An ablative test of silicone-rubber wrapped fuel hose, used in aircraft, exposes the materials to a temperature of 2,000°F for 15 minutes.

The ablative test exposes the hose to flame for 15 minutes. In that time, according to Stratoflex R&D manager Rick Deiss, "The silicone material does char, which we anticipate. After a few minutes under the flame, one layer of ash will drop off, and about five minutes later, another layer will fall." By exaggerating actual service conditions, the test provides a good measure of thermal stability and flame resistance. The hoses are normally rated at 400°F.

Construction of the multilayer fuel line starts with a PTFE tube, over which goes a stainless steel wire braid, then a silicone-rubber jacket. The stainless steel wire braid strengthens the tube and provides a ground for static electricity. "Most of the static electricity is dissipated by a carbon-filled layer within the tube itself," said Deiss.

The inside diameters of the fuel lines range in size from 3/16 to 2 inches, while the outside diameters go from 1/4 to 3 inches. Smaller-diameter hoses are extruded and then cured in infrared ovens. Midrange hoses are cured by steam vulcanization. Larger hoses are wrapped in silicone putty, covered with a nylon shrink tape, then heat cured. After curing, the nylon tape is removed. Stratoflex makes 3,000 to 4,000 feet of hose per week. Individual hose assemblies are generally 10 to 15 inches long.

In addition to resisting fire, silicone-rubber flexes easily. Used in an aircraft engine compartment, these hoses help dampen vibration and absorb mechanical shock.

In a mold made from silicone-rubber compounds, zinc and zinc-aluminum alloys are cast in this spin-casting system, which can be used for small and medium-volume production.

Tekcast's spin-casting system uses molds made from high-temperature silicone-rubber compounds to cast zinc and zinc-aluminum alloys, as well as thermosetting plastics such as polyure-thane, polyester, and epoxy. The molds can be made from existing castings, rapid-prototype models, or any decent samples. A mold might have four or five cavities for large parts or a great many cavities for small parts. Once it is made, the mold is loaded onto the spin casting machine. Then, the molten metal or plastic is poured into the mold as the mold rotates. Spinning forces the casting material into all voids.

One of Tekcast's customers is a remanufacturer of auto parts. The company was seeking a way to mold water pump impellers—25,000 of them—economically and under its own roof. Starting with an original cast impeller to make its mold, the remanufacturer found that it could make multiple-cavity molds for under $100. It also found that it could make as many as 4,000 parts before needing to replace the mold.

Better yet, the water pump remanufacturer eliminated a secondary operation. By placing bushings into the mold cavities before casting, Tekcast's customer no longer needed to press bushings into the impellers after they were cast. Although the same techniques can be applied to conventional die casting and injection molding, Tekcast said it is complex and costly to do so.

Because the molds are made from silicone rubber, they do not last forever. The process is especially suited to production runs of small to medium volumes.

The high light-transmission properties of this diffuser screen suits it for duty in active matrix LCD panels, such as this one that is slated for use in an Army research helicopter.

Tulip Development Laboratory of Milford Square, Pa., has found another application for the diffuser material. Tulip Development Labs builds bezel keyboards and packages active matrix LCD panels found in military and aviation settings. Researchers found that Clarex DR-III, in 2-mm sheets with 65 percent light transmission, balanced the twin demands of light passage and diffusion in the active matrix LCD panel, which they designed and built for an Army research helicopter.

Clarex DR-III material is a polymethyl methacrylate (PMMA) plus additives, which comes with a matte finish on both sides. It can be ordered in 11 light transmission percentages, from 45 to 93, and in thicknesses from 0.012 to 0.080 inch. Filter characteristics are cast directly into the material. The white DR-III material has a color temperature of 6,750°K, which provides the high light transmission and diffusion needed in backlit displays.

Recently, a manufacturer of implantable cardiac pacemakers and defibrillators came to Hudson Medical Products seeking relief for a complex assembly operation. The titanium case the manufacturer was using as a housing for pacemaker electronics required delicate welding to assemble its three parts. That was because its two case halves were symmetrical, with nothing to hold them together during the welding process. The manufacturer had been using a third part—a weld band—to join the two halves before welding them by laser at final assembly. But the maker had to spot weld the band to one of the case halves in an intermediate step, requiring elaborate fixtures. The extra steps added cost.

Hudson Medical Products is used to working with thin wall materials at tight tolerances. Wall thicknesses of 0.010 to 0.020 inch are not unusual; neither are tolerances to +/- 0.0001 inch. Hudson realized that by designing a step into one of the two thin-walled case halves, it could eliminate the weld band and, at the same time, ease assembly. Now the case halves snap together and are sealed in one laser weld operation. Eliminating the spot weld step reduced assembly cost by 20 percent.

A captive fastener solves the concern of a printer manufacturer over free-ranging screws near the power supply. Service technicians benefit, too.

Faced with a problem of the same taxonomy, engineers at Datasouth Computer Corp. in Charlotte, N.C., looked to Penn Engineering & Manufacturing Corp. of Danboro, Pa., for assistance. Datasouth needed to lessen risk of damaged power supplies arising from dropped screws rolling about inside their printer cases. Apart from asking service technicians to be more careful, Datasouth needed a mechanical solution.

Penn Engineering recommended a spring-loaded, self-clinching, captive-screw assembly, complete with a knurled thumbscrew and a combination slot/Phillips drive, as the answer to Datasouth's worries. By specifying the Penn Engineering fasteners, Datasouth not only eliminated its concerns over ruined power supplies, but decreased mean time to repair by taking the hunt-for-lost-screw step out of the service manual.

The fasteners are permanently installed in the mounting brackets after they are stamped out from sheet metal stock. A fastener is inserted through a hole in the bracket, then squeezed with a press. The sheet metal surrounding the fastener deforms into an undercut beneath the fastener head. A serrated clinching ring fixes the part radially. Datasouth uses seven such fasteners in its airline-ticket printer.

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