Effects of Variable Fiber Microstructure in Composite Fused Filament Fabrication on Physical Properties Using High Aspect Ratio Short Fibers
Authors: Nixon Jason R. , Bigio David I. 
Published: March 2016

Polymeric fused filament fabrication technology (FFF), a subfield within additive manufacturing (AM), is becoming a contender for the reintroduction of the small-scale manufacturing of customized consumer products to a mass-production dominated world market. However, before this technology can be widely implemented, there remain significant technological hurdles to overcome. One issue that has been addressed at great length in other traditional polymer manufacturing fields is the inclusion of fillers in the component for physical property enhancement or the introduction of entirely new properties to the matrix material. Experiments conducted in this study examined the inclusion of carbon microfibers (CMFs) into the matrix material prior to the printing process, and the effect of different processing parameters on the final filler structure of the composite parts post printing.

Prior work on microstructural evolution during extrusion in a 3D printer has been conducted computationally to study the effects of extrusion rate, matrix rheology, and nozzle geometry on fiber orientation [1]. It was found that varying the nozzle geometry generated significantly different microstructures, and that the remainder of the parameters could be varied to fine-tune microstructural characteristics. Findings indicated that, by varying the nozzle geometry from a converging to a diverging conical section, microstructures ranging from axially oriented (with respect to the extrusion direction) to radially oriented are theoretically possible. Current work performed on extruders and FFF platforms indicates that during the extrusion process, fibers tend to align very closely to the axis of extrusion in shear flow (i.e. converging or straight dies). However, in some applications, this may not be the most effective filler structure for property enhancement, so there remains interest in exploring methodologies for fiber rotation during extrusion.

For this study, CMFs and acrylonitrile butadiene styrene (ABS) were compounded using a 28mm fully-intermeshing co-rotating twin-screw (CoTSE) extruder. 3D printer feedstock was manufactured in-house. A range of concentrations from 0%wt to 15%wt fabricated and tested. Analysis of the feedstock indicated nearly axial fiber alignment post-manufacture. This feedstock was then used in a Lulzbot TAZ4 printer to manufacture composite tensile testing specimens. Printed composite properties were then identified and compared to neat ABS and bulk composite properties. It was found that using a purely converging die, highly aligned filler structures were produced (with respect to the bead laid by the printer). Using a diverging nozzle, more random filler structures were produced. Improvements in both intra-layer properties were observed using the diverging nozzle geometries to reorient fibers during extrusion. Property improvements were not found to be as high as longitudinal properties for highly aligned filler structures. Using insights gained through these experiments, we are currently working on exploring added functionality for the composites using different types of fillers as well as multi-scale filler combinations.