Entwicklung von Maßnahmen zur Reduzierung der Porosität von endlosfaserverstärkten Halbzeugen für die Additive Extrusion

Kurzfassung

Continuous fiber-reinforced filaments for three-dimensional printing are to be produced in order to be able to design the components to be produced more efficiently. Currently, these filaments exhibit porosity, which is undesirable due to the negative influences caused, for example, by a reduction in mechanical properties and the impairment of thermal properties. In order to achieve the goal of pore reduction, it is necessary to understand and evaluate the influencing parameters of the existing system. The necessary modifications must still allow a continuous process and ,must be integrable into the existing plant. The present work intends to investigate the porosity improvement of continuous fiber reinforced 3D printing filaments. A theoretical characterization of the materials processed in the semi-finished product, carbon fibers and thermoplastic polymers, is carried out, in order to achieve this goal. The various measurement techniques used or considered to characterize these base materials as well as the filaments are discussed. This work focuses on the study of porosity in the manufactured filaments. In order to evaluate them, this work deals with voids, the pores, in a second theoretical section. The term is explained and subdivided, and a description and classification of voids by size and location is given. Various factors influencing the formation of the voids are found in the literature, some of which are decisive for the type of void. These factors are tested in the presented study. The theoretical findings are followed by preliminary tests to characterize the materials and semi-finished products and to check some of the void influencing factors known from literature in order to quantify and, if possible, eliminate their influence. The starting materials and the various filaments are microscoped and subjected to thermal analysis procedures for this purpose. Furthermore, the densities are determined and the polymer rheology is investigated. The findings developed in this phase of work, in combination with the theory included, lead directly to several design adjustments to the production setup as well as parameter engineering opportunities to reduce void content, such as material selection. Within these preliminary tests, a new analysis method for void detection is developed and verified with thermal analysis methods. The new method allows not only the automated evaluation of the void volume content but also the evaluation of the type and location of the voids. Constructively, a temperature control section is used for hydraulic decoupling of two nozzles as well as possible controlled cooling of the filament. In a second step, a nozzle addition is made to the consolidating nozzle in the form of an ironing zone for the resulting filament to relax polymer chains and to reduce stretching. The final adaptation step in this work is to apply low-frequency broadband excitation to the consolidation nozzle to exploit the rheological effects, the change in viscosity when shear is introduced, of viscoelastic polymers. Finally, the different versions are investigated in different parameter configurations of production for both semi-crystalline and amorphous thermoplastics and evaluated with respect to their influence on porosity. It can be seen that the influence of the modification options cannot be evaluated independently. Rather, a change of the void type is shown by the different modifications. These findings lead to an outline of further modification possibilities. The evaluation will conclude with a theory of the interaction of different pore types