Plastics sharpen knife design

Engineering Design Consultants Ltd. recently put a modern twist on the ancient craft of knife making when it applied some engineering rigor to the Black Out knife from Kershaw Corp.

With the help of molded engineering thermoplastics and CAE tools, EDC engineers transformed a sleek design proposal from custom knife maker Ken Onion into a commercial knife that met cosmetic, size, and manufacturability targets. "The initial concept was a far cry from what we ended up with," says Rodger Garner, EDC's chief executive and the knife's lead engineer.

Getting a handle on materials. EDC began the design process by steering Kershaw away from the ABS sheet stock used for the handle exteriors by many knife makers today. Garner instead decided to injection mold the handle from a 30% glass-filled nylon 6. The use of injection molded plastics served cosmetic and ergonomic goals by fostering a contoured, hand-friendly shape that would have been nearly impossible to achieve with flat sheet. "Sheet stock is only capable of very simple shapes without some very complicated machining," he says.

Plastics also helped Garner integrate fastening features into the handles, cutting parts count and improving manufacturability. The handle's plastic shells-one for each side-feature integrated screw inserts and other internal fastening features for attachment of the knife's steel liners, springs, opening mechanisms, and the blade pivot. Beyond cutting more than six parts from the knife, the integrated fastening features allow the two knife halves to come together as sub-assemblies that sandwich the blade to form the assembled knife. This modular approach supports speedy assembly cells, while traditional designs relied on more of a craftsman-style build. "In the past, one guy would follow a knife around the manufacturing floor from start to finish," Garner explains.

Cut down to size. To optimize the handle design without the time-consuming cycle of empirical tests common in the knife business, EDC relied on its CATIA CAD system and extensive CAE work. Stress analyses, for example, helped EDC engineers create a slim package for the knife.

Garner recalls that his customers at Kershaw wanted a finished product that was only a half-inch thick. Yet in traditional designs, the blade, steel liners, and spring-like opening mechanism measure at least 0.60-inch thick by themselves. "That's before the handle shells are even added," Garner says, adding that a tolerance-stack-up analysis of Kershaw's initial design concept showed that knife would have been .25-inch over the target.

Garner managed to trim the overall knife package by thinning the metal liners and designing the ribbed plastic shells to absorb some of the side-to-side forces on the blade. Garner used FEA to optimize the liner thickness as well as the location and size of the ribs. Best of all, it provided answers quickly. "They used to just build models, put them in a vise, and break them," he says.

EDC also conducted a mold filling analysis, using Mold Flow software, to verify the moldability of the design-an important task given the presence of ribs and other delicate internal features.

Slashing costs. All of the virtual prototyping and parts consolidation paid off. According to Garner, the knife went from design to production in about three months-down from the eight months it normally takes. And thanks to its modular assembly and reduced parts count, the knife design doubled manufacturing productivity from 15 knives/hr on a good day to more than 30 knives/hr every day, Garner reports. Best of all, the time-to-production and manufacturing efficiency gains slashed costs. "The knife costs about 25% less to make than previous models," he says.

For more information about design services from EDC: Enter 537