The growing palette of visual-effect plastics may give industrial designers a way to create alluring products. But pity the poor engineers who have to make these materials work in the real world. After all, the very fillers that impart metallic, smoky, pearlescent, or other looks can also make knit and flow lines more pronounced, spoiling the spiffy appearance designers wanted in the first place. Engineers at GE Plastics, which makes Visualfx special-effect plastics, have recently found ways to work around these lines with the help of advanced processing technologies.
No more knit lines. Large electronic products, which increasingly try to catch buyers' eyes with striking aesthetics, illustrate the conflict between cosmetics and molding effects. Because parts such as TV cabinets can be so expensive to paint, these big parts might seem like a natural fit for plastics that can "mold in" the right look. Yet their size and shape also make them susceptible to knit lines that threaten the cosmetic goals, according to Nick Abbatiello, GE's global technology manager for display products.
GE process engineers have been working with sequential valve gating (SVG) as one solution to knit lines. In traditional molding, the multiple gates needed to fill large parts open at the same point in the injection cycle, providing little control over the location of merging flow fronts. Yet open the gates in a carefully timed sequence, as SVG does, and the knit lines can be moved around the part. "That allows you to place weld lines in an inconspicuous location," Abbatiello says. GE recently used SVG to help a customer mold a huge unpainted bezel for a 40-inch television. Made from a light-diffused Lexan polycarbonate, the bezel required high gloss, depth of image, and no gate vestige. "The appearance had to be perfect," Abbatiello recalls, noting that the part's weld lines have become invisible to even a trained eye.
Bye, bye flow lines. With metallic plastics, the chief cosmetic threat comes from the orientation of the tiny metal flakes in the direction of flow, leaving visible lines on the part surface. To keep this excessive flake orientation from occurring, GE uses a pair of processing technologies. One is the SCORIM process, which vibrates the melt during the molding cycle. This vibration of the melt orients the flakes, according to Keith Dunlap, North American program manager for aesthetics. The other technology is proprietary thermal mold coating. It enables a super-fast heating and cooling of the melt in order to provide some control over when and where the flakes freeze in place, Dunlap adds. Used together, these two technologies promise an alternative for painted or metallized parts. Dunlop says a variety of applications are under consideration—from small telecommunications parts to large automotive parts like wheel covers.
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