**1. Introduction**

Fused Deposition Modeling (FDM), also known as Fused Filament Fabrication (FFF), melts thermoplastic filament through a heater and deposits it layer by layer on the platform via a nozzle to form a part. The most significant advantage of FDM is the wide range of molding materials, which includes thermoplastic polymers in general. Sometimes low melting point metals, ceramics, and others materials are also used [1]. Besides, high speed, low cost, pollution-free, and simplicity of the process are also benefits of FDM. Consequently, FDM is emerging as the most widely used and embraced technique of additive manufacturing, which is applied in various fields such as aerospace, automotive, medical, and architecture with rapid growth [2]. However, anisotropic behavior, poor surface quality, and low dimension accuracy are drawbacks of FDM, usually resulting in poor mechanical characteristics of printed components, which dramatically limits the further application of FDM on a large-scale [3].

FDM is a complex process that has a large number of parameters that play different roles in the fabrication. To produce products with good quality and meet requirements for material behavior, it is necessary to evaluate the impact of these parameters on the characteristics. To date, many studies have been conducted to analyze different controllable parameters to achieve desirable properties of parts, including surface roughness [4,5], dimension accuracy [6,7], hardness [8], build time [9,10], and mechanical properties [11–13]. Obviously, mechanical properties are the most fundamental characteristics of FDM printed parts, among which tensile, compressive, and flexural strength are the three most important and concerning properties to the manufacturers and users, which are also the objects of this paper.

FDM involves various parameters that can be classified into three main types: process parameters(raster angle, layer thickness, build orientation, raster width, print speed, infill density, air gap, infill pattern, extrusion temperature); environmental parameters(platform temperature, envelope temperature, humidity, oxygen, etc.); other print parameters(nozzle diameter, material color, filament diameter, etc.). Although these parameters all affect the quality of FDM components, the contributions of which are different. Process parameters are the most commonly analyzed owing to their significant impact on mechanical performance and production efficiency. Actually, several published review papers related to the FDM process parameter are available for interested readers: Gordelier et al. [2], Dey and Yodo [14], Cuan-Urquizo et al. [15], Sheoran et al. [16], Mohamed et al. [17], Popescu et al. [18], Bakır et al. [19], Syrlybayev et al. [20]. These existing literature reviews generally investigate and analyze which process parameters can affect a certain material behavior. Since users of the 3D printer are directly faced with each process parameter, it is necessary and helpful to make them understand how each process parameter affects the quality and characteristics of printed parts at different values. However, to date, no literature review has been reported to explain the influence from the perspective of parameters rather than properties. As a complement, this survey focuses on functions of every process parameter with varying values and discusses the mechanism behind it by amalgamating collusions of existing studies from 2010 to 2021. Some research beyond this range is also included for important topics. This article aims to provide a comprehensive review of the roles of different process parameters in the FDM process, update the recent advances in process parameters optimization for researchers, serve as a resource for newcomers in this field and give directions for anyone wishing to improve the mechanical behaviors of their printed components.

The structure of the paper is organized as follows: Section 2 describes different process parameters and reviews literature related to investigating or improving the mechanical performance of FDM parts. Section 3 contains some key findings of the presented works and highlights concluding results. Section 4 describes difficulties encountered in the improvement of the FDM part characteristics. The last section includes recommendations and perceptions for future work.

### **2. Process Parameters**

The most researched process parameters include air gap, build orientation, extrusion temperature, infill density, infill pattern, layer thickness, raster width, raster angle, and print speed, as shown in Figure 1, which have substantial effects on filament (inter-layer and intra-layer) bonding, and thus influence the mechanical performance of FDM printed components [18]. In addition, interactions of these parameters play a significant role from the perspective of mechanical properties [21,22].

**Figure 1.** FDM process parameters related to toolpath.

### *2.1. Build Orientation*

Build orientation (or part orientation [23], construction/layer orientation [24]) represents how and in which direction the part is generated on the print platform. In fact, build orientation can represent an arbitrary angle with any value [8,25,26], but in most studies, it is regarded as a certain angle with respect to X, Y, and Z-axis [27,28]. Generally, when test specimens are placed horizontally, vertically, and laterally, the build orientation is named as flat, upright, and on-edge, respectively, which is shown in Figure 2. Flat and on-edge are considered parallel to the print platform, while upright is along the direction of normal of the print platform.

**Figure 2.** Build orientation: (**a**) arbitrary angle (**b**) certain angle.

The influences of build orientation on the mechanical performance of FDM components have been extensively researched. Different authors investigated the relationship between various materials and building directions. Wang et al. [7] established building factors with various levels based on analysis of variance (ANOVA). The result verified build orientation in the Z-direction to be the most predominant factor for tensile strength. Lee et al. [29] showed that compressive strength of ABS parts was maximum at 0◦ build orientation. Gorski et.al. [30] noted that tensile strength was maximum at 0◦ for ABS filaments. Moreover, they found the specimen presented brittle behavior instead of ductile behavior as build orientation increased exceeding certain angles. The conclusions were consistent with Ashtankar et al. [25]. Their study reported that tensile strength of ABS specimen decreased, with the increase of build orientation from 0◦ to 90◦ . This trend was also applicable to ultimate compressive strength, which was minimum at 90◦ orientation. In another study, Hernandez et al. [31] experimentally determined both compressive properties and flexural properties were maximum at 0◦ build orientation for ABS P430 filaments. Besides, compressive strength was minimum at 45◦ build orientation. They also deduced that the effect of build orientation on tensile strength of ABS printed parts was insignificant. Bertoldi et al. [32] and Zou et al. [33] experimentally showed that build orientation strongly affected tensile strength and elastic modulus, respectively. Raney et al. [34] evaluated the effects of build orientation and infill density on tensile strength of ABS parts manufactured by a uPrint SE 3D printer, showing that the strength of samples tested against the layers was less than 80% of that tested along the layers.

As for materials other than ABS, Domingo-Espin et al. [35] tested tensile strength of PC parts. This group of researchers proved that tensile strength was maximum at 0 ◦ build orientation. Smith and Dean [36] also pointed out that, compared to bulk material, there was a 45 percent decrease in elastic modulus and a 30 to 60 percent decrease in ultimate tensile strength of PC parts depending on orientation. Zaldivar et al. [37] revealed that FDM materials behaved more as laminated composites with macrostructures than isotropic cast resins, consequently tensile strength, failure strain, Poisson's ratio, coefficient of thermal expansion, and modulus all varied significantly depending on the build orientation of PEI dogbones. Taylor et al. [38] analyzed the flexural behavior of PEI parts with varying build orientation and raster angle experimentally and numerically. Results indicated that modulus and yield strength were influenced by an interaction between these two parameters.

In summary, build orientation significantly affected the mechanical properties, which usually played the predominant role when compared to other parameters [39]. For arbitrary angles, in case of other parameters such as air gap and raster angle are kept constant, the 0 ◦ orientation is preferable, which shows the highest values for maximum tensile strength, compressive strength, and flexural strength. Consequently, flat or on-edge oriented samples usually exhibit inter-layer failure with higher stiffness and strength performance. On the other side, increasing the angle from the build platform results in microstructures that further reduce the volume fraction of extruded fiber material from the primary load direction resulting in lower strength. That is why upright samples showed inter-layer failure with lower stiffness and strength performance.

### *2.2. Raster Angle*

Raster angle (sometimes called raster orientation [40], layer orientation [41], fiber orientation [42], or even pattern orientation [43]) represents the angle of the filament direction with regard to the X-axis (usually load direction) of the platform. The allowed raster angles can vary from −90◦ to 90◦ , and typically used values are 0◦ (axial), 45◦ (cross), 90◦ (transverse), and their combination. For example, −45◦/45◦ (criss-cross) represents the raster printing directions are −45◦ and 45◦ alternately for different layers, as shown in Figure 3.

**Figure 3.** Raster angle: (**a**) 0◦ (**b**) 45◦ (**c**) 90◦ (**d**) −45◦/45◦ .

Ahn et al. [44] applied the Tsai-Wu failure criterion and classical lamination theory to reasonably predict the anisotropic failure model for FDM parts as a function of raster angle. Magalhães et al. [45] suggested that proper choice of raster angles in sandwich specimens could gain in the strength and stiffness of parts, compared to default (45◦ ) FDM configuration. Ziemian et al. [46] and Zhou et al. [47] indicated that the highest tensile strength was obtained at raster angle with 0◦ for ABS and PP-PC composites, respectively, while the specimens with 90◦ raster angle exhibited the minimum strength. Es-Said et al. [40] and Garg et al. [48] drove a similar conclusion for flexural strength as well as tensile strength. Moreover, Ziemian et al. [49] further reported that 45◦ raster specimens in compression were significantly weaker than other raster angles. Based on the analysis of biaxial raster angles, Fatimatuzahraa et al. [50] noted that the structure of 45◦/−45◦provided better flexural strength than that of 0◦/90◦ of ABS built specimens, despite the almost equivalent tensile strength [51]. A similar conclusion for tensile strength was also driven by Diaconescu et al. [52]. Hart and Wetzel [53] explored the fracture properties of ABS parts with different raster angles. Results confirmed that the elasticplastic response of the material depended on the raster angle of printed specimens. In contrast, Arbeiter et al. [54] reported that fracture behavior might be not highly dependent on the raster angle by setting ideal processing parameters of PLA samples.

The interaction of build orientation and raster angle can cause strong anisotropy of the FDM parts, therefore these two parameters are generally studied together. Rohde et al. [12] revealed that ABS and PC samples exhibited strong anisotropy as functions of build orientation and raster angle, respectively. Shear moduli were affected by build orientation rather than raster angle for ABS specimens. The lowest values of modulus of rigidity, ultimate shear strength, and yield shear strength were obtained from on-edge configuration specimens. Durgun and Ertan [23] reported that the build orientation had a more significant influence than the

raster angle on the mechanical behavior of the resulting fused deposition part. Small raster (e.g., 0◦ angle) resulted in increased strength resistance in all component positions. Bellini and Güçeri [55] carried out analytical and experimental approaches to study the effect of build orientation and raster angle on flexural strength and tensile strength of ABS material. Balderrama-Armendariz et al. [56] studied elastic properties in torsion of ABS-M30 samples at different build orientations and raster angles. They characterized that build orientation had an insignificant modification of the response of 0.2% yield strength or ultimate shear strength, while the orientation in YXZ with raster at 0◦ led to improved responses in all measured torsion parameters. Cantrell et al. [57] showed that build orientation and raster angle had a negligible influence on the tensile modulus of ABS specimens. The highest tensile properties and highest shear strength were found in specimens with on-edge orientation and specimens with [+45◦/−45◦ ] flat orientation, respectively for PC material. In addition, the shear modulus was almost the same for all specimens with [+45/−45] raster angle regardless of build orientation. Torrado et al. [58] explored the effects of build orientation and raster angle on mechanical anisotropy. The tensile test results exhibited an equivalency between different sample types. Therefore, the authors recommended horizontal specimens printed with a transversal filling due to its higher reliability, higher accuracy, and simplicity of the printing process. Letcher et al. [59] investigated the relationship between layer number, raster angle, and mechanical properties of ABS printed specimens. Results showed that 0◦ raster orientation yielded the highest strength at each layer number. Furthermore, maximum stress and elastic modulus increased with the increase of the number of layers.

In summary, the relative position of fibers and the axial load causes the specimens to react differently. Raster angles with a higher fraction of specimens oriented along the axis of the load (e.g., 0◦ orientation) exhibit improved tensile and compressive strength of the part, while those that are offset (e.g., 90◦ orientation) exhibit reductions in mechanical performance [60–63]. In the former case, fibers themselves withstood most of the applied load, resulting in inter-layer failure. While for the latter case, bonding between adjacent layers and rasters withstood the load, resulting in trans-layer failure, which is much weaker. A similar trend is applicable to the flexural specimen, which can be regarded as one side experiencing compression while the other side experiencing tension when loaded.
