Plastic Springs? Better than Steel in Some Cases, Especially Medical
N.Y.-based Spring Pioneer Shines Spotlight on Plastic
It seems springs have been metal since tires round, so it is hard to conceive of any variance from that steadfast rule. Enter in Lee Spring's plastic composite LeePTM spring that stretches the boundaries of what springs can endure, surpassing metal springs in many applications because it offers some incredibly interesting characteristics to the design engineer.
Resistance to magnetism. Check. Resistance to corrosion and conductivity. Check, check. Lighter and one-hundred percent recyclable. Check, check, again.
"It's a challenge to break mindsets really," said Lee Spring Application Engineer Evan Guest, who works with customers and speaks to engineers at trade shows, trying to spread the word about the many benefits of using a plastic spring. "Even a lot of people in the spring industry would not think plastic with springs, so it's breaking the status quo and about making people think and creating awareness."
In fact, anyone in the medical industry Guest speaks with, he mentions the LeeP plastic spring, primarily because of its non-magnetic characteristics, as with MRI and X-ray machines. Such equipment is very sensitive to the slightest bit of magnetism from metals and even though certain metal alloys have reduced levels down to just a trace of magnetism, it's still there and still a problem, said Lee Spring CEO Steve Kempf.
"MRI equipment and X-ray equipment as well, are very sensitive to even the slightest bit of magnetism. And all of these machines have moving parts that generally require springs," Kempf said, adding that medical manufacturers are also interested in the plastic springs for disposable syringes. "That's where it's a one-time use and so they want it to be recyclable, and the spring also might come into contact with the fluid that's being injected. So it's a one-two punch," he said, referring to the recyclability and the non-corrosive characteristics of the LeeP plastic springs.
Guest said he has worked with people in the medical tool industry creating equipment for open-brain surgeries, where magnetism with instrumentation is a factor. Operating tools, such as clamps, require springs that can be under force for a long period of time and not interfere with the procedure. The LeeP composite compression springs, manufactured by injection molding, are designed to perform under load with minimum side thrust. The innovative design results in a high-strength-to-weight ratio through efficient use of materials that yield springs made of Ultem PEI resins in six increasing color-coded strengths: red, orange, yellow, green blue, and violet, with violet being the strongest.
The company chose Ultem, by SABIC Innovative Plastics, to devise its springs because the resin has good memory and is relatively strong and stiff and has excellent corrosion/chemical resistance, such as with strong acids, weak bases, alcohols, ethers, inorganic salt solutions, steams, and weak alkalis. Lee Spring chose to injection mold the plastic springs because extruding them limits the strength, 3D printing them leaves weak layers, and machining it out of a plastic block is inefficient. It was a challenge to injection mold the plastic springs because kinks are left where the mold comes apart.
"Ultimately what we were able to design was a unique way of pulling the mold apart which results in no kinks, just a smooth shape. So we overcame the challenge and we filed for a patent on that," Kempf said.
The plastic springs were in development for three years and have been on the market for about three years now, but getting people interested early on in the design process is still key, both Kempf and Guest said.
"We're constantly in dialogue with health care companies and pharmaceutical companies and medical companies trying to figure out ways to use the plastic springs and fit it into their new product development, but it's a challenge. It's not a technology that is widely accepted yet,' Kempf said.
But creating a unique new product is something the medical industry is interested in right now and is where the LeeP plastic springs can fit in, Guest said. "We're targeting the medical industry big time right now because they want to set themselves apart from the competition."
But medical is far from the only industry where the LeeP plastic springs can help improve products, Guest noted, adding that a valve manufacturer for the marine industry is using the plastic springs because they stand up to the corrosiveness of salt water. "They have also been used with tsunami indicators in the ocean. The metal springs traditionally used corroded and they've replaced them with the plastic springs because they're extremely resistant to corrosion of salt water and other strong acids," Kempf said.
"You've got to remember that all metals at some point, no matter what span of time (long span or short term), they are going to rust at some point. They are going to corrode. But people don't like to hear about rust with medical parts - it just doesn't sound right, and designers keep that in mind because if something sits on a shelf for ten years, it starts rusting. Steel has been great for many years, but for some industries there are some inherent limitations that industries just haven't been able to get around," Guest said.
But Guest and Kempf were quick to point out that plastic springs are not ideal for many applications and that certain elements corrode plastic worse than metal. But for certain applications, within the medical industry in particular, plastic springs can be crucial.
And because Ultem is FDA approved, it can come into contact safely with humans and food. And when Guest is at trade shows, if he hears any design engineers mention "delivery system for medical," he immediately brings up the plastic springs. "As an application engineer it's really about creating more dialogue and an understanding. I like to make sure their engineering department has the correct information and line cards on the product. I'll give it to the buyers, but I really want the engineers to start thinking about it and making sure they have the information on hand to develop some curiosity," Guest said.
With 23,000 springs that the company provides off the shelf, including the LeeP plastic springs, the company stresses that the plastic springs function as a compression spring and will perform under load. "The only real difference between this and the traditional metal spring is usually metal springs are made out of round wire and this (plastic spring) has a rectangular cross section, which is just about getting more material in there to give it more strength," Kempf said. "Plastic is inherently weaker than spring steel and we're trying to use the strongest plastic possible and get as much material as we can in per design to increase the strength."
Another advantage to metal is that the plastic is lightweight. This is especially crucial to other industries targeted for plastic springs, such as aerospace and auto industries. By adding that extra material into the design, the company can get high strength relative to weight, Kempf explained.
Many parts go into the interior of planes that utilize springs, such as the air flow valves, seat buckles, and luggage rack releases. "But when you add up all those springs - every single air flow valve, every single seat button, every single seat back literature pocket - that's a lot of metal and can add up to a lot of weight," Kempf said. "So you can save a lot on weight. That's why the airlines are all turning to these composites. It's all about weight savings and fuel efficiency. We've been talking to them about using these composite plastic springs, but it's not high on their list because they're tiny little springs. But when you add them up across the entire fleet, that's a lot of weight over the course of years and that can add up to a lot of savings."
Early on in the Game
With cutting edge innovation, such as the LeeP plastic spring, Lee Spring is focused on being an engineering-driven company that has capabilities ranging from prototype all the way to mass production, Kempf said. Getting involved with OEM design engineers in the early stages of design is key to produce the best and most cost-effective metal or plastic spring, he said. "We have engineers littered throughout our company in every discipline, including sales. We're engineers. We like to concurrently design with our customers," he said, explaining that it is hard sometimes when customers design a product with a cavity for where the spring should go.
"Whereas if we're with them at the design phase, we can work to produce both the most effective design and the manufacturability to make sure it works for them in all ways," Kempf explained. And a lot of spring companies get a blueprint from a customer and just send it to the factory to make the spring. "But we have a team of engineers that will work on perfecting both the design and the manufacturing process to get the customers the best product," he said.
Guest echoed this point and said many times engineers will design everything else in a product before the spring and then try and squeeze the spring into the design. Like a ball point pen, an engineer might make the pen as skinny as possible and not account for the spring that is the essential piece that gets the pen tip to go in and out. "If they don't allow enough room for that to happen, they just spent all their money on all their molds and yet they can't fit the spring in which is actually the life, the nervous center of the product," Guest said.
Headquartered in Brooklyn, N.Y., Lee Spring (www.leespring.com) was founded in 1918 and has five locations spanning the U.S. and four international locations in the U.K., Mexico, China, and India. The global operations enable Lee Spring to handle the high volume production more cost-effectively for the customer. And unlike many other spring companies that specialize in either low-volume production or high-volume production, Lee Spring offers the full range.
"We have the ability to go from the ultimate in low production and pull from a catalogue of springs we can send to you off the shelf immediately, all the way up to millions and millions of springs," Kempf said. "We have the ability to go with a design engineer from day one to prototype his parts all the way through the mid-range production with hundreds and thousands of parts, to ultimate, high-volume production when the part takes off. Unlike the traditional setup where a lot of engineers have to move from one company to another, during the process we can support all the way up from prototype to high-volume production."
Kempf and Guest are hoping word gets out in key industries about the benefits of the plastic LeeP springs in certain applications and hope they make it into high-volume production some day.
"We're doing as much as we can to get them (plastic springs) out there," Guest said. "To make sure we're breaking the status quo, that it's still the same as a steel spring in the function and that it has some different capabilities other than the steel."
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