Enabling non-planar load oriented deposition of carbon fiber reinforced polymers by varying layer height

Abstract

A common research goal for printing carbon fiber reinforced polymers (CFRP) using fused filament fabrication (FFF) has been the deposition along load paths to fully utilize the potential of the highly anisotropic material. Yet, the state-of-the-art solutions for load oriented non-planar slicing and path planning for neat polymers involve the dynamic variation of layer height. This variation is not possible in a single layer for the most commonly used process variant for printing CFRP, towpreg extrusion, because of the fixed ratio of matrix to fiber. This problem can be solved by printing interlayers which roughly double the layer count, introduce weak points, decrease the fiber volume fraction (FVF), and increase manufacturing time. Continuous fiber coextrusion (CFC) offers a possible solution to this problem, as the amount of polymer co-matrix can be controlled. This is possible because of the pre-impregnation of the fiber material, which allows active feed of both fiber and co-matrix. This study aims to investigate the possibility of using continuous fiber coextrusion to dynamically vary layer height during the printing process to enable the load oriented non-planar printing of CFRP. To this end, the process is described, a custom control scheme is mathematically derived, and an experimental plan is presented. The experiments include the printing of coupons to evaluate the minimum and maximum layer heights and the possibility to vary the layer height dynamically. A pipe and a bracket are printed to establish the applicability to manufacturing real-life parts. Micrographs are taken to assess the void content and fiber distribution. Surface roughness is evaluated with white light interferometry. To evaluate the impact of layer height variation on stiffness and strength, a mechanical investigation is performed involving tensile and compressive tests. In conclusion of this study, the possibility of dynamic layer height variation to continuous fiber coextrusion can be confirmed and its application for load oriented non-planar printing is enabled.