Plastics Spring Forward in Medical Devices

Author: 
Bob Michaels
LeeP springs made from Ultem PEI resins have strong load-bearing capacity while solving many problems associated with metal-alloy springs.

Springs are about the most ordinary things in the world. You’ll find them underneath the chassis of your car, on the backside of your garage door, and inside your ballpoint pen. They’re also found in many complex medical device applications. But until now, medical device manufacturers have made metal-alloy-based springs that feature inherent limitations.

In order to address these problems, Lee Spring has developed an alternative to metal helical compression springs that combines the strength of metal with the characteristics of high-performance plastics. Like metal springs, LeeP plastic composite compression springs made from Ultem PEI resins bear loads while minimizing side thrust, according to Subramanya Naglapura, Lee Spring’s global product manager. At the same time, however, they address the inherent weaknesses of metal components, including low corrosion resistance, a high weight-to-strength ratio, high electrical and thermal conductivity, and magnetic properties that interfere with imaging and other ferrosensitive technologies.

“Many plastic materials are relatively weak when compared with spring steel; therefore, traditional spring designs with round cross-sections could not provide sufficient load-bearing capacity for most applications,” Naglapura explains. “Furthermore, while the injection molding process can successfully form accurate and strong plastic parts, it faces challenges in the creation of helical shapes with squared ends and uniformly smooth surfaces.”

In contrast to other plastics, the Ultem PEI used to fabricate Lee Spring’s LeeP components facilitates injection molding for both short and long production runs, Naglapura says. Featuring a slight trapezoid–shaped cross-section to promote manufacturability, these springs have more active material than commonly used round-wire designs. And to maximize the size of their square, flat, load-bearing surface while maintaining a smooth, kink-free design, the springs feature a gradual transition section consisting of variable-pitch coils, active coils with full pitch, and inactive coils. This coil configuration minimizes stress points and enables the injection mold to separate after the part has been formed.

Resistant to an array of chemicals, including strong acids, weak bases, aromatics, and ketones, dielectric-insulating Ultem PEI material is suitable for nonconductive applications, Naglapura remarks. The inert, noncontaminating plastic composite also protects product purity, while its low flammability and low toxicity ensure environmental safety. Designed to fit in standard bore sizes from 0.375 to 1.000 in., the springs feature free lengths from 0.375 to 1.250 in.

“We have been in discussions with several customers about specific applications, including a valve made entirely of plastic that comes into contact with corrosive salt water and a fully recyclable single-use drug-delivery device,” Naglapura says. “But more generally, we see many uses for these springs, including in such target markets as medical instrumentation and imaging and x-ray equipment.”

Lee Spring Co.
Brooklyn, NY
www.leespring.com