Getting a handle on a thrifty screwdriver's new design

Replacing a one-piece, extruded vinyl predecessor, Stanley turned to a South Carolina firm for a molded two-material alternative.

When you front-load a part program with a $1 million-plus investment in technology, every cycle creating every part thereafter must be the picture of efficiency in both cost and production. To ensure a flawless process that creates savings along with parts, you have to start with a high-quality design.

This scenario was laid out to McKechnie Plastic Components (Easley, SC) by The Stanley Works (New Britain, CT), a global manufacturer of hardware and tools. Looking to redesign its Thrifty screwdriver set and replace the existing one-piece, extruded vinyl model, Stanley approached McKechnie for a molded solution. It wanted to spruce up its long-time product, but without giving it a price tag that would make the Thrifty brand name seem out of place.

McKechnie examined the product and opted for a multicomponent solution. Processes like structural foam and gas assist were already in its repertoire, but two-material was not. To fill the need, McKechnie turned to Ferromatik Milacron Europe, which delivered two 450-ton, K-Tec multimaterial presses. With machines in place, McKechnie then focused on part design and, ultimately, the complicated tool needed for an optimized process.

As the Tool Turns

From the start, in cooperation with Milacron, McKechnie worked with German toolmaker Foboha GmbH (Haslach, Germany) and its rotating stack mold technology. Foboha and Milacron are partners on the technology, which is designed to work in conjunction with Ferromatik’s multimaterial K-Tec line.

The rotating stack mold presented McKechnie with an opportunity to create the screwdriver handles at a dizzying rate, using just two 450-ton presses to achieve Stanley’s goal of 35 million to 40 million Thrifty handles within a year. (See “Rotating Center Plate in Mold Turns Some Heads,” November 2000 IMM, pp. 70-72, and “New Turns for Multicomponent Mold Revolution,” November 2001 IMM, pp. 59-60 for initial reports on the mold.) Use of the rotating stack mold also presented some unique challenges to the handle’s design, according to McKechnie’s VP of sales and engineering Steve Olson.

“The biggest challenge involves the shutoffs that you must be able to achieve,” Olson explains. “How can we control the substrate, bring it around, present it to a second set of cavities, and achieve a clean shutoff?”

Foboha’s 90° rotating mold features 128 cavities in all, 32 on each side of the cube. The substrate, molded from a yellow masterbatch polypropylene, is shot in station or side one and then cooled in station two. A black polypropylene, with approximately the same shot size as the yellow substrate, is then overmolded in station three. At station four, finished handles are ejected.

But before the tool could start whirling out parts, there were several problem areas that Olson and McKechnie had to address in the handle’s design phase. Primary among these was achieving sound shutoffs, especially around the handle’s intricate logo.

Signed, Sealed, and Shut off

Using what Olson terms “basic engineering,” many of the shutoff problems were solved. However, the screwdriver handle’s geometry still created a unique challenge, since the tool shuts off not only around the core pin, but also around a molded part.

When the mold rotates, the second set of cavities must shut off on the handle’s three lobes and around the screwdriver’s collar. To overmold the black properly, the handle must be lined up and concentric. If this condition is not met, the shutoff is lost and the black coloring flashes into the yellow portions, which must remain visible for aesthetic reasons.

The most important of those yellow sections is Stanley’s trademark yellow lettering within a black oval. Although a cosmetic concern, in terms of the product’s brand, it was both vital and difficult to achieve.

“The logo becomes the overwhelming, dominant feature of the product,” Olson says, “so one of the things that was challenging was getting the logo to fill properly, while minimizing issues with knitlines or nonfill areas.”

The design team went through four iterations and numerous Moldflow simulations to redirect flow and change how the lettering was supported in the tool. The handle has two gates with the yellow substrate being fed from the end into a 1.5-mm gate. The black overmolding is shot into a dimple for cosmetic reasons and flows through a 1-mm gate. Shutoff pins are employed to prevent drool.

Only after 10 months of weekly teleconferences, visits between Stanley’s U.K. operation and Milacron and Foboha in Germany, and one McKechnie engineer spending the better part of the summer and fall in Germany, did the program come online in early 2002. Through all that, Olson and McKechnie found a design that could do its job at the right price.

“[The final design] was going to achieve the best combinations of processing functionality,” Olson explains, “and of course that overwhelming driver for all of us: cost.”