256 LIQUID COMPOSITE MOLDING to provide cohesion to the fiber architecture during subsequent handling and processing operations.Binding can be achieved mechanically by needling or stitching with a light yarn or roving,but it is more usual to use a chemical adhesive binder.This may be either a thermoplastic or thermoset in the form of a powder,an emulsion or a solution.For systems using polyester of vinyl ester,binders may be categorized by their solu- bility in styrene.A dissolution time of less than 60 seconds corresponds to high solubility,between 60 and 200 seconds indicates medium solu- bility,and over 200 seconds represents low solubility.Relatively low sol- ubility binders result in improved flow characteristics at the expense of prolonged fiber wet-out times.One potential consequence of binder dis- solution is a change in resin viscosity.It has been suggested [2]that the viscosity of a vinyl ester resin may be doubled by the addition of 5%by mass of thermoplastic polyester binder. 2.2.Preforms Preform is an assembly of fibers having the configuration of the part. Once the preform is wetted by the liquid resin and after the resin is cured, a composite part is obtained.Figure 7.8 shows an example of a preform. There are several good reasons to preform the reinforcement before loading it into the mold. Preforms speed up the process and free the mold from everything except loading,injection,in-mold cure,and demolding. FIGURE 7.8 Knitted glass fiber preform for a stiffened T joint (courtesy of Preform Technologies Ltd.).to provide cohesion to the fiber architecture during subsequent handling and processing operations. Binding can be achieved mechanically by needling or stitching with a light yarn or roving, but it is more usual to use a chemical adhesive binder. This may be either a thermoplastic or thermoset in the form of a powder, an emulsion or a solution. For systems using polyester of vinyl ester, binders may be categorized by their solu￾bility in styrene. A dissolution time of less than 60 seconds corresponds to high solubility, between 60 and 200 seconds indicates medium solu￾bility, and over 200 seconds represents low solubility. Relatively low sol￾ubility binders result in improved flow characteristics at the expense of prolonged fiber wet-out times. One potential consequence of binder dis￾solution is a change in resin viscosity. It has been suggested [2] that the viscosity of a vinyl ester resin may be doubled by the addition of 5% by mass of thermoplastic polyester binder. 2.2. Preforms Preform is an assembly of fibers having the configuration of the part. Once the preform is wetted by the liquid resin and after the resin is cured, a composite part is obtained. Figure 7.8 shows an example of a preform. There are several good reasons to preform the reinforcement before loading it into the mold. • Preforms speed up the process and free the mold from everything except loading, injection, in-mold cure, and demolding. 256 LIQUID COMPOSITE MOLDING FIGURE 7.8 Knitted glass fiber preform for a stiffened T joint (courtesy of Preform Technologies Ltd.)