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in-plane permeability, which remained constant throughout the debulk. However, when the debulk temperature increased, permeability decreased with time, as resin gradually infiltrated the dry fiber pathways. The dashed vertical lines mark when the laminate was “impermeable” in the in-plane direc- tion (out of measurement range). When the debulk temperature was >60C, both PW and UD lami- nates became impermeable within $1 h. 3.4. Cure optimization guidelines The permeability data in the previous sections showed that transverse permeability did not increase significantly until the debulk temperature reached 60 C in UD prepregs. The transverse permeability at 60 C was $3-4 orders of magnitude less than the in-plane permeability. To compare the gas removal efficiency of RT and 60 C vacuum holds, models can be used to estimate the time required for air evacuation in both cases. Multiple models have been developed to predict the evacuation of entrapped air in prepregs, and most of them relying on Darcy’s Law [13, 14, 21]. Combining Darcy’s law and the continuity equation, flow in the porous medium can be expressed: K @P r PlrP 1⁄4;@t (5) where P is pressure, K is the permeability, m is gas viscosity and ; is the initial porosity of the laminate. Considering a two-dimensional problem with K in the X and Z direction, Equation (5) becomes [14]: @ P@P @ P@P Kx @x þKz @z 1⁄4;@P (6) l @x l @z @t As the permeability, Kx and Kz, changes during heated or RT debulk, solving the two-dimensional equation requires extensive calculation. The problem was therefore simplified to one-dimensional by assuming that in-plane air evacuation is the sole pathway during RT debulk, while air evacuation ADVANCED MANUFACTURING: POLYMER & COMPOSITES SCIENCE 45 Figure 8. (a) Measured in-plane permeability as a function of degree of impregnation. In-plane permeability versus time dur- ing debulk for (b) PW and (c) UD laminates. The cure cycle used in the model was 2C/min to designated temperatures and hold for four hours.PDF Image | VACUUM BAGGING SUPPLIES Peel Ply Fabric
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