Entrapment and venting of bubbles during vacuum bag prepreg processing

Abstract

During composites manufacturing with partially pre-impregnated fibers (i.e. “prepregs”) in Out-of-Autoclave processes, non-impregnated fabric cross-sections serve as air pathways to evacuate entrapped bubbles of air, moisture, or volatiles. The bubbles trapped within a laminate during processing lead to decreased structural performance. In this work, the motion of resin and bubbles during the processing of a characteristic prepreg is directly visualized in situ. This is performed utilizing a previously developed flow visualization technique under known pressure and temperature conditions. This study investigates the processing conditions under which a bubble succeeds or fails to meet and coalesce with available air pathways in order to escape the laminate. A key finding of this study is that tunable process parameters, such as pressure and temperature, are less important for successful bubble removal as compared to the initial state of resin impregnation in the prepreg. Prepregs with initially high states of resin impregnation will often fail to draw bubbles into air pathways through the center of fiber tow cross sections, whereas prepregs with initially low states of resin impregnation have clear pathways for bubbles to meet local resin flow fronts, coalesce, and escape. The relevant literature on the motion of bubbles in confined spaces is discussed. It is observed that small Capillary number theory (i.e. Ca < 0.01) under predicts the relative velocity of bubbles, and the faster than expected bubble transport is likely due to effects given by the bubble aspect ratio via the fibrous micro-channel geometry.

Fast cure of stable semi-pregs via VBO cure

“…Voids within composite materials reduce mechanical performance, generating stress concentrations that lead to premature failure. Consequently, parts with porosity levels >1% are generally rejected [2]. Autoclaves apply pressure (and temperature) on the laminates during cure that suppress volatile evolution from the resin [3].…”

Recent Industrial Developments of Marine Composites Limit States and Design Approaches on Strength

“…It is worth noting that the specimens were cut out from laminates produced for actual industrial applications. Gangloff et al (2017) showed that imperfections related to air inclusion mainly originate in prepreg production, while the curing process inside the vacuum bag has a scarce influence on the overall quality.…”

Effect of horizontal shift between fabric layers on the meso-scale-void formation in liquid composite molding

“…Therefore, the cladding time Δt a required for resin in the coverage layer to completely clad the mesopore of the present layer is given by Equation 12: According to the position of the coverage layer, the cladding time can be divided into the upper layer cladding time Δt au and the lower layer cladding time Δt al .The total cladding time Δt l can be obtained by Equation 13: 13To accomplish the formation of a meso-scale-void, the in-plane air entrapment and mesopore cladding by resin in the adjacent layers need to be completed firstly. If Δt t > Δt l , the mesopore has been entirely cladded when the in-plane air entrapment, so the trapped air will not change and the meso-scale-void is fully formed, its length can be obtained from Equation (9). If Δt t < Δt l , the mesopore has not been entirely cladded when the in-plane air entrapment occurs, the entrapped air can escape along the thickness direction until the mesopore is fully cladded, the final size of the meso-scale-void can be computed from Equation 14: (14) In conclusion, the length of the meso-scale-void can be obtained from the Equation 15: 153.3.…”

“…Once voids are formed during mold filling, it is difficult to eliminate them completely [9,10]. So studying the effects of preform structure and mold-filling parameters on void formation, and optimizing the process parameters to avoid or decrease the void formation become a significant method to improve the part quality.…”