Pressure autoclave

The first type of bonded design for this application was the beaded doubler panel (Fig. 28). This design was fairly successful at addressing the problems with simple riveted structure but had two primary drawbacks. The area under the beads remained a single thickness sheet and was still prone to fatigue. Reducing the unbonded areas under the beads was not a solution because it reduced the overall stiffness of the panel. Secondly, tooling for these panels was complex and not very robust. Autoclave pressure applied to the beaded areas of the doubler would cause them to collapse, so thick frames were fabricated with cutouts for the beads to protect them. A rubber layer bonded to the surface of the frames... 

Figure 4-2. Jaoketed batoh reaotor. (Source D. B. Gooch, Autoclaves Pressure—Temperature Reactions," Ind. Eng. Chem., 35, 927-946, 1943. Used with permission from the American Chemical Society.)...

Another Methodfor 5 5-Diethyl Barbituric Acid. (This is a scaled down version.) 16 g of clean sodium is dissolved in 300 g of absolute ethanol. To this cooled solution is added 20 g of dry urea and 50 g of diethyl malonic ester (diethyl diethyl malonate). The mixture is heated in an autoclave (pressure cooker, very strong) for 4 to 5 hours at 100-110°. After removing from the autoclave, the mixture is cooled. Upon cooling, the sodium salt of diethyl barbituric acid separates, is filtered off, dissolved in water, and the free acid precipitated by the addition of hydrochloric acid. The acid is filtered and recrystallized from water, using decolorizing carbon, if necessary. Yield Depends on your ability to exclude H2O from the beginning of reaction. 

The transfer of autoclave pressure to the resin in the laminate does not occur hydrostatically because the resin is not enclosed in a constant-volume system. Flow can occur initially both vertically (thickness direction) and horizontally. Furthermore, the network of fibers can also eventually act as a network of springs to which the vacuum bag and bleeder assembly transfer the stress from the autoclave pressure. This stress can then be transferred... 

Figure 6.2 Curing cycle temperature-time profile for typical graphite-epoxy composite in a vacuum bag autoclave process. Autoclave pressure is applied during the 135°C (275°F) hold...

Hinrichs [1] has shown that resin pressures can drop to about 103-117 kPa (15-17 psig) even though autoclave pressures of up to 586 kPa (85 psig) are used. This means that void growth will occur for sufficiently high water contents and the problem of transporting the voids out of the laminate is extremely important. 

Because the actual pressure profile in the resin is as yet unknown, it is assumed that the void experiences a resin pressure of 0.1 atm during Stages 1 and 2, which then increases to 5.78 atm during stages 3-5. The resin never actually experiences the total autoclave pressure, so 5.78 atm represents an upper bound. 

The Dave model considers a force balance on a porous medium (the fiber bed). The total force from the autoclave pressure acting on the medium is countered by both the force due to the springlike behavior of the fiber network and the hydrostatic force due to the liquid resin pressure within the porous fiber bed. Borrowing from consolidation theories developed for the compaction of soils [23,24], the Dave model describes one-dimensional consolidation... 

The resin pressure is almost never equal to the autoclave pressure. If the resin pressure drops due to resin flow, then it may become less than the minimum pressure necessary to prevent void stability and growth. In order to produce quality void-free laminates consistently, accurate resin pressure predictive software is a necessity. 

High pressures are commonly used during autoclave processing to provide ply compaction and suppress void formation. Autoclave gas pressure is transferred to the laminate due to the pressure differential between the autoclave environment and the vacuum bag interior. Translation of the autoclave pressure to the resin depends on several factors, including the fiber content, laminate configuration, and the amount of bleeder used. 

An appreciation of the importance of hydrostatic resin pressure must be developed to understand void growth fully. Because of the load-carrying capability of the fiber bed in a composite layup, the hydrostatic resin pressure needed to suppress void formation and growth is typically only a fraction of the applied autoclave pressure. The hydrostatic resin pressure is critical because it is the pressure that helps to keep volatiles dissolved in solution. If the resin pressure drops below the volatile vapor pressure, then the volatiles will come out of solution and form voids. 

In the early stages of the cure cycle, the hydrostatic resin pressure should be equal to the applied autoclave pressure. As resin flow occurs, the resin pressure drops. If a laminate is severely overbled, then the resin pressure could drop low enough to allow void formation. Thus, the hydrostatic resin pressure is directly dependent on the amount of resin bleeding that occurs. As the amount of bleeding increases, the fiber volume increases, resulting in an increase in the load carrying capability of the fiber bed. 

Step 4 Liquid Continues to Escape, but at a Decreasing Rate Due to a Portion of the Load that Is Now Carried by the Spring (2.5 kg/cm2). This Is Analogous to Bleeding in a Laminate Occurring Rapidly Until the Fiber Bed Starts Supporting a Portion of the Applied Autoclave Pressure. 

The pressure curves also illustrate the horizontal flow process. The resin pressure initially approaches the applied autoclave pressure and then decreases as bleeding occurs. The opposite occurs in the bleeder. The applied vacuum is measured initially, and the pressure increases as resin begins to fill the bleeder. Note that the horizontal pressure gradient is very small for a majority of the laminate but becomes large near the edges. 

To circumvent both of these problems in a production environment, a significant portion of the autoclave pressure should be applied immediately before initiating the heat-up cycle (Refer to Fig. 10.3). For standard epoxy systems, a full vacuum and 5.8 kg/cm2 (85 psi)... 

Changes in convective heat transfer coefficients due to autoclave position and loading scheme are difficult to model. These coefficients are more easily correlated from experimental data. This correlation can be determined from monitoring thermocouples attached to tools or by correlating air flow based on autoclave position. These coefficients are crucial for determining the rate of heat transfer from the autoclave environment into the part. Heat transfer coefficients are also a function of autoclave pressure however, the adjustment for... 

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