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To review, a preform is a preshaped fber form. Its fbers are arranged in one, two or three dimensions in the approximate shape, contour and thickness desired in the fnished composite part. Traditionally, preforms are made in a separate mold and shaping process, not in the fnal part mold. Preforms can be made by spraying discrete chopped fbers combined with a binder over a form; by stacking tackifed continuous fabric plies; by weaving, braiding or knitting shapes, or stitching continuous fber materials; or even by combining several types of continuous reinforcements (see "Learn More," p. 29). Although preforms can be made of prepreg (see "Prepreg preforms for high-rate automotive apps" under "Learn More"), the vast majority are dry fber 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). 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 ofer roll goods with integrally woven three-dimensional structure. No matter the process, dry preforming fxes the fbers in desired orientations, at a predictable fber 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 efcient with the shortest possible mold open time." Te preform type depends on the need. Chopped fber preforms with randomly oriented fbers have isotropic properties. Although it is possible to adjust a spray pattern in a way that aligns and orients fbers to some degree, the load-bearing properties of such preforms are generally limited by the short fber length. For better part properties, continuous fbers are called for, and preforming for highperformance structures typically involves engineering fabrics, such as multiaxials. tion of DRAPETEST, an automated drapability tester, which won a JEC Innovation Award in 2012. To simulate fabric stress during preforming, a motor-driven piston moves upward through a fat circular fabric sample, and the force needed to deform the fabric is measured. 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 fber loops or breakage. An optional triangulation sensor is available to detect larger-scale defects, such as wrinkles, Moerschel explains. 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. DRAPETEST is based on an earlier prototype developed by multiaxial manufacturer SAERTEX GmbH & Co. (Saerbeck, Germany) under a research program funded by the German government. "Te tester gives manufacturers a chance to detect problems like thickening, creasing or bunching in a multiaxial fabric before the reinforcements are included in a preforming program," adds Moerschel. Springback also can cause problems, cautions Buckley. When an engineering fabric is subjected to pressure in the preforming Source (both photos) | AGFM Back to Basics Today preforms can be quite complex, a fact that multiplies processing challenges. For example, fabrics must be manipulated to the desired preform shape, but because glass and carbon fbers don't stretch, fber breakage can cause problems. Conformability, or drapability — how well the fbers 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. "During the preforming process, fber orientations change, which changes the local fber density and thickness," explains Buckley. "Failure to conform to the preform tool creates numerous process and performance problems." Determining conformability is a key issue during preform process design. "Te trend toward production of complex automotive parts requires noncrimp engineering fabrics for efciency, but unanticipated fber shifs during shaping can cause gaps and misalignment," adds Ulrich Moerschel of textile testing instrument developer Textechno (Mönchengladbach, Germany). His frm has developed a way to detect conformability problems with the recent introduc- Visible in these samples are the wrinkles, bunching and even fber breakage that can occur when multiaxial fabrics for preforms are manipulated and stressed during preforming. Testing is needed to determine how "conformable" a fabric is or how easily a fabric can be draped during the preforming process. . CT oCTober 2013 Forming a uniForm preForm 23