Jochen Wellekötter

Although direct Joule heating is a known technique for heating carbon fiber reinforced plastics, it is a yet unexplored heating method for thermoplastic prepregs before back-injection molding. The knowledge obtained from resistance welding, for example, is not directly transferable because of considerably higher heated volumes and more complex shapes. In this study, the governing parameters and process limits are established for this method. The influences of the contacting, the materials used, and the size of the heated part are investigated with respect to the part temperature and heating efficiency. The findings show that the quality of heating is determined by the shape and size of the electrodes. Larger electrodes lead to a more homogeneous temperature distribution. Parts based on woven fabric can be heated more homogeneously because of the existence of intersections between rovings, generating contact between fibers. An increase in part width results in uneven heating behavior.

Although fiber-reinforced plastics combine high strength and stiffness with being lightweight, major difficulties arise with high volume production and the return of manufactured parts back into the cycle of materials at the end of their lifecycles. In a novel approach, structural parts were produced from recycled material while utilizing the so-called injection molding compounding process. Recycled fibers and recycled polyamide matrix material were used by blending carbon and matrix fibers into a sliver before processing. Injection molding was then used to produce long fiber-reinforced parts through a direct fiber feed system. Recycled matrix granules were incorporated into the injection molding process by means of an injection molding compounder to investigate their influences on the mechanical properties of the parts. The findings show that the recycled fibers and matrix perform well in standardized tests, although fiber length and fiber content vary significantly and remain below expectations.

Fiber reinforced plastics (FRP) hold considerable potential for the construction and design of light-weight parts. FRP are rapidly gaining in importance in various industrial sectors. Although this group of materials combines high strength and stiffness while simultaneously being lightweight, major difficulties still arise when it comes to high volume production and the return of manufactured parts back into the cycle of materials at the end of their lifecycle. A new approach follows the idea of producing structural parts through the forming and back-injection molding of preforms with a large portion of recycled fibers and recycled matrix, without drastically compromising the mechanical properties. To achieve this, carbon fibers retrieved out of a recycling process are blended with recycled polyamide fibers and spun into a hybrid yarn. Subsequently these yarns are processed into preforms by means of Tailored Fiber Placement. This process allows for a precise fiber positioning in accordance with the load path. Additionally, back-injection molding allows functional integration into the parts, aiming for weight reduction and lowering installation effort. Recycled carbon fibers and recycled matrix granules are also utilized for the back-injection molding process using an Injection Moulding Compounder to investigate their influence on mechanical properties of the parts.

Climate change and a growing energy demand drive the development of lightweight construction in the automotive industry. The recent focus of interest is on glass and carbon fiber‐reinforced plastics. A resulting increase of production waste and scrap parts requires an efficient recycling. Although thermoplastics are generally suitable for reprocessing, recycled polyamides like PA6 show differing mechanical properties in comparison to virgin material due to molecular degradation processes. To achieve a high recycling rate and to compensate for the reduced mechanical properties, the parts of the test series of this study are filled with glass fibers (20 and 30 wt%). To increase the mechanical properties even further, a single‐stage injection molding compounder is used. A comparison with the standard, two‐stage production process provides further understanding of the complex interactions. The study is carried out with various blends of recycled and virgin PA6 with a recycling rate of 0, 25, 50, and 75%. Both single and double recycled PA6 are used. For the evaluation of the mechanical properties, tensile tests, bending tests and impact bending tests are conducted and assessed by fibers length, fiber–matrix adhesion, and fiber content. The results of all tested recycling ratios show no direct influence of the recycling‐related impairment of the plastic on tensile strength, tensile modulus, bending strength, and impact resistance. Rather, the investigations indicate an expected direct relationship to the fiber content, whereby an increase in fiber content relates to a significant improvement of the mechanical properties. Furthermore, the investigated fiber length distributions show a correlation with the mechanical properties. Therefore, the more gentle material processing of the injection‐molding compounder and an associated reduction in fiber shortening, result in a superiority to the two‐stage production process. POLYM. COMPOS., 40:1731–1739, 2019. © 2018 Society of Plastics Engineers