Liquid Composite Moulding (LCM) techniques are becoming more and more interesting for aircraft manufacturers due to their advantages against traditional prepreg-autoclave processes (reduction of waste, energy and waste toxicity and economic benefits).
The two steps that are involved in the LCM process are first the production of a dry fibre preform and secondly the resin injection. The focus of this project is on the development of advanced production methods for dry fibre preforms. Technical requirements for a preform are:
- Geometrical tolerances and dimensional stability
- Near Net-shape
- Geometrical complexity to form integral shapes
- Possibility to combine multiple sub-preforms to bigger preforms
- No negative influence on the permeability
The currently available consolidation techniques are stitching and binding. Stitching techniques are low energy consumption, but are limited to non-complex shapes. Novel stitching processes have been developed that enable the processing of 3D preforms by means of one side stitching techniques. Further optimization is needed to enable the processing of complex, integral structures. The consolidation of very complex preforms is possible using binding techniques. However, binding shows environmental drawbacks, mainly contamination due to organic volatiles and heating energy, which is needed for the binder activation. Thus, a need for improvement is clear.
For the environmental and economic improvement of the consolidation techniques new techniques will be developed based, on one hand, on novel 3D robotic stitching and, on the other hand, on the use of low temperature activation thermoplastic veils and ultrasonic binder. A demonstration phase will follow, consisting of the manufacturing of different scaled preforms representing skins, stringer and spars sections. Therefore, the braiding technology will also be used to manufacture integral and cost-effective parts. Impregnation tests will then be performed to evaluate the permeability of the obtained preforms. The objective is to be able to scale the techniques to automated serial manufacturing of big preforms (up to 8x3 m).
As agreed upon among the Topic Manager (IAI), the Coordinator (USTUTT) and the Consortium Partner (Tecnalia), May 1st has been chosen as a fixed starting date.
Within project APRIL (Advanced Preform Manufacturing for Industrial LCM-Processes), new promising technologies and materials have been investigated for the manufacturing of carbon fibre reinforced aircraft structures. The research work performed focused on developing a green manufacturing process for aerospace products by introducing ecological and economical preform consolidation techniques for liquid resin infusion processes.
By replacing the currently used, highly energy-consuming and waste-producing autoclave process with textile-based component manufacturing, the individual components can be pre-shaped separately first. Afterwards, they are combined into integral preforms by using different joining techniques like 3D-stitching, thermal binder activation or ultrasonic welding before they are infused with a thermoset resin system. This gradual process allows for applying different techniques, depending on what is the most ecological and economical solution for each step.
First, a large series of different binder materials in combination with braided or Non-crimp-fabric semi-finished products have been examined with regards to their impact on the mechanical properties and their applicability for component manufacturing. It was found that just a small amount of the investigated materials meet all requirements by the aircraft industry. However, suitable combinations could be determined.
Moreover, the robot-assisted braiding technology was introduced for the manufacturing of stiffener parts (T-stringers), in order to higher the energy-efficiency of skin panel manufacturing. By performing a mechanical characterization of the introduced materials, it could be proved that braided components are able to compete with NCF materials regarding their mechanical properties for stiffeners. Braiding also showed a high manufacturing efficiency by integrating the binder application directly into the process and requires an extremely low scrap rate of less than 1 %.
For preforming and joining the different parts, three different techniques were used: 3D-stitching, thermal binder activation and ultrasonic binder welding. It was found that the thermal activation method leads to the most accurate results for stiffener preforming, while ultrasonic welding proved to be an extremely low energy-consuming and fast technique for connecting separate sub-preforms and integrate them into large preform sets.
To transfer the investigated technologies into serial production, several manufacturing process scenarios have been evaluated with regards to their economic and ecological potential. The most promising approach for full scale manufacturing was found by combining the various technologies and materials in a way that allows for high volume production with accurate preform dimensions, low energy consumption and waste production. A promising technique for serial production is the continuous and automated draping of stiffener preforms. A prototype construction has already been developed within APRIL and tested successfully.