Cyanate ester resins offer significant advantages as composite matrices because of their high thermal stability, their improved toughness, low outgassing, low moisture uptake and radiation resistance due to their low dielectric constant. Towards the direction of incorporating these materials in the aerospace industry, a new glass/cyanate ester material system was characterized for use in the plenum of the air-cooling unit of aircrafts. However, the first trials with this new prepreg to manufacture real parts (plenum housing) in an autoclave, highlighted some heterogeneity issues in the reliability and reproducibility of the manufacturing process.
PLENOPTIMUM project aimed to address the reasons that lead to the aforementioned heterogeneities and through a novel Design of Experiments approach identify a number of possible influential factors that affect the manufacturing process (in autoclave and out-of-autoclave) and potentially cause deviations and heterogeneities. For the first time, factors additional to temperature profile and pressure examined according to their effect on the final quality of the composite materials and the actual plenum. Process modelling as well as online curing monitoring technologies assisted the optimisation task and the quest of the optimum curing cycle.
Manufacturing trials for plenum manufacturing in an autoclave as well as out-of-autoclave process took place in the final part of the project taking into account the results of a test campaign specially designed to highlight the effect each one of the identified influential factors has. Guidelines and recommendations to the topic manager and manufacturer that ensured and optimised the manufacturing process are the final outcome.
Both participants that form PLENOPTIMUM consortium have a strong background and experience on processing/manufacturing optimization and have the proper infrastructure to meet the targets set.
PLENOPTIMUM project dealt with the solution of a series of persistent manufacturing problems in the production of the plenum of the air-cooling unit of aircrafts with a new glass/cyanate ester material system. Heterogeneities, pink coloration, and porosity were the most important problems reported leading to an increasing disposal rate of manufactured plenums with all the associated costs (financial, environmental etc). A secondary objective was the feasibility of manufacturing the plenum in out-of-autoclave (OoA) conditions.
The PLENOPTIMUM consortium was formed by the Applied Mechanics Lab, University of Patras (UOP) and FUNDACIÓN TECNALIA RESEARCH AND INNOVATION (TECNALIA) both partners with a strong background on processing-characterization and manufacturing of cyanate ester based materials from their participation in various EC, ESA and national projects.
The team utilised a novel Design of Experiments approach in order to identify a number of possible influential factors which affect the manufacturing process (in autoclave and out-of-autoclave) and cause deviations and heterogeneities. Vacuum level, time to exhaust the vacuum and curing temperature were focused in the first level and an extensive test matrix was built in order to find the influential parameter(s). The quality assessment of the manufactured plates was conducted via fiber volume fraction (Vf), porosity, Tg and ILSS experimental measurements as well as color characterization. The results of the test matrix actually suggested that the manufacturing process is quite robust as far as the aforementioned factors is involved. In the meantime, some key observations from the topic manager coupled with knowledge acquired in the first stages of the project brought into light a factor missed in the first test matrix. The out-life and the storage conditions of the material. This factor was targeted in a second test matrix where the manufacturing trials and associated quality characterization were repeated for material left in room temperature controlled conditions. The results of the second test matrix were quite revealing, offering solid experimental evidence that the storage of the material in room temperature up to 9 days actually solved the problem of non-repetitive heterogeneous manufacturing. Apparently, a major issue in cyanate-ester material systems is high resin flow due to low viscosity when a less than perfect vacuum bag seal is present. In parallel, a physical modeling of the curing cycle offered useful insights into the way viscosity changes during curing towards an optimized curing cycle. Regarding the autoclave process itself, since the factors addressed in the first test matrix proved non-influential, the importance of securing the vacuum in the bagging system via careful sealing was highlighted. Sound seal of the vacuum bag eliminates the need of reduced resin flow and can ensure repeatable high-quality results using a commercial glass/cyanate ester prepreg system.
In the last part of the project, efforts were directed towards the OoA process. The manufacturing trials early on showed that it would be very hard to acquire the desirable compaction and consequently a minimum Vf and low porosity with a material system designed and optimized for autoclave use. Trials in flat plates as well as generic elements with double curvature in an innovative self-heated mold cannot be characterized successful but gave the consortium the impression that with the proper material system it is quite feasible. Finally, manufacturing trials for the actual plenum part in an autoclave process took place taking into account the results of the executed test campaigns.
The main achievements of PLENOPTIMUM project concerned the delineation of the factors that influence the manufacturing process of plenums resulting often in non-acceptable parts. The consortium came out with material as well as process related guidelines to the manufacturer and to the direction of securing the baseline autoclave process. These guidelines with a proper implementation can minimize the percentage of non-acceptable parts. A parallel outcome was the definition of an optimised curing cycle, a result that can lead to energy savings and together with the decrease of rejected plenums to a greening of the whole manufacturing process meeting some of the main goals of Clean Sky initiative. It was calculated based on scientific evidence throughout the project, that ensuring a leakage free vacuum system can optimise the baseline manufacturing process so that only a 2% scrap is reached, then the cost of the plenum could be reduced by 22%. Reducing the process time by means of adopting an optimized curing cycle could further decrease 3.85% of the plenum costs. The total estimated possible manufacturing cost savings if these two actions are taken (ensuring a free leakage system and using the optimized cycle) is an impressive 25% of the actual cost.
The out-of-autoclave process proved a too ambitious goal at this stage. Extensive manufacturing trials did not produce an acceptable quality part but clearly suggest that this can be feasible on the condition that a specially designed material system for this process is designed. Apparently, a successful out-of-autoclave process could really have an important economic and environmental impact so it is a challenge that should be re-addressed in the near future.