Lightweight construction is making waves in all sectors of industry. An increasing number of lightweight structures used in the aerospace and automotive industries are now based on fiber-reinforced plastics. A particularly important field of application is pre-impregnated, semi- finished fiber products, so-called “prepregs”. Prepregs are typically made of glass or carbon fibers, which are available as textile woven- meshes or scrims. The resin in which the fibers are embedded undergoes a continual hardening process making it necessary to store materials at -18°C. The resin content also means that surfaces are highly adhesive. To make it possible to transport, store and process prepregs, they are coated on both sides with a film. The Chemnitz-based Steinbeis Innovation Center for Drive and Handling Technology in Mechanical Engineering joined forces with KVB (the Institute of Construction and Composite Design) and Cotesa GmbH to develop a gripping technology to remove these films. This involved looking more closely into the bond between the protective film and the prepreg.
With the previous process, after the prepreg was cut to size, an operator would pierce the corner of the foil with a cutting knife, turn the corner up and tear the film off by hand. Depending on the strength of adhesion, this process would be more or less long-winded and it involved significant investment of resources.
To establish how strong the adhesion force is between the film and prepegs, the project team developed special testing equipment. This made it possible to examine different carbon fiber-reinforced plastic prepregs with a surface mass of between 110 g/m2 and 650 g/m2, as well as cover films of between 32 g/m2 and 96 g/m2. To investigate whether it would be possible to use conventional gripping technology, the surface topography of the protective films was closely examined visually and the adhesion level was ascertained. One finding of the tests was that the adhesion force was between 2 N/cm2 and 12 N/cm2.
The results of the tests and the need to make a partial separation near the edge were used as the starting point to test conventional gripping technology. The benchmark the experts defined was to be able to remove material safely without damaging embedded fibers. The results soon showed that this was only possible with a vacuum suction device if adhesion was at minimal levels. Since the majority of cover films adhere much more strongly to prepregs, it is not possible to remove films safely with conventional gripping technology.
To develop a specially adapted gripper, the project team leveraged the tendency for adhesive force to be significantly impaired by refrigerants. After bringing the separation device into contact with the protective film, refrigerant is injected through a cannula. A vacuum sucker then partially separates the film from the prepreg. This process has been patented, and the device that has been developed can be used manually or integrated into automated processes.
Together with its partners, the Steinbeis Innovation Center for Drive and Handling Technology in Mechanical Engineering carried out the development of the separation device as part of the Central Innovation Program for SMEs, which is funded by the Federal Ministry for Economic Affairs and Energy.
Prof. Dr.-Ing. Eberhard Köhler
Steinbeis Innovation Center Drive and Handling Technology in Mechanical Engineering (Chemnitz)
su1230@stw.de
Dr.-Ing. Uwe Lauschke
Institut für Konstruktion- und Verbundbauweisen (Chemnitz)
Dr.-Ing. Udo Berthold, Jorg Hüsken
Cotesa GmbH (Mittweida)