Experimental Investigation of Short Fiber Reinforced Thermoplastic Composite Joining in an Integrated Additive Manufacturing and Compression Molding System
Authors: Billah Kazi Md Masum , Kumar Vipin , Rathod Neel , Phadatare Akash , Nuttall David , Smith Tyler , Vaidya Uday , Kim Seokpum , Kunc Vlastimil , Hassen Ahmed Arabi 
Published: August 2024

Abstract
Fiber-reinforced thermoplastic (FRTP) composites have gained popularity within the aerospace and automotive industries due to their high specific strength-to-weight ratio compared to their metallic counterparts. State-of-the-art composite manufacturing technology such as compression molding (CM) is one of the most used techniques to fabricate FRTPs. While compression force allows high consolidation of the matrix and fibers, the orientation of the fibers is random within the matrix. Random orientation of the short fibers yields relatively lower mechanical properties. On the other hand, additive manufacturing (AM) has the ability to deposit composite resin selectively and the inherent shear thinning mechanism allows the majority of the fibers to be oriented along the direction of extrusion. However, the mesoscale porosity within the printed part limits the adoption of high-performance composite applications. The integration of pellet extrusion AM technology with CM has been considered due to its ability to directly fabricate structurally robust thermoplastic composite parts. This hybrid AMCM manufacturing technology developed at Oak Ridge National Laboratory, allows faster production of highly oriented short fiber reinforced composites without meso-scale and micro-scale porosities that are prevalent in a typical fused filament fabrication system. AMCM on a large scale enabled a higher deposition rate, thus fabricating composite parts in a scalable manner. However, the integration of the CM system limits the size of the part. Therefore, the challenge of manufacturing large-scale continuous parts using AMCM remains due to limited mold size.
In this study, we pursued several different joining techniques to enable high throughput by joining multiple parts during production. The main objective was to investigate the tensileshear strength at the joined interface of the two parts. In the state-of-the-art FRTP joining, additional steps within the manufacturing process are required, which leads to low throughput as well as longer cycle time. Our initial approach was to imprint mechanical interlocking features at the interface of the joint in an attempt to improve its strength when compared to a compression-molded butt joint. Acrylonitrile butadiene styrene (ABS) resin reinforced with 20 wt.% short carbon fibers (ABS/CF) composite was printed onto a flat mold with a print dimension of approximately 300 mm by 450 mm panel. 3D-printed polyetherimide imprints were used to create mechanical impressions within the printed composite during the CM process. The CM press was set to exert 150 tons of force for 90 seconds to ensure proper consolidation. The inserts were removed following the compression and the second part was printed alongside the first print. Homogeneity for the print orientation was maintained for both joined parts. We performed mechanical characterization of the joined AMCM composite panels. The tensile strength properties indicated high variability within the mechanical joints due to defects such as porosity at the joining interface. Despite the promising results of joining FRTPs using an integrated AMCM system, incomplete resin flow within the negative impression (cavity) introduced the defects. In addition, optical characterization revealed random fiber orientation and poor matrix-to-matrix diffusion at the joint interface, thus resulting in low strength values. Optimization of processing parameters such as compression force, holding time, and preheating of the joining panels need to be investigated. Optimized and integrated AMCM joining will further the maturation of this fast rate joining technologies thus enabling high volume production of FRTPs on an industrial scale.