Deformation data and images are displayed on a computer, and image analysis technology developed at the Faserinstitut Bremen (FIBRE, Bremen, Germany) enables automatic fabric fault detection. An optional triangulation sensor is available to detect larger-scale defects, such as wrinkles, Moerschel explains. A camera, with appropriate illumination, photographs the sample at intervals during the piston’s travel while the entire sample is rotated so technicians can inspect the surface for gaps and fiber loops or breakage. To simulate fabric stress during preforming, a motor-driven piston moves upward through a flat circular fabric sample, and the force needed to deform the fabric is measured. His firm has developed a way to detect conformability problems with the recent introduction of DRAPETEST, an automated drapability tester, which won a JEC Innovation Award in 2012. “The trend toward production of complex automotive parts requires noncrimp engineering fabrics for efficiency, but unanticipated fiber shifts during shaping can cause gaps and misalignment,” adds Ulrich Moerschel of textile testing instrument developer Textechno (Mönchengladbach, Germany). “Failure to conform to the preform tool creates numerous process and performance problems.” Determining conformability is a key issue during preform process design. “During the preforming process, fiber orientations change, which changes the local fiber density and thickness,” explains Buckley. Conformability, or drapability - how well the fibers of a multilayer fabric shear and change position during shaping, without losing continuity - depends on the fabric type, stitch density, stitch tightness, roving or tow density and whether additional materials (e.g., mats) are added to the preform. For example, fabrics must be manipulated to the desired preform shape, but because glass and carbon fibers don’t stretch, fiber breakage can cause problems. Today preforms can be quite complex, a fact that multiplies processing challenges. No matter the process, dry preforming fixes the fibers in desired orientations, at a predictable fiber volume, and minimizes the hands-on labor required for layup, says Buckley: “It allows you to better achieve a net-shape part, provides uniformity, part to part, and makes the molding process more efficient with the shortest possible mold open time.” Newer, nontraditional preforming concepts that combine thermoplastic tapes and mats are now in the mix as well, and several suppliers, including Sigmatex High Technology Fabrics (Benicia, Calif.), now offer roll goods with integrally woven three-dimensional structure. Although preforms can be made of prepreg (see “Prepreg preforms for high-rate automotive apps” under “Editor's Picks"), the vast majority are dry fiber forms that are subsequently impregnated with resin in a closed mold process, such as resin transfer molding (RTM) or vacuum-assisted resin transfer molding (VARTM). Preforms can be made by spraying discrete chopped fibers combined with a binder over a form by stacking tackified continuous fabric plies by weaving, braiding or knitting shapes, or stitching continuous fiber materials or even by combining several types of continuous reinforcements (see “HIgh-volume preforming for automotive application,” under "Editor's Picks," at top right). Traditionally, preforms are made in a separate mold and shaping process, not in the final part mold. Its fibers are arranged in one, two or three dimensions in the approximate shape, contour and thickness desired in the finished composite part. To review, a preform is a preshaped fiber form. The good news is that equally fast, cost-effective and sophisticated engineered preform technologies are being developed in parallel. Many are developing structural composites in mass-produced vehicles for weight reduction on the strength of recently developed rapid infusion processes designed to meet high auto build rates. These two- and three-dimensional constructions are increasingly capable of reinforcing high-performance structural composite parts, but most have failed to enter the manufacturing mainstream in the automotive industry due to their perceived high cost, the auto industry’s change-averse culture and some difficult-to-surmount engineering hurdles.ĭuring the past decade, however, more stringent fuel economy and emissions standards have overcome automakers’ resistance to change. More recently, engineered preforms have been developed through the use of automated knitting and weaving machinery. For most of that history, however, the vast majority were made with chopped glass fibers directed over perforated metal forms in vacuum-forming processes - think molded transit bus seats, for example. Preforms have been used for almost 80 years in infusion molding processes.
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