This original work investigates the in uence of in ll design, printer selection, and part orientation on the mechanical properties and production cost of parts fabricated using material extrusion additive manufacturing systems. Flexural test specimens are fabricated in both production-grade (Fortus 250mc) and entry-level (MakerBot Replicator 2X) material extrusion systems with varying in ll densities (1 mm to 10 mm spacing between rasters). In addition, solid in ll specimens are printed in three orientations to establish baseline mechanical sti ness and strength. Finite element simulations and a simpli ed analytical model based on Euler-Bernoulli beam theory are developed. Results show reasonable agreement between analytical, simulation, and experimental results; 10-20% and 10-40% deviation for production-grade and entry-level specimens specimens, respectively. There is a 40% reduction in sti ness and strength between the solid XY specimen and 1 mm in ll specimen. As in ll density is further decreased, sti ness and strength asymptotically reduces by 60-70% when compared to solid specimens. This e ect is more pronounced in specimens fabricated using entry-level printers, which indicates that printer selection plays a role in printing highly sparse parts. Cost analysis suggests that up to 40% savings can be achieved with highly sparse structures. However, for structural parts, it is recommended that parts be printed with solid in ll and with the loading direction aligned in the XY plane to achieve high sti ness, high strength, and reasonable cost. Findings from this study show that there is minimal cost savings but high reduction in mechanical sti ness and strength when sparse in lls are used in both production-grade and entry-level printers. Hence, it is recommended that solid in ll should be used in all regions of parts that carry signi cant mechanical stress and sparse in ll be used solely to support internal geometries and overhangs. Keywords: material extrusion, fused lament fabrication, in ll density, printer selection, part orientation 1 Introduction One of the most exciting elds of manufacturing in terms of potential for growth and newfound applica- tions is additive manufacturing (AM). It utilizes layer-by-layer data from a computer-aided design (CAD) model of the part to selectively join materials across multiple layers. This unique approach to manufacturing provides a number of bene ts, including processing of a variety of material systems, little-to-no setup, lack of need for xtures, and economic manufacturing of highly complex, custom, and low volume production parts. These advantages could be attributed to the ever-growing adoption of AM for industrial applications across aerospace, automotive, biomedical, and energy sectors. As listed in ASTM F2792, the seven AM process categories include: material extrusion, vat photo-polymerization, binder jetting, material jetting, sheet lam- ination, and directed energy deposition. This study focuses on material extrusion AM technology. Material extrusion, shown in Figure 1, is also known as fused deposition modeling (FDM) or fused lament fabrication 1 z x(a)(b)(d) (e) (h) (f)(g)(c)Figure 1: Schematic illustration of a material extrusion 3D printer with (a) z-axis motor and screw drive, (b) x-axis motor and belt drive, (c) feedstock lament, (d) extruder carriage assembly with motor, (e) heated nozzle, (f) printed part, (g) build plate, and (h) lament cartridge. (FFF). Feedstock (e.g. lament, pellets) is fed into a heated nozle which is controlled using numeric control (NC) to synchronize material extrusion and extruder movement over a build plate. Feedstocks that can be processed through this process range from thermoplastics, wood, metal, carbon- ber-infused polymers, con- crete, food, and biomaterials. Since the expiration of early patents related to material extrusion, the prices for building, operating, and maintaining material extrusion printers has signi cantly decreased over the past decade [1] and has led to \democratization" of AM [2], with increased adoption in small businesses and schools [3]{[5]. In recent decade, material extrusion has risen to prominence, especially in applications such as education [3], [6], do-it-yourself (DIY) projects, medical implants [7], and rapid prototyping applications that require small-scale custom produced parts [8]. The goal of this work is to evaluate the role of proper in ll selection, part orientation, and in uence of printer selection on optimal in ll density for mechanical part design. Speci cally, this study will investigate the e ect of printer selection (entry-level versus production-grade), build plane orientation, and in ll density on sti ness, strength, and cost of material extrusion AM parts. Four point bending specimens are fabricated using two printers, Fortus (production-grade) and MakerBot (entry-level), and tested to failure to determine e ective exural sti ness and strength. The specimens include solid in lls built in three orientations and sparse in lls of various densities. Finite element analysis and a simpli ed analytical model based on Euler- Bernoulli beam theory are developed to correlate the e ect of in ll density on the e ective exural sti ness. Finally, a cost analysis based on printing time and material consumption is presented to identify implications of in ll density selection based on production-grade and entry-level AM systems. 2 Background Material extrusion AM printers use a heated nozzle to melt and selectively extrude feedstock lament layer-by-layer onto a build plate [2]. The nozzle and build stage are controlled by stepper motors using numerical control. Process planning is conducted by converting a computer aided design (CAD) model of the desired part into NC code for the printer using a \slicer" software. Overhanging surfaces require additional support structures that are removed after build processing. A diagram of a typical material extrusion 3D printer is shown in Figure 1. A major factor in the widespread adoption of material extrusion printers is in the relative ease of installation and operation when compared to other AM technologies. This leads to lower acquisition and operating costs. However, there are still various grades of material extrusion AM systems with variations in machine con guration and processing capabilities. Additional features such as an enclosed heated chamber, high precision stepper motors, in-process monitoring, multiple nozzles, auto-calibration, and dissolvable support structures will increase the cost of material extrusion AM systems but could lead to improvements in overall print quality. Wohlers Associates de nes entry-level printers as AM systems with a price of $5,000 or less. A qualitative comparison of entry-level versus production-grade printers is shown in 2 Table 1: Comparison of typical entry-level and production-grade 3D printers. entry-level printer production-grade printer Size Desktop-sized Refrigerator-sized Enclosure Optional Yes Heated build plate Optional Yes Filament cartridge Loose coil Enclosed canister Cost Low ( $100-5,000) High ( $5,000-750,000) Table 1. Although both AM systems have similar functionality, additional features such as the ones previously mentioned greatly improve dimensional accuracy and reduce part warping due to uneven thermal expansion during the build process [9]. In addition, many feedstock materials are hygroscopic, and moisture in feedstock has been shown to cause bubbles and other print defects [10]. Filaments stored in sealed cartridges in production-grade systems could have better mechanical properties and nish when compared to open spools in entry-level systems. In addition to printer selection, prior studies have shown that multiple process parameters including: material and support selection [11], nozzle temperature [12], uniformity of extruder and bed temperatures [13], part design [13], layer thickness [14], [15], wall thickness [15], in ll pattern [15]{[18], part orientation [19], raster angle [20], and gap between extruded roads [21], will in uence the nal part sti ness, strength, dimensional accuracy, and production time. In particular, in ll density is a process parameter which can be directly controlled by the user and has a strong in uence on part sti ness and strength. Prior studies have shown that parts with sparse in ll density are lighter in weight but more compliant and weaker than dense parts [16], [17], [22]. This leads to a tradeo between material consumption, print duration, and mechanical performance of the part. In summary, it is critical to understand the implications of printer selection, in ll density, production consts, and mechanical performance of parts produced through material extrusion AM systems. 3 Methodology This study focuses on the e ect of in ll density, part orientation, and printer selection on mechanical behavior and production costs of acrylonitrile butadiene styrene (ABS) polymer material extrusion AM systems. An entry-level printer (MakerBot) and a production-grade printer (Stratasys FDM) are used to fabricate four point bending test specimens with varying in ll densities. In addition, solid in ll specimens are printed to evaluate the e ect of build orientation on mechanical behavior and production cost. Alter- nating in ll pattern of 45across each layer is evaluated. A simpli ed analytical model and nite element simulations are used to correlate predicted and experimental results (strength and sti ness) from four point bending experiments. Finally, a cost analysis is presented based on machine usage time and feedstock cost. 3.1 Nominal Specimen Geometry Per ASTM D6272 [23], nominal four point bending specimens consisting of a 0.762 mm oor (solid bottom layer), 0.762 mm ceiling (solid top layer), 0.5 mm wall thickness (contour), and alternating layers of 45 raster in ll of 0.254 mm tall and 0.254 mm thick (total of 20 in ll layers per specimen). The spacing between individual in ll rasters is varied between 1 mm to 10 mm. The overall nominal specimen dimensions are 120 mm long by 25 mm wide by 6.604 mm thick. All specimens are built with the 120 mm 25 mm face along the XY print direction, as shown in Figure 2. In order to ensure that the in ll pattern was consistent across both printers, the in ll geometry was directly modeled in SolidWorks. A partial exploded view of the specimen is shown in Figure 2 and a cross sectional view is shown in Figure 3. In addition, solid specimens with 100% solid in ll, as shown in Figure 4, were printed in all three build orientations (XY, YZ, XZ) on both printers. 3 (a) (b) (c) (d) xyzFigure 2: Four point bending specimen: (a) oor, (b) +45in ll, (c) 45in ll, and (d) ceiling layers. There are ten layers of each in ll type in alternating sequence for a total of 20 in ll layers. bhttop tsideInfill Figure 3: Cross section of bending specimens with in ll, where h= 6:604 mm,b= 25 mm,ttop= 0:762 mm, andtside= 0:5 mm. 4 xyz (a)(b)(c)Figure 4: Solid bending specimens in (a) XY, (b) YZ, and (c) XZ planes. Table 2: Mechanical properties of Fortus and MakerBot feedstock materials. Property ABSplus-P430 MakerBot ABS Ultimate tensile strength (MPa) 33 38 Tensile modulus (MPa) 2200 2320 Elongation at break (%) 6 7 Flexural strength (MPa) 58 59 Compressive strength (MPa) 49 49 3.2 Printer Con guration The Stratasys Fortus 250mc is a production-grade printer with a 254x254x305 mm build volume. Relevant features of the printer include: heated 254x254 mm build plate, heated enclosure with fan, three layer thickness settings (0.178 mm, 0.254 mm, 0.330 mm), dual nozzle extrusion (ABSplus-P430 lament and SR- 30 support material), and proprietary slicing software provided by machine manufacturer (Insight, version 9.0). production-grade specimens are printed using a T14 nozzle tip and a layer thickness of 0.254 mm. File preparation is done using Stratasys's proprietary Insight software. The software automatically selects appropriate temperatures, print speeds, and other process parameters for a given material and with limited user inputs on process conditions and in ll patterns. The MakerBot Replicator 2X is an entry-level printer with a 246x152x155 mm build volume. Relevant features of this printer include: heated 246x152 mm build plate, unheated enclosure with no circulation fan, variable layer thickness (minimum 0.100 mm), dual nozzle extrusion (MakerBot ABS lament), and compatibility with open source slicers (e.g. Cura, Slic3r, MakerBot Desktop). entry-level printer specimens are printed using a 0.4 mm diameter nozzle tip. Build preparation is done using MakerBot's proprietary MakerBot Desktop software (version 3.10). When compared to production-grade systems, entry-level systems (e.g. MakerBot) provides the user with signi cant control over various process parameters and in ll design. In order to normalize production quality across both systems, the layer height is set at 0.25 mm and two contour outer shells are used. By default, the high quality setting in MakerBot Desktop places a raft underneath the geometry. The raft provides structural stability and reduces warping caused by the thermal expansion and shrinking of layers as they are deposited and was manually removed from the specimens after they printed. 3.3 Material Properties The Fortus uses ABSplus-P430 lament feedstock while the MakerBot uses MakerBot ABS lament feedstock, and a summary of corresponding mechanical properties is presented in Table 2. 5 P/2 P/2 P/2 P/2LL = 33.33 mm LS = 100 mm (a) (b)/g4426pin Figure 5: (a) Four point bend test setup. (b) Free body diagram of test setup, where Pis the applied load, LSis the span length, and LLis the load length. 3.4 Bending Test Setup The four point bending tests are performed on an Instron Model 5967 universal testing machine using a 30 kN load cell, shown in Figure 5. Per ASTM D6272, a strain rate of 2.96 mm/mm/min is used across all specimens. The support span LSis set to 100 mm and the load span LLis set to 33.3 mm. The Instron crosshead displacement is used to measure the displacement of the two upper loading pins. The e ective exural modulus Eeffis de ned as the Young's modulus modulus of a solid, homogeneous, isotropic material that would produce the same initial load-displacement response in AM specimens (with the same overall dimensions). Similarly, the e ective exural strength Seffis the maximum bending stress acting on a solid, homogeneous, and isotropic specimen. Based on Euler-Bernoulli beam theory, the load- displacement response for four-point bending is given by pin=P(LSLL)2(LS+ 2LL) 48EeffIeff(1) wherepinis the displacement measured at the upper loading pin and Pis the load applied at both upper loading pins. Since the specimens have a rectangular cross section with width band thickness d, the e ective second moment of area Ieffis given by Ieff=bd3 12(2) Substituting Equation 2 and the relation LS= 3LLinto Equation 1 provides Eeff=5 27P pinL3 S bd3 0:185mL3 S bd3(3) wheremis the slope of the measured load-displacement response in the linear region. It should be noted that this is an estimate based on displacement measurements at the upper loading pin and not at the center of the beam. The e ective exural strength is given by Seff=PmaxLS bd2(4) wherePmaxis the maximum load recorded during experimentation. 6 3.5 Finite Element Analysis Finite element analysis (FEA) is a commonly employed method to predict the exural sti ness of spec- imens, including AM specimens [24]{[26]. The nominal geometry described in Section 3.1 is simulated in Abaqus (version 6.12) using S4R shell elements for the in ll specimens. In addition, an outer shell model consisting of oor, ceiling, and contour walls (i.e. no in ll) is also modeled for reference. In the case of solid specimens, a 3D model consisting of C3D8R brick elements is used, without distinction across multiple build orientations in the simulation. A linearelastic material is used based on Young's modulus presented in Section 3.3. Since Poisson ratio is not provided in the material datasheets, a value of = 0:35 is assumed [27]. Displacement boundary conditions in the vertical direction are applied at the line of nodes to represent the top roller pins in a four-point bend setup. Pinned and roller boundary conditions are applied at the line of nodes for the bottom roller pins. The e ective exural modulus is calculated using Equation 7, where Pandare results from the simulation. E ective exural strength was not simulated due to the lack of material properties in the plastic regime. 3.6 Simpli ed Analytical Solution In addition to experimentation and FEA, a simple analytical model based on Euler-Bernoulli beam theory is developed. In the case of solid specimens, the e ective exural modulus and strength are expected to match the bulk material results. In the case of in ll specimens, the in lls are neglected in the simpli ed model and only the top, bottom, and side shells are considered. The maximum de ection center of a beam under four-point-bending is given by center =P(2L3 S3L2 LLS+L3 L) 96EI(5) wherePis the applied load from two points (with each point contributing P=2),Eis the Young's modulus of the bulk material, Iis the second moment of area, LLis the load span, and LSis the support span, as shown in Figure 5. In order to nd the e ective exural modulus Eeff, the cross section is assumed to be solid, and therefore the e ective second moment of area is given by Ieff=bh3 12(6) wherebis the beam width and his the beam height (thickness). Substituting EeffandIeffinto Equation 5 and rearranging yields Eeff=P(2L3 S3L2 LLS+L3 L) 8bh3(7) Finally, substituting the ratio P=from Equation 5 (which considers the true Young's modulus Eand second moment of area I) into Equation 7 provides Eeff=12EI bh3 =EI Ieff(8) Based on the cross section of the beam shown in Figure 3, the second moment of area ( I) can be written as I=bh3 12(b2tside)(h2ttop)3 12(9) Using nominal cross section dimensions from Section 3.1 and material properties from Section 3.3, the second moment of area is 326 mm4and the e ective exural sti ness is 0 :570E, or 1254 MPa for the Fortus ABSplus-P430 material and 1322 MPa for the MakerBot ABS material. 7 Table 3: Simulated e ective exural modulus values. In llEeff, production-grade (MPa) Eeff, entry-level (MPa) Solid 2170 2288 1 mm 1063 1121 3 mm 1014 1069 4 mm 1009 1064 6 mm 1004 1059 10 mm 1003 1057 None 999 1053 3.7 Print Time and Production Cost Analysis A print time and production cost analysis is performed by using the print time and feedstock volume required, as estimated by Insight and MakerBot Desktop for Fortus and MakerBot, respectively. The unit cost of ABSplus-P430 is estimated at $0.00028/mm3based on $260 per spool with a volume of 922600 mm3, while the unit cost of MakerBot ABS is estimated at $0.000045/mm3based $43 per spool with a volume of 961538 mm3. Machine usage cost rates for Fortus and MakerBot are estimated to be $0.50/min and $0.17/min, respectively. These rates are based on a prior analysis conducted by Baich et al. [16]. A simpli ed model for the total part cost is therefore given by Ctotal=TCusage +VCfeedstock (10) whereCtotalis the total part cost ($), Tis the print time (min), Cusage is the machine usage cost per minute, Vis the feedstock volume (mm3), andCfeedstock is the feedstock cost per mm3. 4 Results and Discussion 4.1 Finite Element Analysis Results for the simulated e ective exural modulus across all in ll densities and build directions are presented in Table 3. A mesh size of 0.12 mm was used for all shell element simulations. Mesh convergence was tested using a smaller mesh size of 0.08 mm, which yielded less than 0.3% di erence in simulated exural sti ness. In the case of solid brick elements, a mesh size of 1 mm was used. Mesh convergence was tested using a smaller mesh size of 0.5 mm, yielding less than 1.4% di erence in simulated exural sti ness. The simulated modulus values are lower than the analytical results due to additional deformation modes in shell elements that are not present in the simpli ed Euler-Bernoulli beam model. The simulations also suggest that the in ll has negligible e ect on the e ective exural modulus. This can be attributed to the distribution of bending stress across both oor and ceiling layers with the in ll acting as a separator between these two surfaces, akin to a sandwich panel structure. 4.2 Four-Point Bending Experiments Three of each specimen type was printed using both the entry-level and production-grade printers. Several representative specimens (partially printed) are shown in Figure 6. Overall specimen length, width, and thickness were measured ( n= 3) with digital calipers before testing. A summary of experimental results for four point bending is given in Table 4. Experimental and simulated e ective exural modulus are presented in Figure 7. Experimental exural strength is presented in Figure 8. Simulated and experimental exural modulus are in reasonable agreement (10-20% error). This error is more pronounced in entry-level specimens as detailed in Section 4.2.2. 4.2.1 E ect of Build Orientation on Mechanical Behavior As observed in Figure 7, FEA simulations overestimate the e ective exural modulus for the solid spec- imens. Since anisotropic behavior due to build direction is not accounted for in the FEA, the deviation is 8 (a) (b) (c) (d)25 mmFigure 6: Representative four point bending specimens with (a) 1 mm, (b) 3 mm, (c) 6 mm, and (d) 10 mm in ll. Top layer (ceiling) was not printed to show in ll. Table 4: E ective exural strength and e ective modulus of elasticity for production-grade and entry-level specimens. S.D. indicates standard deviation. production-grade entry-level In ll Strength S.D. (MPa)Modulus S.D. (MPa)Strength S.D. (MPa)Modulus S.D. (MPa) Solid XY 44.2 0.4 1931 52 47.5 1.0 1814 38 Solid YZ 52.7 2.0 1913 32 49.1 0.7 2034 30 Solid XZ 36.5 0.4 1731 30 26.4 3.4 1734 143 1 mm 25.1 0.8 1085 44 22.7 1.4 1057 29 3 mm 22.8 0.7 1025 18 17.8 2.8 855 25 4 mm 19.9 0.6 916 7 16.9 0.5 834 23 6 mm 18.5 0.8 869 25 13.7 0.7 770 18 10 mm 16.0 0.1 928 45 15.1 0.2 771 32 Production-grade Entry-level FEA Experiment (a) (b) Figure 7: (a) Simulation and experimental results of exural modulus. (b) Percent di erence between simulated and experimental exural modulus. 9 Production-grade Entry-levelFigure 8: Experimental results of exural strength. much larger for the XZ oriented specimen. This anisotropic behavior may be due to voids between adjacent roads that decrease the e ective modulus of the as-printed material [28]{[30]. It may also be caused by changes in melting and re-solidi cation of the lament material itself as it is exposed to the high temper- ature nozzle [31], [32]. In addition, the XY and YZ specimens have continuously deposited raster in lls along the beam axis when compared to XZ specimens with in lls that are perpendicular to the beam axis. These continuously deposited parallel raster in lls carry bending loads more e ectively compared to the perpendicular in lls [15], [33]. Similar trends are observed in the e ective exural strength, where the XZ specimens have the lowest exural strength due to presence of voids and gaps between extruded rasters, which lead to stress concentra- tions [15]. In addition, prior work has shown that factors such as extruder temperature [12], build chamber temperature [34], [35], and print speed [36] a ect the formation of bonds and degree of wetting between adjacent rasters. 4.2.2 E ect of In ll Density on Mechanical Behavior Simulation results correlate with experimental results where the e ective exural modulus decreases with reduction in in ll density. However, this trend is more signi cant in the entry-level system compared to the production-grade system. At very low in ll densities, the FEA simulations predict that the in ll has negligible impact on the e ective exural sti ness. This result is observed in the production-grade specimens, with the 4 mm through 10 mm specimens having nearly the same e ective exural sti ness. However, for the entry-level specimens, the e ective exural sti ness continues to decrease as in ll density decreases. The di erent trends across these two printers could be attributed to geometric distortion and sagging of the ceiling in entry-level systems. The production-grade system has a heated build enclosure with circulation fan to improve convective heat transfer in the build environment, which reduces thermal warping. In addition, when the in ll density is low, the solid rasters in the ceiling are largely unsupported. It is hypothesized that production-grade systems use more optimal process parameters (e.g. speed, nozzle temperature, lament retraction) to reduce warping and geometric distortion in unsupported structures compared to entry-level systems. Finally, overall di erences in printer hardware and printer build quality may also contribute to major variations in dimensional accuracy and reliability [5], [37], which would contribute to lower exural modulus in entry-level specimens. The in ll density also has a dramatic impact on exural strength in both production-grade and entry- level specimens, with lower in ll densities resulting in lower exural strength. These results are expected and agree with other similar studies [38]{[40]. Since the in ll primarily serves as a separator between the ceiling and oor layers, a lower density provides lesser support for these two load bearing layers. entry- level in ll specimens exhibit much lower exural strength when compared to production-grade specimens (except for solid XY specimen), although both material feedstocks have very similar exural and compressive 10 Table 5: Print time and feedstock volume estimates for production-grade and entry-level specimens. production-grade entry-level In ll Print time (min)Volume (mm3)Cost ($)Print time (min)Volume (mm3)Cost ($) Solid XY 46 19600 28.52 76 28130 13.92 Solid YZ 83 19480 46.99 76 25810 13.82 Solid XZ 94 19230 52.42 74 24460 13.43 1 mm 36 10940 21.08 68 16820 12.09 3 mm 30 8800 17.48 51 13230 9.09 4 mm 29 8560 16.91 47 12540 8.39 6 mm 28 8080 16.28 45 12210 8.05 10 mm 27 7750 15.68 41 11460 7.35 Production-grade machine cost Entry-level machine costProduction-grade material cost Entry-level material cost Figure 9: Machine usage cost and material cost for exural specimens. strengths (see Table 2). This is likely due to defects in the specimens caused by aforementioned di erences in build environment, slicer optimization, printer hardware, and printer build quality. This di erence is very pronounced in the solid XZ specimen, where production-grade specimens are 38% stronger than entry-level specimens. 4.3 Time and Cost Analysis A summary of print time, feedstock volume, and total cost is detailed in Table 5. Cost components in both entry-level and production-grade systems are presented in Figure 9. Lower in ll density results in decreases in print time, volume, and production cost. entry-level specimens have longer print time and use more feedstock material due to the presense of a raft. The orientation of the specimens also has a major e ect on print time and production cost in production-grade specimens, with XZ specimens taking 104% longer to print and being 84% more expensive than XY specimens. In the case of entry-level system, there is no noticeable di erences in time and cost for the solid specimens. This indicates that the slicing software for the production-grade system is likely optimizing print speed, lament retraction, and possibly other settings based on build orientation, which can lead to dramatic di erences in print time. 4.3.1 Mechanical Performance versus Cost As shown in Figure 10, analysis of exural sti ness and strength as a function of total production cost identi es clear clusters of specimens that are printer-dependent. All three entry-level solid specimens have very similar costs, but there is a wide variation in mechanical performance between the XZ and YZ 11 Production-grade Entry-level XY YZ XZYZ XY XZ 1 mmXYYZ XZYZ XY 1 mm1 mmXZ 1 mm 3 mm 4-10 mm 3-10 mm4-10 mm3 mm 3-10 mm (a) (b)Figure 10: Total production cost as a function of (a) exural modulus and (b) exural strength for entry-level and production-grade specimens. specimens, which is re ective of aligning solid continuous rasters along the applied loading direction. In the case of production-grade system, the solid specimens have a wide variation in production costs, which is re ective of higher operating costs and print times. In particular, the XZ and YZ specimens are 84% and 65% more expensive than the XY specimen, respectively. However, the YZ and XY specimens have very similar mechanical properties, indicating that printing in the XY plane is preferable for optimal mechanical properties at lower cost. In addition, it can be observed from Figure 10 that the in ll specimens cluster around lower production cost at decreased mechanical performance. It should be noted that the solid XY specimens are 35% and 15% more expensive than the 1 mm in ll specimens for production-grade and entry-level systems, respectively. However, the solid XY specimens are 78% and 67% sti er than the 1 mm in ll specimens for production-grade and entry-level systems, respectively. Similarly, the solid XY specimens are 76% and 109% stronger than the 1 mm in ll specimens for production-grade and entry-level systems, respectively. This major nding indicates that modest increases in production cost can result in substantial increases in mechanical performance, i.e. solid parts could be more optimal than parts with even 1 mm in ll density. Sparser in ll specimens such as the 4 mm through 10 mm specimens generally cluster together in Figure 10, which indicates that there is minimal di erence in cost or mechanical performance between these specimens, especially when compared to the solid specimens. Cost savings, reduction in sti ness, and reduction in strength relative to the solid XY specimen are shown in Figure 11. The XY solid specimen is chosen as the reference specimen since all the in ll specimens are manufactured in the XY plane. As shown in Figure 11, the lowest in ll density can lead to roughly 45% cost savings. However, sti ness is reduced by around 55%, and strength is reduced by around 65%. If cost is the only concern, then printing parts with very low in ll densities can lead to signi cant cost savings. However, if mechanical considerations are important, then printing solid parts oriented with loading in the XY plane leads to excellent sti ness and strength at modest costs. 12 Production-grade Entry-level (a) (b) (c)Figure 11: E ect of in ll type on (a) cost, (b) sti ness, and (c) strength reduction. All values are calculated with reference to the XY solid specimen. 4.4 Implications This study has shown that part orientation, printer selection, and in ll density, part orientation, and printer selection all have an impact on the mechanical behavior and production cost of exural test specimens. These results can be generalized and applied to other parts and geometries printed using material extrusion AM systems. Part orientation has a signi cant impact on mechanical sti ness and strength of parts manufactured using material extrusion AM. Extruded rasters are much more ecient at carrying loads when applied loads and stresses run parallel to the raster extrusion paths. In addition, continuous rasters lead to higher sti ness and strength compared to discontinuous rasters. Therefore, it is extremely important to be cognisant of loading direction with respect to the build direction. Ideally, parts should be aligned such that signi cant load paths lie in the XY plane of the printer. Parts should not be oriented with loading or large features along the build direction, as this leads to de cient mechanical properties and high costs. Proper part orientation is especially important when manufacturing parts on entry-level systems because they lack features such as heated enclosures, leading to worse mechanical properties along the build direction compared to production- grade systems. Printer selection also has an impact on mechanical behavior and manufacturing cost. In general, parts manufactured on production-grade systems have better mechanical properties compared to parts manufac- tured on entry-level systems due to better build environment conditions, printer hardware quality, and slicer optimization. However, production-grade parts are also more expensive, with higher machine usage costs being the most signi cant driver. In addition, production-grade systems have limited options when it comes to in ll geometry, layer height, and other custom options. In comparison, entry-level systems typically give users exibility in choosing and ne tuning process parameters. Many slicing software, such as Cura and Slic3r, are open source, allowing the additive manufacturing community to modify and introduce new fea- tures, in ll geometries, and slicing algorithms. Several entry-level systems, including RepRap and LulzBot, are also open source, allowing users to add and modify hardware to improve printer quality, support new ma- terial systems, and extend printer capabilities beyond the base model. Lower acquisition cost for entry-level systems also makes them much more accessible to schools, small businesses, and hobbyists. Finally, in ll density has a strong impact on mechanical properties, with higher in ll density parts having higher mechanical sti ness and strength. However, there is a signi cant di erence in mechanical properties between solid in ll and 1 mm in ll. For parts under mechanical loading, solid in lls should be used for all regions of the part that carry mechanical stresses. Sparse in lls should only be used in non-structural regions and only for supporting internal geometries and overhangs as they provide negligible mechanical bene ts. While this work only investigates 45alternating raster in lls, other in ll patterns such as ones based on Hilbert curves [41] should be investigated to see if cost reductions can be achieved by reducing extruder 13 travel distance. In addition, other solid in ll types such as stress tensor aligned in ll [42] should also be investigated to see if better mechanical properties can be achieved. 5 Conclusion This work addresses how part orientation, printer selection, and in ll density a ect the mechanical properties and production cost of parts manufactured using material extrusion AM systems. Four point bending specimens with solid and sparse in ll are fabricated using both a production-grade printer and an entry-level printer. The solid in ll specimens are fabricated in multiple build orientations and the sparse in ll specimens are manufactured using several in ll densities. A simpli ed analytical model based on Euler- Bernoulli beam theory and nite element simulations are used to corroborate the experimental results. In addition, a cost analysis based on material feedstock cost and machine usage cost is developed. The results of this work show that: Build orientation has a strong in uence on part sti ness and strength for both entry-level and production- grade AM systems. For production-grade systems, build orientation can dramatically increase the machine usage cost. For best mechanical performance, parts manufactured using material extrusion AM systems should be oriented such that load and stress paths are preferentially aligned in the XY plane of the printer. Parts produced using production-grade AM systems generally have superior mechanical properties compared to ones produced using entry-level systems. However, they are more expensive due to high machine usage costs and less exible in terms of process parameters. entry-level systems, on the other hand, are signi cantly cheaper to acquire and frequently support open source hardware and slicing software to allow precise control of process parameters, which allows the AM community to actively and openly develop new features and tools. In ll density has a signi cant impact on mechanical properties. Switching from a solid in ll to a 1 mm sparse in ll leads to dramatic reductions in mechanical sti ness and strength without much cost savings. Hence, it is recommended that solid in lls be used in all regions of parts that carry signi cant mechanical stress, while sparse in lls be used solely to support internal geometries and overhangs. Several aspects that require further investigation include alternative in ll generation techniques that may lead to superior mechanical properties or lower production costs. In addition, a wider survey of entry-level and production-grade AM systems is necessary to expand beyond the two speci c printers considered in this study. A review by Popescu showed that the generalization of process parameters and results from printer to printer is limited due to wide di erences in how printers implement process parameters and slicing algorithms [40]. Further research is necessary to determine how various process parameters a ect the dimensional accuracy of highly sparse in ll parts and unsupported structures. While the present work has shown that di erences exist in this area with respect to production-grade and entry-level printers, speci c factors have not been conclusively identi ed, primarily due to the lack of openness in the production-grade software. A deeper understanding in this area may help identify what process parameters contribute most to successful part construction and print reliability. Finally, the ideal in ll type, density, and printer orientation for a given part is highly loading dependent. While this work and complementary work by Baich [16] focus on tensile, compressive, and bending loads, more studies are needed on other loading conditions such as shear, torsion, and equibiaxial loading. Con icts of Interest The authors declare no con icts of interest. Acknowledgments The authors would like to acknowledge the Structures and Composites Laboratory (SACL) at Stanford University for providing assistance in performing nite element analysis. Ruiqi Chen would like to thank the 14 National Defense Science and Engineering Fellowship (NDSEG) program as well as the Stanford Graduate Fellowship (SGF) program for their generous and continuous support over the years.