The Guest Editor of this special issue on additive manufacturing of polymeric materials, Prof. Dr.-Ing. Dr. h. c. Mult. Klaus Friedrich, passed away at the end of May 2021. His death is undoubtedly a great loss for the international community of composites.

As a famous scientist of composite materials, Prof. Friedrich published vast amounts of papers and books and book chapters as well, with which he became one of the mostly cited researchers worldwide. In 2005 he was recognized as a “World Fellow” of the International Committee on Composite Materials. The most fascinating part that I learned from him for years was his spirit of continuous exploration and innovation, which kept him active in the latest academic frontiers. In addition to the outstanding scientific achievements, Prof. Friedrich was also well known for his management and leadership abilities. He made tremendous contribution to the international reputation of the Institute for Composite Materials. (IVW) at the University of Kaiserslautern, Germany, when he served as Research Director for Materials Science from 1990 to 2006, and emeritus professor as of 2006.

In my memory, Prof. Friedrich was a very nice and generous gentlemen, always ready for caring others. During my research stay in IVW in 1990s, for example, he gave me full help in every aspect, which made me feel very warm. Since that time, I had continually got valuable advices from him, for which I am always grateful. In 2017 he kindly agreed to prepare a high quality review article entitled “Polymer composites for tribological applications” (https://doi.org/10.1016/j.aiepr.2018.05.001) for the inaugural issue of the journal Advanced Industrial and Engineering Polymer Research, which was critically important for the smooth growth of the new journal. Last year he promised to organize the present special issue by inviting leading experts in the area of 3D printing from different countries, in order to timely reflect one of the cutting-edge progresses of polymer engineering. Even in late March 2021, he communicated with the editorial office on certain technical issues. His work enthusiasm, personality and charm live on.

Unfortunately, Prof. Friedrich couldn't see the publication of the special issue, but it comes out as scheduled under the strong support of his friends and colleagues. There are six invited papers and two contributed papers in the collection. The topics cover preparation of feedstock for 3D printing, development of new fabrication approaches, effects of processing conditions, and applications in various aspects, which are presented in the form of research articles and reviews. Readers may build a panoramic understanding of the exciting field within a relatively short time.

Lastly, I would like to take the opportunity to thank the authors, and dedicate the present issue to Prof. Friedrich in token of gratitude and remembrance.

Continuous lattice fabrication is a newly introduced method for additive manufacturing of fiber-reinforced thermoplastic composites that allows to deposit material where it is needed. The success of this technology lies in a printing head in which unconsolidated continuous fiber-reinforced composite is pulled through a pultrusion die before the material is extruded and deposited out of plane without the use of supporting structures. However, state-of-the-art composite feedstock like commingled yarns shows limits in achievable material quality and part dimensions due to the underlying fiber architecture where thermoplastic fibers are mingled with reinforcement filaments. Hybrid bicomponent fibers overcome these constraints because each individual reinforcement filament is clad in a thermoplastic sheath. This results in absence of time-consuming fiber impregnation steps that would negatively effect void content and material quality.

This study compares the material quality of pultrudates made from hybrid bicomponent fibers to that of commercially available commingled yarns at various processing conditions. Experiments are reported in which polycarbonate composite profiles with a diameter of 5 mm containing 50 vol% to 60 vol% E-glass fibers are pultruded at different die filling degrees, mold temperatures and pultrusion speeds. The results show that the pultrudates obtained from hybrid bicomponent fibers have lower void content than those manufactured under the same conditions from commingled yarns. We assess this to be caused by the difference in consolidation mechanism which in the case of the hybrid bicomponent fibers is dominated by coalescing of the thermoplastic sheaths compared to the Darcian flow-dominated consolidation of commingled yarns.

Ultra-high molecular weight polyethylene (UHMWPE) possesses distinctive properties, but has an extremely low melt flow rate (MFR) of about zero, which makes it unsuitable for processing by standard methods for polymers. The aim of this paper was to investigate the tribological properties of two-component UHMWPE-based composites with different content of isotactic PP. The composites were fabricated by three methods: a) hot pressing of the powder mixtures; b) hot compression of granules; and c) 3D printing (FDM). It was shown that the UHMWPE-based composites obtained by extrusion compounding (hot compression of granules and 3D printing) in terms of the mechanical and tribological properties (wear resistance, the friction coefficient, Young's modulus, and yield strength) were superior to the ones manufactured by hot pressing of the powder mixtures. The most effective was the ‘UHMWPE+20% PP’ composite in terms of maintaining high tribological and mechanical properties and the necessary melt flow rate (MFR) in a wide range of loads. It was recommended as a feedstock for additive manufacturing of complex-shaped products (joint components) for friction units in orthopedics.

Laser sintering of polymers (LS) is one of the most promising additive manufacturing technologies as it allows for the fabrication of complexly structured parts with high mechanical properties without requiring additional supporting structures. Semi-crystalline thermoplastics, which are preferably used in LS, need to be processed within a certain surface temperature range enabling the simultaneous presence of the material in both, the molten and solid state. In accordance with the most common processing models, these high temperatures are held throughout the entire building phase. In the state of the art, this leads to high cooling times and delayed component availability.

In this paper, process-adapted methods, in-situ experiments and numerical simulations were carried out in order to prove that this drawback can be overcome by material-adapted processing strategies based on a deepened model understanding. These strategies base on the fact, that the crystallization and solidification of polyamide 12 is initiated a few layers below the powder bed surface at high temperature and quasi-isothermic processing conditions. Therefore, isothermal crystallization and consolidation behaviour is analyzed by process-adapted material characterization. The influence of temperature fields during laser processing was analyzed in dependence of part cross-section, layer number and fabrication parameters and correlated to the resulting part properties. Furthermore, the possibility to homogenize the parts thermal history by controlling the part cooling is highlighted by a simulational approach. The authors show that the material-dependent solidification behavior must be taken into account as a function of the geometry- and layer-dependent temperature fields and demonstrate a major influence on the material and component properties. From these findings, new processing strategies for the laser exposure process as well as for the temperature control of the build chamber in z-direction arise, which allow for the acceleration of the LS process and earlier availability of components with more uniform part properties.

Locally bendable solid plates were manufactured in a single 3D-printing operation, using a single material, i.e., short carbon fiber reinforced plastic (CFRP). The locally bendable CFRP plates included solid and bendable parts, which were connected seamlessly using double-stepped lap configuration. A parallel cross shape structure and 100% infill structure was adopted for the bendable and solid parts, respectively. The bendability could be controlled by varying the girder angle of the parallel cross shape structure. The bending stiffness was reduced to nearly 98% compared to that of the solid plate. The cyclic bending tests indicated that the locally bendable CFRP plate underwent reversible bending deformation. The bending stiffness decreased by approximately 8–14%. However, visible damage was not observed even after 100 cycles of bending deformation.

Fused filament fabrication (FFF) is an additive manufacturing technology in which a fused filament is deposited in a controlled manner over or adjoining previous deposited filaments, leading to construction of a structural part. This study aims to characterise the extent of fusion bonding between individual filaments using printed single-layer films, which helps understand the process–structure–property relationship, optimising process parameters (i.e. deposition speed and extrusion temperature) involved in the FFF process. For a brittle polylactic acid (PLA) and a ductile polypropylene (PP), single-layer double-edge notched tensile (DENT) specimens with sharp cracks parallel to the deposition direction was fabricated using FFF of different extrusion temperatures (from 200 °C to 260 °C) and deposition speeds (from 30 mm/s to 90 mm/s). The fracture toughness (K_c), defined as the critical stress intensity factor at the critical load for brittle fracture of PLA and the specific essential work of fracture (w_e), as an indicator of the fracture toughness for ductile fracture of PP, were characterised. The results demonstrate that this methodology proved to be an effective tool to identify the effects of process parameters on fusion bonding in the FFF process, showing the strong sensitivity of fracture toughness, either K_C for PLA or w_e for PP, to the extent of fusion bonding between individual filaments.