*2.7. Infill Pattern*

Infill pattern (or print pattern [93]) represents the way how filaments fill and cross the internal space of the printed part, as shown in Figure 6. Different infill patterns usually have different geometrical layouts and complexity, which will affect print time and the material used.

**Figure 6.** Infill pattern: (**a**) linear (**b**) concentric (**c**) hexagonal.

Many filling patterns are available such as hexagonal (or honeycomb), linear, and diamond, as illustrated by Alafaghani et al. [28], in which the commonly used is the hexagonal pattern. Cho et al. [96] compared the influence of PLA samples with different infill patterns and layer thickness on tensile property. They concluded that layer thickness had a higher effect than infill pattern, and the triangle pattern gave the highest mechanical strength and lowest material consumption. Dave et al. [97] investigated the effect of three process parameters: infill pattern, infill density, and build orientation, on the tensile properties of PLA specimens through a full factorial experiment. ANOVA results indicated that infill density was the most predominant process parameter for tensile strength, compared with infill pattern and build orientation. Fernandez-Vicente et al. [98] found that changes in infill

density determined mainly tensile strength of ABS material. At the same time, the influence of the different infill patterns caused a variation of no more than 5% in maximum tensile strength, along with similar behaviors. Akhoundi et al. [99] identified the key factors that influenced tensile and flexural strengths. The input variables, such as infill pattern and infill density, and their relationship with raster angle and void presence, were considered. The result concluded that the highest tensile and flexural strengths were obtained by concentric pattern. They also found that when rasters were deposited at short distances in the Hilbert curve, a high temperature was maintained, which resulted in better fusion and strong bonding between the adjacent rasters. Baich et al. [9] presented the relationship between various infill patterns and different mechanical properties. Statistical analysis revealed that for double-dense infill in all loading conditions, solid infill showed higher strength at the same fabrication cost. Therefore, solid infill was recommended for mechanical applications, in the case of entry-level printers. Moreover, compressive strength increased as the complexity of the infill pattern increased. Nagendra and Prasad [100] revealed significantly linear interactions between infill pattern and other process parameters, such as extrusion temperature, layer thickness, and infill density, on mechanical properties of Nylon/Aramid composite.

In summary, the infill pattern has a complex effect on the mechanical properties of parts produced by FDM owing to a broad spectrum of types. For example, in the hexagonal pattern, each layer lays down on a similar previous layer, the same as the bonding zone. While in the rectilinear pattern, the lay crosses the previous layer at points, which correspond with the bonding zone between each layer. However, the combination of rectilinear patterns in a 100% infill shows higher tensile strength, compared with the honeycomb pattern [98]. Therefore, these results need to be analyzed and explained with caution.

### *2.8. Print Speed*

The print speed (or feed rate [27], print velocity [92], infill speed [101], deposition velocity [102]) represents the speed of the nozzle traveling relative to the print platform. Generally speaking, the lower the print speed, the longer the production time and the better the accuracy of the prints. In comparison, the higher the print speed, the faster parts are produced.

Christiyana et al. [103] produced ABS composite specimens and investigated the role of print speed and layer thickness. It was observed that the maximum flexural and tensile strengths were achieved via setting thinner layer thickness and lower print speed. Similarly, Ning et al. [101] showed that tensile strength of CFRP composites decreased with the increase in print speed. Santana et al. [104] analyzed the factors affecting PLA parts with variations in print speed and extrusion temperature to evaluate the quality of the open-source 3D printer. Based on the value, the print speed and extrusion temperature were irrelevant compared with the mass and modulus of rupture. Kaˇcergis et al. [105] investigated the influence of print speed, platform temperature, and number of layers in the structure printed with PLA and TPU. Experimental results proved that the deformation was strongly influenced by the print speed. By contrast, Li et al. [21] pointed out that air gap played a predominant part in determining tensile strength, followed by layer thickness, and the effect of print speed is the weakest factor. They suggested that smaller values of layer thickness and air gap were preferred if higher tensile strength was needed. Furthermore, print speed could be set relatively higher to improve fabrication efficiency. Lužanin et al. [106] studied the relationship between the maximum flexural force of PLA parts and five process parameters. The input variables were extrusion temperature, infill density, print speed, raster angle, and layer thickness. The optimal parameters setting was maximum levels of infill density and print speed, mid-level of layer thickness, and minimum level of raster angle.

In summary, the effect of print speed on mechanical performance shows a different trend. Generally, lower print speed gives a better bonding and interaction between con-

tiguous filaments, leading to an increase in tensile and flexural strength. However, if the print speed is too slow, the too-long inter-layer cooling time makes just-deposited material cool down at a lower temperature, which disfavors the fusion of the thermoplastics, hence the strength and ductility are affected [107]. On the other hand, rapid print speed could improve the efficiency, but leave not enough time for extrusion materials to plasticize, and the amount of residual stress produced during deposition increases significantly as well [108], which leads to weak mechanical properties. It should be pointed out that the production time is not only affected by print speed but also related to build orientation. Print time decreases as print speed increases for on-edge and flat orientations, while print time remains almost constant for up-right orientation with high-speed values [27].

### *2.9. Number of Contours*

The number of contours (or number of perimeters [109], number of shells [110]) refers to the number of closed roads that are deposited along the edge of the part, as shown in Figure 1. It may range from one to the number of filaments extruded.

Kung et al. [109] studied the influences of three process parameters including number of contours, raster angle, and specimen size. They pointed out that there existed apparent dispersion of the strength for a different number of contours. Interestingly, they also noted that tensile strength of specimens built with 45◦ is greater than those built with 0◦ . According to Mahmood et al. [110], there was a positive relationship between tensile strength and number of contours. In addition, a larger cross-section negatively affected tensile strength of a printed part while keeping the other parameters constant. Croccolo et al. [111] experimentally and analytically dealt with the effect of contouring on the static strength and stiffness of ABS parts. They showed that the larger the number of contours, the greater the elastic modulus and stiffness, and thus the higher the maximum strength. Moreover, with the increase of the number of contours, the percentage of elongation to failure decreased. Griffiths et al. [112] performed an experimental investigation on the tensile property of PLA objects. They utilized a full factorial DOE approach considering building orientation, infill density, number of contours, and layer thickness as parameters. The study concluded that the infill density and number of contours were the only significant parameters that should be maximized for optimization. Lanzotti et al. [61] observed the increase in strength with the number of contours and layer thickness. In particular, the strength increased as the raster angle decreased with a rate that was as greater as the layer thickness increased.

In summary, the number of contours impacts the mechanical properties of the part fabricated. When the number of contours increases, the effect is directly seen in the increase in strength. This is owing to the fact that the load is applied directly on the contour rather than the rasters, therefore a growing contour number causes the raster length and number of rasters to decrease, which will lead to improvement in the performance of the part.

### *2.10. Extrusion Temperature*

Extrusion temperature (or print temperature [82], nozzle temperature [113]) refers to the temperature at which the fibers are heated inside the nozzle during the FDM process. It can influence the fluidity and solidification characteristics of the molten material and control the viscosity of filament extruded from the nozzle.

Deng et al. [82] applied an orthogonal test to evaluate the effects of process parameters such as print speed, layer thickness, extrusion temperature, and infill density, on tensile properties of PEEK components. They demonstrated that more micro-pores and slag inclusion were caused by lower print speed and extrusion temperature, leading to lower strength specimens. Aliheidari et al. [113] designed double cantilever beam specimens of ABS and printed at different extrusion temperatures to study the mode-I fracture resistance. Based on critical J-integral value, the authors stated that the higher the temperature was, the greater number of polymer molecules were inter-diffused at the interface, which resulted in higher resistance to fracture. Rinanto et al. [114] optimized extrusion temperature, infill density, and raster angle to produce prototypes with high tensile strength. The optimization combination was 45◦ of angle, 40% of density, and 210 ◦C of temperature. Among these three parameters, infill density is the most predominant factor. Sun et al. [115] explored the influence of extrusion temperature and envelope temperature on the quality of bonds between adjacent ABS filaments. Statistical analysis proved that both the envelope temperature and variations in the convective conditions within the printer have substantial influences on the mesostructure and the overall quality of the bond strength between rasters. Leite et al. [116] determined the influence of mechanical properties from layer thickness, extrusion temperature, raster angle, and infill density. The best values reported for the sample were higher infill density and extrusion temperature, and lower layer thickness. Sun et al. [117] demonstrated that increasing platform temperature could enhance the PEEK binding force between layers, making the model more excellent mechanical properties. Moreover, low infill density could also improve the performance of the material. Yang [118] observed a decrease in tensile and flexural properties of WFRPC components with an increase in the extrusion temperature, whose trend is opposite to that of compressive strength.

In summary, the extrusion temperature has an important effect on the crystallinity of the material and polymer filament bonding. Thus, the mechanical performance of printed parts will be affected as well. Higher extrusion temperature of the deposited filament gives better inter-layer fusion, which results in higher mechanical properties. However, too high extrusion temperature may cause material degradation or molding failure during deposition, resulting in dimensional inaccuracy and filament deformation [82]. On the other hand, lower extrusion temperature may prevent the material from melting adequately, leading to nozzle clogging. Both of the two cases above will lead to weak mechanical properties of printed parts.

### **3. Results and Discussions**

In an effort to aggregate thorough information on process parameters of the FDM technique and their influence on mechanical properties, we have summarized the research works in the field concisely. Tables 1–11 give an overview of the parameters and mechanical properties of FDM products intensively investigated in the literature. In most existing research, several parameters are studied together. Therefore, the parameter that plays a major role or authors of the research care about most as the basis for classification. For the case of many parameters included, we attribute it to Table 11 (Others). However, for certain process parameters, there is not much research. Therefore, all studies containing this parameter are grouped into its table. As a consequence, the criteria for the aggregation of these tables are not strictly unique. Since there is much scattered data and information, interested readers are encouraged to review the references provided according to their interests. The key findings of this survey are summarized below:


Small raster angles (e.g., 0◦ ) will contribute to load-bearing due to filament lying along the loading direction. On the other hand, they will also lead to long rasters, which result in stress accumulation and hence weak bonding [22]. However, the final effect is that a small raster angle ensures the best tensile, compressive and flexural strength, proving that the former one plays a dominant role.

• Optimal parameter values obtained are just in theory, which should be reconsidered and adjusted in practice. According to the conclusion obtained in the former section, thinner layer thickness can help reinforce the tensile strength of the part, which, however, costs more due to more material and time usage for producing [136,137]. Consequently, a compromise needs to be made between improving property and reducing cost.


### **Table 1.** Build direction.


### **Table 2.** Raster angle.

### **Table 3.** Layer thickness.



### **Table 4.** Infill density.

### **Table 5.** Infill pattern.


### **Table 6.** Air gap.



### **Table 7.** Print speed.

### **Table 8.** Number of contours.


### **Table 9.** Extrusion temperature.


### **Table 10.** Raster width.



### **Table 11.** Others.

### **4. Research Shortcomings and Challenges**

This paper reviewed the literature concerned with the effects of various process parameters on mechanical performance by investigating their individual/combined effect. Despite the achievements of the current work, this section describes the major challenges and shortcomings of recent research.

### *4.1. Diversity of Materials*

In most presented research, influences of materials and printers are neglected insignificantly, in fact. From tables, it can be seen that there is a variety of materials for FDM, among which ABS and PLA are the two most widely studied. Other few known materials such as PC [35,57], PEI [37,79], PEEK [138], and Nylon [139] occupy only a small part of the research, not to mention PP [68], PPSF [75], PETG [140,141], or composite materials [93,142]. Therefore, conclusions about process parameters of most studies are obtained from ABS and PLA, which may be not applicable to other materials. For example, negative air gaps are preferred to enhance tensile and flexural behavior for ABS, as demonstrated by multiple works [11,73]. However, for structural materials such as PEI, a minus air gap is not recommended. As this material is processed at high temperature, and zero air gap is sufficient to improve mechanical properties flexural strength by adjusting other parameters, which can reduce the loss of dimensional accuracy and surface quality, caused by the usage of a negative air gap [80]. It should also be noted that materials from different suppliers differ in quality [141]. Moreover, even though the same material from the same source in different colors can lead to variation in properties. For instance, Wittbrodt et al. [143] reported that colors influenced the crystallinity percentage of polymers, and thus impacted the strength, which could not be deemed a low level of significance [44,121]. Therefore, research in a wider variety of materials will contribute to understanding the effect of process parameters better and help overcome shortcomings of FDM.

### *4.2. Variety of Printers*

There exist a wide range of machines from different manufacturers, as presented in the tables. Although samples are from the same material, they may have different properties when printed by other printers [144]. For instance, Tymrak et al. [63] found that ABS parts in a 0◦ orientation had elastic moduli around 1900 MPa and tensile strengths nearing 30 MPa by RepRap printer, which was higher than that in similar studies from different commercial printers, with moduli varying between 1000 and 1700 MPa, and tensile strengths ranging from 10 to 18 MPa [55,71]. The influence of 3D printers on the mechanical property of FDM parts is definite and obvious. However, there is still a lack of adequate and specific means to measure or evaluate this impact. An effort should be made to identify standard and test methods that could be used to validate FDM machine performance.

### *4.3. Difference in Results*

Since FDM is a complex process, it is difficult to replicate the experiment completely from others, which may lead to different or even opposite conclusions. For example, Dawoud et al. [73] showed that an air gap with a negative value could improve the mechanical property. On the contrary, Mohamed et al. [17] claimed that a positive air gap facilitated the spread of semi-molten materials between the gaps, which led to stronger structures. This phenomenon is more apparent when it involves multi-parameter optimization. As another example, Panda et al. [26,133] investigated process parameters (air gap, build orientation, raster angle, layer thickness, raster width) for mechanical properties of ABS parts. Experiments were conducted using a central composite design and part swarm optimization, respectively. However, the optimum process parameters obtained were different from that by Rayegani et al. [84]. In a word, samples with the optimal combination of parameters may have similar strength to those under the opposite parameters setting. That is why it is difficult to evaluate the role of a specific parameter in a multi-parameter combination.
