Highly loaded fiber filled polymers for material extrusion: A review of current understanding

This review documents the current state of the art of highly filled (HF) polymer composite systems used in additive manufacturing (AM), with a core focus on short and continuous fiber filled composites for use in material extrusion (MatEx) AM. Current state of the art composite materials systems have been succesfully incorporated into the MatEx process with loading percentages up to approximately 45 vol. %. Further increasing the loading percentage would afford significant mechanical improvements that could enable structural performance, however there are currently a number of limitations that need to be addressed. Viscosity limitations associated with MatEx processing of HF composites originate from the low processing pressure inherent to filament feed MatEx processes. Restrictive hot end nozzle geometries (high volume contractions) increase required processing pressure and potential for nozzle clogging failures. The increased viscosity of the HF composite reduces interlayer contact and adhesion during the printing process, resulting in increased void spacing. Mechanical limitations of HF composites processed using MatEx are also reviewed. Flow induced fiber orientation and its influence on viscosity and mechanical anisotropy are investigated. Fiber breakage as well as strain to break limitations are reviewed. Lastly, the geometric packing limitations of fibers within a circular bound nozzle are reviewed. This review takes an in-depth look at recent advances in addressing these challenges, and discusses opportunities for realizing MatEx processing of HF composite systems with significantly improved performance.