Design News for Mechanical and Design Engineers

Computerized weapons will bring more pilots home

High-powered computers, GPS systems, and image processing technology result in versatile, stand-off weapons

By Laurie Ann Toupin, Associate Editor

The above is a fictional, yet potential scenario, as the SLAM ER will be the Navy's first air-launched cruise missile to allow the pilot to send updated target information while the missile is in flight.

After Desert Storm in 1991, Navy and Marine pilots returned with a wish list of ideas to increase their odds of hitting their target and yet come home safely. The military implemented some of those, such as the upgraded Tomahawk Cruise Missile (see Design News, 1/18/99, pg. 37) during the recent Desert Fox campaign. Others, such as the SLAM ER and JDAM will debut this summer.

The pilot releases the SLAM ER more than 150 nautical miles away from the target. The pilot watches the missile’s progress through continuous images fed back via an infrared camera located in the bomb’s warhead. Based on this data, the pilot can alter the weapon’s

At a length of 172 inches, weight of 1,500 lb, and with a 150 nautical mile range, Boeing's Standoff Land Attack Missile Expanded Response (SLAM ER) doubles the capability of its SLAM predecessor in just about everything-- range, altitude, warhead penetration, and the distance from which the pilot can control it.

The original SLAM appeared on the military scene in Desert Storm as the first Navy stand-off missile, one that could be air-launched out of range of area defenses. This keeps the FA-18 pilot far enough away from the target that anti-air defenses won't reach the aircraft. "This is a far cry from when the pilots had to fly directly over the target they hoped to hit," says Boeing's Jack Rose.

After launching the SLAM ER, the pilot flies to a standoff range of 150 nautical miles or more (see figure 1). As the missile nears the intended target, a datalink on the SLAM ER transmits video from an infrared camera back to the F/A-18.

Next, the pilot uses the Stop Motion Aimpoint Update (SMAU) to select the exact target he wants to hit. "We wanted to make it extremely easy for a pilot to pick out a target and precisely hit an aim point," says Kurt Mizgate of Boeing. Instead of trying to hit a moving target, the missile freezes a picture of the intended area and transmits it to the control aircraft. The pilot then slides a cursor over the exact point targeted. Because the missile is still flying while the image is frozen, the missile must project the original position forward in time. The Boeing SMAU patent-pending process and Terminal Kalman Filter are the enabling technologies.

"This substantially improves the user friendliness of the process," says Rose. "The newest generation of pilots grew up on video games. So when they get into the cockpit, it's old news to them. The more computerized it is, the happier they are."

Another benefit, the pilot knows immediately if the mission was successful because he actually sees it go into a door or window of a target. This is particularly important for assessing target and collateral damage.

Smart flying. With a Global Positioning System (GPS) aided inertial navigation system on board, the SLAM ER negotiates hills, mountains, rock outcroppings and other obstacles. Known as Adaptive Terrain Following (ATF), the missile hugs the ground and calculates its best altitude while in flight from stored digital maps of the countryside, eliminating intensive mission planning sessions prior to takeoff.

"We've talked to Navy pilots who have chased these missiles during test flights and they thought they had another pilot out there flying over the terrain," says Mizgate.

The Adaptive Terrain Following technology could be adapted, says Rose, as a ground or collision avoidance technology for a commercial aircraft.

Double the penetration. Engineers also added a new warhead. "We put in a titanium warhead developed at the Navy Air Weapon Center in China Lake, CA," says Mizgate. The titanium warhead penetrates more than double the thickness of concrete than its predecessor. In addition, titanium fragments are much more effective.

Computers were the enabling technology for all of the guidance improvements. We've got a flight computer that's about a hundred times more powerful than the baseline SLAM missile," says Mizgate. High-powered computers adapted for the missile used to be extremely expensive and large; now they are inexpensive and small.

The JDAM. "We need all-weather precision-guided munitions. Work with me and my staff to make it happen," wrote General McPeak, Chief of Staff of the U.S. Air Force during Desert Storm. Too many pilots were returning to base with their bombs intact because of poor visibility.

McPeak's plea was the beginning of JDAM, the Joint Direct Attack Munition, which recently debuted. It isn't a "new" bomb or missile, costing the government hundreds of thousands of dollars each. Instead, JDAM is a $15,000 guidance system, strapped onto an existing 2,000-lb free fall bomb--converting it into a near-precision smart weapon. In comparison, the unit cost of each cruise missile used in Desert Fox cost more than $750,000.

This tail kit gives the pilot the capability to drop a missile from high altitudes with great accuracy, regardless of weather, extends the range of the weapon to 15 miles, and provides the opportunity to drop multiple weapons at once, each programmed for its own target. "So instead of talking about how many sorties it takes to kill a target," says Carl Avila, JDAM program manager of Boeing Aircraft & Missile Systems Group, "we're asking, how many targets can we attack on one aircraft sortie?"

In order to achieve high performance within the strict cost constraints, Boeing relied heavily on commercial technology.

Boeing worked with existing bombs, some dating back to World War II. The concept: add a low-cost guidance tail kit and a set of aerodynamic surfaces, to these standard free- fall bombs, and presto--a high-tech, precision-guided weapon appears.

The JDAM is just that: a small, low cost, GPS-aided inertial guidance system with an on-board mission computer and a tail actuator or servo system for control. Military ordinance personnel also "strap" on small aerodynamic surfaces called strakes--simple aluminum aero-surfaces or small wings--around the circumference of the bomb to provide additional aero-stability. "Strap is really a misnomer," says Avila. "The strakes are attached through an interlocking mechanical system, aluminum hooks and toggle T-bolts that are placed and then torqued around the bomb's shell."

Three components make up the core of the guidance and control unit: a low-cost ring laser gyro-based inertial measurement unit and a 12-channel GPS receiver, both integrated with a mission computer. "For flight control, we have a tail actuator system," says Avila. "There are four fins on the tail end of the bomb: one completely fixed serving as the vertical stabilizer; and three independently steerable, actuator-controlled control surfaces."

In an operation that takes less than 10 minutes, work crews break open a kit and attach the system to the bomb.

Engineers at Boeing in St. Charles, MO, are currently working on an extended range version of JDAM where, instead of strakes, they will add a retractable wing kit to the bomb. The wings will unfold after a pilot releases the missile. Avila expects a 40-nautical-mile release range.

Presently, the JDAM is integrated on the B-52, B-1B, B-2, F/A-18C/D, and F-16 aircraft. Boeing is testing the units for the F-15E. Plans are to integrate the weapon on the F-22 and Joint Strike Fighter, F/A-18E/F, and F-117. The same software is interchangeable with any bomber or fighter for the Air Force, Navy, or Marines.

What this means to you

• GPS in embedded systems

• IR imaging

• ground proximity detection

• stop motion technology

For a complete list of suppliers check out the DN Website at: www.designnews.com 

Rethinking industrial drives

When General Electric Industrial Control Systems set out to re-engineer how low- and medium-voltage Insulated Gate Bipolar Transistor (IGBT) ac drives are designed, manufactured, and packaged, the company first turned to its customers. What, exactly, would they want?

"Up until then," recalls Lincoln Fujita, manager, systems architecture, "our design focus had been primarily internal"--a trap, he notes, many big companies fall into. Customer input--gathered via surveys and "many pointed questions asked by GE application engineers and field personnel"--could be summarized in a common request: Customers wanted ac drives with "dc drive reliability."

To assist engineers in designing the new Innovation^TM Series ac drives to Six-Sigma levels, GE applied simulations, predictive design, and statistical methods to identify quality and reliability issues--not at the end of the production cycle where defects are easy to see and costly to fix, but during early design phases when changes are most economical.

Completely sealed, two-phase heat pipes replace liquid or air cooling systems. The result is a simple, low-noise, low-maintenance system capable of fitting in half the space of previous

Gaining more power per cubic meter of space, yet matching the reliability standards of a less-complex dc system, Fujita claims, forced the integration and development of the following design features:

Laminated bus. Two bus bars running close together tend to induce current in each other, a phenomenon known as parasitic inductance. This, in turn, reduces a drive's power factor and can lead to overheating, a bad situation if the drive is being applied to existing motors where there is little thermal margin.

Innovation Series ac drives avoid this problem. "Rather than delivering dc power to the IGBTs via discrete buses throughout the drive," Applications Engineer James Zayechek explains, "we put them all together as a specially insulated laminated bus."

Not only does the laminated bus assembly make all the power connections within each phase of the load inverter to eliminate internal power cabling, it reduces parasitic induction 75%. This, in effect, gets rid of efficiency-robbing inverter snubbers as well as the associated circuit boards, resistors, capacitors, and inductors--all high-maintenance items in other ac drives.

In addition, the laminated bus structure can be physically removed as a single assembly, allowing complete access to all of the power cells. As a result, the mean-time-to-repair for the inverter is less than 30 min.

Rack-mounted control cards. "Maintenance people will tell you that power connections and controller connections represent the major source of problems in a drive," Zayechek says. "The more connections there are, the greater the chance of failure."

As an example, Zayechek points to variations in air flow over a block of ribbon cables. Such fluctuations, he claims, can cause the cables to flex back and forth, eventually breaking electrical connection.

The Innovation Series ac drive does away with most cables and connections. All the controls, feedback, and bridge interfaces for the entire drive are located on a single control rack--a VME Eurocard-style rack which accepts plug-in circuit boards that utilize surface-mount technology.

Four basic control cards constitute the assembly:

• A DSP card performs all motor control functions including high performance speed and torque regulation.

• The bridge interface card carries out all gate firing commands and also provides a path for voltage, current, and temperature feedback.

• A fiber-optic-coupling card isolates the controls from the power electronics to improve noise immunity and increase user safety.

• The I/O card routes all signals from the field to a common point on the drive; terminal blocks for both customer I/O and for the switchgear interface are DIN rail mounted for easy access. An optional application controller/LAN card can be added.

Heat pipe technology. While heat-pipe cooling systems have been successfully applied by the computer industry to cool chips, the Innovation Series ac drives represents their first application to industrial controls. Here's how the new system works:

Conventional drive systems use liquid cooling or high-velocity air to remove heat from the IGBTs. As a result, they require either space-intensive plumbing, reservoirs, and pumps, or noisy blowers. In contrast, the heat pipe has no moving parts, uses less than a spoonful of heat transfer medium, and occupies the same footprint as the IGBTs. Noise level is 70 vs. 90 dB for the previous-generation drives.

Heat enters the pipe at the chill plate which bolts directly to the IGBT assembly. Vaporization of the heat transfer medium creates a pressure gradient, forcing the vapor toward the pipe's condenser end.

As the working medium condenses, the latent heat of vaporization is released; a wick returns condensate back to the chill plate by capillary action. By maintaining the chill plate at a relatively constant 40C, the heat pipe protects the IGBTs from thermo-cycling, thereby ensuring long life.

Innovative ideas such as heat pipe cooling, a laminated bus, and control cards, concludes Fujita, permit fewer mechanical parts, the elimination of power cabling, and less wiring and connections. For users of GE's Innovation Drive, Fujita adds, this means fewer points for failure, "which is what Six-Sigma quality is all about."

National Manufacturing Week features special event lineup

Chicago--National Manufacturing Week (NMW) is around the corner. There's a complete lineup of special events and exhibits scheduled for this year's show, which will take place from March 15^th through the 18^th at McCormick Place, Chicago.

On the opening day, Sports Day Monday, engineers can meet champion race car driver Al Unser Jr, who will be giving autographs. His PENSKE race car will be on display all week in booth 9473.

U.S. Commerce Secretary William M. Daley will speak at the National Assn. of Manufacturer's (NAM's) featured-speaker program on Tuesday, part of Industrial Distribution Day^TM. The event, which will take place in Ballroom A in McCormick South from noon to 2:00 pm, is free and open to all badge holders.

Wednesday is Space Exploration Day. NASA will present an event-wide keynote presentation, "Providing Solutions for the Manufacturing Industry." The presentation, which will take place in room N229 McCormick North from 9:00 to 10:00 a.m., is free and open to all badge holders. NASA will display technologies coming out of the aerospace program available to U.S. companies. Engineers will highlight the technologies and discuss how companies can access them. Such technology has been used in the development of televisions, cars, tires, saw blades, cameras, and medical-imaging equipment.

FOR DAILY UPDATES on all show activities during National Manufacturing Week, check out www.manufacturing.net/nmw99 

For more information on National Manufacturing Week, visit www.manufacturingweek.com 

Booth Business

This year, Design News' booth will feature an array of innovative technologies recently featured in the magazine (some in 1999!).

Who could forget the splash Titanic made as it hit movie theaters around the world? Definitely not the engineers behind the scenes who built the movie set, which used hydraulics to raise and lower the model (DN, 1/19/98, p. 58). A scale model of the Titanic will be featured in the booth, complements of Parker Hannifin Corp.

Design News also reported on an attachable rear-shock for mountain bikes, designed by BikeControl^TM (DN 4/6/98, p. 90). With this contraption, BikeControl says bike fanatics no longer have to purchase a new bike to get full suspension. A mountain bike that features its product, Shockster APS, will also be on display.

The luxury of home technology is transferring to the automobile. The past year bought news of a dashboard-mounted PC (DN, 4/6/98, p. 35). The "Net" vehicle from Delphi Delco will be on display in the DN booth. The automobile features an auto PC from Microsoft and Clarion Corp.

 The 12^th annual Design News Engineering Achievement Awards Banquet that honors top design engineers is scheduled for Tuesday evening, March 16^th.

Aluminum/nylon sunroof ring gives way to olefin

Rochester Hills, MI--Webasto Sunroofs Inc. has replaced an aluminum and nylon-reinforced plastic for the outer fitting ring of its aftermarket sunroofs. The result: a one-step, injection-molded plastic ring that requires no shaping, welding, or secondary assembly process. And, because the resin can be colored on press, no painting is required.

For the project, Webasto selected a 20% glass-reinforced material, Hivalloy® G XPA062, from Montell Polyolefins (Wilmington, DE) for the AirOne^TM Inbuilt sunroof series. The general-purpose, styrene-based olefinic resin alloy offers exceptional low-temperature impact compared to many traditional engineering thermoplastics, according to Rodney Johnson, program director at Webasto. Other key properties include: chemical- and moisture-resistance, low mold shrinkage, and low density.

"Traditional nylon-reinforced plastic rings tend to warp, bend, and fade after extensive exposure to the sun," Johnson adds. "Hivalloy exhibits some of the best shape change versus temperature change (CLDE) readings we have seen during its resin testing."

Webasto uses the new rings for a variety of sunroof replacements, with two new versions in the works to complete its automatic and manual sunroof offerings in the AirOne series.

Mold design purrs

Billund, Denmark--The LEGO® Primo Cat with its cute smile and gently arching back appears quite friendly and benign. But its design wasn't so friendly to LEGO engineers. After spending 300 man-hours to develop the product surfaces in a solid modeling environment and still missing the original design intent, LEGO design engineers wanted to leave that cat at the pet store. "This prompted us to search for better alternatives for creating product surfaces that are required for the mold cavity designs," says Torben Rasmussen, LEGO design engineer.

Surfacer from Imageware (Ann Arbor, MI) came to the rescue. Using Surfacer in conjunction with a scanning device to capture the digital representation of a clay model of the LEGO cat, engineers built surfaces over the digital model to create a complete free-form model. The entire process took approximately 20 hours. "Our team was able to prove a significant time reduction in the mold design process; more importantly, we achieved the desired design," says Rasmussen.

LEGO uses Surfacer to design all of its handmade models. In one process, toy originals are created in clay or PUR foam, scanned, designed in Surfacer, then imported into the CAD system. In another, free-form surfaces are created directly from curves in Surfacer on the computer. This allows LEGO engineers to quickly model complex shapes such as human-like wigs and hair. Once the surfaces are complete, they are brought into the CAD system and sewn into a solid. A LEGO designer subtracts the solid element into the mold forms. When finished, the complete CAD design of the mold is delivered to manufacturing, where the actual/exact geometry is milled/spark eroded into a mold.

With Surfacer, LEGO can handle the most complex, free-form modeling scenarios within a position tolerance of 0.0001 mm. And their engineers are purring with a 50% time savings in developing free-form surfaces.

QUICKER QUERIES Go to www.designnews.com/info    for more information on the technologies in this section

Teamwork streamlines TV production

Yokohama, Japan--In today's world, consumers shop for television sets that feature high quality and cutting-edge technology with an affordable price tag. To meet these demanding requirements, TV makers must maximize their expertise in production to overcome and exceed these customer demands.

The Victor Company of Japan, Ltd. (JVC) provides an example of how worldwide teamwork can bring about such results. To produce TVs for North America and the Far East, JVC partnered with injection molder Meiki De B.C. S.A. De C.V. to open a new manufacturing site in Tijuana, Mexico. The objective: streamlined production processes.

"We work hand-in-glove with Meiki in the production of enclosures that not only are thinner, stronger, and can accommodate larger tubes, but that meet the demands of our customers," says Henry Nagaoka, JVC's general manager, purchasing department. The screens range in size from 13 to 36 inches.

Selecting a supplier with a durable and versatile resin to help JVC and Meiki deliver a distinctive product rounded out the teamwork effort. The company chosen: BASF's Polystyrene Business Group (Mount Olive, NJ) and its Avantra^TM ES 8550 non-deacbromated, styrenic polymer resin. In fact, BASF developed the high-impact, high melt-flow, flame-retardant styrenic polymer especially for such demanding applications as supporting the weight of the picture tube and other TV components during various phases of production.

Meiki has six injection-molding machines that range in size from 220 to 1,600 metric tons, and plans to add another 1,600 metric-ton press. It uses the BASF polystyrene to produce 5,000 TV pieces per day, including the front cabinets, rear covers, and terminal boards.

"Sometimes we experience quality problems with other resins," says Nagaoka. "Avantra produces high-flow, fewer defects, reduced scrap, and, because it has less gas, cuts down on the tool deposits left in the molds."

JVC's specifications also require a smooth surface achieved with the right mold and resin during the assembly process. "If the surface is not very smooth, the suction cup of the lifting mechanism can't form an airtight seal. This may damage a completed TV when it is being placed in the box," Nagaoka adds.

Based on the teamwork of Meiki's molding, BASF's resin, and JVC's stringent quality control standards, JVC continues to win over TV buying customers. It plans to extend that teamwork to the production of digital TVs and units with flat screens.

Sixty-ton beast clocks 40+ mph

Vernon Hills, IL--Komatsu America International Co.'s newest addition to its dump-truck family has a maximum gross vehicle weight of 211,860 lb. The truck carries up to a 60-ton payload, depending on customer-selected options.

"HD465-5 is an extremely productive machine," says Komatsu dump truck product manager Lee Haak. "It can be fully loaded to 60 tons in three passes with a Komatsu WA700-1 wheel loader and still achieve travel speeds up to 43.5 mph."

Its powerful 715 flywheel hp Komatsu SAA6D170E engine complies with EPA and CARB standards, and is used in other Komatsu trucks so that many service and parts items are interchangeable. The same engine is used in a Komatsu wheel loader, dozer, and excavator--an obvious advantage at multiple-machine job sites.

A Komatsu-designed seven-speed transmission with K-Atomic technology (Komatsu Advanced Transmission with Optimum Modulation Control System) smoothes shifting, improves operator comfort, and extends the life of drive-train components. Direct drive in every range results in good fuel economy, according to Komatsu.

"It's more maneuverable than other trucks in the same class," Haak says. "A MacPherson-strut suspension connected to a lower A-frame equips the truck for sharp tire-cut angles, giving the operator a 27-ft, 11-inch turning radius."

Komatsu uses a "human first" approach to engineering. The result is enhanced operator comfort to minimize fatigue and simplified operation to boost productivity. Electronic displays make it easy to monitor the truck's operation and fluid and pressure levels. The truck's box-section construction extends frame life and improves reliability.

Rubber and silicone viscous-damping mounts attenuate vibration and noise in the two-door, walk-through cab. "An electronically controlled heater, air conditioner, tinted glass, intermittent wipers, and turn signals provide an automotive-styled interior," Haak says, "with tilt steering and adjustable seats with headrests included." Moreover, the fully hydraulic steering system gives fast steering response and requires light effort--even at low engine rpm.

Driver and passenger seat belts, multiple braking systems, and an automatic back-up steering system increase operator safety. Air-over-hydraulic, dry-caliper disc brakes in front and oil-cooled, continuously lubricated, multiple-disc brakes on the rear axle extend brake life and reduce maintenance.

A separate spring-applied, air-released parking brake engages automatically if the truck loses air pressure. In addition, separate brake circuits between the rear and front axles help maintain braking even if one axle fails. And if the engine quits, a battery-driven backup system lets the operator steer the truck to a stop. An optional automatic retarder allows the operator to select the maximum safe operating speed on downhill, loaded hauls to limit acceleration on steep grades.

3D solids software wins speaker race

Canoga Park, CA--Harman/JBL wanted to win a bid from Digital Equipment Corp. (DEC) to provide multimedia speakers for a new computer system. They had the speakers--they needed a case. Harman called RKS Design. The hitch: RKS had only 12 weeks to finish the assignment.

RKS completed the industrial design in two weeks. But before it was finished, they began engineering. RKS produced prototypes while simultaneously beginning the tooling process.

Using SolidDesigner from CoCreate (Fort Collins, CO), RKS engineers produced a stereolithography model from the 3D CAD data to validate the design and produce silicon molds to prototype the speaker. The parts, along with the electronics, were assembled so JBL could test the acoustic properties of the design and present a working prototype to DEC. JBL won the contract. Engineers used the final 3D CAD data to cut tooling. Production began 12 weeks after the first conceptual drawings.

Ravi Sawhney, president of RKS Design, says, "Beyond the speed of completion, we were able to develop a visually dynamic product that would differentiate itself in a highly competitive market. I credit both the RKS team, which is able to conceptualize products, and the 3D solid modeling software, which allows our team to solidify those ideas."

Some design firms use history-based 3D solid modeling systems, which can make design changes more difficult because every step is dependent on all the previous steps, he says. Designers have to virtually start over when making changes that were not anticipated up front. "With the exception of SolidDesigner, the current generation of modelers are fully dependent on the history--the steps taken to create their models. SolidDesigner gives us no such limitations. We can make changes at any time. The system is completely flexible and user-friendly."

SolidDesigner can also import CAD models from other CAD software programs, including history-based systems, says Sawhney. Once loaded into CoCreate's software, the source of data is no longer an issue.

"In developing a product, a number of issues must be dealt with. We narrow the focus on what the client needs for ergonomics, manufacturing costs, assembly, and serviceability," says Sawhney. "On top of that, we include the competitive environment, pricing, and product positioning strategy. SolidDesigner allows us to give form and substance to those issues which allow them to be evaluated. The beauty of the tool is that we're able to look at a product on screen in real time and make industrial design decisions. We're able to solve a mechanical problem that may be impeding the execution of a design.

"With SolidDesigner, we can look at so many different design alternatives so quickly that it's become for us a virtual shop, where we can create products entirely in cyberspace," he says. "Other programs may allow you to do that too, but because this is not a history-based modeler, you have no limitations to explore radical design alternatives."

Polymer clears the way for emergencies

Lieshout, The Netherlands--REFRA Tecnics BV has designed a simple, but ingenious clip that, when applied onto the edge of a door, prevents it from closing. The clip could prove to be a life-saving tool for firefighters, as well as an invaluable aid for police, ambulance, and hospital services involved in household emergency situations or when fighting fires. The REFRA Controller's® fluorescent strips also serve as a route marker to help lead people out of dark or smoke-filled buildings.

The company's design engineers looked for a material with outstanding thermal performance and impact resistance when designing the device. Several materials were evaluated before they selected CARILON Polymers from Shell Chemicals (Houston) for the application. Having a Deflection Temperature Under Load (DTUL) of 100C at 1.8 MPa, and a notched Izod impact strength of 20 kJ/m² at 23C, the aliphatic polyketone (PK) material offers toughness and ductility over a wide temperature range. In addition, it features a high elongation at a yield of 25%, while demonstrating good hydrolytic stability and broad chemical resistance.

Extensive testing with CARILON showed the molded clips worked effectively in high temperatures, and they didn't lose their grip or break when the door was slammed. Already, the device has won acceptance in hospitals across the Netherlands, and is becoming increasingly popular in the UK, U.S., Germany, and Japan.

Ear seals overcome severe use

Canton, OH--Performance expectations of today's high-end cushions can present a real design challenge, as Evans Industries Inc., a producer of some 60 or more types of ear cushions found out. Some of these products, such as a new silicone gel-filled ear seal, are so complex they require a tough, versatile, high-performance material that functions under tough on-the-job conditions. Evans found that material in a thermoplastic polyurethane (TPU) from Stevens Urethane (Holyoke, MA).

"Headsets are used in all kinds of applications, each of which have different technical requirements," explains John Ebert, director of sales at Evans. "For example, in such applications as commercial and military aviation, an effective ear seal must block external noise to ensure clear communications and safety."

Not only are these specialized cushions expected to last up to five years, but they must endure hot and cold temperature extremes and long-term exposure to skin oil, an aggressive substance. For such applications, Evans generally puts a TPU cover over a foam pad to give the cushions added durability and toughness, says Ebert.

The Stevens TPU has a tensile strength of up to 10,000 psi. It also has broad-spectrum chemical resistance and temperature flexibility down to -40F. These features made it the material of choice for can't-fail applications, Ebert notes.

Evans begins the manufacture of its gel-filled ear seals with a 10-mil sheet of transparent or black TPU, then vacuum forms it into a female die, during which the sheet thins out to 5 mil. The gel-filled TPU sets at room temperature. A 15-mil, heat-sealed backing completes the seal.

"At 5 mil, the skin material has to be extremely tough to contain the gel and not burst under the pressure and abuse the seals typically encounter," Ebert adds. "TPU is the only option given the strength and durability we needed in this thin-skin application. Equally important, TPU remains impervious to the silicone gel, which can migrate through materials such as vinyl."