Lamination, autoclave

Figure 4.12 The lay up of a thick TGDDM epoxy (Hercules 3501-6) graphite laminate autoclave run showing the positions of the dielectric sensors and thermocouples...

Some high performance laminates consisting of carbon fiber webs and epoxy resins are cured in autoclaves. An autoclave is a pressure chamber in which the pressure is appHed hydrostatically. 

Two resin systems based on this chemical concept are commercially available from Shell Chemical Company/Technochemie under the COMPIMIDE trademark COMPIMIDE 183 (34) [98723-11-2], for use in printed circuit boards, and COMPIMIDE 796 [106856-59-1], as a resin for low pressure autoclave mol ding (35). Typical properties of COMPIMIDE 183 glass fabric—PCB laminates are provided in Table 8. COMPIMIDE 183 offers a combination of advantageous properties, such as a high glass transition temperature, low expansion coefficient, and flame resistance without bromine compound additives. 

Figures 10 and 11 show a manufacturing application in which a resin s flow properties are measured in-sltu at a particular point in a thick laminate during cure in an autoclave. The sensor was inserted on the tool surface and in the center of a thick 192 TGDDM graphite epoxy laminate. Figure 10 shows the noise free raw data taken by the center sensor during cure in the 8x4 foot production size autoclave. Using the procedures described, the ionic mobility was measured at both the tool surface and the center of the thick laminate. In Figure 11, the sensor values of a show a 10 to 20 minute time lag in the point of maximum flow on the surface versus the laminate s center. Measurements of a versus q, as shown in...

Figure 10. Raw data from the sensor in the middle of a 1 thick 3501-6 graphite epoxy laminate during cure in a production size autoclave.

Figure 11. Log (1/ct) vs. time for a thick graphite laminate (192 ply) catalyzed epoxy monitored during cure in the autoclave.

Figure 4.6 Plot of e" x a> versus time of the sensor output at the sixty-fourth ply of the thick epoxygraphite laminate during cure in the autoclave...

Figure 4.13 is a plot of the viscosity determined by the sensor at each of the four positions in the thick laminate during cure in the autoclave. The FDEMS sensor data show that the middle ply achieves its first viscosity minimum 20 minutes after the plies on the surface of the... 

Autoclave processing is a process in which individual prepreg plies are laid up in a prescribed orientation to form a laminate (Fig. 5.9). The process involves consolidation of the laminate, which generally results in a three-dimensional flow field. Similar to the IP process the fiber bed is not stationary in the AP process hence, its movement has to be specifically considered when the appropriate conservation equation for this process are developed. If it is assumed that the resin has a relatively constant density (i.e., the excess resin is squeezed out before the gel point is reached) then the appropriate conservation of mass equation for this consolidating system is Equation 5.12. 

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... 

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. 

For thick epoxy laminates processed in the autoclave, voids once formed and stabilized can only be removed by dissolution or by resin flow. Furthermore, resin gradients are deleterious to structural laminates. These two key phenomena make an understanding of resin transport vital to the development of any processing model. 

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. 

As the thickness of the laminate increases, the strength of this thermal spike and the degree of thermal lag during heat-up increases. Figure 8.8 shows the results for a 62.5-mm (500 ply) laminate of the same material. Now the center-line temperature never reaches the autoclave temperature during the first dwell, and the thermal spike during the second dwell is nearly 135°C. The thermal spike is directly related to the release of internal heat during cure. The thermal lag is a manifestation of the low thermal diffusivity of polymer matrix composites. 

Figure 8.7 Centerline and autoclave temperature histories during cure of a 25-mm (200 plypthick AS4/3501-6 laminate...

During the processing of composite materials in a hot press or an autoclave, the laminate is usually kept flat until cure is complete. If the platen surfaces are assumed frictionless, the effect of the constraints is to require that the curvatures K] and k2 be zero throughout cure. To develop the elastic solution under these constrained conditions, the laminated plate theory may be used with conditions of N = 0 and jc, = 0. The resulting midplane strains are given by... 

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