A chopped glass preform is typically made by spraying chopped fiber and a liquid or powder binder onto a part-shaped screen. A vacuum is pulled to hold the fibers in place until heat or ultraviolet (UV) light melts or reacts the binder. These binder systems, however, require application streams and equipment separate from the glass chopping stream, which can make it difficult to control the fiber/resin mix. Inconsistencies between the fiber and resin streams can impact key variables, including "mat weight,"or areal density, and tensile strength, which must be sufficient to prevent fiber "wash"during resin injection. And binders can adversely affect part strength by interfering with the final resin/fiber bond. "The 'holy grail' of directed fiber preforming has been a perfect binder that will hold the reinforcing fibers together without adversely impacting the process and part,"says Steve Morris, senior R&D project engineer at PPG Industries (Pittsburgh, Pa.).
Sotira (a division of Sora Composites Group, Changé, France), in collaboration with automakers Ford Motor Co. (Dearborn, Mich.) and Aston Martin (Gaydon, U.K.), has adopted PPG's trademarked PREFORMance, a new roving that commingles E-glass filaments and proprietary thermoplastic fibers (6 to 7 percent by weight). This "string binder"reportedly has eliminated binder-related problems during production of structural body panels. Because the thermoplastic binder is part of the roving, control over the amount of applied binder is built in, and transfer efficiency is said to be nearly 100 percent. Further, the binder is dispersed more uniformly throughout the preform. Here's why: Morris says microscopic studies have shown that the thermoplastic fibers adhere to the glass fibers only at small discrete contact points, like a fine spider web, which means they don't interfere with resin/fiber bond.
One of several automotive fabricators now using the new roving to make preforms, Sotira is a licensee of Ford's Programmable Preforming Process (F3P), a robotic system that can create body-part-sized, net-shape preforms in less than four minutes. Preforms are subsequently converted via resin transfer molding (RTM) to produce parts for two Aston Martin models: door opening rings, decklid surrounds, sills and decklids for the DB9 coupe and tailgate surrounds and tailgate inners for the V8 Vantage.