Casting Study: Two-Piece Housing

When Black & Decker decided to introduce a new manual staple gun, designers and engineers started with a clean slate. Their goal was to integrate efficiency, simplicity, and ergonomics into the new tool. The result is a radical design that moves the handle pivot point from the front of the gun to the rear. Accommodating all these changes is a two-piece housing. In developing the housing, engineers strove to achieve very specific goals. Since competitive hand staplers are fabricated from steel, engineers wanted the new design to have a comparable feel and texture. This ruled out a plastic housing. The engineers also designed for operational simplicity, which means fewer parts. Reducing the high piece count of traditional staplers required a housing with a significant level of detail. B&D engineers also wanted good wear properties and impact resistance.

Last, but not least, an attractive, functional surface finish was required for acceptance in the consumer market. The light metals–aluminum and magnesium–were ruled out for functional reasons. Die cast aluminum does not provide the required dimensional precision. Magnesium offered the detail and precision, but not the impact strength. Designers then turned to zinc die casting. The net shape of die-cast two-piece housing satisfies all required mechanical, functional and finishing objectives. Besides functioning as the housing, the two cast halves provide the lever pivot points, spring attachment points, cam surfaces, staple guides and feed guides. As a result of the new firing mechanism and the housing’s cast-in details and combined functions, the PowerShot stapler has about 50 percent fewer parts than the leading competitive model. The die casting process also allowed the engineers to style the stapler. Not limited by the confines of stamping design, the PowerShot staple gun incorporates contoured lines and contemporary styling. Zinc’s ductility is a further asset after assembly. The housing is held together with tri-lobed self-tapping screws. These screws take advantage of zinc’s material flow that functions as a stop against the lobe profile and prevents the screw from backing out.