Spatial variation of void morphology in resin transfer molded E-glass/epoxy composites

Abstract

Due to their light weight and high performance, fiberreinforced composites are being increasingly used in various commercial products in aerospace and automotive industries. Emerging new methods such as liquid composite molding (LCM) and resin transfer molding (RTM) offer an economic, versatile, and efficient means for manufacturing geometrically complex, large composites. RTM process consists of injecting a thermosetting polymer into a mold cavity preloaded with a dry reinforcing preform, which is most often made of E-glass fibers. In RTM, voids and dry spots are reported to originate primarily from mechanical entrapment during mold filling [1, 2]. This entrapment is believed to arise from uneven movement of flow front due to differences in permeability between the fiber tows and the surrounding pore space. Furthermore, several process parameters such as injection pressure, vacuum assistance, fibers surface treatment, and resin viscosity have been shown to influence the overall voidage, as well as the void morphology defined by the size and shape of voids within the composite [3]. Adverse effects of voids in composites have been well documented. For example, a void content as low as 1% is known to result in a decrease in strength up to 30% in bending, 3% in tension, 9% in torsional shear, and 8% in impact [4]. In addition, void presence is shown to affect both the rate and the equilibrium level of moisture absorption [5]. Voids with irregular shapes are also known to induce early crack formation [6]. However, detailed morphology analyses together with spatial variation of voids in resin transfer molded composites are not available. In this work, we present the radial and through-the-thickness variation of void morphology in a resin transfer molded, E-glass/epoxy composite disk. A resin transfer molded composite disk having 152.4 mm diameter is fabricated using a mixture of epoxy resin EPON 815C and curing agent EPICURE 3282 (Shell Chemicals). The molding is achieved by injecting the mixture into a center-gated mold cavity at a constant flow rate of 5.32 cm3/s. The mold cavity is preloaded with 4 layers of E-glass preform (FiberGlast part #250) yielding a fiber volume fraction of 18.1% in a 3.84 mm thick disk. The transient inlet pressure recorded during mold filling is presented in Fig. 1. During filling, inlet pressure