Autoclave processing for composites

HUBERT, McGill University, Canada andG. FERNLUND and A. POURSARTIP, The University of British Columbia, Canada 

Abstract This chapter discusses the state-of-the art of autoclave processing of polymer composites. First, the architecture and typical formulation of a process model applied to the autoclave process is presented. Then, the steps for autoclave process development using the proposed modelling approach are discussed. In particular, the thermal management during the process and the dimensional control of the final part are reviewed in more detail. 

Key words autoclave processing, process modelling, flow, compaction, heat transfer, cure, dimensional control, residual stresses, cure cycle. 

Vacuum bag Breather Bleeder Vacuum plug Sealant 

Many of the shortcomings of the intelligent process control approach can be addressed through the use of autoclave process modelling. Using 

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Hubert P, Femlund G, Poursartip A. Autoclave processing for composites. In Advani S, Hsiao K-T, editors. Manufacturing techniques for polymer matrix composites (PMCs). Cambridge (UK) Woodhead Publishing Limited 2012. 

LeClair, S.R., Abrams, F.L., Lagnese, T.J., Lee, C.W., Parks, J.B. Qualitative Process Automation for Autoclave Cure for Composite Parts, (1987) AFWAL-TR-87-4083, AFWAL/MLTC, Wright-Patterson AFB Ohio... 

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. 

The stability, growth, and transport of voids during composite processing is reviewed. As a framework for this model, the autoclave process was selected, but the concepts and equations may be applied equally effectively in a variety of processes, including resin transfer molding, compression molding, and filament winding. In addition, the problem of resin transport and its intimate connection with void suppression are analyzed. 

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...

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... 

Dave, R., Kardos, J., Dudukovic, M. Process Modeling of Thermosetting Matrix Composites A Guide For Autoclave Cure Cycle Selection, American Society for Composites, First Technical Conference, Dayton, OH, Oct. 1986... 

Abrams, F., et al. Qualitative Process Automation for Autoclave Curing of Composites, AFWAL-TR-87-4083, Interim Report for the Period 15 October-15 May 1987... 

The state of the art for many polymer processes is a simple time-temperature recipe for the process equipment. This is often a satisfactory solution for processes in which the process equipment temperature and the part temperature are nearly identical. Temperature control for pultrusion of thin cross-sections, for instance, can be satisfactorily based on the die and barrel temperatures. In processes where there is a major difference between the setpoint and the local conditions, it is now becoming more common to implement supervisory controllers which account for these differences. The autoclave curing of composites, for instance, may have large temperature lags between the autoclave and the part as is shown in Figures 15.3... 

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. 

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