Hot Forming Instead of Compression Molding/Press Forming
High-performance structural parts based on glass, carbon and aramid fibers find application as body and interior components in vehicles, but also as structural elements in aircraft. Creating complex geometries from the flat prepreg inserts requires flexible forming processes and tooling. Compression molding/hot pressing is often employed for such parts. However, diaphragm-based forming equipment offers considerably greater flexibility than presses. Large radiant heaters or hot-air blowers first warm the prepreg throughout. Following this, the material is formed with the aid of flexible rubber membranes (diaphragms) mounted to a frame on the forming mold.
With the single-diaphragm (SD) technique, the mold holding the preform can simply be slid into the machine. In contrast, with the double-diaphragm (DD) technique, the preform must first be secured between the two diaphragms before it is positioned over the mold and forming takes place. The DD technique offers benefits regarding productivity, since it permits shorter preheating and forming phases. This has been confirmed by investigations conducted recently at Airbus Deutschland.
What is decisive for high-quality results is that the layers (plies) of fibers be able to move in relation to one another in order to produce a curved geometry from a flat geometry. The tests demonstrated that especially high intra-ply shearing is achieved with the DD technique, because the material positioned between the two diaphragms is heated throughout uniformly prior to forming and undergoes forming in a second step only afterwards. Curing of the prepreg material is the rate-determining step in the production process. The time the tooling is occupied can in this way be shortened by decoupling the forming and curing steps.