3.1.2 Resin Transfer Molding of Thermoset Composites Resin transfer molding(RTM)of composite laminates is a process wherein the dry-fiber preform is infiltrated with a liquid polymeric resin and the polymer is advanced to its final cure after the impregnation process is com- plete.An extensive review of the resin transfer molding process can be found in Reference [12].The process consists of four steps:fiber preform manufac- ture,mold filling,cure,and part removal.In the first step,textile technology is typically utilized to assemble the preform.For example,woven textile fabrics are often assembled into multilayer laminates that conform to the geometry of the tool.Braiding and stitching provide mechanisms for the creation of three-dimensional preform architectures. Typically,a thermosetting polymer of relatively low viscosity is used in the RTM process.There have been applications for thermoplastic polymers,but they are rare.Pressure is applied to the fluid polymer to inject it into a mold containing the fiber preform,and the mold may have been preheated.The flow of the fluid through the fiber preform is governed by Darcy's Law [12], wherein the velocity of the flow is equal to the product of the pressure gradient,the preform permeability,and the inverse of the polymer viscosity. Clearly,the lower the polymer viscosity,the greater the flow rate,and the greater the permeability,the greater the flow rate.Note also that because the fiber preforms typically exhibit different geometries in the three principal directions,permeability is a tensor and exhibits anisotropic characteristics. That is,for a given pressure gradient,the flow rates in three mutually ortho- gonal directions will differ.Flow through the thickness of a fiber preform that contains many layers of unidirectional fibers will be quite different than flow in the planar directions.In addition,the permeability of the preform depends on the fiber volume fraction of the preform.The greater the volume fraction, the lower the permeability.It is important to vent the mold to the atmosphere to remove displaced gases from the fiber preform during the mold filling process.Otherwise trapped gases will lead to voids within the laminate. After the polymer has fully impregnated the fiber preform,the third step occurs:cure.This step will begin immediately upon injection of the polymer into the mold and will occur more rapidly if the mold is at an elevated temperature.As the cure of the polymer advances to the creation of a cross-link network,it passes through a gelation phase wherein the polymer viscosity increases and transforms the polymer into a viscoelastic substance, where it possesses both viscous and elastic properties.As this process pro- ceeds and the cross-link network continues to grow,the instantaneous glass transition temperature of the polymer increases.Finally,vitrification of the polymer occurs when its glass transition temperature exceeds the laminate temperature.Should gelation or vitrification(or both)occur prior to com- pletion of mold filling and preform impregnation,the resulting laminate will not be fully impregnated. The viscosity of most polymers is highly dependent on temperature and polymer cure kinetics are controlled by temperature as well.Therefore,heat 2003 by CRC Press LLC3.1.2 Resin Transfer Molding of Thermoset Composites Resin transfer molding (RTM) of composite laminates is a process wherein the dry-fiber preform is infiltrated with a liquid polymeric resin and the polymer is advanced to its final cure after the impregnation process is com￾plete. An extensive review of the resin transfer molding process can be found in Reference [12]. The process consists of four steps: fiber preform manufac￾ture, mold filling, cure, and part removal. In the first step, textile technology is typically utilized to assemble the preform. For example, woven textile fabrics are often assembled into multilayer laminates that conform to the geometry of the tool. Braiding and stitching provide mechanisms for the creation of three-dimensional preform architectures. Typically, a thermosetting polymer of relatively low viscosity is used in the RTM process. There have been applications for thermoplastic polymers, but they are rare. Pressure is applied to the fluid polymer to inject it into a mold containing the fiber preform, and the mold may have been preheated. The flow of the fluid through the fiber preform is governed by Darcy’s Law [12], wherein the velocity of the flow is equal to the product of the pressure gradient, the preform permeability, and the inverse of the polymer viscosity. Clearly, the lower the polymer viscosity, the greater the flow rate, and the greater the permeability, the greater the flow rate. Note also that because the fiber preforms typically exhibit different geometries in the three principal directions, permeability is a tensor and exhibits anisotropic characteristics. That is, for a given pressure gradient, the flow rates in three mutually ortho￾gonal directions will differ. Flow through the thickness of a fiber preform that contains many layers of unidirectional fibers will be quite different than flow in the planar directions. In addition, the permeability of the preform depends on the fiber volume fraction of the preform. The greater the volume fraction, the lower the permeability. It is important to vent the mold to the atmosphere to remove displaced gases from the fiber preform during the mold filling process. Otherwise trapped gases will lead to voids within the laminate. After the polymer has fully impregnated the fiber preform, the third step occurs: cure. This step will begin immediately upon injection of the polymer into the mold and will occur more rapidly if the mold is at an elevated temperature. As the cure of the polymer advances to the creation of a cross-link network, it passes through a gelation phase wherein the polymer viscosity increases and transforms the polymer into a viscoelastic substance, where it possesses both viscous and elastic properties. As this process pro￾ceeds and the cross-link network continues to grow, the instantaneous glass transition temperature of the polymer increases. Finally, vitrification of the polymer occurs when its glass transition temperature exceeds the laminate temperature. Should gelation or vitrification (or both) occur prior to com￾pletion of mold filling and preform impregnation, the resulting laminate will not be fully impregnated. The viscosity of most polymers is highly dependent on temperature and polymer cure kinetics are controlled by temperature as well. Therefore, heat TX001_ch03_Frame Page 44 Saturday, September 21, 2002 4:51 AM © 2003 by CRC Press LLC