Thermal Characterization and Improvement of Curing Stage in Resin Transfer Molding Process

Introduction

The importance of materials from industry requires the use of composite materials with good properties whose design respects the environment and contributes to sustainable development with the maintenance of a growing economy. Composite materials are known for their rigidity, their high strength and shaping of complex parts and reduction of the number of interfaces (Saad et al. (2011).), they take an increasingly important place in the industry. Already known and used in several fields of application: Aeronautics, automobile, health, sport …

Among the shaping of thermoset matrix composites, resin transfer molding (RTM), this latter is considered one of the most promising techniques available today, it is able to make a large part complex with a high mechanical performance, tolerance, dimensional, narrow and very high finish. The manufacturing of a part by the RTM process can be freely divided into four main steps (Choi et al., 1998): the manufacturing of fiber preform, the filling of the mold, the curing reaction and the release of the part. The cure process irreversibly transforms the soft fiber/resin matrix to a hard structure component. It is a critical step in that the temperature and cure histories, and their spatial variation within the layup cross section, during the process directly influence the final quality of composite products (Roger et al., 1987; Wang et al., 2018). At the initiation of a typical process, the outer layers of the laminate, which are subjected to the external heating, cure more rapidly, than the inner layers, whereas, as the cure progress, temperature of the inner layers may exceed these of the outer layers due to the exothermic cure reaction and low thermal conductivity of the composite. Uncontrolled polymerization may cause undesired and excessive thermal variation that could induce microscopic defects in the network structure of the matrix phase, and macroscopic defects such as voids, bubbles and debonded broken fibers (Halpin et al., 1983; Kenny et al., 1989). Processing of polymeric composites is based on thermoset matrixes therefore requires optimization of the cure cycle parameters as well as adequate formation of the reacting system as a function of the geometry of the parts. Mallick has studied the effect of cure cycle time, temperature, preheating and post-cooling on mechanical properties of continuous as well as chopped glass fiber reinforced polyester and vinyl ester systems. Internal heat generation due to curing reaction causes high thermal gradients across the thickness, the flexural and interlaminer shear strengths are strongly dependent on the mold cycle time (Mallick et al., 1993). Barone and Caulk studied the influence of the applied heat on the curing process of epoxy resin and proposed a thermo chemical model based on a two-dimensional heat conduction equation with internal heat generated by the exothermic chemical reaction (Barone et al., 1979).

Several attempts have been made to reduce this thermal gradient by optimizing the energy cycle. We can cite as not exhaustive, the work of Choi et al. (1998) who handled with a temperature profile that represents a constant temperature range, they therefore observed the disappearance of this gradient which is argued by the low heat transfer in this time interval, so the temperature will not exceed its value at the mold wall. Vincenza et al. (2002), who carried out a dimensional study of the energy equation, assumed in their study that the mold is completely saturated and only interested in the curing stage.