Medical Component Manufacturer Meets Design Challenges with Innovative Engineered Compounds

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Bearings

By formulating proprietary materials to withstand a variety of chemicals, fluids, and high- and low-temperature environments, and then molding and machining tight-tolerance specialty parts from these materials, the company provides cutting-edge solutions for the medical and semiconductor equipment industries.

While keeping up with current trends in medical machinery and equipment, engineers at Trelleborg Sealing Solutions need to know the latest high-tech developments in the engineered rubbers and plastics that are used for these applications. Fortunately, the company is part of a large corporation that has chemists in research and development departments all over the world. Quite often, the firm's chemists will develop materials on their own for a particular application, adding to the company's list of over 2,000 active compounds for precision seals, bearings, and tubing.

Trelleborg Sealing Solutions is a division of Trelleborg AB, headquartered in Sweden. Its West Coast headquarters for medical parts is situated in Torrance, California, about 20 miles south of Los Angeles. True to its credo of being a "global company that acts locally," Trelleborg has nine marketing companies throughout the United States and Canada to service North America. The Sealing Solutions division also operates a semiconductor segment that makes seals and bearings for most of the big chip-making equipment manufacturers, including Applied Materials, Lam Research, and Novellus.

The medical segment of Trelleborg's Sealing Solutions division manufactures a wide variety of seals, bearings, and tubing for medical products that include pumps, cylinders, actuators, diagnostic equipment, dialysis machines, drug delivery systems, home-care oxygen compressors, orthopedic products, endoscopes, blood separators, and fluid analyzing equipment.

"I think one of our strongest areas is material compound development because of our R & D facilities worldwide," explains Jerry Zawada, medical devices segment manager. "Our expertise processing rubber and plastic materials, specifically PTFEs (polytetrafluoroethylenes), is extensive since we pioneered some of the materials and processes in the 1950s. We have developed quite a few proprietary compounds using PTFEs, PEEK (polyetheretherketone), perfluoros (perfluorocarbons), fluorocarbons, silicones, EPDM (ethylene-propylene diene monomer), and nitriles. And we have specialized expertise in bonding PTFE to rubber and plastic and metal parts."

Design and Engineering Considerations

At its regional marketing facilities, Trelleborg enlists the services of its application engineers and sales engineers to assist clients in those geographic areas. The application engineers work with clients to design and engineer a part, and then work with manufacturing engineers at their plants.

"Our engineers first look at our part portfolio to see if a standard part will work," says Zawada. "If not, we can design a custom part for a client. For example, if a client wants to flow a fluid through some type of device, we can add seals and bearings to the device and create tubing manifolds. Our design engineers can handle a project from its inception, and in fact, we like to be involved from the very beginning to make sure a design is manufacturable. We can make 3D models for most parts, and can check stresses and component extrusion ahead of time using FEA (finite element analysis)."

When solving a design challenge, an initial requirement would be meeting federal FDA and USP Class 6 guidelines to ensure that nothing harmful will leech out to the human body. The seal, bearing, or tubing, therefore, must stand up to chemicals and fluids like saline solution, blood, or pharmaceuticals. And the parts must have a wide range of fluid handling capabilities, should not degrade easily, and must be able to handle stringent friction requirements.

Wear testing is also very important. Trelleborg begins by asking a variety of questions about the application, including the types of fluids to be processed, pressures, temperatures, and part sizes. Surface finishes are also extremely important with medical devices and equipment. Consequently, the smoothness of the surface that the bearing or seal will be rubbing against is taken into account. Lastly, engineers must decide if an existing material will work or if they need to create one for the application.

Rust- and Corrosion-resistant Thermoplastic Bearings

Thermoplastic bearings are an advantageous addition to a piece of medical machinery because they are inert near critical fluids. Although they don't have a high load capability, thermoplastic bearings are less expensive and won't rust or corrode. Therefore, they can be used in areas where fluids could attack a metal substance. In many medical situations, either saline solution or blood passes through the machine, so plastic bearings hold up well in these environments. In other environments, like in an MRI machine, there can't be any metal near the machine, since it is basically a very large electronic magnet. So the bearings Trelleborg supplies for the MRI machines are made of low-friction thermoplastics.

Trelleborg is also involved in producing seals and bearings for hydraulic cylinders and actuators that are used on electronically-controlled hospital beds. In one instance, a patented Turcite® Slydring prevents metal-to-metal contact between the piston and the bore. Turcite® is a reinforced PTFE, capable of handling moderate side loads while providing good friction-reducing properties. The Slydring added damping (noise reduction) to the system and removed the stick-slip, thereby providing the performance the customer was requesting.

"These hospital beds often have multiple cylinders that are used to lift the patient up and down," Zawada explains. "We provide seals and bearings for several different types of bed cylinders and also produce them for cylinders for exercise equipment. In addition, our plants manufacture seals and bearings for a wide array of different types of pumps, as well as tubing for peristaltic pumps. And our company has been at the forefront of technology in helping to bring electronically-controlled shock absorbers to the prosthetic limb market. The challenge with the shock absorbers is to develop a zero-leak sealing system while limiting friction and stick-slip forces."

The company's Sealing Solutions division also builds bearing components for blood separators—machines that are used to separate a patient's blood platelets by means of a spinning centrifuge. Quite often, after a surgical operation, medical personnel will inject the separated platelets into the incision area, which helps the healing process. This device separates the platelets by using a bearing component to provide rotary fluid exchange between the collection tray and the separator unit. The bearing operates at room temperature and rotates at 1,680 rpm without coming into direct contact with the blood. In one situation involving a blood separator, a client wasn't getting the field-performance results that it wanted from another company's device.

"We developed thermoplastic Durobal® roller bearings for this application encased in a specialized enclosure," Trelleborg's manager recalls. "The existing bearing not only experienced unacceptable field failures, but it was too costly. The Durobal® bearing design consisted of PBT (polybutylene terephthalate) inner and outer portions with a PBT compounded cage and 302 stainless steel balls. Our design enabled our parts to perform better in the field. So significant cost savings were achieved by going to a near net-shape molded part and then machining in the critical dimensions—versus the fully-machined original. We saved the customer over 50% per part and additional savings from reduced field failures."

Durable, Leak-free Seals

Trelleborg also manufactures a low-friction PTFE-based Turcon® seal for a bone drill used in dental or medical surgery. The compound has very low-friction capabilities and tends to wear very well. Rubberized materials, on the other hand, would soften and have a tendency to degrade as a result of the high temperatures—anywhere from 250 to 300 degrees Fahrenheit—emitted by these drills.

The PTFE materials are also used in cryogenic applications to seal liquid nitrogen and oxygen. Cryogenic applications typically require seals to hold pressures of up to 100 psi, and temperatures from -300 degrees Fahrenheit to 150 degrees Fahrenheit. One PTFE seal is for a quick-disconnect coupling used for portable oxygen units that convert liquid oxygen into a breathable form, according to Zawada. The seals can withstand cryogenic temperatures and are FDA-compliant, leak-free, and durable, while repeatedly exhibiting low friction.

The Sealing Solutions division also handles rapid prototyping, in which a dozen or so machined plastic prototypes can be turned around in a few days. In some cases, the company can go from 3D drawing to prototype part in one day, using a Z Corporation Rapid Prototype machine to fabricate the part out of a grainy, starch material. It's not a very durable material, but a client can use it to visualize how the actual part will look.

Trelleborg's medical parts are manufactured at one of three ISO 9001-certified plants in the United States: Hudson, Mass.; Broomfield, Colo; and Ft. Wayne, Indiana. The plants manufacture rubber seals using injection and compression molding for a variety of engineered compounds, including Trelleborg's own Isolast® compound. These perflouro compounds are used when elastomers are needed to seal hostile chemical and processing environments, such as anesthesia gases. Most of the PTFE and UHMWPE (ultra-high molecular weight polyethylene) seals are made by compression molding a piece of material, and then machining parts on a CNC lathe. The machining helps hold tight tolerances down to +/- 0.001-inch. After the machining and molding processes, finishing processes and assembly work are performed in-house.

Anti-microbial Tubing is New Frontier

Silicone and other materials are also extruded to make tubing with precise tolerances. Zawada says that silicone tubing tolerances can be held down to +/-0.003-inch on the ID and with wall thicknesses down to 0.007-inch, +/-0.002-inch.

"One area we're working on right now, which is very new, is anti-microbial tubing," Zawada reports. "We add silver to the compound, which attracts the bacteria and then kills it. This might be used for a catheter that is inserted into a patient for an extended period of time, so it needs to be kept as free of bacteria as possible.

"One of the areas we handle routinely—that many of our competitors do not handle—is overmolding silicone tubing with liquid-injection-molded silicone to add T and Y connections. This eliminates the use of tie straps and other fasteners that wouldn't be as stable as a molded connection, and it prevents leakage. Another thing we're able to do on the inside diameter is make the whole part one precise dimension with a completely smooth surface throughout to prevent friction loss and bug traps."

High-performance liquid chromatography (HPLC) pumps are high-pressure plunger pumps used in analytical chemistry equipment. They handle a wide range of liquids, including abrasive saline solutions, and operate in the 3,000- to 6,000-psi range. Hence, the seals need to remain leak-free in spite of high pressures, while sealing on very smooth sapphire plunger rods.

One engineering challenge for a client's HPLC pump brought together several different solutions requiring minimal friction and long life. "First off, we had to make sure that the expensive plunger rods had a long life, so our seal could not scratch them at all," Zawada affirmed. "Our solution for the pump was to use our Variseal™ FW with an extended heel, Zurcon® Z80, and heavy-load Slantcoil® spring, all of which is our own proprietary technology. The seal, made from UHMWPE, provided excellent wear resistance and long life, while the Slantcoil spring delivered the force necessary to create a consistent leak-free seal with minimal friction. We were able, therefore, to reduce their maintenance costs by thousands of dollars."

Chemical Compatibility, Tolerances Are Major Issues for Semiconductor Seals and Bearings

Trelleborg's semiconductor segment manager once worked for a company that performed work for the aerospace industry. He said they routinely worked on parts for the Space Shuttle and the Space Station. "The aerospace parts had paramount importance and critical capabilities, but the tolerances on those parts were nowhere near the requirements for the semiconductor industry," insisted Mark Russell, Trelleborg Sealing Solutions' semiconductor segment manager. "In fact, I think we have to turn out tighter tolerances than any industry in the world."

Trelleborg's seals and bearings for semiconductor machinery are designed, engineered, and manufactured—whether they are machined or injection molded—based on customer applications. A variety of requirements, ranging from chemical compatibility to temperature and pressure, friction and load, and travel speed, must be considered. "One of the most impressive things about our company is that I think we do a great job of analyzing a client's application," says Russell. "We're not interested in looking at an existing seal that may not be working properly. We want to understand the part's application, the hardware, and other factors and then engineer our own part."

Another reason why tolerances are important in semiconductor work is that chip-making machinery operates in a vacuum environment. The lower the vacuum, the cleaner the environment has to be. The smallest contaminant can cause problems for a particular circuit or even ruin the entire chip. In 1992 and 1993, particle sizes were measured in sizes of 5 to 10 microns, according to Russell. In 1995, sub-microns started to be used for chip making. Now it's based on nanotechnology.

"In one situation, a chipmaker uses a 300mm processing wafer that holds hundreds of chips on it," Russell explains. "Across the surface, about 12 inches, they control the flatness on the wafer to less than a half- micron. So our seals and bearings must also be very precise. Quite often, our bearings are used to position or index a chip wafer in a very precise way, usually in microns or sub-microns, so our components have to able to work precisely when moving their parts into position."

Another big strength is Trelleborg's ability to formulate and work with many different engineered rubber and plastic compounds. In semiconductor chip manufacturing, chemical compatibility is imperative because processing chemicals are very exotic. Therefore, the company must use special materials for its parts.

"One of the problems with metals is that the processing chemicals want to attack the metal, so they have to be coated with special coatings," Russell maintains. "These coatings aren't always ideal from an environmental standpoint, which add additional costs to the process. Plastics are better for these types of applications because of the chemical resistance, and the added benefit of minimal part weight."

For more information on Trelleborg Sealing Solutions, visit www.tss.trelleborg.com

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