Projects

Reproducible pre-form fabrication for textile-reinforced plastics

Simulation zur Festlegung der Fixierungslinien
© Fraunhofer IWS Dresden

Simulation zur Festlegung der Fixierungslinien

Task

To extend the range of application for fiber-reinforced plastics (FRP) in the automotive industry, new technologies, low-cost manufacturing processes and design concepts are required, since they are at present the method of choice for complex, sculptured parts in the dry technique of pre-form fabrication. Particularly complicated part geometries demand significantly enhanced production accuracy, as well as reproducibility of the cutting, handling, stacking and compounding processes. The combination of various surface formation techniques and fiber-reinforcement orientations in the preform design only delivers the expected results if dimensioning is supported by exact knowledge of the material characteristics in blanking, shaping and manufacturing and furthermore enhanced by quality assurance measures.

The current production sequence only partially fulfils the quality requirements needed for high-tech areas. Threads and, particularly selvedge threads, are shifted or drop out of the area compound. When inserting the blanks in spatially formed setting fixtures or molding dies, the area must be shaped by shearing. Loads that are hard to quantify and stresses result in sometimes significant, undesired deviations in geometry.

Solution

Reference geometry
© Fraunhofer IWS Dresden

Reference geometry

The main objective of the project was a fundamental proof that complex textile preforms with/ without thermoplastic components can be produced for different requirements with high reproducible quality. A significant reduction of shaping due to structure-attachment measures should be avoided.

For this reason, local solidification was intended, which counteracts dropping out of the textile prepregs during cutting and subsequent stacking and shaping procedures. This was expected to overcome limitations on the shearing characteristics.

To achieve this, mechanical and thermal alternatives based on the use of modifiable, surface-structured thermoplastic and/or bondable threads or coatings, as well as the use of adhesives and adhesive non-woven fabrics or tapes, were investigated. The effect could be triggered by the cutting or handling process.

Results

Finished preform at a mold tool
© Fraunhofer IWS Dresden

Finished preform at a mold tool

To bring reinforcement structures without rework into a specific 3D component shape thereby preliminarily considering further load, the blank was designed directly based on the model geometry. For this purpose, the blanks were generated from the 3D geometry by means of inverse calculation methods. To tryout the adherence with adhesives, glass fiber fabrics and the matrix materials PET and PP were chosen as reference materials. Apart from manual adhesive application with varying application nozzles, the adhesives were applied by robots.

The experiments proved that adhesion problems could be avoided with the adhesives ultimately chosen when the reference materials were locally treated. Tests of the shearing characteristics proved that drapability was not limited significantly, when the reference blanks were attached by linear fixation.

When using fabrics made of GF/PP hybrid threads, the laser is also suitable for local adhesion fixation. After optimized application, the preform perfectly matches the desired part geometry (excluding the preform thickness, where a significant change in part thickness is observed) and is inherently stable.

This way, local fixation of the structure clearly improves the preform quality and shapes the essential preconditions for an increase in the level of automation over the whole process chain for perform production.

The research project ”Reproducible production of preforms for textile reinforced plastics“ described hereunder was performed on behalf of the German Federation of Industrial Research Associations (AIF) under the registration code 151 29 BR.