Pittsburgh—Who says true beauty can't be skin deep? Bayer Corp. has developed new technology that infuses the surface of thermoplastic parts with color. Called Aura, this patent-pending process represents a big departure from traditional molded-in color and could change the economics of coloring short-run, made-to-order, and mass-customized products. "We start with a clear or white part and add the color downstream of the molding press," says John Skabardonis, Bayer's manager for consumer markets.
Aura's infusion process takes place in a heat bath containing an aqueous dispersion of dye and a proprietary, polymer-friendly surfactant. Skabardonis explains that the heat opens up the polymer matrix, allowing the surfactant to transport the dyestuff into the plastic. Depending on the type of plastic and the process conditions, Aura colors penetrate up to 10 mils deep. Parts stay submerged for anywhere from a few seconds to five minutes, with immersion time, the processing temperature, and the type of thermoplastic plastic determining the final hue.
Bayer researchers have successfully applied the new process to the company's core engineering plastics, including PC, ABS, SAN, PC/ABS, PC/PET, nylon, and TPU. "It works with every material in our portfolio," says Skabardonis, adding that Bayer has used Aura for parts as small as screws and as big as 4- x 8-ft sheets. Compared to their uncolored counterparts, pieces put through the Aura process haven't shown any measurable differences in their physical or mechanical properties. "The properties remain essentially the same as those of the base resin," Skabardonis says. "If you start out with a polycarbonate part, you end up with a polycarbonate." He attributes the consistency of mechanical properties to the fact that the colorant remains so close to the surface and, even then, constitutes a relatively minor portion of the surface compared to the much larger polymer molecules. And because Aura parts actually absorb the color, the dimensions of the part don't change as they could with a thick coating.
Parts colored by the Aura process can achieve a variety of aesthetic looks—including transparent, translucent, and opaque—that closely mirror what's available with traditional molded-in color. "We use the same sorts of colorants as pre-compounded resins, except that we introduce them after the molding process instead of before," Skabardonis says. And another process may also see use by adding a splash of color to existing products without having to modify the tooling or molding machines. The process can also create a two-tone effect by selectively etching away the top layer of resin along with the applied color.
Driving down cost. Where Aura may really shine, though, is when it comes piece-part costs. Owlia acknowledges that the material costs for Aura match those of comparable pre-compounded colors. But she argues that Aura nevertheless could reduce the cost of color dramatically by eliminating those costs associated with inventory and frequent molding machine changeovers. These in-process costs typically stand out at low production volumes. "But Aura can even color a single part economically," she says. "You wouldn't normally want to do compounding and clean out the molding machine for just one part." She expects that in practice, though, Aura will address production volumes starting at about 100 pieces.
THis chair started life as a clear plastic part, but a new process for imparting color to individual parts after the molding process gives it a blue hue.
Likely applications for Aura span a range of components in which short life cycles or a desire to mass customize will keep production volumes of any given color relatively low—even though total production volumes could be in the thousands. Skabardonis cites consumer electronics, house wares, and automotive interiors as good candidates for Aura. It may also see use in cosmetic prototyping. Designers can dip a part to create a new look on the fly, and the process is also reversible. Parts that have gotten too dark can be lightened with a second trip to the bath. "There's a lot of room for creativity," Skabardonis says.
Not for every part. The process, however, does have some limitations. "Aura isn't a cure-all," Owlia concedes. Parts that have high production volumes and unchanging color needs, for example, would usually be more economically served by pre-compounded resins. And so would parts with certain special effects. Aura can't yet impart metameric or metallic effects—though the latter can still be accomplished by compounding the metal flakes in the base resin. Finally, the Aura process currently does not have FDA approval for food contact uses. Owlia says that these last two limitations will disappear as Bayer continues to develop Aura technology. The process may one day do more than just color parts; Bayer researchers have already begun work on using Aura to add functional attributes, such as scratch resistance, to plastic parts.
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