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Figure 1. Schematic of in situ monitoring method set-up. In practice, air removal is usually accomplished by a room temperature vacuum hold (debulk), typ- ically hours, before initiating high-temperature cure. While edge-breathing is readily achieved for small and flat laminates, pathways are often occluded in large and/or complex geometries [7]. For large and/ or complex parts, prepreg manufacturers generally recommend debulks for 16h or more, which becomes the longest and thus rate-limiting step for VBO production of large parts [8]. For some com- mon geometric constraints, such as embedded ply drops, edge-breathing can be problematic and/or virtually impossible. A recent study [9] reported that similar quality was achieved in laminates cured using a four-hour debulk at 50C versus those cured with a 16-hour room temperature debulk prior to heated cures. They concluded that debulking at an intermediate temperature (50 60 C), a so-called “super-ambi- ent” dwell (SAD), reduced the cure cycle time with- out compromising laminate quality. However, the mechanisms operative in super-ambient dwell were not provided. The increase in debulk temperature has a com- plex effect on air removal. On one hand, the decrease in resin viscosity increases resin flow, which generally enhances air removal. On the other hand, increased resin infiltration can also occlude pathways for air evacuation. The interplay between these factors introduces added complexity to the process. Thus, a clear understanding of the effects of pre-cure dwell temperature on air evacuation must be acquired to establish science-based guidelines for cure cycle optimization. Permeability describes gas (and liquid) transport through a porous medium, and is thus an important process variable affecting air evacuation. The intrin- sic permeability of a material is determined solely by the solid matrix, and is independent of the fluid properties and flow mechanisms, while the effective gas permeability of composites is affected by both material and process parameters [10]. In-plane per- meability of prepregs has been studied extensively [11–14], albeit mostly at room temperature. However, the initial in-plane permeability values for typical VBO prepregs are on the order of 1014 m2 [11, 12]. In contrast, the through-thickness perme- ability is generally 3 and 4 orders of magnitude less than the in-plane permeability [10, 12]. Kratz et al. [10] and Tavares et al. [15] both investigated the through-thickness permeability during cure and reported an increase in through-thickness perme- ability at high temperature due to the decrease in resin viscosity. However, no observations have been reported on the effects of debulk temperature on air evacuation, and no comparison is available to iden- tify which air evacuation pathway (in-plane or transverse) is more effective during this period. In this study, the removal of inter-ply air during heated debulk was investigated using an in situ monitoring method (reported previously) [16]. The effective gas permeability of prepregs in both in- plane and through-thickness directions was investi- gated to understand the effects of temperature on gas transport. Tow impregnation during heated debulk was predicted using a model developed by Centea et al. [17]. The relationships between gas permeability, resin properties, and tow impregnation as functions of time and temperature are estab- lished. Cure optimization guidelines are provided by a comparison of evacuation times at different debulk conditions. 2. Experimental 2.1. Materials The prepregs selected for this study featured a toughened epoxy resin (CYCOM 5320-1, Solvay) and two carbon fiber beds: IM7/12K unidirectional (UD) tape with fiber areal weight of 145 g/m2 and a resin content of 33% by weight, and a T650-35 3K plain weave (PW) with areal weight 196 g/m2 and a resin content of 36% by weight [5, 6]. Neat resin film (CYCOM 5320-1) was also used, with areal weight $92 g/m2 and thickness $50 mm. 2.2. In situ monitoring method To observe inter-ply air evacuation and entrapment during debulk, a custom-built experimental setup afforded in situ visual observations of in-plane bub- ble motion [6, 16]. A perforated resin film with con- trolled pore size and distribution was prepared and laid up against the glass window of an oven, fol- lowed by four layers of prepreg plies and standard consumables, shown in Figure 1. The resin film intentionally introduced air into the lay-up, replicat- ing the condition of air bubbles that are inevitably trapped between prepreg plies during lay-up. ADVANCED MANUFACTURING: POLYMER & COMPOSITES SCIENCE 39PDF Image | VACUUM BAGGING SUPPLIES Peel Ply Fabric
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