Correlating Large-Format AM Print Parameters to Fiber Length and Mechanical Performance of Reinforced Polymer Composites

This paper aims to correlate processing conditions to fiber length and mechanical properties in fiber-reinforced composites on the Big Area Additive Manufacturing (BAAM) system at Oak Ridge National Laboratory. The processing of fiber-reinforced composites has a significant influence on their microstructure, which dictates the properties of the final product. The effect of processing is poorly documented in systems like the BAAM, leaving significant opportunities to improve the mechanical performance of printed structures. In this work, fiber length distributions from pelletized feedstock were compared against those of specimens extruded under different processing speeds. The mechanical strength of each specimen was evaluated to correlate processing speed to fiber length and mechanical properties. Experimental results showed that fiber length decreases slightly with increasing screw speed. Mechanical performance was not found to be affected by the decrease in fiber length. This research will guide future modifications to hardware design and print parameters to maintain fiber length and maximize mechanical performance. Introduction Reinforcing fiber is essential to large-format polymer additive manufacturing [1]. However, the effect of processing conditions on fiber-reinforced composites in large-format additive manufacturing has not been well documented. The BAAM uses a single screw plasticating extruder to rapidly deposit large quantities of material in the printing process. Figure 1 shows an example diagram of a plasticating extruder [2]. Pellets are fed into the flights of a screw through a hopper, and then are melted through compression caused by the screw. This creates a torturous environment for the fibers, which partially defines the mechanical properties of the extruded material. Fortunately, this process is almost identical to injection molding, where there have been numerous investigations concerning the influence of processing conditions of fiber reinforced plastics, and the resulting mechanical properties of the material. Wolf et al demonstrated that conditions which increase shear decrease average fiber length [3]. Wolf took fiber length distributions from different regions within the extruder, and under different processing conditions including screw speed, temperature, and die diameter. Wolf found that gentler processing conditions (slow screw speed, higher extrusion temperature, larger die diameter) result in the preservation of the fiber length. Turkovitch et al. studied the fracture of glass fiber through the length of the extruder and at different screw speeds. The findings were the same as Wolf; fibers progressively break through extrusion, and fibers breakage increases with screw speed [4]. Yilmazer et al. examined the effects of processing conditions on fiber length 771 Solid Freeform Fabrication 2021: Proceedings of the 32nd Annual International Solid Freeform Fabrication Symposium – An Additive Manufacturing Conference