# Experimental and Numerical Study of Vacuum Resin Infusion for Thin-Walled Composite Parts
## 2 Vacuum Infusion of Large CFRP Parts: Experiment  
In order to understand how the dry spots are formed during resin front advances in the porous preform, the experimental vacuum infusion processes for two different CFRP panels were carried out. At the manufacturing of both panels, the carbon fiber fabric CC245 with weaving twill 4/4, mass density 240 g/m\({}^{2}\) and thickness 0.28 mm +/- 15% was used. To speed up the filling, Gerster HPM tape was laid in accordance with the selected method. A semi-permeable membrane VAP3D-C2003 was used to effectively remove air while retaining resin in a preform. The infused epoxy compound SR1710 with hardener SD8822 (Sicomin) had viscosity \(\sim\)0.1 Pa\(\cdot\)s at 25 \({}^{\circ}\)C. The air dissolved in resin was evacuated before the start of the process in the special vessel Vacmobiles RT 19/11 where low air pressure was supplied by the vacuum station Vacmobiles 20/2 (Vacmobiles Lim., Auckland, New Zealand). The first thin-walled rectangular panel of dimensions (\(800\times 700\)) mm with thickness 1.5 mm was an 8 layered cross-ply symmetric balanced laminate. Laying of HPM, hoses for pumping out the vacuum and resin injection was carried out along three sides around the perimeter of the panel. The vacuum infusion process was carried out at the ambient temperature 19 \({}^{\circ}\)C; the averaged preform temperature in the filled region and resin front position recorded every 5 min (see Figure 1). The dynamics of the preform resin infusion is illustrated by a series of photographs in Figure 2.  
Figure 1: The time histories of the resin front advances and preform’s temperature in the filled area. Figure 2: _Cont._  
The propagation speed of the resin front began to decline sharply after 40 min from the start of the process. Obviously, this is caused in part by the polymerization reaction of the resin, accompanied by an increment in temperature (see Figure 1), partly by the closure of the resin fronts encountered, as a result of which the conditions for the penetration of vacuum to the dry preform region surrounded by the resin worsened (see Figure 2d). After 2 h and 20 min, the movement of the resin ceased. A gradual decrease in the front velocity in the time interval 0-40 min is in full agreement with the observations [3, 8, 27, 35, 39, 40]. Photograph of the ready infused panel is shown in Figure 3, where the formed dry spot is clearly visible. It arose as a result of the closure of the resin front, called in paper [12] as the inner front, i.e., surrounding the void area with resin in the inner part of the preform. Much more difficult in terms of ensuring the quality of impregnation is the tail panel of a light aircraft with an overall size of more than 2 m. At the assembly of the technological packet, 4 layers of carbon fabric are laid sequentially over the entire area of the tooling, then foam plates (see Figure 4a). The space between these foam plates is filled with carbon fabric ribbons (8 layers), and the whole sandwich is covered with 4 layers of carbon fabric, a semi-permeable membrane (see Figure 4b) and a vacuum bag film that is sealed around the perimeter. The resin feed channel is shown in Figure 5 by red lines. The resin front propagation in time is present in Figure 6, and its positions at the different time instants of the infusion process are shown together in Figure 7.  
One can see in Figure 7 that within 1 h and 45 min, the resin front did not reach the forward edge of the panel and the corner at its rear. This caused the fatal defect of the panel, which is shown in Figure 8 after polymerization. The experiment showed the formation of dry spots as a result of stopping the outer front, as described in [12]. Figure 3: Defected CFRP panel after curing with dry spot. Figure 2: The sequence of the snapshots, which demonstrate the resin front propagation in the vacuum infused rectangular CFRP panel.