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Forming and Modular Integration of Thermoplastics

Periodic Reporting for period 2 - FORMIT (Forming and Modular Integration of Thermoplastics)

Reporting period: 2017-12-01 to 2019-05-31

The main objective of the FORMIT project is to develop a modular tooling to integrate a wing upper skin and an external wing leading edge with several T-Shape and omega stringers. Skins will be laminated on the integration tool by the Topic Manager with Automated Fiber Placement technology and stringer will be produced with an innovative continuous forming process patented by Applus+ Laboratories. Both structures will be made of carbon fiber embedded in PEEK thermoplastic matrix.
The conjunction of these two manufacturing technologies and thermoplastic composite enables an out-of-autoclave production process and curing final structure with low cost and high production rates. This project is a step forward in the development of thermoplastic aero-structures to fulfil the requirements derived from manufacture aeronautical components for Clean Sky 2 “on-ground” and “in-flight” demonstrators of a Regional Aircraft FTB2.
At the end of the project, the maturity of the glide forming technology applied to thermoplastic composites has been demonstrated. Several full scale laboratory demonstrators have been developed capable to produce full size parts with representative T stringer geometries on a laboratory environment. All this reasons lead us to consider achieved a technology readiness level (TRL) between 4 and 5. Regarding integration tools, a new concept based on induction has been proposed and preliminary tested.
Along the development of the Formit project, several technical objectives have been achieved on different areas. Relevant advancements on manufacturing processes, tooling design and parts manufacturing have been obtained.
In order to manufacture the desired T shape stringers, several production processes have been developed. All the processes have achieved a maturity level enough to be used on the manufacturing of real parts with representative geometry. They are the following:
- Thermoplastic prepreg layup process
One of the Applus glide forming process characteristics is that is able to form a full thickness layup from flat to a final geometry in one step. A new stacking process has been developed to manufacture cohesive flat layups, involving cutting, positioning and spot welding on the key positions of the layup.
- Thermoplastic L shape stringer automated continuous forming process
The automated continuous forming process developed to form the L shape stringers of the project has been an adaptation of the previously existing Applus+ Glide forming technology, used for epoxy prepregs, to the much higher processing temperatures, around 400ºC, required for the thermoplastic material. Nearly all the elements involved on the forming (heating blankets, forming tool, forming effector, etc.) have been radically changed. Finally, a complete forming process has been defined, with a complete new set of parameters and tools, which is able to deliver good quality L shaped thermoplastic stringers.
- Thermoplastic T shape stringer assembly process
The technical solution adopted is very innovative. Self-heated tools have been implemented, allowing for a energy and cost efficient solution. A several stages process has been developed able to assembly complete T stringers of 1.2 meters with a representative geometry and good quality.

A total amount of 156 L shaped stringers have been manufactured. The first batch of 106 stringers were used to develop and improve the L shape forming bench, achieving an evolution on the surface quality and an improvement on the geometry reproduction of the part. The other 50 L stringers produced were used on the development of the assembly process. A total amount of 50 T shaped stringers were produced, achieving good quality parts with good compactation degree and geometry. A batch of parts was used for a test campaign performed by the Topic manager, who validated and accepted the T-stringer parts.

Regarding integration tools, the development carried out by Sertec started with conceptual designs based on the use of heating resistances as heat source, these resistances were placed in a vacuum and heating chamber below the machined surface of the table. This concept showed deformation problems for the poor thermal performance due to resistor placement inside a vacuum chamber. After that, the concept proposed for the integration tool switched to a solution where the resistors where embedded inside the top metallic plate to improve heat transmission. After some thermal simulation this concept was discarded due to the excessive technical difficulty to machine the long holes where the resistors where housed.
A new heating concept/system was proposed, based on the heat being generated by currents inside the table material (by means of the Joule effect), in order to minimize the thermal conduction and therefore improve the efficiency of the system. Among the possible solutions, induction was chosen. To test its feasibility, different thermal CAE analysis were carried on, and physical test demonstrator was built as a proof of concept. After having some promising results on the analysis and tests carried on, the development of a pre design for the final assembly tool was started and validated by simulation.

Work conducted in the project has allowed the development of a complete glide forming bench implementing a variety of solutions to face the challenge of very high process temperatures, around 400ºC. The process has been able to manufacture good quality L shaped thermoplastic stringers. A subsequent innovative process to assembly L shaped stringers to obtain T stringers has been also developed and implemented successfully. The parts obtained have been validated through a test campaign. This demonstrates the maturity of the glide forming technology applied to thermoplastic composites. Several full scale laboratory demonstrators have been developed capable to produce full size parts with representative geometries on a laboratory environment. All this reasons lead us to consider achieved a technology readiness level (TRL) between 4 and 5.

Regarding the development of the integration tools, the definition of the conceptual design of the wing upper skin full integration mould has allowed SERTEC to face the main technical issues derived from large dimensions of the demonstrator. After analysing of several solutions based on resistance heating, which didn’t give satisfactory results, a new technological solution based on induction was proposed to overcome deformation issues, and was preliminarly tested in laboratory test. A pre-design for the final integration tool was done and validated through simulation.

This situation lets the consortium be optimistic about the potential impacts of the project, since the use of thermoplastic composites allows reducing CO2 emissions and environmental impact along the whole lifecycle of an aircraft thanks to:
- Manufacturing process with reduced energy consumption since autoclave curing is not necessary
- Lighter aerostructures and, therefore, minor fuel consumption during operation
- Material easily recyclable.
T stringer assembly tool on closed position before being wrapped with the vacuum bag
T stringer after being assembled
L stringer after being formed
L stringer forming bench previous to the beginning of a forming test