Tough part needs a tough process

The chuck on this professional-grade cordless drill required a material stronger than plastic. TXM fit the bill, offering both strength and light weight.When Black & Decker approached Thixotech Inc. (Calgary, AB), the leading maker of small household appliances and power tools was looking for an edge for its new line of professional cordless drills. Manufactured by B&D under the DeWalt brand name, these professional tools used by building contractors get some heavy-duty use and therefore must meet tougher standards than power tools designed for home or personal use.

First, B&D looked at materials that could withstand the rigors of the professional contractor. It outlined specifications for the various components of the Terminator cordless drill and matched those specs with materials that would meet the requirements, explains John Tesensky, president of Thixotech. Plastic was chosen for the housing, but the chuck component had to be stronger and lightweight—something thermoplastic couldn’t offer in that application. That’s where Thixotech brought in its capability to injection mold magnesium components through the registered Thixomolding (TXM) process.

“The characteristics of strength and light weight—the weight of magnesium vs. steel or other metals—were the first factors that had B&D seriously considering magnesium for this application,” says Tesensky. “The next question was, could we use Thixomolding given the tolerances established for the part? Obviously, the answer was yes.”

B&D benchmarked TXM and Thixotech against aluminum diecasting and more conventional methods to compare costs, explains Tesensky. “[B&D] wanted what it felt was a superior product with greater strength and light weight, and we were very cost competitive.”

The plan was to design the three different gear case housings and one gear case cap using magnesium to reduce weight and, using TXM, reduce production costs via net-shape molding.

Problems with conventional hot chamber magnesium diecastings highlighted the advantages offered by TXM: good dimensional consistency, shot to shot; little or no machining required to achieve tight tolerances; and good, consistent surface cosmetics and aesthetics.

Tool Design For TXM
Designing molds for TXM is not unlike designing molds for thermoplastic molding. Requirements such as uniform wall thickness and avoiding thick-to-thin-to-thick cross sections are critical. Also necessary are adequate radiuses in all of the part’s corners and employment of proper gating techniques so the part can be molded without secondary machining, notes Charles Van Schilt, chief technology officer for Thixotech.

Gating, in particular, is critical. The gates must be larger to accommodate the high-viscosity magnesium slurry and ensure proper flow. Thixotech uses a vacuum venting system to facilitate flow. “We don’t use overflows like diecasting,” notes Van Schilt. “We use vacuum venting to help fill the die [mold] because we’re going in at very high speeds and otherwise we can’t get the air out fast enough. To go in at the high speeds required, a vacuum system is necessary to partially evacuate the air so we’re not fighting air to fill the part.”

One of the problems caused by the design was a blind area in the top of one part that made getting a water-soluble wax mold spray into the area a challenge. Mold spray is necessary to establish a barrier between the molding material and the mold itself to prevent soldering in the mold. A Rimrock 410 Reciprocator spray system with a custom spray manifold assembly was used to eliminate this potential problem.

Fixes on the Fly
The Black & Decker project took about 15 months from concept to full production runs, which Thixotech performs. Parts were shipped six to seven months after getting the order, but Van Schilt says R&D continued on the molds during production ramp-up. Because of design changes, there was no time for preproduction runs to debug the molds.

Another challenge to the program came from the thermal front, where fluctuating process temperatures wreaked havoc. “This is still a fairly immature technology,” says Van Schilt. “We didn’t understand that the machines are not capable of running long production without human intervention in the area of the heating system.”

To solve that problem, Thixotech process technicians monitored the solids more carefully, as solids loading rates heavily influence process temperature. High solids content tends to reduce temperature; conversely, low solids content tends to increase it.

Thixotech found 5 to 30 percent solids loadings, which caused the unwanted temperature fluctuations. “Now we monitor that and keep the temperature within a narrow window, which reduced the scrap rate from about 30 percent at one time to less than 10 percent now,” Van Schilt says.

Thixotech’s ability to mold parts in multicavity tools helped B&D be cost effective with the new cordless drill product line. One component is produced in an eight-cavity mold; the others are made in four-cavity molds. Currently, the company molds about 50,000 parts/week for the B&D project.

Tesensky says that Black & Decker is having “outstanding acceptance” in the market for the new cordless drill, and Thixotech is swamped with parts orders. “From our point of view, we’re committed to this process and want to see it do well,” he adds. “We look at solid applications that we feel can bring benefits such as thin walls, complex shapes, and tight tolerances—shoe-box size or smaller—and the Black & Decker project fit all those.”