High-Performance Composites

MAR 2013

High-Performance Composites is read by qualified composites industry professionals in the fields of continuous carbon fiber and other high-performance composites as well as the associated end-markets of aerospace, military, and automotive.

Issue link: https://hpc.epubxp.com/i/110847

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Page 36 of 67

TherMal Press Curing Autoclave alternative come into full physical contact with and apply even pressure to the part to avoid these potential pressure "dead zones." Modeling for multi-element tooling For an initial demonstration project that involved a benchmark industrial part, Vistex designed the molds using a SolidWorks computer-aided design (CAD) package from Dassault Systèmes (Waltham, Mass., and Vélizy, France). The curing mold was CNC-machined from solid billet aluminum. Its back side was configured to accept a 300W silicone membrane heater with etched foil elements, held in place by a thin aluminum cover plate, explains Kuppers. (A second, hollow aluminum mold was constructed from the same data, and outfitted with a membrane heater, for the comparative study described later.) To make the matching base mold, a second CAD model was created, assuming an initial uniform rubber mask with a thickness of approximately 0.5 inch/1.3 cm. Then, using COSMOS finite element analysis (FEA) software, also from Dassault Systèmes, this second model was used to simulate the mold set in compression and yielded the normal pressure distribution on the surface of the curing mold. Pressure data from the model was analyzed within MATLAB from MathWorks (Natick, Mass.), using Vistex's proprietary shape optimization techniques. The MATLAB code generated revised mold and mask geometries and mask thicknesses that were imported back into SolidWorks, notes Dr. Daniel Walczyk, professor of mechanical engineering at CATS. This procedure was part of an iterative process that optimized the rubber mask to ensure that pressure would be Thermally insulative Base Mold ForMed CoMPosiTe laMinaTe rubber Mask Insert in position Source: Vistex + release Film Bleeder/Breather Temperature-controlled Tool Mold essentially equal across the mold face. Although the initial simulation showed that the surface pressure on the curing mold deviated, from minimum to maximum, by as much as 34 percent, the iterative process reduced the variation significantly to 13 percent. Additional modifications included extending the rubber insert beyond the part edges to ensure no abrupt drop-off in pressure. "To compress the prepreg layup evenly, especially in a shaped, 3-D part, the rubber mask has to vary in thickness," says Kuppers, noting, "In general, the rubber layer will be thicker where the part layup This diagram shows the general concept behind Thermal Press curing. The rubber insert enables the application of uniform pressure across a part's surface. is thicker, to ensure consolidation, and thinner at the edges, but the final shape is determined by the part shape, through the modeling." For the demonstration, the base mold was CNC-machined from a block of laminated red oak, an inexpensive material with an auto-ignition temperature well above the maximum curing temperature for the prepreg. Its shape was based on the modeling previously described. Then the rubber mask was cast by mixing a two-part silicone rubber material at a maximum working temperature of 315°C/598°F and pouring it into the Source: Vistex The Thermal Press curing (TPc) method, developed by Vistex composites (howes cave, N.Y.): a heated aluminum curing mold, a production kayak paddle, a rubber insert and an unheated base mold machined from epoxy tooling board are stacked on the right side of the photo. The computer shows screen shots produced by the company's proprietary shape-optimization modeling techniques, which optimize the rubber mask insert for uniform pressure across the part. at lower left are examples of other benchmark parts. Specialized Elastomeric Tooling (SET) Seeking pressure uniformity The red arrows indicate the points at which applied pressure was measured by sensors during paddle cure. a 5.5 percent disparity in pressure across the part was observed. march 2013 | 35

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