Moldability of Tackified Fiber Preforms in Liquid Composite Molding

In this work, the effectiveness of using a reactive (bismaleimide) and a non-reactive (polymethylmethacrylate) tackifier in controlling preform springback is investigated using a 6k, 4HS woven graphite fiber mat. Effects of tackifier on preform permeability and fiber wetting are also addressed. Preform springback occurs when the force exerted by the compressed fibers, upon release of an externally applied load, is greater than the "holding" force provided by the tackifier. The latter depends upon the tackifier modulus and the wetted surface area of the preform. The surface area of the pre-form covered by the tackifier can be either interor intralayer, depending upon the rheological properties of the tackifier and the preforming conditions. For the same tackifier concentration, better springback control is achieved when tackifier is inside the fiber tows. Moreover, since mold filling is governed primarily by resin flow between the fiber tows, permeability or flow resistance is not affected much by intra-layer coverage of the tackifier. Blocking of the interstitial gaps within the fiber tows may, however, affect fiber wetting by lowering the capillary pressure.