PSI - Issue 35

Mohammad Reza Khosravani et al. / Procedia Structural Integrity 35 (2022) 59–65 M.R. Khosravani and T. Reinicke / Structural Integrity Procedia 00 (2021) 000–000

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The material extrusion technique is one of the most popular 3D printing processes which showed some advantages such as mold-less, low cost, minimum waste, and the ability to produce complex structural elements. Evidently, 3D printing based on FFF technique involves matrix and reinforcement. Matrix is a base material which holds the fibers for long term serviceability. Matrix plays an important role and protects the fibers from external forces. Using fiber improves mechanical performance of the part such as tensile and load bearing capacity. The fundamental working procedures of FDM and FFF are the same to the other 3D printing processes and consists of creating 3D model, slicing, and printing. At the first stage, a 3D model should be designed in a computer aided design (CAD) platform. Later, the prepared STL files must be sliced and the G-code would be uploaded as feed in a 3D printer. Generally, for fabrication of continuous fiber composites based on fiber integration into the part, three di ff erent methods can be considered. In the first type, there is one conventional print head. In this case, there is incorporation of fiber before 3D printing process, and filament itself is a composite. In the second type, the incorporation would be in the print head. In such cases, two materials would be combined when they pass through the extruder (single nozzle). In the third type, there are two di ff erent entrances for thermoplastic polymer and continuous fiber. In this case, two independent extruders, each one with an independent nozzle are required. It is worth noting that mechanical behavior of the printed parts depends not only on the amount of the fiber, but also on the method in which the fibers are integrated into the matrix. Fig. 1 schematically shows 3D printing process of continuous fiber reinforced composites with single and dual nozzles. Using 3D printer with two separate extrusion nozzles for matrix and reinforcing fiber, facilitates the design and fabrication process.

Continuous fiber

Continuous fiber

Thermoplastic polymer

Nylon filament

Fiber spply coil

Fiber nozzle

Extrusion head

Nylon nozzle

Nozzle

Fiber spool

Filament supply coil

Z

Nylon spool

X

Y

Building platform

Building platform

Fig. 1. Schematic representation of 3D printing of continuous fiber composites, single nozzle (left), and dual nozzles (right).

Although the described techniques have been used in di ff erent industrial projects and research works, there are some issues which require further attempts. For instance, structural elements produced by FFF technique indicated low interlayer and intralayer fracture properties (Young et al., 2018). Similar to other 3D printing techniques, there are di ff erent factors which have significant e ff ects on the mechanical performance and quality of the 3D-printed components. They are categorized into three groups: (a) preparatory factors, (b) printing parameters, and (c) post processing factors. Geometry of the model and slicing technique are examples of the preparatory factors. Several parameters such as print speed, raster angle, fiber orientation, layer thickness, nozzle temperature, and infill pattern are printing parameters. There are di ff erent post-processing methods (e.g., heat treatment and polishing) which have influence on the quality of the end-use products. Due to the early stage of the development, there are limitations in the available designs of printer heads in printing of continuous fiber composites. In detail, there is no robust design of printer head. Moreover, the printable materials with fast curing mechanism are limited. Considering current constraints, it seems further investigations are required in order to provide a robust platform for 3D printing of continuous fiber composites.

3. Specimen preparation

In this study, all specimens were fabricated using a Mark Two Desktop 3D printer provided by Markforged . It is driven by a web interface developed by Markforged based on the continuous fiber reinforcement. The specimen geometry was created according to ASTM D638 using Type I geometry (ASTM D638, 2014). The test coupon ge-