News

Handle with care - Thermoplastic holds superflammable flares

Dec. 8, 2005

Thermoformed containers and lids made from conductive, fire-resistant thermoplastic sheets assure safe handling of highly combustible pellets used in countermeasure decoy flares.

Lawrence Kren

Handle with care Thermoplastic holds superflammable flares

Armtec Defense Products Co. produces eight different decoy flares for aircraft such as this AH64-D helicopter. Safe handling requires antistat lids and containers of vacuum-formed Kydex GND, a conductive, fireresistant thermoplastic alloy sheet that prevents staticelectricity buildup during the manufacture and transport of the flares highly combustible magnesium blend.

U.S. Air Force C-17 transport plane releases decoy flares. Their high-intensity IR radiation draws heatseeking missiles away from aircraft. Photo courtesy of Defense Visual Information Center.

Armtec Defense Products Co., Coachella, Calif., makes the flares from a mixture of magnesium and other chemicals that is extruded and then pelletized. The magnesium mixture burns intensely and is extremely hazardous if mishandled, says Linda Swope, Armtec director of flare programs. Static electricity can ignite the pellets as they are hand-loaded into the flares metal canister.

To keep technicians safe, Armtec, a subsidiary of Esterline Technologies Corp., Bellevue, Wa., uses antistatic Kydex GND alloy sheets that have a surface resistivity of 105 Ω/sq and meet UL Standard 94 V-0 flame requirements. The thermoplastic from Kleerdex Co., Bloomsburg, Pa., remains conductive after being thermoformed into deep-drawn containers and lids by Formed Plastics Inc., Carle Place, N.Y. The Armtec parts range from 25-in.-diameter, 2.75 in.-high flare-composition-storage and 29-in.-diameter, 6-in.-high mixing-vat lids to 18-in.-diameter, 24-in.-high blend-pot and 14-in.-diameter, 17-in.-high slurry-pot covers.

Kydex GND sheets are thermoformed on a single-station model 84SP pressure-forming machine from Maac Machinery, Carol Stream Ill., using a special snap-back vacuum-forming technique. After thermoforming they are rigid (flexural modulus of 270 kpsi), resist most chemicals, and have uniform wall thicknesses and high detail, says Formed Plastics president Pat Long. The sheets mount on a clamping frame and transfer into a heating station where ceramic heaters uniformly warm sheet tops and bottoms.

The sheets move to a forming station once they hit the correct temperature where a prestretch box pushes against the sheet bottom. A vacuum pulls the sheet into the box in a bubblelike shape. An electric eye controls the depth of this prestretch bubble. When the bubble reaches the correct depth, the upper platen descends to drop a male mold into the bubble. A vacuum is drawn on the male mold and the sheet snaps back onto it. The parts are removed after cooled with high-speed fans.

The snap-back technique assures even distribution of material thickness, Long says. Each lid requires a different sized sheet. For example, the mixing-vat-container lid blank is 36-in. square and 0.312-in. thick while the blend-pot cover blank is 42-in. square and 0.375-in. thick. The flare composition-storage and slurry-pot blanks are from 0.25-in.-thick sheets that are 34 34 in. and 30 30 in., respectively.