Shorter aerospace composite parts production cycle using induction heating and computer simulation

This article initially was published in Industrial Heating


Aerospace manufacturers rely heavily on the autoclave to ensure that their products meet porosity goals. For large parts of thermoplastics (like skins, spars, and integrated structures), this has always been the best way to achieve the compaction and consolidation pressure required, to drive out any volatiles. However, recently aerospace manufacturers are researching the options of producing thermoplastic composites in an Out-of-autoclave press consolidation technique as it reduces the production cycle from 4-7 hours to 10-30 minutes for small and mid-size parts.


For the last three decades, the manufacturing of aerospace structural composites has been using autoclaves to achieve high-quality reproducible parts, including high fiber volume fractions and low porosity. While autoclaves have worked well, manufacturing composites within an autoclave is accompanied by high acquisition and operation costs due to the necessity of a specialized heated pressure vessel to suppress the formation of voids. Furthermore, the capacity of autoclaves limits the size and design of parts, and the production rate is primarily affected by autoclave availability. As a result, there has been an increasing interest in the development of alternative techniques. Out-of-autoclave (OoA) technology is solving these problems for some applications.



What is Out-of-autoclave?


In the broadest sense, the OoA method includes any composite production technology that doesn’t use an autoclave such as microwave heating, induction heating, laser heating, hydroelastic hydroforming, and resistive heating of carbon fibers or carbon nanotube fillers in composites. OoA consolidation provides many advantages over traditional manufacturing methods - relatively short cycle times, efficient heating and cooling, and low initial capital expenditure. Other advantages of OoA technologies are the potential for a higher degree of part integration: for example, the co-molding of skin, stiffeners, and spars in the same operation, which leads to an overall reduction in the total process cycle, with increased production rates.


Additionally, specially-formulated and designed prepregs that can be cured in an oven (out-of-autoclave, or OoA prepregs) have been recently developed to remove the need for an autoclave. The OoA curing can achieve the equivalent thermal and mechanical properties of OoA composites, as the conventional curing method. Moreover, the OoA composite curing process provides a significant reduction in electrical energy consumption by two orders of magnitude.


“We will pursue OoA manufacturing for as many parts as is feasible in the future,” comments GKN Aerospace (Redditch, Worcestershire, U.K.) director of technology, Rich Oldfield. “We see it as a way to bring efficiency to the factory, including an entirely different workflow due to cellular manufacturing vs. large parts queued up for an autoclave.” GKN also sees this flexible manufacturing, which is less reliant on traditional manufacturing monuments, as critical to the task of achieving the forecast 60 to 70 percent composites content on the next generation 737 and A320 single-aisle jetliner programs. (source: https://www.compositesworld.com/articles/out-of-autoclave-prepregs-hype-or-revolution)


Induction heating method


Induction heating can replace the traditional autoclave method for heating and curing large prepreg parts or heating molds for thermoplastics or resin injection. It is used to achieve the necessary temperature of the mold, but it also can guarantee better control over the curing processes, including local heating to accommodate parts with different thicknesses, eliminating hot and cold spots providing fast and homogeneous heating. Even though an integrally heated tool costs slightly more than a standard tool, the benefits of greatly reduced cycle time and freedom from monument constraints deliver an overall economic gain. This technology also is much more conducive to higher-rate manufacturing.


The general benefits of induction heating: