André Schlink

Methods for evaluating the orientation of carbon fibers in reinforced plastics vary in complexity and are application specific. Here, we report an algorithm that quickly evaluates in-plane fiber orientation based on determining the correlation coefficient of adjacent regions in microscopy images. The result is not the fiber orientation of individual fibers, but the principal fiber orientation of small image areas. This method is applicable to large areas due to its low computation time and captures varying fiber orientations, making it suitable for the study of injection molded samples with complex geometries. A great advantage is that no information about the fibers or the matrix, or their combination, is needed prior to the analysis. This approach is also suitable for samples with surface defects. Demonstrations of this technique are given for Polyamide 6 and Polypropylene with 30 weight % carbon fibers of different fiber lengths.

The increasing demand for renewable raw materials and lightweight composites leads to an increasing request for natural fiber composites (NFC) in series production. In order to be able to use NFC competitively, they must also be processable with hot runner systems in injection molding series production. For this reason, the influences of two hot runner systems on the structural and mechanical properties of Polypropylene with 20 wt.% regenerated cellulose fibers (RCF) were investigated. Therefore, the material was processed into test specimens using two different hot runner systems (open and valve gate) and six different process settings. The tensile tests carried out showed very good strength for both hot runner systems, which were max. 20% below the reference specimen processed with a cold runner and, however, significantly influenced by the different parameter settings. Fiber length measurements with the dynamic image analysis showed approx. 20% lower median values of GF and 5% lower of RCF through the processing with both hot runner systems compared to the reference, although the influence of the parameter settings was small. The X-ray microtomography performed on the open hot runner samples showed the influences of the parameter settings on the fiber orientation. In summary, it was shown that RCF composites can be processed with different hot runner systems in a wide process window. Nevertheless, the specimens of the setting with the lowest applied thermal load showed the best mechanical properties for both hot runner systems. It was furthermore shown that the resulting mechanical properties of the composites are not only due to one structural property (fiber length, orientation, or thermally induced changes in fiber properties) but are based on a combination of several material- and process-related properties.

The fiber type, orientation of the fiber, fiber-matrix adhesion, and the fiber length are very important for the performance of a short fiber reinforced plastic. Hybrid reinforced polybutylene terephthalate and reference compounds were tested using tensile, Charpy impact, and three-point bending mechanical tests. The interaction of regenerated cellulose fiber and glass fiber was investigated using a polybutylene terephthalate matrix at a fiber volume content of 10%. The ratios of each fiber type was varied. The compounds were pultruded with an extrusion die to have an even fiber length of 3 mm after granulating. In a second step, the specimens were injection molded for mechanical testing. The results were compared to the rule of hybrid mixtures (RoHM) prediction. It was shown that the results of the hybrid reinforced compound were close to the RoHM prediction. The Charpy impact tests show a high positive hybrid effect. The fiber length shows an interaction that is dependent on the ratio of each fiber type.

Short glass fiber reinforced plastics (SGFRP) offer superior mechanical properties compared to polymers, while still also enabling almost unlimited geometric variations of components at large-scale production. PA6-GF30 represents one of the most used SGFRP for series components, but the impact of injection molding process parameters on the fatigue properties is still insufficiently investigated. In this study, various injection molding parameter configurations were investigated on PA6-GF30. To take the significant frequency dependency into account, tension–tension fatigue tests were performed using multiple amplitude tests, considering surface temperature-adjusted frequency to limit self-heating. The frequency adjustment leads to shorter testing durations as well as up to 20% higher lifetime under fatigue loading. A higher melt temperature and volume flow rate during injection molding lead to an increase of 16% regarding fatigue life. In situ X-ray microtomography analysis revealed that this result was attributed to a stronger fiber alignment with larger fiber lengths in the flow direction. Using digital volume correlation, differences of up to 100% in local strain values at the same stress level for different injection molding process parameters were identified. The results prove that the injection molding parameters have a high influence on the fatigue properties and thus offer a large optimization potential, e.g., with regard to the component design.