Wood: Newest Electronic-Enclosure Material

Mass-produced electronics enclosures rarely incorporate any wood. And if they do, that wood usually falls into one of two categories-faux or imitation. But real wood, as well as a wide variety of other decorative materials, could soon become a more popular way to dress up molded plastic parts, thanks to technology developed by Inclosia Solutions.

Inclosia, a business unit of Dow Plastics that designs and supplies electronics enclosures, two years ago introduced its EXO method for adding decorative skins to plastic housing components. The company started with fabrics and then moved onto real and synthetic leathers. Last month, it expanded its materials line-up with the introduction of wood and metal skins. "Wood in particular gives these enclosures a real hand-crafted look similar to what you might find on a classic wooden boat or very expensive car," says Tom Tarnowski, a mechanical engineer and Inclosia's business manager.

But make no mistake; these new wood- and metal-skinned enclosure components do lend themselves to mass production. In fact, Tarnowski says the process generally won't make sense until production volumes reach at least 20,000 pieces. That's because EXO technology boils down to a two-shot injection-molding process that requires sophisticated tooling, molding machines, and processing know-how. While the two-shot molding itself isn't new, Inclosia has developed some proprietary innovations that allow the process to work with relatively delicate fabrics and veneers. To take one example, Tarnowski says the company has found ways to design around the CTE mismatches between the plastic substrates and decorative skins. "Our expertise involves knowing how to tailor the overmolding process to the various material combinations and part geometries," he says.

Whatever the decorative material, the EXO process consists of three key steps: It starts with the robotic or manual placement of a precut fabric, wood, or metal insert inside the mold cavity. The first shot of >> plastic, typically a rigid thermoplastic like PC/ABS, is then injected onto the back of the decorative material. The second shot of plastics, usually an elastomer, finally encapsulates the edges of the first two materials, creating the finished part. Tarnowski notes that the encapsulation shot can be either proud or flush relative to the skin, and the size of the encapsulation can vary depending on styling goals.

The EXO process does have a few variations, depending on the type of decorative material and the shape of the part. For example, some material combinations require an adhesive or adhesion promoter between the decorative skin and the first shot of plastics. In all cases, the typical bond strengths between the decorative skin material and its substrate exceeds 10 pli, Tarnowski says.

Working with skins

The part geometries possible with EXO's latest decorative materials tend to relate more to the usual constraints of thin-walled plastic parts than to the wood or metal. For example, Tarnowski says that "the practical size limits for the EXO process are based on the realities of injection molding." And he's reluctant to rule out too many design possibilities this early in the game. "Every time I say, 'We can't do that,' someone in our lab figures out a way," he says. In general, though, flat components with absolutely no curved edge for the encapsulation represent the worst scenario.

Still, the choices of decorative skin material do impose a few design constraints. Wood veneers, for instance, do best in parts without deep draws or complex curves. And EXO parts with wood have a minimum total thickness of 1.3 mm-a figure that includes the veneer, underlying plastic, and any adhesives.

Parts skinned with thin metal sheets or even foils can go even thinner-down to as little as 1 mm. And unlike the wood veneers, some of the metal skins can be preformed to allow deeper draw parts and more complex curves. Metal skins may also provide some benefits beyond the purely decorative. Inclosia researches are looking into the possibility of using the EXO method to provide EMI shielding. "EXO metal can be used for shielding but may require grounding to the electronic components," Tarnowski says, adding that there could be other functional uses related to wear and friction.

EXO metal could play a structural role too, perhaps enabling thinner plastic wall sections. As Tarnowski explains, the stiffness of an EXO part is determined primarily by the plastic substrate material and design. "Stiffness comes from the plastic," he says. "In fact, the plastic helps avoid the oil-can effect that you would get from using a thin metal alone." But he goes on to argue that thin metal skin can contribute to the plastic part's structural performance-because the addition of metal skin increases the distance from the structure's neutral axis and thus improves its bending stiffness.

With the metal and wood capabilities launching just last month, Inclosia has not yet delivered any enclosures that sport the new skin materials, but the company has worked up a variety of prototypes and models. During a recent meeting, Tarnowski handed out working USB "thumb drive" prototypes with wood and metal trim. And the company has likewise constructed wood- and metal-clad models of notebook computers and mobile phones. Tarnowski says the latest EXO technology is currently in the hands of a few large consumer electronics makers, and he expects that the first wood-trimmed products will appear by the end of 2005.