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46 W. HU AND S. NUTT Figure 9. Air evacuation time vs laminate length. Comparison of air evacuation time between room tempera- ture vacuum hold and 60C vacuum hold. instantaneously upon application of vacuum (discussed in Section 3.2), and as number of plies increases, the delay time increases. The dashed lines showed that the laminate critical length increases to 184 mm and 910 mm for a 4- and 8-ply laminate, respectively, when taking delay time into account. The plot also indicates that for thin UD laminates, a vacuum hold at 60C prior to cure can be more effective when the laminate length exceeds a threshold value. However, note that as the number of plies increases, the through-thickness permeability decreases sharply and can eventually go to zero (no gas flow in the through- thickness direction), even at elevated temperature. For PW prepreg debulking at 60C, the transverse permeability of a 4-, 8-, and 16-ply laminates consist- ently reached $2 1014 m2, a level comparable to the initial in-plane permeability ($6 1014 m2), while the transverse permeability for a 24-ply laminate exhibited slightly a lower permeability ($2 1015 m2). The evacuation efficiency for in-plane and trans- verse gas removal can be compared using the follow- ing equation, derived from Equation (7) 2 tx1⁄4Lx Kz (8) tz Hz Kx Equation (9) indicates that provided Lx/Hz > 5.5, a heated debulk at 60C is more effective than a RT debulk. In practice, laminate length is almost always 2-3 orders of magnitude greater than laminate thick- ness, so a 60C debulk will greatly increase the effi- ciency of air evacuation in PW laminates. 4. Conclusions The effects of debulk temperature on inter-ply air removal and gas permeability of VBO prepregs were investigated. Results showed that debulk temperature (resin viscosity) and fiber bed architecture are critical factors for through-thickness air transport. Reducing resin viscosity promotes inter-ply air removal via through-thickness gas transport while concurrently impeding the in-plane gas transport due to increasing tow impregnation. In situ monitoring observations confirmed that inter-ply air evacuation during 60C debulk was more effective than room temperature debulk. This method can be applied more widely to assess mechanisms of void formation and migration, as well as revealing bubble behavior under different cure conditions and with different prepreg formats. The permeability data were used to guide the devel- opment of more effective cure cycles for prepregs under VBO conditions. For this PW laminate, results showed that if laminate length/thickness ratio >5.5, a debulk at 60 C can be applied to reduce total process- ing time. However, in UD prepregs, transverse air evacuation relies on a continuous network of random, occurs exclusively via through-thickness pathways during heated (60 C) debulk. These assumptions are used for estimation because during RT debulk, the transverse permeability of 2- and 4-ply UD lam- inates were 5-6 orders of magnitude less than the in-plane permeability, and decreased with increasing time. Besides, during a 60 C debulk, tow impregna- tion reaches full saturation in about an hour. The solution for one-dimensional gas transport can be expressed as [21]: l ;L2 t1⁄4P0 K 0:9ln ; (7) In this equation, permeability K can be in-plane, Kx, or through-thickness, Kz, and L can be the length to breathing edge (half the laminate length) or the thickness of the laminate, accordingly. Initial porosity ; was calculated from the cross-section micrographs of uncured UD laminates, while ;F, the final porosity, was assumed to be 1%. With all the constants determined, the vacuum hold times required for RT and for 60C were estimated for different part sizes (L) using Equation (7). Figure 9 shows the vacuum hold times required for room temperature and 60 C air evacuation. Transverse permeability data for 2- and 4-ply lami- nates were used to estimate the air evacuation time required for 4- and 8-ply laminates, assuming that perforated release film was applied to both sides of the laminates. The solid black line shows air evacu- ation time during RT debulk as a function of laminate length. The solid blue and red lines show the time required for air evacuation at 60 C for 4- and 8-ply laminates. The figure shows that if the laminate length exceeds 48 mm for a 4-ply laminate, or 460 mm for an 8-ply laminate, debulking at 60 C is more efficient than at room temperature. The delay time should also be considered to estimate air evacuation time, as pres- sure drop (i.e. air evacuation) does not occur !1 1 ;F 0:6PDF Image | VACUUM BAGGING SUPPLIES Peel Ply Fabric
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