  Citation: Zhang, H.; Lei, X.; Hu, Q.; Wu, S.; Aburaia, M.; Gonzalez- Gutierrez, J.; Lammer, H. Hybrid Printing Method of Polymer and Continuous Fiber-Reinforced Thermoplastic Composites (CFRTPCs) for Pipes through Double-Nozzle Five-Axis Printer. Polymers 2022, 14, 819. https:// doi.org/10.3390/polym14040819 Academic Editor: Lilia Sabantina Received: 1 December 2021 Accepted: 16 February 2022 Published: 20 February 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). polymers Article Hybrid Printing Method of Polymer and Continuous Fiber-Reinforced Thermoplastic Composites (CFRTPCs) for Pipes through Double-Nozzle Five-Axis Printer Haiguang Zhang 1,2,3, * , Xu Lei 1 , Qingxi Hu 1,2,3 , Shichao Wu 1 , Mohamed Aburaia 4 , Joamin Gonzalez-Gutierrez 5 and Herfried Lammer 6 1 Rapid Manufacturing Engineering Center, Mechatronic Engineering and Automation of Shanghai University, Shanghai 200444, China; YamaS@shu.edu.cn (X.L.); huqingxi@shu.edu.cn (Q.H.); seedo@i.shu.edu.cn (S.W.) 2 Shanghai Key Laboratory of Intelligent Manufacturing and Robotics, Shanghai University, Shanghai 200072, China 3 National Demonstration Center for Experimental Engineering Training Education, Shanghai University, Shanghai 200444, China 4 Competence Center Digital Manufacturing and Robotics, University of Applied Science Technikum Wien, Höchstädtplatz 6, 1200 Wien, Austria; aburaia@technikum-wien.at 5 Institute of Polymer Processing, Montanuniversitaet Leoben, Otto Gloeckel-Strasse 2, 8700 Leoben, Austria; joamin.gonzalez-gutierrez@unileoben.ac.at 6 Kompetenzzentrum Holz GmbH, Altenberger Straße 69, 4040 Linz, Austria; h.lammer@wood-kplus.at * Correspondence: haiguang_zhang@i.shu.edu.cn Abstract: The most widely used 3D process, fused deposition modeling (FDM), has insufficient interlayer adhesion due to its layer-by-layer forming method. A support material is also essential for the hollow parts and cantilevers. Moreover, the polymer materials used have limited mechanical properties. These issues have restricted the application of FDM in high-performance fields. Con- tinuous fiber-reinforced thermoplastic composites (CFRTPCs) have high mechanical properties and have recently become the focus of research in the field of 3D printing. This paper, using pipe parts as an example, proposes a hybrid of pure polymer (pure PLA used) and CFRTPC (flax fiber pre- impregnated filament) material to develop a printing method based on the outstanding mechanical properties of CFRTPC material. After studying the printing path planning algorithm, the CFRTPC filament is laid along the axial and radial directions on the surface of the polymer base to improve the printed parts’ properties. The method feasibility and algorithm accuracy are verified through the development of five-axis printing equipment with a double nozzle. Through the printed sample’s tensile, compression and bending tests, the results show that the tensile, compressive and bending properties of PLA pipe can be significantly enhanced by laying CFRTPC filament along the axial and radial directions of the pipe. To summarize, the introduction of CFRTPCs greatly improved the mechanical properties of the printed parts, and the implementation of our method provides an effective way to solve the defects of the FDM process. Keywords: 3D printing; continuous fiber-reinforced; 5-axis printer; printing path 1. Introduction Three-dimensional printing (3DP), also called additive manufacturing (AM), plays a key role in high-tech areas such as bioengineering [1] and aerospace [2] and is increasingly gaining attention. One of the most popular and widely used AM processes is fused deposition modeling (FDM) [3]. Polylactic acid (PLA) and Acrylonitrile Butadiene Styrene (ABS) are the most commonly used filaments in FDM because they are cheap, readily available and environmentally friendly. However, with increasing demands on product performance, high-performance plastics can also meet the increased requirements, but fiber materials perform even better. People are therefore turning their attention to composites in Polymers 2022, 14, 819. https://doi.org/10.3390/polym14040819 https://www.mdpi.com/journal/polymers