Finding the Right Fit with Elastomers

Rubber Fabrication

Often the best, and sometimes the only, solution to a challenging manufacturing puzzle involves the use of custom rubber parts, components, or assemblies.

By Mark Shortt
Editorial Director, Design-2-Part Magazine

Say you need a tight, custom-shaped seal for a product you're manufacturing. Though you may have options for which material to use, you have a pretty good idea of what will work best in most situations. Or, you might be responsible for a multi-part component that provides cushioning while withstanding harsh chemical environments. Again, you've got choices, yet you know what one of the parts will be made of. But when you're looking for a part that bends without cracking, while surviving prolonged exposure to sunlight, you know you've got an easy decision. When flexibility and functionality are needed in a single part, rubber is often the best answer.

"Because of the flexible and forgiving nature of rubber and sponge and the wide variety of types, it is an excellent material for a wide variety of applications," says John Savickas, president of Interstate Gasket, Leicester, Massachusetts. The many applications for precision rubber parts include custom seals, grommets, tubes, bushings, diaphragms, valves, gaskets, and washers. They also include O-rings, anti-vibration isolation mounts, and rubber parts bonded to metal or plastic substrates. Custom rubber parts are used in numerous markets, such as the automotive, medical, electronics, aerospace, defense, fluid power, plumbing, computers, and printing industries.

Rubber FabricationOne of the biggest advantages of using rubber parts is that custom compounds can be created to meet a wide variety of specifications related to chemical and abrasion resistance, flexibility, and resistance to extreme heat or cold, according to Dale Wolford, A & D Rubber Products Company, Inc., Stockton, California. Also, rubber can be molded into virtually any shape and, because of its elastomeric quality, "has the ability to form and seal better than most materials," says Steve Shapiro, president, California Sealing Devices, Northridge, California. As a result, Shapiro says, rubber compounds can be formulated to meet customers' unique design requirements.

Choices of rubber (elastomer) compounds range from natural rubber to high-performance synthetic elastomers such as silicone and fluorocarbon-based elastomers. Elastomers such as fluorocarbon - typically used in gasoline, diesel fuel, and petroleum oil applications - and nitrile (often used with jet fuel and petroleum-based hydraulic fluid and power steering fluid) exhibit superior oil resistance, as do fluorosilicone and polyacrylate. Silicone rubber, on the other hand, does not have high oil resistance, but combines a number of performance characteristics that are said to be unique.

According to William Stockwell, president of Stockwell Elastomerics, Inc. (Philadelphia, Pa.), silicone rubber's combination of resilience, high temperature stability, and general inertness are unavailable in any other elastomer. Silicones are generally unaffected by extended exposure to temperatures from -100F to 500F, and are also unaffected by aging and degradation from sunlight and other environmental exposure, he notes. Because of these unusual properties, silicone rubber products are used in a variety of industries in a number of interesting ways, he says. Finished part configurations include molded components, extruded profiles, and die-cut components.

"Silicone rubber products are used in medical diagnostic equipment such as glucose monitoring devices, home renal therapy (kidney dialysis) units, blood analyzing units, incubators, and infusion pumps," says Stockwell. "They are also used in analytical instrumentation, chromatography, moisture analysis, and other special testing equipment that is used to support the food and pharmaceutical industries."

Silicone rubber is readily available in closed-cell sponge, open- and closed-cell foam, and solid sheets, as well as in a variety of adhesives and gels, Stockwell adds. Fillers can be added to make silicone rubber electrically or thermally conductive, and thus able to solve design issues requiring components that provide EMI shielding, ESD grounding, or thermal dissipation. In addition, portable electronic items, such as rugged hand-held computing devices and data acquisition devices, can be made more "drop-proof" by using silicone sponge. The reason, Stockwell explains, is that silicone can maintain resiliency over a broader temperature range than can traditional shock isolation foams. Furthermore, new liquid silicone rubber (LSR) technology has made many silicone foam and sponge products flame retardant, and thus able to meet the requirements of UL94V0 and UL94HF-1. Flame retardant properties, he says, are often specified for parts used in telecommunications base stations, computer and peripheral hardware, and aerospace components.

Another advantage of rubber is that its physical properties allow it to absorb tolerance in various applications. From a design perspective, however, parts such as rubber seals and diaphragms should be considered engineered parts and, as such, should not be used to accommodate wide tolerance variations, says Edward Schmidt of Tempron Products Corp., Milford, Massachusetts.

"In designing a rubber part, close attention should be paid to the environment in which the part is expected to function," says Schmidt. "Is it used at elevated or low temperature, or exposed to oils, grease, solvents, acids, or bases? What are the physical properties required of the rubber? Proper material selection can overcome almost any single requirement, but as multiple requirements are placed upon the rubber, it becomes more difficult to find a material that will meet all of the requirements."

Despite its many appealing characteristics, rubber is not the best choice for every application. Rubber is not suited to temperatures above 700F, for example. In some semiconductor sealing applications or ultra high-vacuum sealing applications, for example, even fluorocarbon elastomers or perfluorelastomers are unable to withstand the aggressive chemistries or high temperatures. For applications such as these, a metal seal would be preferred.

Although few materials exhibit the mechanical properties of thermoset rubber, thermoplastics are sometimes an effective replacement in applications that do not require the mechanical propertiesor heat and chemical resistancethat rubber offers, says Chris Muir of California Gasket and Rubber Corp., Gardena, California. Also, the comparatively longer cycle times of rubber molding, along with some secondary operations, make it difficult for rubber to compete with the component prices of plastic, says Dave Mabie, senior project and marketing manager, Hi-Tech Rubber, Inc., Anaheim, California. Another limiting factor is that the rubber industry still lags behind the plastics industry in manufacturing automation, Mabie says.

Many OEMs today are looking to eliminate the secondary step of assembling rubber and plastic subassemblies, says Mabie. As a result, the ability of contract manufacturing firms to over-mold or co-mold rubber to plastic, especially in high volumes, is becoming increasingly popular. According to Mabie, Hi-Tech is keeping pace with this trend by offering services that include the over-molding of rubber to substrates such as plastic, metal, and other rubbers. Another company staying abreast is California Gasket and Rubber Corporation, which has the ability to vertically integrate a full range of rubber manufacturing services to provide complete subassemblies. Besides providing die cutting and metal stamping, and rubber molding, the company can bond rubber to metal, cloth, or plastic, and provide secondary machining, assembly, and marking.

Recent innovations utilized by more progressive custom rubber molders include injection molding (as opposed to older transfer and compression processes) and liquid silicone injection molding, which is becoming increasingly popular. Flashless molding techniques are also on the upswing.

"Certainly, one of the greatest innovations in our industry in the past few years has been rubber injection molding presses," says Rick Norman, CEO of R&R Rubber Molding, Inc., South El Monte, California. "Injection molding was once the haven only of plasticmolders, but now, with innovative elastomeric compounds and injection presses, custom rubber parts may be efficiently manufactured in large quantities." R&R uses five injection molding presses to manufacture both prototype and production-quantity rubber parts.

Very high precision on part dimensions, better integrity and uniformity of the rubber without voids, and low manufacturing cost for high volumes are comparative advantages of using rubber injection molding as opposed to other rubber manufacturing processes, according to John Gill, engineering manager at Shercon Rubber Group, Santa Fe Springs, California. However, parts that exceed the size limit, and compounds with very high viscosity do not lend themselves well to rubber injection molding, he says.

According to Gill, increasingly sophisticated product designs and more demanding material requirements have led product manufacturers to seek out rubber specialists who can satisfy their unique needs. Instead of looking for "molding companies," OEMs are interested in technologically advanced partners who will use their expertise and experience to add value to the OEM's rubber part design, Gill says. "The mechanical and chemical design of rubber products, along with their tooling, is extremely complex," he says. "Our expertise with 3D solid modeling and custom product design encourages people to outsource to us. We offer compatibility with customers' design software for an easy exchange of ideas and data, along with rapid prototyping techniques that will accelerate the design-to-manufacture process."

Manufacturing Challenges and Solutions

Shercon, a contract manufacturer that services OEMs in the automotive, appliance, medical, electronics, and coating industries, recently solved a problem for a client that was experiencing difficulty with a one-way valve commonly known as a "duckbill valve." The technical challenge was to produce a design that would allow air to pass through a hole one way, but stop it from returning through the hole once it had gone through. Because the rubber valve needed to fit a hole approximately 0.156 inch in diameter, the design and production required a small, highly accurate mold cavity.

"This item is a high-volume precision part and, as such, lends itself to injection molding," Gill explained. "Our procedure was to determine the operating conditionsboth mechanically and chemicallyto which the molded part would be subjected. Once we knew the environment and operating conditions, we used a concurrent engineering technique to formulate a rubber compound, generate an acceptable mechanical design, and develop a production plan, so that we were positive that we could efficiently produce the item over a high-volume production run."

Creating a design that could be sustained over a high-volume production run was a major challenge. Accurate control of several aspects of the partsuch as controlling the flash and accurately positioning a cut in the itemrequires several additional operations before, during, and after the molding process. The company met the challenge by a strict process control and inspection procedure and by producing several jigs and fixtures for positioning the items as they were being de-molded.

California Sealing Devices has an ISO 9002 quality system and capabilities that include the bonding of various elastomers to exotic substrates and metals. Recently, a customer of the firm was designing a new diaphragm for a valve. According to Steve Shapiro, president, the client wanted the flexibility of an elastomer, but the chemical compatibility and purity of Teflon. The diaphragm had three components: a stainless steel screw, a rubber diaphragm, and a Teflon face.

"Other companies took these components, drilled a hole in the center of the rubber and Teflon, and ran the screw through them to complete the seal," explains Shapiro. "We took a unique approach. We welded a washer onto the top of the screw head to give it better support, then molded the screw into the rubber and, at the same time, bonded the rubber to the Teflon. We produced one continuous seal that exceeded our customers' expectations." According to Shapiro, the job required the coordinated effort of six disciplines to achieve the desired result.

R&R Rubber Molding, Inc. became involved in an unusual project recently. One of the firm's customers was using an existing, straight-length commercial silicone molded part, and splicing it to form a vacuum lip seal for a semiconductor application. Although the concept had been approved, the technique failed to achieve requirements for repeatability and electrostatic discharge (ESD), according to R&R's Rick Norman. Complicating matters was the fact that the application had not been completely tested in the field prior to production, and was subject to an unusually large number of variables, says Norman. The application requires that the seal, 'under load,' establish contact between a platform and the wafer test equipment, Norman explains. In addition, the lip seal utilizes a special compound that requires durability, excellent memory, and conductivity.

First, R&R's sales engineers recommended that molding the seal as one piece would be more practical and achieve optimum results. But how to achieve the proper lip seal angle and length was the general concern, Norman says. After numerous phone calls and meetings between manufacturing engineers and R&R's engineering sales staff, a design of the proper part configuration was achieved. This prompted the customer to praise R&R for its "responsiveness and ability to expedite under critical conditions," Norman said.

A & D Rubber recently took on the technical challenges of manufacturing a shipboard bellows, a part that requires extreme flexibility to move in three separate directions at once. The part also needs to be able to resist severe weather conditions. One of the firm's customers had designed and built a bellows that would last only 10 to 15 hours under the extreme flexural stress, according to A&D's Dale Wolford. As designed, the part was extremely expensive to manufacture, and replacing the bellows was also difficult because it required hundreds of hours of downtime while the ship was dry docked. A&D's goal was to design and produce a part that would flex and take the stress in all three directions, give a longer service life, and be easily replaced while the ship was under way, says Wolford.

To meet these objectives, the company first custom formulated materials to meet the temperature, flexibility, and stress requirements, then redesigned the bellows to allow for functioning, simultaneous movement in all three directions. A&D also created a secondary part that is used to vulcanize a replacement section, so that an old part can be removed and the new part installed and sealed while the ship is under way. While the part's cost remained relatively unchanged, its useful life increased from 10-15 hours to two years before recommended change-out, an improvement that Wolford attributes to sound engineering and effective communication with the customer.

"The cost savings in operational use were tremendous," said Wolford. "All of the benefits came from discussing the part's requirements and carefully evaluating all of its functions. Had the first rubber company talked with the customer, the redesigned part would probably never have been required."

Several years ago, one of Tempron's major customers asked the company for assistance in designing a diaphragm for a mechanically driven, chemical metering pump. The challenge, says Tempron's Edward Schmidt, was to design a PTFE-faced diaphragm, backed with fabric-reinforced rubber that would withstand pumping at up to 300 psig line pressure, mat a full output of 144 strokes per minute, 24 hours per day, for 365 days. At full output, the diaphragm would need to withstand more than 75 million cycles per year. Tempron's initial prototypes provided approximately 50 million cycles, prompting the company to make an adjustment to the design that allowed it to meet the customer's criteria. The company followed with a second, minor change that enabled the diaphragm to last for more than 200 million cycles without mechanical degradation, Schmidt said.

Rubber FabricationInternational Rubber Products (IRP) supplies precision parts for automotive applications, in addition to custom molding parts for manufacturers of medical devices and electronics components. The ISO 9002-certified company also molds diaphragms and valve seats for air and water valves. IRP provides innovative, cost-effective solutions to OEMs through its three product lines: Viking Custom Engineered Rubber Components, Mikron Fluid Power Components, and Wagner Precision Roller Components. Its innovative business model provides OEMs with a superior combination of product performance, reliability and value.

IRP's product lines include:
  •  Viking: Custom Engineered Rubber Components for medical, aerospace and commercial manufacturers
  •  Mikron: Fluid Power Products for automotive, construction and heavy industrial uses
  •  Wagner: Precision Roller Components for commercial and industrial product manufacturers

Hi-Tech Rubber, Inc. molds using silicones, liquid silicones, EPDMs, nitriles, fluorelastomers, and thermoplastic (TPE and TPU) materials. "Our molding techniques include injection, flashless, transfer, and compression," says Dave Mabie.

California Gasket & Rubber molds a variety of custom parts, including automotive fuel regulator parts, heavy-duty diesel replacement parts, and parts for fluid power controls, pumps, and connectors. An example of the company's vertical integration is its ability to manufacture composite seal plates. A metallic carrier plate is cut to shape, and bolt holes and fluid paths are filled. A groove is then milled for the seal element, which is bonded in place. Cal Gasket designs the mold to fit the plates, which are cut in house. A new CNC milling center machines the plates as needed. The plates are primed, molded, deflashed, inspected, marked, and packaged to specification in the Cal Gasket facility.

Kismet Products, Inc., Perry, Ohio, manufactures rubber extrusions for a diverse customer base, including the passenger railcar industry and the home appliance and architectural markets. To complement its extruding capabilities, the company offers secondary operations that include the ability to injection mold corner gaskets and perform butt splicing. "What this does is offer the customer an added option for a complete gasket and it does not require any additional work for them," says Kismet's Dale Morvaji. "We also have the capability to run long continuous jobs for our customers on our microwave curing line." The microwave line180 feet in lengthcures, cools, and completes the rubber extruding process in one step. The first stage cures the outer shell of the profile (forms a skin). This keeps the product from distorting its shape, according to Morvaji. The second stage then cures the inside of the rubber and the third stage cools the part so that it is ready for packaging. After the cooling stage, Kismet can coil, reel, or cut the material to the customer's desired length. Two people operate the line, including the cutting, coiling, or reelingone at the extruder and one at the end of the line. "The benefit to our customers is that they receive a finished product, which is cleaner than steam-cured and more cost effective as well," says Morvaji.

Stockwell Elastomerics' core competence is the custom fabrication, molding, and precision die cutting of silicone rubber and related high-performance elastomers. Its services include liquid injection molding (LIM), with large-shot capability, to support the custom molding requirements of the medical diagnostic, analytical instrumentation, and communications and navigation industries. The company can use this capability to mold relatively low quantities of larger gaskets and cushioning pads.

Interstate Gasket carries a full line of elastomer products and other materials, and prides itself on creative ways of recommending, processing, and solving customers' challenging requests for industrial components. Interstate offers large flat bed die cutting, capable of parts up to 73 inches or more, as well as miniature precision stamped parts. The company, located in Leicester, Mass., employs rotary and high-speed automatic equipment for long-run competitive projects. For items outside of the die cutting category, Interstate provides laser and waterjet cutting. "Each specific application must be looked at with great care because of the nature of all the different operating conditions," says John Savickas. "These include temperature, pressure, exposure to chemicals, sunlight, and other environmental conditions."

Some of the firm's OEM customers manufacture pumps, valves, (biotech), and filtration and diagnostic equipment. Other clients manufacture products for the chemical processing, pneumatic, scientific, and water treatment industries. "We typically work to standards that have been developed by the Rubber Manufacturers Association," Savickas says. "However, many times customers require a closer tolerance and creative ways are employed to achieve this. We recently had a project that required a flat gasket tolerance of +/- 0.003" thickness. The calendered RMA tolerances were +/- 0.012" thick, so we engaged a grinding process for the sheet rubber that was able to achieve the desired thickness."

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