Developments in lightweight construction can lead to significant savings in energy and costs in transportation. Thermoplastic prepregs are becoming increasingly established, especially for the interior lining and thermal insulation of refrigerated containers and truck bodies.
To produce these unidirectional (UD) fiber-reinforced tapes, Karl Mayer Technische Textilien GmbH, Chemnitz/Germany, a subsidiary of the Karl Mayer Group, Obertshausen/Germany, has developed the new Sim.Ply machine. The innovative machine produces thermoplastic fiber-reinforced tapes and uses plastic in the form of films. As part of an inline process, the fibers are spread and impregnated with the plastic film by applying pressure and increasing the temperature.
However, fiber composite structures with multiaxial fiber orientations are in demand for many lightweight construction applications. Therefore a project has been put into place offering manufacturing technologies for thermoplastic prepregs made from multiaxial materials and alternative raw materials for the thermoplastic matrix.
The Cop Max 5 multiaxial warp knitting machine was used in the project work to produce reinforcing structures with flexible fiber orientations for thermoplastic prepregs. Initially, the commercially available glass fibers were homogeneously spread on the UD 700 fiber spreading assembly unit to form 12"-wide (30.5 cm) fiber tapes.
The spread tape was then fed to a Cop Max 5 machine. The multiaxial warp knitting machine is actually designed for using carbon fibers with finite weft insertion. The use of textile glass fiber is now being tested for the first time.
Another innovation was the polymer combination material for the composite solution, using nonwovens rather than a polymer film, in contrast to Sim.Ply. In cooperation with a nonwovens manufacturer, the suitability of these products as a raw material for the plastic component in thermoplastic fiber composites was investigated. The nonwoven polymer consisted of polypropylene (PP) hot-melt nonwovens. The nonwoven fabric is very easy flowing when melted and the individual glass fibers are very well embedded. In general, nonwoven polymers offer advantages over their closed film counterparts thanks to their good air permeability, since the air can escape better from the fiber-plastic composite to be produced.
The PP nonwovens for this project work had an area weight of approx. 45 g/m². The fiber material was fed into the Cop Max 5 as a layer above and below an intermediate layer of spread glass with a fiber orientation of 45° or 60° angles. The individual layers of the sandwich structure were then stitch-bonded. When choosing the right warp knitting yarn, using a classic polyester (PET) variant proved favorable, because PET yarn has a higher melting point than the PP hot-melt nonwovens. This means it can withstand the subsequent impregnation process, thus ensuring the fibers are fixed permanently. The angular positions are not shifted while passing through the impregnation section. Initial investigations completed in the field of nonwovens production yielded positive results.