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Magnesium and elastomer join forces against the elements

Magnesium and elastomer join forces against the elements

Beaverton, OR -When joined in an overmolding process, diecast magnesium and thermoplastic elastomers don't seem to have what it takes for a happy union. After all, as-cast magnesium parts can't hold the tight dimensional tolerances they need to serve as an injection molding insert. Machine tighter tolerances into the magnesium part, and manufacturing costs skyrocket. Then consider all the tooling problems arising from dissimilarities between the two materials. This materials marriage just isn't worth the effort. Or is it?

A new electronics enclosure from Tektronix refutes conventional wisdom by successfully combining a polyurethane elastomer with diecast magnesium. More than just adding the kind of supple covering sported by so many consumer products, this uncommon materials combination helped Tektronix engineers improve the enclosure's performance and consolidate parts. Despite the complexity of the overmolding project and the need for additional tooling, the new case's streamlined component count resulted in a 20% savings compared to similar plastic cases, according to Jim McGrath, the senior mechanical engineer and project leader responsible for the enclosure. "The project was driven by functionality rather than cost, but we also wound up with a more cost effective case," he says.

Developed for the new NetTek line of modular testing equipment, the main enclosure consists of three overmolded magnesium parts and measures 9x12x3 inches overall. Separate test modules plug into this base unit, which will ultimately serve as the platform for a collection of testing tools. "The base unit allows the customer to interchange modules and add to the rear of the case depending on testing requirements," McGrath explains. The first module, for example, houses an "optical time domain reflectometer" used in fiber-optic analysis work. Each module adds one more elastomer-covered magnesium part to the two found on the main unit.

The diecast parts are produced in a hot-chamber process by Chicago White Metal (Bensenville, IL) and are rich with features-including a set of holes and gripper teeth that form mechanical attachment points for the elastomer. To make them suitable for injection molding, the parts even get some finish machining (to plus or minus 0.002 inch) in those areas where the magnesium part serves as a molding shut off. For the elastomer, mechanical engineer Steve Lyford picked a 70 Shore A grade of Desmopan polyurethane from Bayer Corp. (Pittsburgh). He chose this material over other thermoplastic elastomers because it offered both good flow and adhesive properties-the latter to permit bonding with the NetTek's PC/ABS access doors and labels.

Fighting the good fight. The two enclosure materials together fight what McGrath describes as an "engineering battle." Because it has to protect delicate electronics innards from rough treatment in the field, the NetTek case has to satisfy tough impact requirements without sacrificing electromagnetic shielding, environmental resistance, and thermal-management goals.

Magnesium beat out engineering thermoplastics, the traditional choice for portable enclosures, on four counts. For one, it imparted ruggedness. McGrath notes that the ten-pound unit has to survive a battery of physical tests, including a 48-inch drop test onto concrete. "Many of the plastics traditionally used for housings would just disintegrate," he says. For another, the magnesium adds a bit of torsional rigidity to protect the NetTek's glass touchscreen display. And magnesium also did a better job meeting thermal management requirements that call for a heat dissipation capacity of 6 watts per module. "That would have been a stretch for most plastics," he says, noting that the magnesium easily handles 8 watts per module with good internal heat sinking to the outer case. Finally, magnesium provided electromagnetic shielding without the need for expensive coatings or gasketing materials.

The polyurethane elastomer makes a contribution too. Beyond its obvious ergonomic contribution as soft-touch grip, it also absorbs some of the impact during the drop test. And surprisingly, it even helps a bit with the thermal management, with McGrath reporting that the elastomer-covered case remains a degree or two cooler than the magnesium alone. He attributes that unexpected improvement to the surface area and roughness added by the elastomer, which averages about 0.070 inch but thickens into a set of bumpers around the outside of the case.

But the elastomer's greatest contribution has come from function integration. McGrath can reel off a list of design features that have been incorporated into the overmolding. To take two examples, it provides a mounting bed for the NetTek's display, and it forms the living hinges that attach the unit's access doors. It also serves as built-in weather sealing, which in turn helps protect users from the potentially unhappy consequences of using electrical equipment in wet environs. "The elastomer helps protect the users from hazardous voltages," says McGrath.

All these benefits didn't come easy. It took two years to bring the project to completion. Building diecast tools and injection molds that work together took up most of that time. At one point, the magnesium inserts even needed to be hand-fit to the cavity of the overmolding tool. "All our risk in this project came from integrating parts from the two processes," McGrath says. But the pay-off has been worth the effort. "We set out to create a superior package for our instruments," he says. "And we did."

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How to make the marriage work

A diecast magnesium part doesn't easily work as an overmolding substrate. In fact, it probably won't work at all without some careful initial planning. "We couldn't have integrated the overmolded plastics at the tail end of the project," says Jim McGrath, the lead mechanical engineer on the NetTek analyzer.

Early in the process of creating this new enclosure-long before anyone even looked at a piece of steel, much less cut one-McGrath tapped the expertise of his entire supply base. Three different tool makers, a CNC programmer, two injection molders, the material supplier, and the diecaster all consulted on the project. "They helped us address our manufacturing problems up front," he says.

And many thorny manufacturing problems did at times threaten the project's success. Just take a look at McGrath's checklist of design-for-manufacturing considerations:

Casting tolerances. Ensure that the diecasting process can provide tight enough tolerances to serve as repeatable cast insert in the overmold tool. "The nominals may be okay, but it's the part-to-part variation you should be concerned about," McGrath stresses.

Diecasting flatness and shut-offs. Determine the flatness spec needed for shut-offs between the die-cast insert and the steel overmolding tool. Also determine how tight a fit you need for repeatable shut-offs in the face of plastic injection pressure levels that can hit as much as 15,000 psi. "If the overmolding flashes, your process will be out of control," McGrath notes. And poor control can be costly.

Parting-line tolerances. When designing the overmold tool, compensate for the inherently larger parting-line tolerances from the diecasting process. According to McGrath, Tektronix accounted for that by machining parting line regions-from an as-cast tolerance of around +0.015 inch down to plus or minus 0.002 inch needed for a good shut-off.

Insert fitting. Tightly fit the insert on the core-side of the overmold tool. Says McGrath, "The high injection pressures will crack the die cast insert if it's not properly supported during overmold injection." Tektronix had to get a fit in the 0.005 to 0.010 inch zone.

Overmold gating: Establish the optimum gate location for the elastomer to flow over the comparatively cool die cast insert. To take some of the trial-and-error out of this step, Tektronix prevailed on its materials supplier, Bayer Corp., to perform mold-filling simulations early in the design process.

Thermal effects: Compensate for the fact that the die cast part will heat up in the overmolding tool and expand. "In our design, the front case expanded as much as 0.020 inch, which can pretty much eat up any tolerances," McGrath says.

Elastomer shrinkage: Compensate for the urethane elastomer shrinkage as the overmolded part cools down.

Ejection: Finding the right ejector configuration for the overmolding tool had a "critical effect" on the cosmetic appearance of the enclosure, McGrath reports.

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