February 2, 1998

9 Min Read
Elastomers STRETCH design performance

Got a design problem that calls for an exceptionally flexible material? How about one that requires extreme resistance from oils and chemicals? Perhaps you need a material that can withstand the abuse from various sterilization procedures or is compatible with both drugs and body tissues. Or do you need something that can be overmolded with a tough material to provide a soft touch?

Elastomers can provide the solution to these and many other design dilemmas. Here's a sampling of recently debuted elastomeric materials, plus a look at one or two that will influence the materials scene in the near future.

Elastomer adds soft touch to keyboard

How do you make a piece of interactive office equipment not only feel comfortable, but look as though it is something you could cuddle-up with in your easy chair? The solution came easy for design consultant Herbst Lazar Bell Inc. (Chicago)--a thermoplastic rubber.

Commissioned by Compaq Corp. to design an infrared, full-size cordless keyboard that interfaces with a TV/PC monitor, Herbst identified ergonomics and aesthetics as the key challenges to developing the new console. The keyboard integrates mouse controls, a standard trackball, and battery-powered infrared transmitter to provide full functionality, without having to be tethered to the host. Users can interface with their Web TV systems while lying on the couch, reclining in an easy chair, or sitting at a desk.

The keyboard had to be lightweight and attractively contoured to fit the human form. The under side of the board rests comfortably, without slipping, on the user's lap. And the concave front edge wraps neatly around the user's waist. The batteries also are centered to help balance the device while in use. Soft elastomeric pads, made from Santoprene thermoplastic rubber supplied by Advanced Elastomer Systems, L.P. (Akron, OH), cushion the hands that interface with the keyboard in many ways.

To account for the anticipated pressure points, for example, the design incorporates a soft, low-durometer rubber grade of Santoprene into four places on the keyboard--the two wrist rest areas and around the trackball and select buttons. The thermoplastic rubber not only has a nice feel, but it is durable enough to protect against wear.

Other padding options were considered. First, Herbst looked at neoprene, but felt the material was "too soft." Then it considered leather or vinyl. The materials proved too costly and called for low-volume production.

The keyboard also needed to look comfortable. The soft pads visually direct the user's hands to the best place to work from. The Santoprene rubber also allowed Herbst to color differentiate the keyboard and its padded areas.

The device transmits keyed-in information to a receiving unit plugged into a CPU or Web box from up to 15 ft away. To insure a clear signal no matter how users angle themselves, the keyboard has three IR transmitters around its perimeter. The unit also automatically powers on and off in response to the users input. It is compatible with PC-AT, PS/2, Novell Netware, and Windows 95. Wired magazine was so impressed with the design that it named the keyboard one of its top 100 must-have products.

Overmold wins scanner customers

Retail point-of-sale bar code scanners take a lot of abuse from drops, bumps, and spills. But the QuickScan(TM) 6000 hand-held laser scanner has survived repeated five-ft drops onto concrete and kept counting away. The device's durability originates from the design of its outer housing.

The concept stemmed from PSC Scanning Inc. (Eugene, OR) design engineer Craig Bontly, whose interest in a multi-shot consumer product gave him the idea for the housing. He felt a multi-shot application--the first shot a durable resin, the second a rubber-like resin--would produce a more durable part and eliminate the need for a separate rubber-boot component and its subsequent assembly.

However, PSC had some concern that the design would drive the final product's cost beyond what the market would pay. Still, company officials felt the multi-shot concept would be integral to the success of the program. "We felt that to succeed in our market, we had to strike a balance between cost, performance, and style," Bontly explains. "We needed a value-price, durable scanner that had both an aesthetic and a functional appeal."

PSC brought the problem to injection-molder Phillips Plastic Corp. (Hudson, WI). Phillips technicians determined a lower price could be achieved, without losing the properties PSC sought, through insert molding.

In general, for lower volumes, insert molding can produce a two-shot part with properties similar to multi-shot molding. On the one hand, it is less expensive to create tooling for insert molding. On the other, there is increased handling and two separate processes involved, which raises part price. (With multi-shot technology, both shots are injected into the same mold during the same cycle.)

To make insert molding practical for this application, Phillips got involved early in the design stage. "PSC had a good concept design for the QS6000," notes Steve Schultz, Phillips program manager. "We suggested ways to make an impact on the manufacture of the plastic components."

The QS6000's housing guards against liquid and dust infiltration. It is insert molded with a soft-touch overmold that provides external anti-skid and stabilizing features. The part has a styling groove for overmold shut-offs, preventing the mismatch of parts, while enhancing the finished appearance.

The second shot provides a shock-absorbing mount for the scanner's internal circuit boards. This design eliminates the component parts needed for isolating the circuitry.

To meet PSC's tight deadline, Phillips Short Run Solutions operation pursued a bold plan for tooling. Normally, when a tool is designed for insert molding, the first shot is tooled before the second. The toolmaker can then use parts from the first mold to verify part fit in the second tool. However, this process takes anywhere from 18 to 22 weeks.

To reduce leadtime, the scanner's two mold cavities were tooled concurrent-ly. In just 10 weeks from their release, the tools, including texturing, were ready for their initial run by ShortRun Solutions.

Elastomers flex catheter's utility

Since their introduction in 1950, central venous catheters (CVCs), their makers, and their users have experienced high rates of infection, blood clotting, and insertion complications. To overcome these problems, medical equipment manufacturers continuously seek new materials for the development of these instruments. However, one manufacturer, B. Braun Medical Inc. (Bethlehem, PA), solved all three dilemmas using thermoplastic polyurethanes (TPUs) for its CVCs.

The CVCs are inserted into a cen-tral vein to aid in the administrationof drugs, fluids, blood samples, andcentral venous pressure (CVP) measurements. They are used in virtually all hospital surgeries.

Originally, the CVCs were made from polyvinyl chloride and fluoropolymers, materials stiff enough for easy insertion, but often associated with a high incidence of blood clotting once in the body. In an effort to reduce the incidence of clotting, engineers used flexible materials, such as silicone and polyurethane. However, since these materials were not rigid enough, insertions became more difficult.

"What we needed was a material that would be rigid enough to provide physicians with easy insertion, but would then soften enough in the body to accommodate a patient's comfort and safety," explains Mike Connelly, product director for vascular access with B. Braun Medical. The company turned to ISOPLAST engineering thermoplastic polyurethanes and PELLETHANE thermoplastic polyurethane elastomers from Dow Plastics (Midland, MI).

Multi-lumen CVCs consist of two or more (typically three) lumens within a single catheter. A triple-lumen model has three fluid paths that allow a physician to administer up to three solutions at a time. These lumens are connected by a junction hub made from PELLETHANE to separate extension tubes or "pigtails." The pigtails, also made with PELLETHANE, remain outside of the body and are connected to the drug/fluid supply or diagnostic systems by a pigtail hub made from ISOPLAST.

"A multi-lumen catheter allows the simultaneous infusion of incompatible drugs," says Jim Roma, B. Braun materials engineer. "A TPN solution can be administered through one lumen, while the other lumens remain available for the administration of drugs or other fluids and for drawing a blood sample. CVCs made with the elastomers help ensure there is no chemical crossover in the catheter."

ISOPLAST resins combine the toughness and dimensional stability of amorphous resins with the performance and chemical resistance normally associated with semi-crystalline resins, explains Scott Blanchard, senior development chemist for Dow Plastics. "On the other hand, PELLETHANE elastomers exhibit exceptionally smooth surfaces and good hydrolytic stability," Blanchard adds, "and they come in a broad range of durometers (80 Shore A to 75 Shore D) to bridge the gap between flexible elastomers and more rigid conventional thermoplastics."

Stethoscope won't discolor uniforms

Stethoscopes made by Welch Allyn/Tycos Inc. (Arden, NC) feature a variety of colors for the device's injection-molded components. The objective: coordinating the color of the instruments with a medical staff's uniforms. The problem: the precolored materials originally used for the Harvey ELITE(TM) stethoscopes bled color onto the uniforms.

Alliance-Carolina Tool & Mold turned to specialty compounder RTP Co. (Winona, MN) for help solving the problem. Within a week of the initial request, RTP colorists had color samples to the molder for testing. In determining which resin and pigment systems would work best for the project, the colorists selected a precolored RTP 1500 Series polyester thermoplastic elastomer. Less than a month later, the parts were in production using a special RTP precolored compound.

"The material achieves all of the needed characteristics," explains Steve Schneider, Alliance-Carolina materials manager. It also helped solve another critical problem. "The previous materials resulted in scrap rates near 30%," Schneider adds. "The new material has reduced that rate to less than 10%."

In addition, since the stethoscopes must routinely be cleaned with alcohol, an inorganic pigment systems, is needed for all colors. "Our finished parts are submerged in alcohol for 24 hours to ensure the colors don't bleed," Schneider notes. "The RTP compound holds up to the test."

What this means to you

- Elastomers offer engineers the opportunity to make their designs more flexible and ergonomically pleasing, yet withstand greater impacts.

- Many elastomers provide medical-device designers a material that can resist harsh chemicals and sterilization procedures.

- Color-coordinated elastomers will match or contrast with other primary product colors.

- Overmolded elastomers provide a cushioned soft touch to products.

- Switching to elastomers often can cut part, material, and assembly costs.

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