Fending off an attacker takes nerves of steel, quick reflexes, and maybe a bit of luck. It can also take a good engineering team, as Walter Cardwell found out when he invented a new kind of self-defense spray.

His new Spitfire system, which has more than 30 patented features, packs replaceable pepper-spray canisters into a quick-draw housing. "Other sprays on the market are really designed for offensive use," Cardwell says, explaining that they use bulky canisters and often require cumbersome actuation procedures. "You can't aim and spray them fast enough for a typical self-defense situation."

The Spitfire, by contrast, is designed for speed: At three-and-a-half inches long, it hangs on a key chain when not in use, but it quickly separates when needed, thanks to a ball-and-socket break-away feature in its plastic end cap. And unlike most other self-defense sprays, the Spitfire canister and nozzle are both oriented in the direction of fire for the sake of intuitive aiming. "If you can point your finger, you can aim the Spitfire accurately," Cardwell says. Most important, the Spitfire also features an innovative trigger mechanism to make firing easy-but not too easy. Cardwell came up with a simple sliding firing button with a built-in safety. Push it forward and down in one fluid movement to release the spray. But spring pressure on the button and the roughly 8 lbs of force needed to overcome the canister's custom-designed aerosol valve keep the spray from firing off accidentally, says Cardwell.

Before he could take his Spitfire from the drawing board to the streets, Cardwell and his industrial design firm, Design Edge, first needed some help tweaking the initial design to improve manufacturability. So they turned to Phillips Plastics (Phillips, WI), whose engineers not only improved overall moldability but also eased what at first shaped up to be a difficult assembly process.

Taking aim at assembly. Joining the two halves of the Spitfire housing presented a challenge right from the start because of materials selection. Designed both to be visually appealing and to provide a good grip, the Spitfire housing is injection molded from a PC/ABS and then selectively covered with an 80 Shore A Hytrel elastomer (from DuPont) in a two-shot molding process. This use of an overmolded elastomer at first wreaked havoc with the sonic welding process that brings the housing halves together. As Phillips project engineer Dave Munkwitz explains, when the sonic welder transfers energy through the part, the elastomer melts faster than the rigid material, producing visual and functional defects. To solve the problem, Phillips engineers recommended a change in the welding horn shape so that it would only come into contact with the rigid material. "From our experience we knew we had to avoid the overmolded areas on the Spitfire," he says.

Further complicating welding matters, the Spitfire's assembly process put the sliding button in place prior to welding-in contact with the case halves. At first, Phillips suggested the common solution in these cases: mold the button and case from dissimilar materials that weld at a different frequency. Though workable, this solution would have required additional color matching and procurement steps. So Phillips engineers instead focused on how part design changes could help channel the welding energy within the part to bypass the button. "We added saw tooth features to the housing to act as energy directors," says Munkwitz. When the energy goes through these features, they have a tendency to weld there first, leaving the sliding button alone. "The energy goes through the parts in such a way that there is no energy directed to the button," he says.

Welding relief. Changing the part de-sign also averted other welding problems. For example, Phillips engineers had to carve out a relief area along the joint near the nozzle in order to stop the front of the housing from deflecting during the welding process and interfering with the path of energy into the parts. "We were getting a poor-quality joint in that section until the change," Munkwitz says.

And they had to fine tune the size and location of the assembly posts that align the two housing halves and hold the part together during the welding process. They did so in order to reconcile a manufacturing paradox: On the one hand, the case halves need a precise, tight fit because they have internal threads that mate with the cap and also hold the canister in place; on the other hand, the sonic welding process, which relies on sliding friction to create a joint, required some relative movement between the two housing halves. "We had to find a happy medium," Munkwitz says. "We went through quite a few iterations to vary the alignment posts to achieve the best sonic weld without compromising the thread alignment."

All the work done to facilitate the welding process has paid off, according to Cardwell. "At first, we couldn't weld the things," he says. Yet for the next generation of housings, welding won't be a problem at all. With more engineering support from Phillips, Cardwell's next generation of Spitfire sprays will move away from welding altogether-and use a hand-assembled snap fit design instead.