Konferenzbeiträge

Increasing demands in the fields of high-precision force measurement and weighing technology require an ever-higher measurement resolution, a larger measurement range, a lower measurement uncertainty, and traceability to a natural constant. Load cells using the compensation principle have the potential to fulfill these requirements. To enhance the measurement resolution and decrease the measurement uncertainty, the residual stiffness of the compliant mechanism in use needs to be compensated. Due to a lack of solutions in the state of the art, a novel monolithic stiffness-compensated mechanism for measurements according to the compensation principle was developed. Simulations show a stiffness reduction to 0.2%of the initial value, a theoretical force resolution of 31 pN, and applicability for any orientation in the gravity field. Experimental investigations on a prototype confirmed the existing potential. However, further optimization of the mechanism is required to negate the effects of manufacturing deviations.

Continuous fiber reinforced thermoplastics (CFRT) are composite materials consisting of continuous fibers and a thermoplastic matrix and offer outstanding mechanical properties, low densities, short cycle times and recyclability. CFRT can be classified into unidirectional tapes and sheets utilizing various semi-finished textiles as reinforcement. CFRT sheets are of interest for area measured products or multiaxial loads. Various discontinuous and semi-continuous methods to prepare CFRP sheets are described in the literature. All these methods either feature high cycle times or high investment costs and require double melting of the polymer, e.g., first to produce a polymer film and second to produce the CFRT sheet. An energy efficient alternative to produce CFRT sheets is extrusion, which allows to spare one melting step. A twin-screw extruder melts the polymer, which is then conveyed by a melt pump to the film extrusion dies and applied to both sides of the semi-finished textile, which is wetted and consolidated using a calendar. Due to the high melt viscosity and the line load at the calendar the major challenge is to achieve full void-free impregnation of the semi-finished textile. The mechanical properties of a CFRT sheet are determined by fiber and void volume content. Hence, the influence of the processing conditions on the fiber and void volume content as well as the mechanical properties were examined applying a parametric study of the die temperature, the haul-off speed, and the gap between the calendar rolls. The properties of the extruded CFRT sheets were compared to compression molded sheets. The fiber volume content was directly adjusted by the haul-off speed and the extruder throughput. An increasing die temperature lowers the melt viscosity and results in an increased fiber volume content. Scanning electron microscopy shows complete macro impregnation between the fiber bundles but not completely wetted individual filaments within fiber bundles.