Composite materials autoclave process

Vacuum Bagging/Autoclave, Lecture Slides, Course ENG 4793 Composite Materials and Processes, Georgia Institute of Technology, www.ME.GaTech.edu/jonathan.colton/me4793/slides.html (2003)... 

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... 

Part heat-up rate during autoclave processing can dramatically influence final part quality. At least three variables can affect the autoclave heat-up rate for composite parts (1) tool material and design, (2) the actual placement of the tool within the autoclave, and (3) the autoclave cure cycle used. Recommendations for the design of an individual tool are fairly obvious and well understood in industry (e.g., thin tools heat faster than thick tools materials with a high thermal conductivity heat faster than those with lower thermal conductivity and tools with well-designed gas flow paths heat-up faster than those with restricted flow paths [e.g., tools... 

The rest of the chapter deals with the hydration chemistry of Portland and composite cements at temperatures outside the range 15-25 C, including that of autoclave processes, and with specialized uses of cements in casing oil wells and in making very high strength materials. 

In order to reduce the volume stream and to optimize the efficiency of the following processing units the plastics have to be compacted mechanically before heat/pressure treatment starts. Bales of compacted wet plastics reach bulk densities of about 450 kg/ m3, they are put into moulds which will be transported into autoclaves. During the autoclave process plastics are heated and plastified under saturated steam conditions. Autoclave process is within a wide range independent from changing compositions of the plastics. The process temperature (about 170°C) does not provoke decomposition of plastics, used for household packaging materials. Hydrothermal autoclave conditions lead to further compaction and a stable product (bulk density up to 700 kg/m3) of the mixed different kinds of plastics. Fig. 1 shows a compacted plastic block after finishing the autoclave process. 

The advantage of optical methods for industrial process control is that they are not subject to electrical interference and have a high bandwidth for information transfer. The theory of fibre-optics and examples of prototype and laboratory-based systems were described earlier in Section 3.4. The use of NIR combined with chemometrics as described in Section 6.10.1 is an example that requires fibre-optics and has the advantage that the components may be made of quartz or even glass and still operate successfully. It is thus possible to treat the optical fibres as disposable items. For example, in autoclave processing of composites, it is possible to leave the fibre embedded in the part and use the optical fibre for subsequent assessment of the condition of the material. 

Thermoset based composite laminates are generally produced bj Autoclave/Vacuum Degassing Lamination Process (38, 39). The characteristics of this inocess are shown in Fig. IS. In this process, pr eg plies of desired shape are laid up in a prescribed orientation to form a laminate. The laminate is covered with successive layers of an absorbent material (glass bleeder fabric), a fluminated film to prevent sticking, and, finally, with a vacuum bag. The mitire system is placed upon a smooth metal tool surface into an autoclave, vacuum is a Ued to the bag and the temperature is increased at a constant rate in order to promote the resin flow and polymerization. The autoclave process will be used along this section as a case study to describe the influence of the matrix characteristics on the processing behavior of hi performance conqmsites. 

As discussed earlier, a finished rubber formulation is composed of many different materials. These materials bring a certain amount of retained moisture to the rubber batch additionally, each may have its own implications in the formulation s capacity to absorb water. This can occur directly from the environment, but more often occurs during the sterilization cycle. Most closures are steam sterilized through an autoclave process. This process pushed steam into the matrices of the rubber. The composition of the rubber will have a direct impact on the amount of moisture which is absorbed and retained. 

The aerospace industry has recognized the major benefits associated with fiber-rein-forced composite materials [1]. The more popular techniques available for composite production are the traditional wet layup or autoclave and resin transfer molding. Efforts to further reduce processing time and improve part quality have focused on improved process control. To date, this has been based on off-line techniques. The need for online cure monitoring is widely recognized, and this will require the development of suitable in-mold sensors. The Engineering Composites Research Centre has investigated and concentrated on the specific problems encountered in the aerospace industry. 

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