3-D Molded Circuit Makes the Switch

Literally a standout, one of the finalists in the recent SPE Automotive Division's Most Innovative Use of Plastics competition gives molders a window on the state-of-the-art in 3-D molded circuitry. After a decade, the technology continues to evolve, and pioneers at Mitsui/Pathtek (Rochester, NY) are still at the head of the pack. This time, they've teamed up with Chrysler and Amoco to produce a two-shot molded circuit for the '96 Minivan's HVAC blower switch. In the process, tooling costs tumbled, weight dropped 10 percent, and the switch lost four metal parts.

The switch contains three assemblies that snap together: a base that contains the 3-D circuit; a housing; and a knob. Mitsui/ Pathtek created tooling for and molded the base assembly, while the other two ABS parts were molded by Precision Plastics (Columbia City, IN). Mitsui/Pathtek refers to its process as two-shot molding for MIDs (molded interconnect devices).

Meredith Suhr, VP marketing for Mitsui, told IMM that this part represents the first time a high current (17A) will be continuously applied to a 3-D molded circuit for an automotive application. Since the ability to carry current is directly related to the cross-sectional area of the copper, Suhr explained, designers set out to increase copper thickness without taking up extra real estate. Rather than flat "traces" or circuit paths, this part contains a raised circuit so that copper coats three sides instead of one.

For the first shot, Mitsui uses a mineral/glass-filled PES (Radel from Amoco) with a catalyst added to initiate the electroless copper plating process. The first shot is then overmolded with a second shot of an ABS/PS alloy (Mindel from Amoco) leaving the first shot exposed only where plating, and thus circuitry, is desired. The molded part is then treated to enhance adhesion of copper to plastic.

After copper plating, an electroless nickel plating covers all exposed copper. All electrical connector features are part of the molding/plating process, including the light socket and male connector pins that mate to the vehicle wiring harness. Six silver wiping contacts currently riveted to the base are targets for future elimination also using electroless copper. By going to a 3-D plastic design, Mitsui was able to eliminate the stamped metal busses and connector pins. In total, the design got rid of four metal parts. As a result, the MID saved $100,000 in tooling costs associated with the metal stampings. A comparison of the old manufacturing process vs. the new (see Table below) verifies other economies.

As far as the technology is concerned, Suhr reports that, among electronics suppliers, demand for the 3-D molded circuits is escalating. Working in a concurrent engineering format, the blower switch involved talents for roughly two years at Eaton Corp., which supplies the completed assembly, Precision Plastics, Amoco, Chrysler, and Mitsui. On the other hand, computer giant Microsoft recently collaborated with Mitsui on a switch mount for the company's new SideWinder 3-D Pro gaming joystick; that project took six months from design through production of multicavity tooling.