the efficiency of our product is now much higher," said Chelgren. "We could see from the computer modeling what the benefits were going to be. I don't think we could have developed a product nearly as good as the one we got without that assistance."

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