Fused Particle Fabrication 3-D Printing: Recycled Materials’ Optimization and Mechanical Properties

Michigan Technological University Digital Commons @ Michigan Tech

Department of Materials Science and Department of Materials Science and Engineering Publications Engineering

8-1-2018

Fused particle fabrication 3-D printing: Recycled materials’ optimization and mechanical properties

Aubrey Woern Michigan Technological University

Dennis Byard Michigan Technological University

Robert B. Oakley re:3D Inc.

Matthew J. Fiedler re:3D Inc.

Samantha Snabes re:3D Inc.

See next page for additional authors Follow this and additional works at: https://digitalcommons.mtu.edu/materials_fp

Part of the Electrical and Computer Engineering Commons, and the Materials Science and Engineering Commons

Recommended Citation Woern, A., Byard, D., Oakley, R. B., Fiedler, M. J., Snabes, S., & Pearce, J. M. (2018). Fused particle fabrication 3-D printing: Recycled materials’ optimization and mechanical properties. Materials, 11(8). http://dx.doi.org/10.3390/ma11081413 Retrieved from: https://digitalcommons.mtu.edu/materials_fp/181

Follow this and additional works at: https://digitalcommons.mtu.edu/materials_fp Part of the Electrical and Computer Engineering Commons, and the Materials Science and Engineering Commons Authors Aubrey Woern, Dennis Byard, Robert B. Oakley, Matthew J. Fiedler, Samantha Snabes, and Joshua M. Pearce

This article is available at Digital Commons @ Michigan Tech: https://digitalcommons.mtu.edu/materials_fp/181 materials

Article Fused Particle Fabrication 3-D Printing: Recycled Materials’ Optimization and Mechanical Properties

Aubrey L. Woern 1, Dennis J. Byard 1, Robert B. Oakley 2, Matthew J. Fiedler 2,3, Samantha L. Snabes 2,3 and Joshua M. Pearce 3,4,5,* ID 1 Department of Mechanical Engineering–Engineering Mechanics, Michigan Technological University, Houghton, MI 49931, USA; [email protected] (A.L.W.); [email protected] (D.J.B.) 2 re:3D Inc., 1100 Hercules STE 220, Houston, TX 77058, USA; [email protected] (R.B.O.); [email protected] (M.J.F.); [email protected] (S.L.S.) 3 Department of Material Science and Engineering, Michigan Technological University, Houghton, MI 49931, USA 4 Department of Electrical and Computer Engineering, Michigan Technological University, Houghton, MI 49931, USA 5 Department of Electronics and Nanoengineering, School of Electrical Engineering, Aalto University, 00076 Espoo, Finland * Correspondence: [email protected] or joshua.pearce@aalto.fi; Tel.: +01-906-487-1466



Received: 21 July 2018; Accepted: 9 August 2018; Published: 12 August 2018


Abstract: Fused particle fabrication (FPF) (or fused granular fabrication (FGF)) has potential for increasing recycled polymers in 3-D printing. Here, the open source Gigabot X is used to develop a new method to optimize FPF/FGF for recycled materials. Virgin polylactic acid (PLA) pellets and prints were analyzed and were then compared to four recycled polymers including the two most popular printing materials (PLA and acrylonitrile butadiene styrene (ABS)) as well as the two most common waste plastics (polyethylene terephthalate (PET) and polypropylene (PP)). The size characteristics of the various materials were quantified using digital image processing. Then, power and nozzle velocity matrices were used to optimize the print speed, and a print test was used to maximize the output for a two-temperature stage extruder for a given polymer feedstock. ASTM type 4 tensile tests were used to determine the mechanical properties of each plastic when they were printed with a particle drive extruder system and were compared with filament printing. The results showed that the Gigabot X can print materials 6.5× to 13× faster than conventional printers depending on the material, with no significant reduction in the mechanical properties. It was concluded that the Gigabot X and similar FPF/FGF printers can utilize a wide range of recycled polymer materials with minimal post processing.

Keywords: 3-D printing; additive manufacturing; distributed manufacturing; open-source; polymers; recycling; waste plastic; extruder; upcycle; circular economy

1. Introduction With the introduction of the self-replicating rapid prototyper (RepRap) 3-D printer [1–3], the costs of additive manufacturing (AM) with 3-D printers have been reduced enough to be accessible to consumers. This has allowed for the emergence of a distributed manufacturing paradigm in AM [4–6], where 3-D printing can be used to manufacture products for the consumer and by the consumer directly, with significant savings compared to the purchasing of mass-manufactured products [7–12]. The emerging support for this model in the business literature [13–15] is in part due to the exponential rise of free digital design file sharing for 3-D printed products [12], which ranges from sophisticated scientific instruments [16–20] to everyday toys for children and cosplayers [10]. Regardless of the

Materials 2018, 11, 1413; doi:10.3390/ma11081413 www.mdpi.com/journal/materials Materials 2018, 11, 1413 2 of 19 sophistication of the product, high return on investments (ROIs) can be enjoyed based on the download substitution values using commercial polymer 3-D printing filament [21,22]. Commercial filament, however, is marked up significantly (e.g., >5×–10×) over the cost of the raw polymers, which limits the cost savings and thus, the deployment velocity of distributed manufacturing to further increase the rate of accessibility of AM [23]. The negative effects of the high costs of filament are most notable for large format 3-D printers (those with a build volume that is greater than a cubic foot), which can process several kg of polymer in a single print lasting over 24 h. One method of overcoming these cost barriers is to use a means of distributed plastic recycling, involving upcycling plastic waste into 3-D printing filament with a recyclebot (an open source waste plastic extruder [24]). Previous research on the life cycle analysis of the recyclebot process found a 90% decrease in the embodied energy of the filament from the acquiring, processing of the natural resources, and the synthesizing compared to traditional filament manufacturing [25–27]. This allows for the tightening of the loop of the circular economy [28] because it enables real distributed recycling that eliminates nearly all energy use and pollution from transportation. Many recyclebot versions have been developed including open source variations from the Plastic Bank, Filastruder, Precious Plastic, Lyman, and Perpetual Plastic, as well as fully commercial versions including the Filastruder, Filafab, Noztek, Filabot, EWE, Extrusionbot, Filamaker (also has a shredder), and the Strooder, Felfil (OS) [29]. Most recently, a “RepRapable Recyclebot” has been demonstrated [30], where most of the machine’s parts can be 3-D printed from waste plastic themselves. Several polymers have been successfully recycled as single component thermoplastic filaments, such as polylactic acid (PLA) [30–34], high-density polyethylene (HDPE) [24,35,36], acrylonitrile butadiene styrene (ABS) [28,36,37], elastomers [9], as well as composites (e.g. waste wood [38] and carbon fiber reinforced [39]). Unfortunately, each time a polymer is heated and extruded (whether it be in the recyclebot filament making process or during conventional fused filament fabrication (FFF)/fused deposition modeling (FDM) 3-D printing), the mechanical properties are degraded [31,32,40,41]. This effectively limits this process of recycling to 5 cycles [31,32] without the use of blending virgin materials or adding materials for mechanical property reinforcement. To reduce the number of melt/extrude cycles of recycled plastic that is used for 3-D printing, one option is to eliminate the need for filament and print directly from pellets, flakes, regrind, or shreds of recycled plastic, which will be referred to as particles here. Several 3-D printers using fused particle fabrication (FPF) (or fused granular fabrication (FGF)) have been designed to accomplish this in the academic [42–44], hobbyist [43–45], and commercial systems [46–50]. These systems have been tested with virgin pellets, however the mechanical properties of FPF printers using recycled polymer particles of various shapes and sizes has not been reported, which limits the ability of engineers to fabricate load bearing products from recycled waste using FPF 3-D printing. In this study, the open source Gigabot X [51], which is a large scale recycled plastic 3-D printer, was used to fabricate and test the mechanical properties of parts that were made using FPF to fill this knowledge gap. To establish a baseline, virgin PLA pellets were analyzed first and were then compared to four recycled polymers: (1) PLA regrind of 3-D printed parts (the most common 3-D printed plastic), (2) recycled ABS pellets (the second most common 3-D printed plastic), (3) recycled polyethylene terephthalate pellets (PET, the most common waste plastic [52]), and (4) recycled polypropylene chips (PP, the second most common waste plastic [52]). First, the material size characteristics were quantified using digital image processing. Then, a power and nozzle velocity matrix printing test were completed to determine the optimum print speed and temperature settings for a given polymer feedstock. During this phase of testing, any problems with bed adhesion and warping were identified and resolved. Third, a set of ASTM type 4 tensile bars were printed and were pulled to confirm the mechanical properties of the plastic when it was printed with a pellet drive extruder system and was compared with the results of past work with FFF/FDM 3-D printers, while noting the number of melt cycles. Materials 2018, 11, 1413 3 of 19

2. Materials and Methods

2.1. Materials Virgin 4043D PLA pellets were obtained from Nature Works LLC. PLA regrind was obtained from a mixture of failed 3-D prints from various sources of filaments and was ground with an open source grinder [53]. PP regrind was provided by McDonnough Plastics. Northwest Polymers and CiorC provided ABS and PET recycled materials. The size characteristics of the particles for each starting material were quantified using digital imaging and the open source Fiji/ImageJ [54].

2.2. FPF 3-D Printer A prototype Gigabot X [55] was used to print the materials. The prototype used an extruder system that was closely related to an industrial thermoplastic extruder, however it was scaled down and mounted as the extruder head of the 3-D printer. The extrusion screw was designed with an increasing diameter down the length of the screw with a ratio of 2.5:1 from the start to the end [55]. The motor driving the extrusion screw was a NEMA 23 stepper motor with a planetary gearbox with an 18:1 reduction. This ran off an external stepper driver [55]. The hopper was 3-D printed to allow for ease of modification and optimization of the design during testing. The extruder was heated by four 60-watt 24-volt heater cartridges that were placed in groups of two in strategic locations on the barrel [55]. Figure1a shows a Gigabot X (the parts are labeled along with the heating zones) and Figure1b shows the details of the extruder in a cutaway computer aided design (CAD) model with the major components labeled. Full details of the design can be found at reference [55]. 3-D models were sliced with Slic3r [56] and the printer was controlled with Marlin Firmware [57]. The testing procedures were then developed for an FPF platform to allow for the data to be collected efficiently for each plastic. These data can be collected in a couple of working days and provide information on the optimum settings regardless of what type of plastic is used. This is beneficial for expediting the continued work on this or similar platforms with new polymers, additives, and composites.

2.3. Optimization of Temperature and Speed The power vs. velocity matrix testing was accomplished by first identifying the upper and lower bounds of the printable temperature range of a given plastic through literature research, as summarized in Table1.

Table 1. 3-D printable temperature range for the two most common 3-D printed polymers and the two most common post-consumer waste polymers.

Material Temperature Range ◦C Source PLA 160–200 [58] ABS 200–250 [59] PET 200–240 [60] PP 170–250 [61] Materials 2018, 11, 1413 4 of 19 Materials 2018, 11, x FOR PEER REVIEW 4 of 19

Extruder Motor

Hopper

Heat Zone 2

Heat Zone 1

Barrel

(a)

(b)

Figure 1. (a) The open source Gigabot X with the major components labeled. (b) Details of the Figure 1. (a) The open source Gigabot X with the major components labeled. (b) Details of the extruder. extruder.

Materials 2018, 11, x FOR PEER REVIEW 5 of 19

2.3. Optimization of Temperature and Speed The power vs. velocity matrix testing was accomplished by first identifying the upper and lower bounds of the printable temperature range of a given plastic through literature research, as summarized in Table 1.

Table 1. 3-D printable temperature range for the two most common 3-D printed polymers and the two most common post-consumer waste polymers.

Material Temperature Range °C Source PLA 160–200 [58] ABS 200–250 [59]

Materials 2018, 11, 1413 PET 200–240 [60] 5 of 19 PP 170–250 [61] Heating zones 1 (tip of the nozzle) and 2 inside the barrel were then set to every combination of temperaturesHeating zonesin the 1printable (tip of the temperature nozzle) and range 2 inside at an the appropriate barrel were resolution then set toto everycapture combination the entire spectrum.of temperatures A series in of the double printable lines temperature were then printed range atat aneach appropriate temperature resolution combination to capture at print the speeds entire fromspectrum. 5 to 50 A seriesmm/s ofin double 5 mm/s lines increments were then [62]. printed The atprinted each temperature lines (example combination in Figure at 2) print were speeds then massedfrom 5 toon 50 a digital mm/s scale in 5 mm/s(+/− 0.01 increments g) and the [ 62line]. masses The printed were linescompared (example to the in theoretical Figure2) wereline mass. then Themassed objective on a digitalof this test scale was (+/ to− find0.01 both g) and the theoptimum line masses print speed were comparedand the temperature to the theoretical settings linefor themass. extruder The objective for each of this3-D test printing was to material. find both The the optimum printsettings speed were and determined the temperature both settingsby the temperaturefor the extruder settings for each that 3-Dhad printing the lowest material. standard The deviation optimum in settings terms of were the determinedline weight bothfor a byset the of linestemperature and the settings print speeds that had across the lowestall of the standard temperature deviation settings, in terms which of the resulted line weight in the for heaviest a set of linesline masses.and the print speeds across all of the temperature settings, which resulted in the heaviest line masses.

Figure 2. Materials Sample Matrix Test (polylactic acid (PLA) shown). Figure 2. Materials Sample Matrix Test (polylactic acid (PLA) shown).

A single walled vase test [63] was used to determine the ideal settings for the flow percentage and theA single actual walled extrusion vase testwidth [63 once] was the used print to determine speed and the the ideal temperature settings for zone the flow settings percentage had been and confirmed.the actual extrusion To conduct width this once test, the a single print speedwalled and hollow the temperature cylinder with zone known settings theoretical had been dimensions confirmed. wasTo conduct printed. this Once test, the a singleprint was walled completed, hollow cylinderthe specimen’s with known mass was theoretical recorded dimensions and was compared was printed. to theOnce theoretical the print wasmass completed, of the component the specimen’s using massthe specific was recorded density and of wasthe plastic compared that to was the theoreticalused. The methodmass of theof similar component triangles using was the then specific used density to determine of the plastic the appropriate that was used. flow Thepercentage. method ofOnce similar the flowtriangles percentage was then was used successfully to determine calibrated, the appropriate digital calipers flow percentage. (+/−0.005) Oncewere used the flow to find percentage the printed was wallsuccessfully thickness. calibrated, This value digital was calipersthen loaded (+/− into0.005) the were slicing used software to find the(Slic3r) printed so that wall the thickness. printed Thisline widthvalue wascould then be loadedcorrected into for the in slicing the slicing software software. (Slic3r) so that the printed line width could be corrected for in the slicing software. 2.4. Tensile Testing 2.4. Tensile Testing

Tensile testing was first completed on PLA using the ASTM D638 Type 1 and Type 4 standard tensile bars. For PLA, Types 1 and 4 were used to examine any potential difference in mechanical performance due to the smaller geometry of the Type 4 tensile test bars, as has been done before with more conventional FFF based 3-D printing [64]. This was a point of concern due to the relatively large size of the printer nozzle diameter (1.75 mm) compared to the smallest dimension of the Type 4 tensile bars (2 mm). The remainder of the tensile testing was completed using ASTM D638 Type 4 standard tensile bars. The bars were printed at ideal print settings that were found during matrix testing at 100% infill. The infill pattern was set to 0 degrees or “perpendicular” with respect to the long axis of the tensile bars. The samples were then pulled until failure using a 5000 lb. load cell (Model LCF455). The strain data was captured using a 1-inch Epsilon extensometer (Model E95691, Error: +/−0.0276 mm). Materials 2018, 11, x FOR PEER REVIEW 6 of 19

Tensile testing was first completed on PLA using the ASTM D638 Type 1 and Type 4 standard tensile bars. For PLA, Types 1 and 4 were used to examine any potential difference in mechanical performance due to the smaller geometry of the Type 4 tensile test bars, as has been done before with more conventional FFF based 3-D printing [64]. This was a point of concern due to the relatively large size of the printer nozzle diameter (1.75 mm) compared to the smallest dimension of the Type 4 tensile bars (2 mm). The remainder of the tensile testing was completed using ASTM D638 Type 4 standard tensile bars. The bars were printed at ideal print settings that were found during matrix testing at 100% infill. The infill pattern was set to 0 degrees or “perpendicular” with respect to the long axis of Materials 2018, 11, 1413 6 of 19 the tensile bars. The samples were then pulled until failure using a 5000 lb. load cell (Model LCF455). The strain data was captured using a 1-inch Epsilon extensometer (Model E95691, Error: +/−0.0276 mm). 3. Results 3. Results 3.1. Materials Size Distribution Digital3.1. Materials images Size of Distribution the input polymer materials are shown in Figures3–7 for virgin PLA, recycled PLA, ABS,Digital PET, images and PP, of the respectively. input polymer In eachmaterials Figure, are shown the inset in Figures shows 3–7 the for particle virgin PLA, size recycled distribution for thePLA, material. ABS, PET, The and virgin PP, respectively. PLA had the In each most Figure, uniform the particlesinset shows as the well particle as the size most distribution spherical for shape whichthe provided material. theThe bestvirgin feeding. PLA had The the recycledmost uniform polymers, particles however, as well as had the most much spherical larger size shape distributions. which The regrindprovided polypropylene the best feeding. and The regrind recycled PLA polymers, were however, the least had consistent. much larger Both size of thedistributions. reground The plastics consistedregrind of polypropyleneflakes, dust-like and particles, regrind PLAand large were chunks,the least all consistent. of which Both surprisingly of the reground fed nicely plastics into the augerconsisted system withoutof flakes, jammingdust-like particles, or miss-feeding. and large Itchunks, was concluded all of which that surprisingly pellets with fed areasnicely smallerinto the than auger system without jamming or miss-feeding. It was concluded that pellets with areas smaller than 22 mm2 could feed into the auger regardless of shape. 22 mm2 could feed into the auger regardless of shape.

Figure 3. Virgin PLA pellet size distribution. Figure 3. Virgin PLA pellet size distribution. Materials 2018, 11, x FOR PEER REVIEW 7 of 19

Figure 4. Reground 3-D printed PLA size distribution. Figure 4. Reground 3-D printed PLA size distribution.

Figure 5. Recycled acrylonitrile butadiene styrene (ABS)) pellet size distribution.

Materials 2018, 11, x FOR PEER REVIEW 7 of 19

Materials 2018, 11, 1413 7 of 19 Figure 4. Reground 3-D printed PLA size distribution.

Figure 5. Recycled acrylonitrile butadiene styrene (ABS)) pellet size distribution. Figure 5. Recycled acrylonitrile butadiene styrene (ABS)) pellet size distribution. Materials 2018, 11, x FOR PEER REVIEW 8 of 19

Figure 6. Recycled as well as the two most common waste plastics (polyethylene terephthalate (PET) Figure 6. Recycled as well as the two most common waste plastics (polyethylene terephthalate (PET) size distribution. size distribution.

Figure 7. Recycled polypropylene (PP) flake size distribution.

3.2. Optimal Printing Temperatures and Velocities For each polymer, a matrix was generated for the range of the operating temperatures for the two temperature zones as well as the nozzle velocities from 5 to 50 mm/s. The difference between the theoretical and the actual mass of the line is shown as a function of the nozzle speeds. The color coding on the charts represent how different each actual line mass is compared to its ideal mass. The farther away from the ideal, the redder the number becomes. For the main part of the chart, zero would be ideal, and the farther away from zero, the worse the sample was and the redder it is. For the average and standard deviation section, the more green the number is, the closer the average was to its ideal mass. The standard deviation tells us how much each mass varied with one another. The ideal temperature combination is one that has a low average difference and a low standard deviation.

Materials 2018, 11, x FOR PEER REVIEW 8 of 19

MaterialsFigure 6.2018 Recycled, 11, 1413 as well as the two most common waste plastics (polyethylene terephthalate (PET) 8 of 19 size distribution.

Figure 7. Recycled polypropylene (PP) flake size distribution. Figure 7. Recycled polypropylene (PP) flake size distribution. 3.2. Optimal Printing Temperatures and Velocities 3.2. Optimal Printing Temperatures and Velocities For each polymer, a matrix was generated for the range of the operating temperatures for the For each polymer, a matrix was generated for the range of the operating temperatures for the two two temperature zones as well as the nozzle velocities from 5 to 50 mm/s. The difference between the temperature zones as well as the nozzle velocities from 5 to 50 mm/s. The difference between the theoretical and the actual mass of the line is shown as a function of the nozzle speeds. The color theoretical and the actual mass of the line is shown as a function of the nozzle speeds. The color coding coding on the charts represent how different each actual line mass is compared to its ideal mass. The on the charts represent how different each actual line mass is compared to its ideal mass. The farther farther away from the ideal, the redder the number becomes. For the main part of the chart, zero away from the ideal, the redder the number becomes. For the main part of the chart, zero would would be ideal, and the farther away from zero, the worse the sample was and the redder it is. For be ideal, and the farther away from zero, the worse the sample was and the redder it is. For the the average and standard deviation section, the more green the number is, the closer the average was average and standard deviation section, the more green the number is, the closer the average was to its to its ideal mass. The standard deviation tells us how much each mass varied with one another. The ideal mass. The standard deviation tells us how much each mass varied with one another. The ideal ideal temperature combination is one that has a low average difference and a low standard deviation. temperature combination is one that has a low average difference and a low standard deviation.

3.2.1. Virgin PLA Figure8 shows the results for virgin PLA. The ideal temperature settings were found to be 160 and 200 ◦C for heat zones 1 and 2, respectively. In examining the general trend across the matrix, also note that higher zone 1 temperatures, with respect to zone 2, results in a higher standard deviation for a given line test. This has been corroborated by initial literature research [65]. This phenomenon is attributed to the specific density differential of the plastic across zones 1 and 2, creating a stabilizing back pressure at the extrusion die. Materials 2018, 11, x FOR PEER REVIEW 9 of 19 Materials 2018, 11, x FOR PEER REVIEW 9 of 19 3.2.1. Virgin PLA 3.2.1. Virgin PLA Figure 8 shows the results for virgin PLA. The ideal temperature settings were found to be 160 and Figure200 °C 8for shows heat zonesthe results 1 and for 2, respectively. virgin PLA. InThe examining ideal temperature the general settings trend acrosswere found the matrix, to be 160also andnote 200 that °C higher for heat zone zones 1 temperatures, 1 and 2, respectively. with respect In examining to zone 2,the results general in trenda higher across standard the matrix, deviation also notefor a that given higher line test.zone This 1 temperatures, has been corroborated with respect by to initial zone literature 2, results research in a higher [65]. standard This phenomenon deviation forMaterialsis attributed a given2018 ,line11 to, 1413 thetest. specific This has density been corroborateddifferential of by the initial plastic literature across zones research 1 and [65]. 2, creating This phenomenon a stabilizing9 of 19 isback attributed pressure to theat the specific extrusion density die. differential of the plastic across zones 1 and 2, creating a stabilizing back pressure at the extrusion die. Temperature (°C) Speed Zone 2: 160 170 180 190 200 160 170 180 190Temperature 200 160 170 (°C) 180 190 200 160 170 180 190 200 160 170 180 190 200 Speed(mm/s) ZoneZone 2: 1:160160 170 160 180 160 190 160 200 160 160 170 170 170 180 170 190 170 200 170 160 180 170 180 180 180 190 180 200 180 160 190 170 190 180 190 190 190 200 190 160 200 170 200 180 200 190 200 200 200 (mm/s)5 Zone 1: 1601.17 160 0.97 160 0.67 160 0.27 160 0.27 170 0.67 170 -0.03 170 -0.13 170 0.67 170 -0.03 180 1.57 180 0.97 180 1.07 180 0.17 180 -0.33 190 1.37 190 0.37 190 -0.03 190 -0.83 190 -0.03 200 -0.83 200 -0.03 200 -0.23 200 -0.03 200 0.27 510 1.170.67 0.97 -0.03 0.67 -0.13 0.27 0.17 0.27 0.27 0.67 0.47 -0.03 -0.23 -0.13 -0.23 0.67 0.67 -0.03 -0.13 1.57 0.67 0.97 -0.03 1.07 -0.63 0.17 -0.93 -0.33 -0.53 1.37 0.57 0.37 -0.43 -0.03 -0.73 -0.83 -0.83 -0.03 -0.73 -0.83 -0.93 -0.03 -0.63 -0.23 -0.03 -0.03 -0.63 0.27 -0.93 1015 0.670.37 -0.03 -0.03 -0.13 -0.13 0.17 -0.03 0.27 0.17 0.47 0.47 -0.23 -0.23 -0.23 -0.23 0.67 0.47 -0.13 -0.23 0.67 0.07 -0.03 -0.43 -0.63 -0.73 -0.93 -0.83 -0.53 -0.53 0.57 0.07 -0.43 -0.43 -0.73 -0.73 -0.83 -0.73 -0.73 -0.73 -0.93 -0.73 -0.63 -0.73 -0.03 -0.13 -0.63 -0.73 -0.93 -1.03 1520 0.370.57 -0.03 -0.03 -0.13 -0.23 -0.03 -0.13 0.17 -0.03 0.47 1.07 -0.23 -0.13 -0.23 -0.23 0.47 -0.33 -0.23 -0.13 0.07 0.47 -0.43 -0.03 -0.73 -0.33 -0.83 -0.63 -0.53 -0.63 0.07 0.57 -0.43 -0.03 -0.73 -0.63 -0.73 -0.73 -0.73 -0.63 -0.73 -0.73 -0.73 -0.43 -0.13 0.37 -0.73 -0.43 -1.03 -0.83 2025 0.570.97 -0.03 0.07 -0.23 -0.23 -0.13 -0.13 -0.03 0.07 1.07 1.07 -0.13 -0.03 -0.23 -0.23 -0.33 -0.33 -0.13 -0.23 0.47 0.77 -0.03 -0.03 -0.33 -0.33 -0.63 -0.43 -0.63 -0.33 0.57 0.97 -0.03 0.07 -0.63 -0.43 -0.73 -0.53 -0.63 -0.43 -0.73 -0.63 -0.43 -0.33 0.37 0.47 -0.43 -0.33 -0.83 -0.63 2530 0.970.97 0.07 0.07 -0.23 -0.03 -0.13 -0.03 0.07 0.07 1.07 1.17 -0.03 -0.03 -0.23 -0.33 -0.33 -0.43 -0.23 -0.03 0.77 1.17 -0.03 0.17 -0.33 -0.13 -0.43 -0.33 -0.33 -0.33 0.97 1.07 0.07 0.07 -0.43 -0.23 -0.53 -0.43 -0.43 -0.23 -0.63 -0.63 -0.33 -0.03 0.47 0.77 -0.33 -0.03 -0.63 -0.43 3035 0.970.97 0.07 0.17 -0.03 -0.23 -0.03 0.17 0.07 0.07 1.17 1.37 -0.03 0.47 -0.33 -0.03 -0.43 -0.23 -0.03 -0.13 1.17 1.07 0.17 0.67 -0.13 0.07 -0.33 -0.13 -0.33 -0.33 1.07 1.07 0.07 0.67 -0.23 -0.23 -0.43 -0.33 -0.23 -0.23 -0.63 -0.43 -0.03 -0.03 0.77 0.77 -0.03 -0.03 -0.43 -0.33 3540 0.971.37 0.17 0.57 -0.23 -0.13 0.17 -0.13 0.07 0.07 1.37 1.17 0.47 0.67 -0.03 -0.13 -0.23 -0.13 -0.13 0.07 1.07 1.07 0.67 1.07 0.07 -0.03 -0.13 -0.23 -0.33 -0.33 1.07 1.27 0.67 0.77 -0.23 -0.03 -0.33 -0.33 -0.23 -0.03 -0.43 -0.53 -0.03 0.27 0.77 0.67 -0.03 0.27 -0.33 -0.23 4045 1.371.27 0.57 0.57 -0.13 -0.13 -0.13 -0.13 0.07 -0.03 1.17 1.27 0.67 0.57 -0.13 -0.13 -0.13 -0.13 0.07 -0.13 1.07 1.37 1.07 0.87 -0.03 0.07 -0.23 -0.43 -0.33 -0.23 1.27 1.17 0.77 0.87 -0.03 0.17 -0.33 -0.33 -0.03 0.17 -0.53 -0.43 0.27 0.37 0.67 0.77 0.27 0.37 -0.23 -0.03 4550 1.271.37 0.57 0.87 -0.13 -0.13 -0.13 -0.43 -0.03 -0.13 1.27 1.17 0.57 0.67 -0.13 -0.13 -0.13 -0.13 -0.13 -0.33 1.37 1.17 0.87 1.27 0.07 0.47 -0.43 -0.43 -0.23 -0.43 1.17 1.27 0.87 0.97 0.17 0.27 -0.33 -0.23 0.17 0.27 -0.43 -0.53 0.37 0.47 0.77 0.77 0.37 0.47 -0.03 0.47 50 1.37 0.87 -0.13 -0.43 -0.13 1.17 0.67 -0.13 -0.13 -0.33 1.17 1.27 0.47 -0.43 -0.43 1.27 0.97 0.27 -0.23 0.27 -0.53 0.47 0.77 0.47 0.47 Average 0.97 0.32 -0.07 -0.04 0.08 0.99 0.17 -0.18 0.01 -0.13 0.94 0.45 -0.05 -0.42 -0.40 0.94 0.29 -0.26 -0.53 -0.26 -0.64 -0.11 0.42 -0.11 -0.37 AverageAbs. Value of Avg.0.97 0.97 0.32 0.32 -0.07 0.07 -0.04 0.04 0.08 0.08 0.99 0.99 0.17 0.17 -0.18 0.18 0.01 0.01 -0.13 0.13 0.94 0.94 0.45 0.45 -0.05 0.05 -0.42 0.42 -0.40 0.40 0.94 0.94 0.29 0.29 -0.26 0.26 -0.53 0.53 -0.26 0.26 -0.64 0.64 -0.11 0.11 0.42 0.42 -0.11 0.11 -0.37 0.37 Abs.Standard Value Deviation of Avg. 0.97 0.33 0.32 0.37 0.07 0.25 0.04 0.19 0.08 0.12 0.99 0.31 0.17 0.36 0.18 0.08 0.01 0.40 0.13 0.11 0.94 0.42 0.45 0.56 0.05 0.50 0.42 0.31 0.40 0.12 0.94 0.39 0.29 0.49 0.26 0.34 0.53 0.22 0.26 0.34 0.64 0.16 0.11 0.39 0.42 0.39 0.11 0.39 0.37 0.48 Standard Deviation 0.33 0.37 0.25 0.19 0.12Best 0.31 0.36 0.08 0.40 0.11 0.42 0.56 0.50 0.31 0.12 0.39 0.49 0.34 0.22 0.34 0.16 0.39 0.39 0.39 0.48 TemperatureBest TemperatureSettings Settings FigureFigure 8. 8. PLAPLA Virgin-Difference between the theoretical and actual actual mass mass of of the the line-speed line-speed Figuretemperaturetemperature 8. PLA matrix. matrix. Virgin-Difference between the theoretical and actual mass of the line-speed temperature matrix. Observing the trend in the average line weight speeds across the full temperature spectrum, line Observing the trend in the average line weight speeds across the full temperature spectrum, line weightObserving is maximized the trend at printin the speeds average of line around weight 20 speedsmm/s, asacross shown the infull Figure temperature 9. At this spectrum, print speed, line weight is maximized at print speeds of around 20 mm/s, as shown in Figure9. At this print speed, weightthe volumetric is maximized deposition at print rate speeds of the ofprinter around is 3520 mmmm/s,3/s. asIn showncomparison, in Figure a traditional 9. At this FFF print printer speed, at the volumetric deposition rate of the printer is 35 mm3/s. In comparison, a traditional FFF printer thea print volumetric speed of deposition 50 mm/s, with rate printof the settings printer of is 0.235 mm 3layer/s. In heightcomparison, with a 0.4a traditional mm nozzle, FFF will printer average at at a print speed of 50 mm/s, with print settings of 0.2 mm layer height with a 0.4 mm nozzle, will a print speed3 of 50 mm/s, with print settings of 0.2 mm layer height with a 0.4 mm nozzle, will average at 4 mm /s. This3 translates to an 8.75x increase in the speed of the 3-D printed part formation. ataverage 4 mm3 at/s. 4 This mm translates/s. This translates to an 8.75x to increase an 8.75x increasein the speed in the ofspeed the 3-D of printed the 3-D printedpart formation. part formation. 3.0 3.0 2.5 2.5 2.0 2.0 1.5 1.5 1.0 Average Mass (g) Mass Average 1.0 Average Mass (g) Mass Average 0.5 0.5 0.0 0.0 0 10 20 30 40 50 60 0 10 20Print Speed 30 (mm/s) 40 50 60 Print Speed (mm/s) Figure 9. Virgin PLA–Average mass as a function of the print speed. Figure 9. Virgin PLA–Average mass as a function of the print speed. Figure 9. Virgin PLA–Average mass as a function of the print speed. 3.2.2. Reground 3-D Printed Parts PLA 3.2.2. Reground 3-D Printed Parts PLA 3.2.2. Reground 3-D Printed Parts PLA The reground PLA was printed with the optimized settings that were found for virgin PLA.

It printed well considering it had different types, colors, sizes, and shapes of particles being fed into it. The extruder did a good job of mixing the polymer inside the auger because the prints came out a consistent dark green color. Mechanical testing was conducted and it was found that the recycled PLA had a similar tensile strength to the virgin PLA, with the average being 38 MPA compared to 39 MPA for the virgin. This compares to studies by Cruz Sanchez et al. [31,32] where they showed that Materials 2018, 11, x FOR PEER REVIEW 10 of 19

The reground PLA was printed with the optimized settings that were found for virgin PLA. It printed well considering it had different types, colors, sizes, and shapes of particles being fed into it. The extruder did a good job of mixing the polymer inside the auger because the prints came out a

Materialsconsistent2018 ,dark11, 1413 green color. Mechanical testing was conducted and it was found that the recycled10 of 19 PLA had a similar tensile strength to the virgin PLA, with the average being 38 MPA compared to 39 MPA for the virgin. This compares to studies by Cruz Sanchez et al. [31,32] where they showed that thethe tensiletensile strengthstrength ofof PLAPLA degradeddegraded byby aboutabout 2%2% afterafter thethe firstfirst cycle.cycle. ItIt waswas foundfound thatthat thethe tensiletensile strengthstrength differencedifference betweenbetween thethe recycledrecycled PLAPLA andand thethe VirginVirgin PLAPLA thatthat waswas printedprinted onon thethe GigabotGigabot XX waswas aboutabout 2.5%.2.5%. ItIt waswas discovereddiscovered thatthat therethere waswas littlelittle differencedifference printingprinting regroundreground PLAPLA versusversus printingprinting withwith virginvirgin PLA.PLA. UsingUsing thethe samesame optimumoptimum settingssettings resultedresulted inin tensiletensile strengthsstrengths thatthat wouldwould bebe expectedexpected fromfrom thethe additionaladditional melt/print melt/print cyclecycle [[31,32].31,32].

3.2.3.3.2.3. RecycledRecycled ABSABS PelletsPellets ApproximatelyApproximately 33 mmmm recycledrecycled ABSABS pelletspellets werewere verifiedverified forfor printingprinting withwith FPFFPF typetype 3-D3-D printers. ◦ TheThe resultsresults ofof thethe speedspeed matrixmatrix testingtesting areare shownshown inin FigureFigure 10 10.. TemperatureTemperature settingssettings ofof 240240 °CC andand ◦ 240240 °CC werewere identifiedidentified asas thethe optimumoptimum temperaturetemperature settingssettings forfor heatingheating zoneszones 11 andand 22 onon thethe printer,printer, ◦ andand thethe bedbed temperaturetemperature waswas setset toto 90 90 °C.C.

Temperature (°C) Speed Zone 2: 210 210 210 210 210 220 220 220 220 220 230 230 230 230 230 240 240 240 240 240 250 250 250 250 250 (mm/s) Zone 1: 210 220 230 240 250 250 240 230 220 210 210 220 230 240 250 250 240 230 220 210 210 220 230 240 250 5 -0.26 -0.23 0.05 -0.08 -0.07 -0.20 -0.16 -0.15 -0.29 -0.26 -0.36 -0.22 -0.38 -0.17 0.04 -0.03 -0.18 -0.30 -0.28 -0.33 -0.31 -0.22 -0.03 -0.16 -0.26 10 -0.25 -0.16 0.02 -0.12 -0.11 -0.23 -0.20 -0.22 -0.26 -0.25 -0.27 -0.14 -0.26 -0.26 -0.07 -0.06 -0.17 -0.39 -0.28 -0.20 -0.22 -0.22 -0.22 -0.22 -0.28 15 -0.17 -0.05 0.02 -0.16 -0.15 -0.22 -0.17 -0.23 -0.22 -0.25 -0.31 -0.16 -0.25 -0.23 -0.17 -0.04 -0.22 -0.42 -0.10 -0.18 -0.20 -0.19 -0.26 -0.18 -0.31 20 -0.11 -0.12 -0.12 -0.25 -0.16 -0.05 -0.24 -0.31 -0.22 -0.16 -0.31 -0.24 -0.26 -0.19 -0.21 -0.14 -0.21 -0.32 -0.24 -0.16 -0.19 -0.17 -0.23 -0.22 -0.27 25 -0.20 -0.13 0.02 -0.27 -0.22 -0.08 -0.24 -0.28 -0.20 -0.19 -0.32 -0.31 -0.19 -0.18 -0.24 -0.22 -0.19 -0.29 -0.33 -0.26 -0.31 -0.24 -0.25 -0.28 -0.21 30 -0.28 -0.10 -0.02 -0.31 -0.23 -0.13 -0.14 -0.19 -0.24 -0.34 -0.39 -0.25 -0.27 -0.27 -0.23 -0.23 -0.19 -0.24 -0.37 -0.26 -0.34 -0.25 -0.20 -0.25 -0.20 35 -0.30 -0.22 -0.05 -0.21 -0.23 -0.11 -0.14 -0.26 -0.32 -0.36 -0.36 -0.23 -0.29 -0.30 -0.14 -0.25 -0.22 -0.28 -0.28 -0.30 -0.29 -0.32 -0.29 -0.26 -0.21 40 -0.23 -0.26 -0.16 -0.14 -0.19 -0.13 -0.14 -0.17 -0.32 -0.39 -0.31 -0.30 -0.30 -0.30 -0.18 -0.26 -0.29 -0.30 -0.14 -0.34 -0.28 -0.35 -0.35 -0.25 -0.22 45 -0.21 -0.23 -0.18 -0.21 -0.21 -0.20 -0.22 -0.25 -0.30 -0.31 -0.32 -0.37 -0.29 -0.25 -0.11 -0.26 -0.22 -0.29 -0.20 -0.31 -0.41 -0.16 -0.34 -0.28 -0.22 50 -0.26 -0.22 -0.19 -0.19 -0.21 -0.17 -0.16 -0.21 -0.34 -0.24 -0.36 -0.38 -0.31 -0.22 -0.15 -0.26 -0.22 -0.24 -0.24 -0.36 -0.32 -0.10 -0.28 -0.22 -0.19

Average -0.22 -0.17 -0.06 -0.19 -0.18 -0.15 -0.18 -0.22 -0.27 -0.27 -0.33 -0.26 -0.28 -0.23 -0.14 -0.17 -0.21 -0.30 -0.24 -0.27 -0.28 -0.22 -0.24 -0.23 -0.23 Abs. Value of Avg. 0.22 0.17 0.06 0.19 0.18 0.15 0.18 0.22 0.27 0.27 0.33 0.26 0.28 0.23 0.14 0.17 0.21 0.30 0.24 0.27 0.28 0.22 0.24 0.23 0.23 Standard Deviation 0.05 0.07 0.09 0.07 0.05 0.06 0.04 0.05 0.05 0.07 0.03 0.08 0.05 0.04 0.08 0.09 0.03 0.05 0.08 0.07 0.06 0.07 0.09 0.04 0.04 Best Temperature Settings FigureFigure 10. 10.The The Recycled Recycled ABS-Difference ABS-Difference between between the the Theoretical Theoretical and and the Actual the Actual Mass Mass of the of Line-Speed the Line- TemperatureSpeed Temperature Matrix. Matrix.

The optimum print speed settings were experimentally found to be 15 mm/s, as can be seen in The optimum print speed settings were experimentally found to be 15 mm/s, as can be seen in Figure 11. It should be noted, however, that the total range of the line weights shows minimal Figure 11. It should be noted, however, that the total range of the line weights shows minimal insensitivity to the print speed, as is noted by the smaller range (1.57–1.64 g) of line weights. At 15 insensitivity to the print speed, as is noted by the smaller range (1.57–1.64 g) of line weights. mm/s, the volumetric flow is 26.25 mm3/s. Comparing this speed to a normal FFF printer at 4 mm3/s, At 15 mm/s, the volumetric flow is 26.25 mm3/s. Comparing this speed to a normal FFF printer at the FPF system can print 6.5x faster in ABS. 4 mm3/s, the FPF system can print 6.5× faster in ABS.

Materials 2018, 11, 1413 11 of 19 Materials 2018, 11, x FOR PEER REVIEW 11 of 19 Materials 2018, 11, x FOR PEER REVIEW 11 of 19

1.66 1.66 1.65 1.65 1.64 1.64 1.63 1.621.63 1.62 1.61 1.61 1.6 1.6

Average Mass (g) Mass Average 1.59

Average Mass (g) Mass Average 1.581.59 1.58 1.57 1.57 0 10 20 30 40 50 60 0 10 20Print Speed 30 (mm/s) 40 50 60 Print Speed (mm/s)

Figure 11. Recycled ABS-Average mass as a function of print speed. FigureFigure 11.11. RecycledRecycled ABS-AverageABS-Average massmass asasa afunction functionof ofprint printspeed. speed. 3.2.4. Recycled PET Pellets 3.2.4.3.2.4. RecycledRecycled PETPET PelletsPellets Material testing was conducted on ~4 mm recycled PET pellets. The results of this testing can be Material testing was conducted on ~4 mm recycled PET pellets. The results of this testing can foundMaterial in Figure testing 12. It wasshould conducted be noted on that ~4 mmthe idealrecycled temperature PET pellets. settings The results of 220 of°C this and testing 230 °C can were be be found in Figure 12. It should be noted that the ideal temperature settings of 220 ◦C and 230 ◦C identifiedfound in Figure for zones 12. It1 shouldand 2. Abe bed noted temperature that the ideal of 100 temperature °C was found settings to achieveof 220 °C the and best 230 adhesion °C were were identified for zones 1 and 2. A bed temperature of 100 ◦C was found to achieve the best results.identified for zones 1 and 2. A bed temperature of 100 °C was found to achieve the best adhesion adhesionresults. results. Temperature (°C) Speed Zone 2: 200 200 200 200 200 210 210 210 210 210Temperature 220 220 (°C) 220 220 220 230 230 230 230 230 240 240 240 240 240 (mm/s)Speed ZoneZone 1: 2: 200200 210 200 220 200 230 200 240 200 240 210 230 210 220 210 210 210 200 210 200 220 210 220 220 220 230 220 240 220 240 230 230 230 220 230 210 230 200 230 200 240 210 240 220 240 230 240 240 240 (mm/s)5 Zone 1:0.27 200 0.23 210 0.00 220 0.02 230 0.19 240 0.08 240 0.02 230 0.02 220 0.20 210 0.06 200 0.17 200 -0.14 210 -0.15 220 -0.14 230 0.04 240 -0.08 240 -0.16 230 0.11 220 0.49 210 -0.16 200 -0.22 200 -0.22 210 -0.27 220 -0.21 230 -0.08 240 105 0.080.27 0.15 0.23 0.22 0.00 0.28 0.02 0.42 0.19 0.30 0.08 0.09 0.02 0.15 0.02 0.06 0.20 -0.21 0.06 0.29 0.17 -0.06 -0.14 0.09 -0.15 0.02 -0.14 0.09 0.04 0.09 -0.08 -0.08 -0.16 0.14 0.11 0.24 0.49 -0.08 -0.16 0.04 -0.22 -0.18 -0.22 -0.27 -0.27 -0.09 -0.21 -0.04 -0.08 1510 0.060.08 0.31 0.15 0.27 0.22 0.13 0.28 0.12 0.42 0.20 0.30 -0.22 0.09 -0.08 0.15 0.02 0.06 0.22 -0.21 0.06 0.29 0.13 -0.06 0.02 0.09 0.12 0.02 0.15 0.09 0.21 0.09 0.00 -0.08 0.12 0.14 0.13 0.24 -0.02 -0.08 0.06 0.04 0.04 -0.18 0.06 -0.27 0.13 -0.09 -0.12 -0.04 2015 -0.020.06 -0.04 0.31 0.12 0.27 0.04 0.13 0.11 0.12 0.21 0.20 0.21 -0.22 0.14 -0.08 0.04 0.02 0.04 0.22 0.02 0.06 0.16 0.13 0.16 0.02 0.08 0.12 0.22 0.15 -0.18 0.21 0.21 0.00 0.14 0.12 -0.02 0.13 0.21 -0.02 -0.08 0.06 0.06 0.04 -0.27 0.06 -0.04 0.13 0.00 -0.12 2520 -2.42-0.02 -0.13 -0.04 0.06 0.12 -0.02 0.04 0.18 0.11 0.17 0.21 -0.06 0.21 0.13 0.14 0.06 0.04 -0.19 0.04 -0.16 0.02 -0.06 0.16 0.06 0.16 0.02 0.08 -0.02 0.22 0.04 -0.18 -0.02 0.21 0.11 0.14 -0.02 -0.02 0.04 0.21 0.09 -0.08 -0.12 0.06 0.00 -0.27 0.11 -0.04 -0.04 0.00 3025 -2.42-2.42 0.17 -0.13 0.24 0.06 0.20 -0.02 0.04 0.18 0.16 0.17 0.26 -0.06 0.08 0.13 -0.12 0.06 -0.10 -0.19 -0.06 -0.16 0.04 -0.06 0.12 0.06 0.22 0.02 0.21 -0.02 0.28 0.04 0.21 -0.02 0.00 0.11 0.04 -0.02 -0.09 0.04 -0.02 0.09 0.11 -0.12 0.02 0.00 0.31 0.11 0.24 -0.04 3530 -2.42-2.42 -0.17 0.17 -2.42 0.24 0.00 0.20 0.04 0.04 -0.06 0.16 0.00 0.26 -0.21 0.08 -0.04 -0.12 -0.23 -0.10 -0.12 -0.06 -0.06 0.04 0.02 0.12 -0.08 0.22 0.15 0.21 0.15 0.28 0.04 0.21 0.08 0.00 -0.04 0.04 -0.04 -0.09 0.19 -0.02 0.06 0.11 -0.26 0.02 0.13 0.31 -0.11 0.24 4035 -2.42-2.42 -2.42 -0.17 -2.42 -2.42 0.02 0.00 0.06 0.04 -0.06 -0.06 -2.42 0.00 0.06 -0.21 -0.12 -0.04 -2.42 -0.23 -0.24 -0.12 0.09 -0.06 -0.17 0.02 -0.06 -0.08 0.04 0.15 0.17 0.15 -0.16 0.04 0.00 0.08 -0.12 -0.04 -0.24 -0.04 0.04 0.19 -0.18 0.06 -0.02 -0.26 0.12 0.13 0.06 -0.11 4540 -2.42-2.42 -2.42 -2.42 -2.42 -2.42 -0.04 0.02 0.04 0.06 0.21 -0.06 -2.42 -2.42 0.04 0.06 -0.15 -0.12 -2.42 -2.42 -0.23 -0.24 -0.12 0.09 -0.11 -0.17 0.15 -0.06 -0.20 0.04 -0.04 0.17 -0.25 -0.16 0.02 0.00 -0.06 -0.12 -2.42 -0.24 -0.08 0.04 -0.19 -0.18 0.08 -0.02 0.00 0.12 0.09 0.06 5045 -2.42-2.42 -2.42 -2.42 -2.42 -2.42 -2.42 -0.04 0.04 0.04 0.22 0.21 -2.42 -2.42 -0.04 0.04 -2.42 -0.15 -2.42 -2.42 -2.42 -0.23 -0.04 -0.12 -0.10 -0.11 0.06 0.15 0.08 -0.20 0.09 -0.04 -0.15 -0.25 0.11 0.02 0.06 -0.06 -2.42 -2.42 -0.09 -0.08 -0.26 -0.19 -0.10 0.08 0.12 0.00 0.02 0.09 50 -2.42 -2.42 -2.42 -2.42 0.04 0.22 -2.42 -0.04 -2.42 -2.42 -2.42 -0.04 -0.10 0.06 0.08 0.09 -0.15 0.11 0.06 -2.42 -0.09 -0.26 -0.10 0.12 0.02 Average -1.41 -0.67 -0.88 -0.18 0.12 0.14 -0.69 0.03 -0.25 -0.77 -0.27 -0.01 0.00 0.04 0.07 0.07 -0.04 0.08 0.07 -0.52 -0.01 -0.09 -0.10 0.06 0.00 Abs.Average Value of Avg. 1.41-1.41 0.67 -0.67 0.88 -0.88 0.18 -0.18 0.12 0.12 0.14 0.14 0.69 -0.69 0.03 0.03 0.25 -0.25 0.77 -0.77 0.27 -0.27 0.01 -0.01 0.00 0.00 0.04 0.04 0.07 0.07 0.07 0.07 0.04 -0.04 0.08 0.08 0.07 0.07 0.52 -0.52 0.01 -0.01 0.09 -0.09 0.10 -0.10 0.06 0.06 0.00 0.00 StandardAbs. Value Deviation of Avg. 1.23 1.41 1.15 0.67 1.26 0.88 0.75 0.18 0.11 0.12 0.11 0.14 1.13 0.69 0.10 0.03 0.73 0.25 1.09 0.77 0.73 0.27 0.10 0.01 0.11 0.00 0.10 0.04 0.11 0.07 0.13 0.07 0.14 0.04 0.05 0.08 0.17 0.07 0.95 0.52 0.11 0.01 0.13 0.09 0.14 0.10 0.13 0.06 0.10 0.00 Standard Deviation 1.23 1.15 1.26 0.75 0.11 0.11 1.13 0.10 0.73 1.09 0.73 0.10 0.11 0.10 0.11 0.13Best 0.14 0.05 0.17 0.95 0.11 0.13 0.14 0.13 0.10 TemperatureBest TemperatureSettings Settings Figure 12. Recycled PET-difference between the theoretical and the aActual mass of the line-speed FiguretemperatureFigure 12.12. RecycledRecycled matrix. PET-differencePET-difference betweenbetween thethe theoreticaltheoretical andand thethe aActualaActual massmass ofof thethe line-speedline-speed temperaturetemperature matrix.matrix. Figure 13 shows that when printing with recycled PET, speeds between 5 mm/s and 30 mm/s Figure 13 shows that when printing with recycled PET, speeds between 5 mm/s and 30 mm/s haveFigure little difference 13 shows thatin terms when of printing average with mass. recycled When PET,printing speeds past between 30 mm/s 5 mm/s however, and 30the mm/s mass have little difference in terms of average mass. When printing past 30 mm/s however, the mass havesubstantially little difference drops and, in termsin some of cases, average when mass. the temperatures When printing were past low, 30 mm/sthe extruder however, motor the would mass substantially drops and, in some cases, when the temperatures were low, the extruder motor would substantiallyskip and stall, drops causing and, the in print some to cases, fail. when the temperatures were low, the extruder motor would skip and stall, causing the print to fail. skip and stall, causing the print to fail.

Materials 2018, 11, 1413 12 of 19 Materials 2018, 11, x FOR PEER REVIEW 12 of 19 Materials 2018, 11, x FOR PEER REVIEW 12 of 19

3.00 3.00 2.50 2.50 2.00 2.00 1.50 1.50 1.00 Average Mass (g) Mass Average 1.00 Average Mass (g) Mass Average 0.50 0.50 0.00 0.00 0 10 20 30 40 50 60 0 10 20Print Speed 30 (mm/s) 40 50 60 Print Speed (mm/s)

Figure 13. PET-Print speed effect on the printed line. Figure 13. PET-Print speed effect on the printed line. Figure 13. PET-Print speed effect on the printed line. 3.2.5. Recycled PP Chips 3.2.5.3.2.5. RecycledRecycled PPPP ChipsChips Reground PP chips were used to verify PP as a valid print material. The results of the matrix RegroundReground PPPP chipschips werewere usedused toto verifyverify PPPP asas aa validvalid printprint material.material. TheThe resultsresults ofof thethe matrixmatrix testing can be found in Figure 14, where the optimum print temperatures were found to be at 230 ◦°C testingtesting cancan bebe foundfound inin FigureFigure 1414,, wherewhere thethe optimumoptimum printprint temperaturestemperatures werewere foundfound toto bebe atat 230230 °CC and 250 °C◦ for heat zones 1 and 2, respectively. andand 250250 °CC forfor heatheat zoneszones 11 andand 2,2, respectively.respectively. Temperature (°C) Speed Zone 2: 170 170 170 170 170 190 190 190 190 190Temperature 210 210 (°C) 210 210 210 230 230 230 230 230 250 250 250 250 250 (mm/s)Speed ZoneZone 1: 2: 170170 190 170 210 170 230 170 250 170 170 190 190 190 210 190 230 190 250 190 170 210 190 210 210 210 230 210 250 210 170 230 190 230 210 230 230 230 250 230 170 250 190 250 210 250 230 250 250 250 (mm/s)5 Zone 1:0.22 170 -0.21 190 -0.08 210 0.12 230 0.28 250 0.09 170 0.19 190 0.22 210 0.26 230 0.36 250 0.05 170 0.12 190 0.13 210 0.25 230 0.23 250 0.02 170 0.05 190 0.04 210 0.10 230 0.22 250 0.02 170 0.04 190 0.08 210 0.12 230 0.04 250 105 -0.150.22 -0.21 -0.21 -0.18 -0.08 -0.06 0.12 -0.02 0.28 0.12 0.09 0.18 0.19 0.20 0.22 0.26 0.26 0.29 0.36 0.11 0.05 0.13 0.12 0.15 0.13 0.25 0.25 0.23 0.23 0.09 0.02 0.11 0.05 0.12 0.04 0.17 0.10 0.20 0.22 0.12 0.02 0.06 0.04 0.09 0.08 0.14 0.12 0.15 0.04 1510 -0.43-0.15 -0.45 -0.21 -0.24 -0.18 -0.08 -0.06 0.11 -0.02 0.11 0.12 0.17 0.18 0.22 0.20 0.24 0.26 0.27 0.29 0.11 0.11 0.17 0.13 0.16 0.15 0.26 0.25 0.24 0.23 0.06 0.09 0.18 0.11 0.17 0.12 0.16 0.17 0.22 0.20 0.16 0.12 0.16 0.06 0.13 0.09 0.14 0.14 0.15 0.15 2015 -0.50-0.43 -0.66 -0.45 -0.23 -0.24 0.00 -0.08 0.12 0.11 0.08 0.11 0.16 0.17 0.20 0.22 0.22 0.24 0.26 0.27 0.12 0.11 0.14 0.17 0.14 0.16 0.26 0.26 0.21 0.24 0.09 0.06 0.16 0.18 0.17 0.17 0.17 0.16 0.23 0.22 0.14 0.16 0.13 0.16 0.13 0.13 0.16 0.14 0.17 0.15 2520 -0.63-0.50 -0.49 -0.66 -0.14 -0.23 -0.04 0.00 0.06 0.12 0.11 0.08 0.17 0.16 0.22 0.20 0.20 0.22 0.26 0.26 0.12 0.12 0.18 0.14 0.17 0.14 0.25 0.26 0.26 0.21 0.14 0.09 0.15 0.16 0.20 0.17 0.20 0.17 0.25 0.23 0.12 0.14 0.14 0.13 0.15 0.13 0.17 0.16 0.18 0.17 3025 -1.66-0.63 -1.66 -0.49 -0.05 -0.14 -0.04 -0.04 0.05 0.06 0.08 0.11 0.13 0.17 0.18 0.22 0.19 0.20 0.25 0.26 0.12 0.12 0.17 0.18 0.18 0.17 0.23 0.25 0.24 0.26 0.13 0.14 0.13 0.15 0.16 0.20 0.19 0.20 0.20 0.25 0.11 0.12 0.15 0.14 0.14 0.15 0.12 0.17 0.15 0.18 3530 -1.66-1.66 -1.66 -1.66 -0.03 -0.05 -0.06 -0.04 0.02 0.05 0.06 0.08 0.12 0.13 0.15 0.18 0.19 0.19 0.21 0.25 0.10 0.12 0.14 0.17 0.17 0.18 0.22 0.23 0.24 0.24 0.14 0.13 0.16 0.13 0.20 0.16 0.20 0.19 0.18 0.20 0.11 0.11 0.15 0.15 0.13 0.14 0.13 0.12 0.16 0.15 4035 -1.66-1.66 -1.66 -1.66 -0.08 -0.03 0.03 -0.06 -0.02 0.02 0.05 0.06 0.09 0.12 0.12 0.15 0.15 0.19 0.19 0.21 0.10 0.10 0.13 0.14 0.17 0.17 0.21 0.22 0.24 0.24 0.14 0.14 0.16 0.16 0.21 0.20 0.19 0.20 0.19 0.18 0.10 0.11 0.13 0.15 0.14 0.13 0.14 0.13 0.18 0.16 4540 -1.66-1.66 -1.66 -1.66 -0.02 -0.08 -0.03 0.03 0.00 -0.02 0.02 0.05 0.08 0.09 0.11 0.12 0.16 0.15 0.17 0.19 0.12 0.10 0.12 0.13 0.17 0.17 0.18 0.21 0.21 0.24 0.11 0.14 0.16 0.16 0.19 0.21 0.18 0.19 0.17 0.19 0.11 0.10 0.11 0.13 0.15 0.14 0.12 0.14 0.20 0.18 5045 -1.66-1.66 -1.66 -1.66 -1.66 -0.02 0.07 -0.03 0.09 0.00 0.02 0.02 0.05 0.08 0.13 0.11 0.13 0.16 0.17 0.17 0.11 0.12 0.12 0.12 0.19 0.17 0.18 0.18 0.21 0.21 0.03 0.11 0.11 0.16 0.18 0.19 0.18 0.18 0.20 0.17 0.11 0.11 0.09 0.11 0.16 0.15 0.15 0.12 0.20 0.20 50 -1.66 -1.66 -1.66 0.07 0.09 0.02 0.05 0.13 0.13 0.17 0.11 0.12 0.19 0.18 0.21 0.03 0.11 0.18 0.18 0.20 0.11 0.09 0.16 0.15 0.20 Average -0.98 -1.03 -0.27 -0.01 0.07 0.07 0.13 0.18 0.20 0.24 0.11 0.14 0.16 0.23 0.23 0.10 0.14 0.16 0.17 0.21 0.11 0.12 0.13 0.14 0.16 Abs.Average Value of Avg. 0.98-0.98 1.03 -1.03 0.27 -0.27 0.01 -0.01 0.07 0.07 0.07 0.07 0.13 0.13 0.18 0.18 0.20 0.20 0.24 0.24 0.11 0.11 0.14 0.14 0.16 0.16 0.23 0.23 0.23 0.23 0.10 0.10 0.14 0.14 0.16 0.16 0.17 0.17 0.21 0.21 0.11 0.11 0.12 0.12 0.13 0.13 0.14 0.14 0.16 0.16 StandardAbs. Value Deviation of Avg. 0.71 0.98 0.64 1.03 0.47 0.27 0.06 0.01 0.09 0.07 0.03 0.07 0.05 0.13 0.04 0.18 0.04 0.20 0.06 0.24 0.03 0.11 0.02 0.14 0.02 0.16 0.03 0.23 0.02 0.23 0.04 0.10 0.04 0.14 0.05 0.16 0.03 0.17 0.02 0.21 0.03 0.11 0.04 0.12 0.02 0.13 0.02 0.14 0.04 0.16 Standard Deviation 0.71 0.64 0.47 0.06 0.09 0.03 0.05 0.04 0.04 0.06 0.03 0.02 0.02 0.03 0.02 0.04 0.04 0.05 0.03 0.02 0.03 0.04Best 0.02 0.02 0.04 TemperatureBest TemperatureSettings Settings Figure 14. Reground PP-Difference between the theoretical and the actual mass of the line-speed temperatureFigureFigure 14. 14. Reground Reground matrix. PP-Difference between the theoretical theoretical and and the the actual actual mass mass of of the the line-speed line-speed temperaturetemperature matrix.matrix. When examining Figure 14, it should be pointed out that failures occurred when printing at too When examining Figure 14, it should be pointed out that failures occurred when printing at too low ofWhen a temperature. examining The Figure extruder 14, it motor should stalled be pointed because out of that the high failures viscosity occurred of the when plastic printing at those at low of a temperature. The extruder motor stalled because of the high viscosity of the plastic at those temperatures.too low of a temperature. To prevent damage The extruder to the motormachine, stalled the test because was halted of the highat the viscosity first sign of of the an plasticextruder at temperatures. To prevent damage to the machine, the test was halted at the first sign of an extruder stall.those temperatures. To prevent damage to the machine, the test was halted at the first sign of an stall. extruder stall. As seen in Figure 15, average line masses as a function of print speed, 30 mm/s was identified as the optimum print speed. Although, it should be pointed out that considering the overall magnitude of the mass scale, the weight is relatively insensitive to the print speed. At these settings, however, the volumetric printing speed is about 52.5 mm3/s, or has a deposition rate that is ~13 times faster than traditional FFF printing.

Materials 2018, 11, 1413 13 of 19 Materials 2018, 11, x FOR PEER REVIEW 13 of 19

1.80

1.75

1.70

1.65 Average Mass (g) Mass Average 1.60

1.55 0 10 20 30 40 50 60 Print Speed (mm/s)

Figure 15. Recycled PP-Average mass as a function of print speed. Figure 15. Recycled PP-Average mass as a function of print speed. As seen in Figure 15, average line masses as a function of print speed, 30 mm/s was identified as 3.3. Mechanical Testing the optimum print speed. Although, it should be pointed out that considering the overall magnitude of theThe mass mechanical scale, the propertiesweight is relatively for PLA thatinsensitive is printed to the with print FPF speed. are comparable At these settings, to what however, has been reportedthe volumetric in standard printing FFF speed type printers is about where 52.5 themm tensile3/s, or strengthhas a deposition was found rate to bethat 47.55–50.23 is ~13 times MPa faster [66]. Thethanaverage traditional strain FFF at printing. the break (mm/mm) was 0.02 with a standard deviation of 0.01 for Type 1 and 0.02 with a standard deviation of 0.01 for Type 4. In addition, the modulus (MPa) was 2815 for Type 1 (SD3.3. Mechanical 166.4) and 2135Testing (SD 273.5) for Type 4. Notably, peak stress for the Type 2 and Type 1 tensile bars was 43.35The mechanical (+/−5.89) andproperties 39.43 (+/ for− PLA5.22) that MPa, is whichprinted is with close FPF enough are comparable to consider theto what Type has 4 tensile been barsreported as being in standard representative FFF type ofthe printers printed where mechanical the tensile properties strength here. was Recycled found to ABS be had47.55–50.23 a peakstress MPa of[66]. 26.40 The MPa average (SD strain 1.78), recycledat the break PET (mm/mm) had a peak was stress 0.02of with 40.63 a standard MPa (SD deviation 2.41), and of recycled 0.01 for PPType had 1 aand peak 0.02 stress with of a 23.82standard MPa deviation (SD of 0.49). of 0.01 for Type 4. In addition, the modulus (MPa) was 2815 for TypePLA 1 (SD has 166.4) a bulk and tensile 2135 strength (SD 273.5) of 16for to Type 103 MPa 4. Notably, [67], whereas peak stress 3-D printed for the PLA Type is 56.62 and MPa Type [68 1], 50–57tensile MPa, bars dependingwas 43.35 (+/−5.89) on the color and [39.4369]. In (+/−5.22) this study, MPa, it was which found is close that theenough virgin to PLA consider Tensile the Strength Type 4 wastensile on bars average as being 39 MPa, representative which is comparable of the printed tothe mechanical average thatproperties was found here.with Recycled FFF/FDM ABS had style a printers.peak stress The of lower26.40 MPa than (SD average 1.78), result recycled can bePET explained had a peak by stress the large of 40.63 nozzle MPa size (SD of 2.41), the Gigabot and recycled X and thePP had fact a that peak it cannotstress of print 23.82 small MPa objects (SD of that 0.49). are less than 20 mm by 20 mm without introducing gaps in complexPLA has geometries. a bulk tensile The strength tensile bars of 16 had to 103 visible MPa holes [67], atwhereas the neck 3-D region printed where PLA the is printer56.6 MPa would [68], skip50–57 over MPa, because depending the geometry on the wascolor too [69]. tight In for this the study, slicer toit printwas it.found This that resulted the invirgin breaks PLA happening Tensile outsideStrength of was the on extensometer, average 39 MPa, near thewhich neck is regioncomparable of the to bar, the corrupting average that any was elastic found modulus with FFF/FDM data. style ABSprinters. has aThe bulk lower tensile than strength average ofresult 20 to can 73 be MPa explained [70], whereas by the large 3-D printednozzle size ABS of ranged the Gigabot from aroundX and the 25 fact to 32 that MPa it cannot [71], 28.5 print MPa small [68 ].objects In this that study, are theless recycledthan 20 mm ABS by was 20 foundmm without to have introducing an average tensilegaps in strength complex of geometries. 26 MPa. This The falls tensile into thebars range had visible of FFF/FDM holes at style the printers,neck region meaning where that the printing printer fromwould granules skip over does because not directly the geometry reduce thewas tensile too tight strength. for the slicer to print it. This resulted in breaks happeningPET has outside a bulk Tensileof the Strengthextensometer, of 47 to near 90 MPa the neck[72], whereas region of 3-D the printed bar, corrupting PET ranged any from elastic 27 to 45modulus MPa [73 data.]. In this case, recycled PET was printed and the testing concluded that the average tensile strengthABS was has 40a MPa.bulk tensile Comparing strength this of to FFF20 to printed 73 MPa PET, [70], it iswhereas actually 3-D on theprinted high end,ABS meaningranged from that thearound recycled 25 to nature32 MPa of [71], the 28.5 plastic MPa did [68]. not In have this anystudy, significant the recycled impact ABS on was the found strength to have of the an material. average tensilePP strength has a bulk of 26 tensile MPa. strengthThis falls of into 4 to the 369 range MPa of [74 FFF/FDM], whereas style 3-D printers, printed meaning PP ranged that from printing 27 to 36from MPa granules [75]. PP does was not shown directly to have reduce an averagethe tensile tensile strength. strength of 24 MPa when it was printed with the GigabotPET has X. a Thisbulk is Tensile on the Strength low end ofof both47 to the90 MPa bulk tensile[72], whereas strength 3-D and printed the FFF PET 3-D ranged printed from tensile 27 strength,to 45 MPa which [73]. couldIn this be case, explained recycled by havingPET was more printed thermal and cycles the testing from numerous concluded recycling that the processes average andtensile the strength purity of was the PP,40 MPa. which Comparing are unknown this with to thisFFF specificprinted plastic.PET, it Thisis actually underscores on the the high need end, for meaning that the recycled nature of the plastic did not have any significant impact on the strength of the material.

Materials 2018, 11, 1413 14 of 19 better material ingredient tracking [76] and expanded recycling symbols [77] as the recycling of 3-D printed parts becomes more widespread.

4. Discussion The advantages of printing with recycled particles rather than filament include lower costs, because of the lower cost of the starting material, and it is also easier to print large objects (e.g., where more than 1 spool of material is required) [78]. This has caused a recent surge in research that is related to 3-D printing with pellets including new printer designs: using a double stage screw [79], derivative of a metal injection molder [80,81], those based on RepRap technology [82], and industrial robots for 3-D printing [83], as well as a large range of polymers including conventional filament materials and recycled materials [82] as well as conductive polymer composites [84] and flexible materials [85]. These studies all indicate that FPF 3-D printing will play a larger role in the future of the 3-D printing industry and are corroborated by the results of this study. The Gigabot X successfully printed with a wide range of particle sizes and distributions, which opens up a large array of starting materials beyond high-quality (e.g., uniform size and spherical) pellets. It can be concluded that the Gigabot X and similar FPF printers with good auger-barrel tolerances (+/−0.025 mm) can handle a wide range of polymer inputs, including recycled materials with minimal post processing (i.e., only cleaning and grinding/shredding). In addition, due to the ease of extrusion and the ability to print in a wide distribution of particle sizes, this system is an ideal candidate for upcycling waste plastics and the development of unique plastics co-polymers and composites. The mechanical testing using tensile strength indicated that FPF did not degrade the polymer properties (e.g., similar to FFF), however, future work should also consider testing other mechanical properties such as compression, impact resistance, fracture toughness, creep testing, fatigue testing, and flexural strength. This will result in additional potential applications of recycled polymer FPF. The ability to define multiple heating zones in the FPF extrusion system as in the Gigabot X is useful for multiple reasons. First, by establishing a pre-heating zone prior to extrusion, this helps to both transfer the required thermal energy to the plastic (given the high material flow rate) and to achieve sufficiently low viscosities for printing. The low viscosity also reduces motor wear from over torqueing and allows for faster material throughput during purging cycles. Another benefit of having multiple heating zones is to allow for more custom temperature profiles. Some plastics experience more shear heating than others. Therefore, having a descending temperature gradient would allow the temperature at the inlet to be higher to start melting and mixing the pellets as soon as possible to allow for the best throughput, while simultaneously setting the die heater at a lower temperature to prevent degradation from overheating in the shearing (metering) section. Changing the screw geometry or the compression ratio for every plastic would be very unsustainable, so being able to have control over how the different plastics flow while they are inside the extruder without having to replace the screw, comes down to controlling speed and temperature profiles. The large format FPF type printer has unique abilities to print large components at high mass flow rates, resulting in dramatically shorter print times for large components. This is due to the ability to easily print with large nozzle sizes (1–2 mm). This is particularly useful in the manufacturing of large, functional components with non-critical surface smoothness criterion. Some applications include custom furniture, complete sporting goods equipment, large research tools, breathable casts and other health care products, agricultural processing equipment, OEM components, construction applications, and on demand fixtures or jigs. There are, however, some limitations with this prototype system, including lower than normal FFF resolution in the x-y-plane (1.75 mm diameter minimum due to the nozzle diameter) and further limitations are imposed on the component size due to the high heat transfer rates from the large (comparative to traditional FFF printers) contact area of the printer’s hot-end, meaning that parts that are less than 20 mm by 20 mm cannot be printed reliably. Useful future work would consider other Materials 2018, 11, 1413 15 of 19 mechanisms for providing FPF beyond the system that has been described here. The Gigabot X is also currently lacking in any sort of part cooling system, which is a common feature on most FFF printers to assist in the cooling of the extruded plastics. This allows the printer to print complex geometries, such as overhangs of plastic, without losing dimensional accuracy or creating visual blemishes on the surface of the print. Without this, Gigabot X is limited in the geometries that it is capable of printing accurately. Finally, the prototype provides approximately six hours of continuous feed before manual replenishment. If this technology is to be scaled commercially, users would require much longer printing sessions and an automated feeding system. Future work is needed to quantify the environmental benefits of using FPF over conventional FFF/FDM with both conventional filament as well as recycled filament. Several plastic pellet types and sizes from multiple vendors were used in this research. Significant variability in the size and the uniformity of the sourced pellets was observed between the different vendors. Further research on this hardware is needed to support supplier variability using non-uniform flake and/or pellets. Finally, a detailed cost analysis is needed to quantify the economic benefits of utilizing this approach.

5. Conclusions The Gigabot X system presents a uniquely robust open source solution to large format FPF printing. Notable with this design was the ability to print a large variety of polymers at dramatically lower print times when compared to traditional FFF type printing. As recycled plastic and pellets are less costly than filament, this system succeeds in lowering the economic barriers to the fabrication of large format, high value, plastic components, which has been an unfulfilled gap in the open source, distributed manufacturing design space. The tensile strengths of the printed parts for traditional FFF/FDM prints and the FPF prints are comparable for all of the polymers that were tested when the melt/print cycles are taken into account. This means that there will be no need to sacrifice part strength by using FPF systems. More studies need to be conducted to determine how the layer adhesion compares between an FFF and an FPF system. Finally, a novel methodology was tested to successfully operate any FPF system (and the Gigabot X in particular) to experimentally determine the best 3-D printing parameters for new polymers. The line test was developed to show the best temperatures to print the new polymer regardless of type or particle size distribution. The single walled vase test was used to determine the theoretical vs actual mass and to get a flow percentage that would calibrate the extruder. The vase test was also used to determine the actual extrusion width to be put into Slic3r to prevent overlapping lines or under-extrusion. These simple tests can be performed over the course of a couple days for any unknown polymer and, once completed, should give the correct settings for successful prints.

Author Contributions: Conceptualization, J.M.P.; Data curation, A.L.W. and D.J.B.; Formal analysis, A.L.W., D.J.B., R.B.O., M.J.F., S.L.S. and J.M.P.; Funding acquisition, S.L.S. and J.M.P.; Methodology, A.L.W., D.J.B. and J.M.P.; Project administration, S.L.S.; Writing-original draft, A.L.W., D.J.B. and J.M.P.; Writing-review & editing, A.L.W., D.J.B., R.B.O., M.J.F., S.L.S. and J.M.P. Funding: This research was funded by NSF SBIR Phase I grant number: 1746480 and the WeWork Global Creator Award. Conflicts of Interest: Robert B. Oakley, Matthew J. Fiedler and Samantha L. Snabes are employees of re:3D, which manufacturers the Gigabot X that was used in this study. All of the data was collected by the authors from MTU, who have no conflict of interest. The funders played no part in the design of the study, in the collection, analyses, or interpretation of the data, in the writing of the manuscript, or in the decision to publish the results.

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