Model-Based Optimal Control of Polymeric Composite Cure in Autoclave System

The fabrication of high loaded aircraft polymeric composite parts requires tools such as heated closed-mold to which a pressure is applied at the correct gelation stage of the resin. The curing process includes the placing of a premixed compound in the mould, a pre-warming to the resin viscosity reaches a minimum, next applying of pressure to remove the gas bubbles and excess resin, and finally consolidation of resin at elevated temperature to its full polymerization. During matched-die molding, at which the mould is heated by the pressurization gas inside an autoclave or by built-in electrical resistance elements, it is particularly important that pressure is applied at the correct gelation stage of the resin within the cured body. To match the independently controlled pressure and mould heating we propose a model-based control approach, which use the mathematical model for epoxy-based thick-walled composite structure cure in the form of coupled kinetic equation of the resin with the heat transfer equation. This model takes into account an exothermal heat, change of the heat capacity and heat transfer coefficient at the phase transition of resin from liquid to gel and further to the solid state, where warmed pressurized gas in autoclave should provide the most uniform temperature field within the cured composite body. We formulate the control law for the inward heat flow as a function, which depends on four parameters determined at the solving of the optimization problem. To optimize this control law we restore the Pareto frontier for the minimized standard deviations of degree of cure and temperature averaged over the cured body. Finally, we compare the abilities to ensure the high quality curing results in the closed moulds heated by autoclave and by the discrete set of heaters positioned along the length of the die.