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Complex Cores for New Utilities

Periodic Reporting for period 2 - CoCoNut (Complex Cores for New Utilities)

Reporting period: 2021-01-01 to 2022-07-31

CoCoNut (Complex Cores for New Utilities) is a project that is part of the Clean Sky 2 program.
The challenge is to design and implement an innovative solution to make a complex, hollow composite part for a critical aircraft structure, via a method compatible with high production rates.

In existing processes for manufacture of composite landing gear parts with complex hollow internal geometries, there is significant time wastage and part complexity associated with use of metal cores, which need to be machined and sectioned, increasing production time and cost. Improving the manufacturing process allows for more simplified, cost-effective, and environmentally friendly serial production of these parts.

The proposed solution is to use an internal core based on a water-soluble material, which can be easily removed from the finished part by washing. The core material is environmentally friendly and recyclable.

The overall project objective is to demonstrate the efficacy of a water-soluble internal core material for fabrication of a structural composite part with internal cavity, meeting the design requirements, withstanding the required process conditions, and showing an advantage over the existing technology in terms of manufacturing time and cost.

• A stable, reliable, repeatable method of manufacturing water-soluble cores was developed. The result was the production of 3 water-soluble mandrels, each conforming to the complex geometry requirements, allowing for successful fabric lay-up and installation in the RTM tool.
• Techniques for sealing the water-soluble mandrels were developed, tested, and applied. The 1st mandrel sealing material, PET shrink tube, did not withstand RTM injection conditions, the main problem being the temperature resistance of the sealing. Therefore, multi-layer sealing candidates with more robust design, including both shrink tube and wrapped Teflon tape, were selected for sealing the 2nd and 3rd mandrels.
• Improvements in the sealing resulted in substantial improvements in the results of the fabrication of the 2nd and 3rd parts. In the case of 3rd part production, the mandrel sealing was much more effective, resulting in minimal resin infiltration, allowing for rapid and effective demolding and wash-out, resulting in a smooth internal part surface. Final part dimensions were mostly within dimensional tolerances, and no delamination was detected.
• Sealing quality is of paramount importance to the success of RTM injection. Failure of the sealing results in resin infiltration into the mandrel, impeding full and effective washout of the water-soluble mandrel.
• Localized internal heating was implemented at thick areas of the part(where heating rates are slowest), via steel end-rings, to ensure the entire part reaches the injection and curing temperatures.
• Production of the parts shows that RTM production of structural aircraft parts with complex, hollow internal geometry using water-soluble mandrels is feasible, thus validating the technological concept behind the CoCoNut project.
The work performed during the period of reporting covered in this report included the following main activities:

1. Manufacturing of Water-soluble Mandrels
The mandrel-manufacturing effort required procurement of a core-casting tool (mold) for fabricating water-soluble mandrels based on the internal geometry of the demonstrator part design.
Production of each mandrel followed the following procedure:
- Casting of mandrel raw materials onto an inner steel shaft inside a mold
- Sealing of the water-soluble mandrel
3 mandrels were thus produced, composed of the water-soluble "CavusCore" material, comprised of glass beads and a polymer binder.
The 1st mandrel was sealed with a single layer of PET, and then integrated with the steel end-rings in preparation for lay-up and RTM injection.

2. Manufacturing of 1st Part
The production sequence of the 1st part was as follows:
- Cutting of carbon-fiber-reinforced polymer (CFRP) fabric plies, and layup on mandrel
- Installation of fabric preform into RTM tool
- Resin injection & curing
- Demolding of finished part from tool
- Wash-out of water-soluble mandrel from hollow interior of finished part
Due to a rupture in the shrink-tube mandrel sealing during RTM injection, resin infiltrated into the mandrel, so it could not be fully removed from the finished part by water dissolution.

3. Selection of Improved Mandrel Sealing Candidates.
Production of the 1st part showed that shrink tube sealing alone would not be sufficient to withstand the RTM conditions (6 Bars at 180°C). Therefore, the mandrel sealing had to be modified to a more robust design.
Sealing candidates were selected, according to criteria such as drapability and heat and mechanical resistance.
Accordingly, the following multi-layer sealing options were chosen for the remaining two mandrels:
- 2nd mandrel sealing sequence: Mandrel --> Spirally wound Teflon tape --> PET shrink tube --> lengthwise wrapped Teflon tape
- 3rd mandrel sealing sequence: Mandrel --> Shrink tube --> Spirally wound Teflon --> Lengthwise wrapped Teflon tape

4. Manufacturing of 2nd and 3rd Parts, using Mandrels with Improved Sealing Design
RTM production of the 2nd and 3rd parts resulted in substantial reduction in resin infiltration.
- During 3rd part production, very little resin infiltration was observed. Effective and rapid demolding and wash-out were achieved, with smooth internal part surface.

5. QA Testing
- Dimensional control of parts: Most of the measurements fall within the required tolerance.
- Non-destructive testing (NDT): Ultrasonic testing of the 3rd part detected no delamination.
- Microscopy images indicate low porosity.

6. A final activity involved an investigation of compatibility of production process with serial production of structural CFRP parts (including cost evaluation).

Dissemination activities & exploitation perspectives:
- Project website
- Project exhibited online on EU Horizon 2020 'CORDIS' website
- Plans for exhibiting demonstrator part and project results at upcoming conferences / exhibitions.
- Pending further improvements to mandrel sealing (beyond framework of CoCoNut project), public data will be uploaded to a public repository ('Zenodo'), and patent application submitted.
Demonstration of feasibility of the technological concept marks a successful R&D project milestone.
Further maturity of the technology to allow for compatibility with a serial production effort requires further modifications to the mandrel sealing, and automation of the fabric preform manufacturing and layup processes.
The Consortium members plan for further collaboration, beyond the current project framework, for further R&D toward a low-cost, reliable, repeatable, robust, rapidly applied mandrel sealing design, compatible with a serial production effort.
Such a result, coupled with fiber preform automation, has future potential regarding serial fabrication of hollow, structural aircraft parts, improving TRL from 3-4 to 5-6, and reducing manufacturing time and cost of serial production by at least 15%.