RepRapable Recyclebot: Open source 3-D printable extruder for converting plastic to 3-D printing filament

Abstract In order to assist researchers explore the full potential of distributed recycling of post-consumer polymer waste, this article describes a recyclebot, which is a waste plastic extruder capable of making commercial quality 3-D printing filament. The device design takes advantage of both the open source hardware methodology and the paradigm developed by the open source self-replicating rapid prototyper (RepRap) 3-D printer community. Specifically, this paper describes the design, fabrication and operation of a RepRapable Recyclebot, which refers to the Recyclebot’s ability to provide the filament needed to largely replicate the parts for the Recyclebot on any type of RepRap 3-D printer. The device costs less than $700 in mate rials and can be fabricated in about 24 h. Filament is produced at 0.4 kg/h using 0.24 kWh/kg with a diameter ±4.6%. Thus, filament can be manufactured from commercial pellets for


Hardware in context
In 2015, world-wide plastic production was 322 million tons per year and is growing 3.86%/ year [1]. Both landfilling [2] and incineration [3] of plastic create well-established health and environmental issues [4,5]. Rather than follow a linear model of materials use, a circular economy model can be used to provide sustainability by separating economic growth from resource consumption [6,7]. Thus, recycling, is now established in the circular economy as the optimum treatment of postconsumer plastics [8]. Unfortunately, there can be significant environmental impacts from the collection and transportation of relatively low-density waste plastics to collection centers and reclamation facilities for separation and reconstruction in traditional recycling [9,10]. In addition, in developing regions (and even in some developed economies) the labor for this recycling is provided by waste pickers, which collect post-consumer plastic in landfills, among other places, far below poverty-level wages [11][12][13][14]. To reduce the embodied energy of transportation needed for centralized recycling [15], while at the same time potentially improving the financial situation of waste pickers a distributed recycling paradigm has been proposed [14][15][16][17].
One method of distributed plastic recycling is to upcycle plastic waste into 3-D printing filament with a recyclebot, which is an open source waste plastic extruder [18]. Previous research on the life cycle analysis (LCA) or the recyclebot process using post-consumer plastics instead of raw materials, showed a 90% decrease in the embodied energy of the filament from the mining, processing of natural resources and synthesizing compared to traditional manufacturing [19,20]. In addition, the recyclebot provides the potential for consumers to recycle plastic in their own homes to save money by offsetting purchased filament [19][20][21]. Recyclebots are also useful for laboratory and industry prototyping research as failed prototypes can be recycled into filament for future work. Many versions of recyclebots have been developed by both companies (e.g. Filastruder) as well as individuals (e.g. Lyman) [22] including open source versions from the Plastic Bank, Precious Plastic, and Perpetual Plastic. There are also several commercial versions of the recyclebot including the Filastruder, Filafab, Noztek, Filabot, EWE, Extrusionbot, Filamaker (also has shredder) and the Strooder, Felfil (OS), which all could potentially be used for waste plastic. Additionally, there are several examples of commercialized recycled filament (e.g. Filamentive, Fila-cycle and Refil). However, most filament research as well as production is still accomplished with large-scale extruders inappropriate for distributed recycling. These systems range from $6000 to tens of thousands of dollars for manufacturing level extrusion lines that can produce a few kg/h. The small extruders on the market as well as the freely posted designs suffer from one or more of the following deficiencies: 1) not open source (thus, do not provide adequate control and customizable features needed for laboratory work [23,24], 2) do not have adequate control (e.g. single speed), which is needed for non-uniform feedstocks of waste plastic, 3) are made from components that are not robust enough to handle contamination as well as composite waste, 4) demand machining experience and access to equipment often unavailable for DIY systems, 5) have high costs, 6) have slow extrusion rates, 7) have limited temperature ranges so cannot do some thermopolymers, 8) do not have a reliable form of process observation (e.g. filament diameter monitoring).
Although some polymers have been successfully recycled as single component thermoplastics such as PLA [25][26][27][28], HDPE [18,29,30], ABS [21,30,31], elastomers [32] as well as waste wood composites [33] and carbon fiber reinforced composites [34]. This early work, however, hardly begins to scratch the surface of the potential to use distributed methods to recycle a much longer list of polymers as well as composites made up of multiple distributed waste streams [35]. There is a tremendous potential to further improve the feed stocks as well as recycling 3-D printed parts themselves [36].
To assist researchers meet this potential, a recyclebot designed for fused filament-based 3-D printer filament research is described here that takes advantage of both the open source hardware methodology [37,38] and the paradigm developed by the open source self-replicating rapid prototyper (RepRap) 3-D printer community [39][40][41]. Specifically, this paper describes the design, fabrication and operation of a RepRapable Recyclebot -an open source 3-D printable waste plastic extruder, which can provide the filament to largely replicate the machine that produces it.

Hardware description
The RepRapable Recyclebot is an open source [42] compact form of a single screw thermopolymer extrusion system found all over the world in many plastic manufacturing facilities. It is a horizontal built extruder with a 5/8 00 screw. The design includes an interchangeable hopper, heating zone, channel for cooling, a puller, and a spooler with traversing mechanism. Also included is a light sensor [43], designed by Mulier, for accurately measuring filament diameter. The design is intended to maximize 3-D printable parts that can be fabricated from filament the RepRapable Recyclebot produces. The RepRapable Recyclebot also uses open source electronics. It is controlled by a single LCD screen and knob just like commercial open source RepRap-based 3-D printers (e.g. the Lulzbot Taz). Also, just like many of the types of RepRap 3-D printers, the RepRapable Recyclebot is controlled with a Ramps 1.4 Arduino Shield. The machine can be built for less than $700 in material costs in roughly 24 h.
The desktop-size low up-front cost RepRapable Recyclebot not only can be largely fabricated from its own output on any form of RepRap-based 3-D printer, but also differs from other open source (or proprietary) 3-D printer polymer extruders on the market because of attributes including: i) all parts are easily sourced from local hardware stores or online, no specialty or machined parts are necessary, ii) adjustable hopper size and shape with emptying feature, iii) aluminum cooling path provides more even cooling than passive or forced-air only methods, which also guides the filament into the puller reducing bad extrusions and providing operators with better control over filament made with sub-optimal feed stocks, iv) adjustable traverse for different spool sizes and v) modular design allows for ease in upgrading (e.g. water bath cooling, pelletizer, or injection molding add-on). These attributes make it ideal as a research system for those investigating novel filaments. This design offers the researcher far more control than most other systems as it is modular and could be turned into many different devices or augmented more easily than others to fit specific needs of the researchers in the lab (e.g. adding higher rated heaters to extrude engineering grade plastics). Such modifications or upgrades are relatively easy on this system, whereas particularly for proprietary system making modifications can be challenging or impossible.
The RepRapable Recyclebot enables manufacturing of custom filament in the laboratory for material science studies of new materials and recyclability studies, as well as research on novel applications of fused filament based 3-D printing reduces costs of 3-D printing supplies to approximately 2.5 cents per kg if waste plastic is used allows for easy and inexpensive repair, reconfiguration and upgrading of the system provides a method of distributed recycling (upcycling) in the lab, business, home or by professional waste pickers for profit.

Design Files Summary
Design file name File type Open source license Location of the file

Bill of materials
It should be noted that a mixture of metric and Imperial units is used here. Metric units are used whenever possible and all STL files are in mm, however, for all of the standard parts purchased from U.S. hardware stores the original measurement units used are given using inch ('') units.
File names for the STL, 3-D printable parts start with the assembly they are a part of and the number of prints needed is at the end of the file name. The costs for Table 2 are derived from filament costs of $0.025/g for PLA and $0.086/g for NinjaFlex, these 3-D printing materials can be purchased from most 3-D printer supplies vendors. It should be noted this is the commercial cost of polymer filament. If recycled plastic made from the RepRapable Recyclebot is used the costs drop to $0.025/kg (or 1000 times less than commercial polymer filaments). Parts were printed with the following settings shown in Table 3. Table 2 with the appropriate material on a fused filament fabrication (FFF) based 3-D printer that can do both hard plastic and elastomers such as a RepRap or Lulzbot Taz 6 with FlexyStruder head. The printer settings that are listed are required to give the parts sufficient strength and durability for extended use on the Recyclebot. Fabricators should be sure that all parts have good layer adhesion after they are done printing. This means that there should be no malformed or missing layers visible to the human eye as summarized in [44] for determining if parts intended for mechanical loading are of acceptable quality. The vast majority of the parts for this system do not undergo significant mechanical stresses. Any parts that appear insufficiently strong should be reprinted before assembly. Table 1 and all components in Tables 4 and 5. 3. Place parts into piles associated with the particular assembly following tips for assembly 4. ''Machine" the parts that need it, which involves either drilling or angle grinding. 5. Assemble each sub-part assembly 6. Screw onto wood 7. Wire 8. Program 9. Turn on and tune 10. Make filament 11. Share your results with the academic and RepRap communities Follow these steps for a successful build of a RepRapable Recyclebot. The build instructions are broken into modules where you build each assembly separately and then mount it to the board.

Tip #1 Heat set inserts
Using a soldering iron turned to 343°C (650°F), press heat inserts into the plastic when told to do so in the instructions in the next section. It helps to use the side of the soldering iron. Do not insert the tip inside of the heat insert, it will get too hot, and the heat insert will come out when the iron is removed. Sometimes the inner hole may need to be drilled out with a small drill bit to clean out the plastic that melted into the hole. A properly placed heat insert is shown in Fig. 1 flush mounted in the 3-D printed part.

Tip #2 bearing insert
Use a vice or vice grips to press fit bearings into plastic parts as shown in Fig. 2.

Tip #3 thumb screw
Use a vice and an 8 mm socket to push a thumb wheel onto a bolt to convert it into a thumbscrew for easy disassembly (see Fig. 3).

Tip #4 layout parts
Find every part needed for each individual assembly and organize them into groups as shown in Fig. 4.

Mechanical assembly
Starting with the extrusion assembly, this is where the pellets are turned into filament. It consists of a hopper, barrel, auger and nozzle. The board will first be laid out to find where the motor brackets will be screwed down, then the rest of Table 1 Tools and their uses required for assembly. Note to makers, all of these tools are commonly found in makerspaces or garages and the more expensive tools such as the angle grinder can be eliminated with more simple hand tools such as a hacksaw and pliers. the extruder will be placed on top of those. First, the feet should be screwed to the bottom of the board, and then leveled to provide a nice flat surface that will not slide around while working.

Extrusion assembly
The extrusion assembly shown in Fig. 5 is broken into the following parts:   The feet were designed in order to maintain an elevated height of the wood plank over the surrounding base. Ninjaflex was chosen as the material due to its higher degree of friction resulting in a ''rubbery" texture allowing a safe and nonslip contact surface.   3. Insert M3X16mm bolt through board feet screw mount X4.stl 4. Screw M3 nut on bolt, tighten so nut fits into plastic part, hexagon already cut out. 5. Screw all 4 screw mounts into the wood 6. Screw on Ninjaflex feet, flip board over and adjust height so that all feet are on the table.

II. Motor brackets
Motor brackets hold the motors to the assembly.       Fig. 8) 4. Do the same for the second template 13 ¼ 00 from the end of the board (see Fig. 9) 5. Insert the (4) M5X65mm Bolts through the bottom of the wood and printed template with an M5 Washer on it.
Then Screw (4) M5 nuts (see Fig. 10). 6. Place motor Bracket on top of Bolt pattern on board. Screw down all (8) M5 nuts to secure the motor brackets (see Figs. 11 and 12) III. Barrel The barrel material was chosen to be steel for its durability and much higher melting point. The use of nichrome wire and uniform spacing is critical for optimal heating and for predicting the internal environment in the barrel as material flows through. Kapton Tape is utilized to protect the wires, prevent them from fusing together, as well as lock them into place. The nozzle size plays a significant role on the material output during filament production, a larger hole will allow more material to flow at a lower pressure. A 2 mm hole was found to be ideal for 1.75 mm diameter filament production.

Power supply
The power supply is where the mains voltage from the wall is converted to 12 V to power the control panel. It has an entrance on the left side and exit on the right side for wires. The cover should be installed before operating the Recyclebot.
The Power Supply Assembly shown in Fig. 28

Cooling
The cooling assembly consists of an angled aluminum piece with fans blowing down to cool both the filament and the aluminum. The fans are adjustable to different locations and also the speed is adjustable through the software. As hot filament material is extruded out of the nozzle, if the temperature control is correct, it has a slightly viscous rheology. As that material touches the thermally conductive aluminum trough the outside of the filament solidifies completely and becomes hardened. However, since polymers in general are poor thermal conductors the interior of the filament maintains a much lower viscosity and a higher temperature. This combination allows aluminum, as long as it is properly cooled, to be a non-stick surface for near molten filament material. Additionally, the filament is capable of being pressed by the rollers, elongated without cracking and spooled with a severe curve and tensioning force while maintaining its continuity.  The Cooling Assembly is broken into the following parts shown in Fig. 33

Control Box
The control panel houses all the electronic controls for the entire Recyclebot. It fits everything perfectly, but is heavy with wires once everything is installed. It is recommended to use hot glue on all the terminals to keep the connections sturdy over the life of the machine. Some filing on the switch holes may be required to fit in the rocker switches, they were designed to fit snugly (see Fig. 38).     The Control Box Assembly is broken into the following parts:  Fig. 41). 8. Mount the solid-state relay (under the heat controller in the Figure), and both reversing switches to the front plate. Then mount the motor controllers to the bottom plate and mount the RAMPS to the 2 mounting holes sticking out of the bottom plate. Everything is mounted with M3X10 or M3X16mm bolts (see Fig. 42) 9. Attach Top Plate with LCD Screen, to the rest of the control box with M3X10mm bolts threading into the heat inserts as placed before (see Fig. 43). 10. Attach the separate control panel assembly to the back plate already mounted on the board. Keep the sides off until the wires are organized (see Fig. 44). 11. Finally, once the wires have been organized, mount the sides on with M3X10mm bolts threading into the heat inserts as set before (see Fig. 45).

Puller
The puller is a crucial part of the RepRapable Recyclebot, it determines the diameter of the filament. The faster it is pulling, the smaller the diameter will be, the slower it is pulling, the larger. When making filament, users must manually tune this motor speed to get the correct diameter. This should only need to be done once per material, but the diameter might change due to external variances like temperature swings or the hopper misfeeding, so users may want to monitor the diameter while manufacturing the filament. This becomes more important when working with non-uniform feedstocks and composites (see Fig. 46).
The Puller Assembly is broken into the following parts:

Diameter sensor
The diameter sensor automatically reads the filament diameter at every location as it is extruded. The sensor consists of a PCB that was designed by Filip Mulier, an LED, and a printed enclosure for it. In the future, an upgrade for the RepRapable Recyclebot would be to have a feedback loop to determine how large the diameter is and then change the puller speed to maintain a uniform diameter.
The Diameter Sensor Assembly shown in Fig. 58 is broken into the following parts:

Roller guide
The roller is essential for keeping tension in the filament for spooling and to guide it straight through the diameter sensor. The filament guide is adjustable to increase or decrease tension. The filament should be run through the diameter sensor and underneath the roller guide.
The Roller Guide Assembly (Fig. 63) Fig. 64. 3. Press Roller half 2 on other side until assembly is tight as in Fig. 65.     2. Place previously built assembly onto the guide wood mount and tighten another 5/8 00 nut as shown in Fig. 68. 3. The guide is adjustable up and down by loosening and tightening this nut. 4. Place on full assembly as shown in Fig. 69.

Traverse assembly
This assembly is winds filament onto an empty spool evenly and prevents the filament from overlapping. If the filament is not spooled tightly, it will not be able to fit an entire kilogram of plastic. The system works by using a speed-controlled DC motor to turn a threaded rod that has a nut running along it. The nut is attached to the cart with a guide roller on it. A slider switch is attached to the wood and the cart moves back and forth to throw the switch. The switch will change the direction of the DC motor and therefore change the direction of the cart. The limits of this system are that it uses rubber bands to overcome the force to throw the switch. This could be upgraded in the future to use a more robust system of flipping the switch to change the motor speed. Currently, the speed needs to be manually tuned to find the correct travel distance for the speed that the system is currently extruding at. Future work could automate this process.
The Traverse Assembly shown in Fig. 70 is broken into the following parts:  Fig. 71 3. Mount 12 V DC motor onto the other bracket as shown in Fig. 72 4. Mount both brackets to the board and align with a speed square. Screw in with (8) wood screws (Fig. 73). 5. Slide on smooth rod through bottom holes. Secure with (2) m3x10 bolts (Fig. 74). 6. Start assembly of the traversing cart. Place 3/8 00 Nuts into each side of the cart, one of them will stick out about ¼ 00 .
Screw in rod to make sure smooth rotation, if it is not smooth, push in or pull out the nut and try again. The two nuts are to avoid backlash (see Figs. 75-77). 7. Place 3.5 00 Rod through the top hole of the cart (Fig. 78). 8. Follow the directions above for creating the roller guide shown above (see Fig. 79). 9. Remove rod, slide through bearing and then re-thread the rod onto the car assembly, thread the rod until it reaches the motor. 10. Install the coupler with (4) M3X10mm Bolts and Nuts (see Figs. 80 and 81). 11. Drill a 3 mm hole into the DPDT switch (Fig. 82). 12. Mount the switch in Traverse_Switch Housing_x1.stl (Fig. 83). 13. Screw on slider to the switch as shown in Fig. 84.             14. Add M3X16mm bolts to each side. These are where the rubber bands will wrap around to provide more force to throw the switch 15. Mount the switch and slider guides to the board (Fig. 85). 16. Wrap rubber bands through the cart and back to each of the M3X16mm bolts. Experiment with whichever ones work the best at throwing the switch. The final assembly is shown in Fig. 86.

Spooler
The spooler is the system that winds the filament onto the spool. Once wired correctly, the speed and direction can be varied to accompany different filament sizes and extrusion speeds. It has a built in ''clutch" system from using a smooth belt. This is to prevent the spooler from pulling the filament through the entire system and making the diameter inconsistent. The empty spool can be loaded by unscrewing one of the hubs and placing the new spool onto the threaded rod and tightening the hub back on.
The Spooler Assembly (Fig. 87) is broken into the following parts: i. Spool (Recycled) ii.     Fig. 88). 5. Screw back bearing tower to board 6. Press another bearing into the opposite bearing tower as shown in Fig. 89. 7. Align both bearing towers with the threaded rod and attach the Spooler_Cross Beam_x2.stl 8. Screw front bearing tower onto board 9. Remove threaded rod. 10. Screw in printed tower cross beam with 4 M3 X10 bolts on each side of the 2 upright bearing towers as shown in Fig. 90. 11. Melt M3 heat inserts into the large pulley   Fig. 91. 13. Press a nut into both of the spooler hubs (Fig. 92). 14. Add the spooler hub to the threaded rod shown in place and bare in Fig. 93 and assembled in Fig. 94.   15. Finally add an empty spool and secure it with the final spooler hub (Fig. 95) and mount on main assembly (Fig. 96).

Electric wiring
The wiring is setup in two separate systems. The 110 V system that runs the heating element, and the 12 V system that runs the auger motor, RAMPS/Arduino board, DC motors, motor controllers, diameter sensor and LCD.
The 12 Volt power runs from the power supply to the RAMPS board, which is then distributed to the LCD, diameter sensor and auger motor. Separate wires are run from the power supply to each of the motor controllers. The 110 V system has wires connected to the 110 V ports on the power supply. The 110 V runs directly to the heater controller and relay for the heater. The frame is grounded to the power supply ground. The ground is connected to one of the bolts holding on the floor flange of the barrel. The entire system is switched from the wall outlet with an emergency stop switch to cut power in case of malfunction.
The electrical wiring involves the following steps:  1. Remove the cover from the controller box and the RAMPS board. 2. Place DDRV 8825 motor drivers on x, y, z, and Eo output pins on the RAMPS board as shown in Fig. 97. 3. Wire the heater controller following Fig. 98. 4. Wire the temperature sensor following Fig. 99. It should be noted that for the thermistor, longer wires need to be soldered to it so that it will reach to the control box. Finally, care should be taken to make sure the thermistor leads do not cross, which results in a bad reading.  A wiring diagram for the entire system is shown in Fig. 100.

5.
Wire the remaining electronic components following Table 5 and Fig. 100.

Software
Download and open the Arduino Software (https://www.arduino.cc/). Download the software for the RepRapable Recyclebot from ref [42]. Open the program folder and make sure each of the program files open as tabs in the Arduino IDE environment. Plug in the Arduino using a USB cord to your computer. Next go to Tools > Board > Mega 2560. Make sure the proper port is selected for the Arduino. Finally hit the Upload button and make sure there are no errors. You should now see the main menu of the Recyclebot on the LCD screen. Unplug from your computer before powering up the machine using its own power.

Operation instructions
Tools needed to operate the system are shown in Table 6.

Basic overview of recycling plastic waste
After classifying and separating the plastic to ensure that roughly equivalent melting points are used simultaneously (unless making some form of composite), it is cleaned, shredded into small pieces, and dried to control moisture content. The amount of cleaning will depend on the waste stream. However, in general the plastic should be cleaned to remove all visible contaminants (e.g. using mild soap and water to remove food particles from food containers). After cleaning if necessary the feedstock is dried and then shredded. To improve the filament quality, it is also possible to sieve the pieces into uniform size batches to help maintain the consistency of the feed rate and the resultant uniformity of the extruding filament. This can be accomplished with 3-D printed sieves. Shredding can occur using an office shredder [18], as well as commercial granulator, shredder (e.g. Shredbuddy or Filamaker shredder used in project Seafood to recycle ocean waste) or with the use of an open source shredder [45]. Waste plastic shards, powder or pellets are fed into the recyclebot through the hopper, and transported to the heating pipe by the cut-off auger, which is driven by a motor. The plastic is compressed and melted in this heating pipe and can be extruded through the nozzle to form filament for fused filament fabrication-based 3-D printing. More complex processes can be followed to make composites following ref [33]. However, most thermoplastics partially degrade during the recycling processes, as the heating-extruding cycle weakens the polymer [25,26]. This leads to a fixed number of iterations (e.g. 5x) before they must be chemically processed or additional virgin polymer, or less-recycled polymers must be added to enable the mechanical strength needed for some applications. To provide the foundation of a circular economy [21], considerably more work is needed in this area and the RepRapable RecycleBot enables this research to take place for low capital costs.

Safety
For safe operation of the RepRapable Recyclebot, please be aware of the following warnings: Burn Warning -The barrel, heat sink and nozzle of the extruder is hot during operation avoid touching these areas to prevent burns. Pinch Warning -Do not stick fingers or tools into the extruder hopper while screw is turning, the screw can pull tools or fingers towards the extruder inlet. Also, avoid sticking fingers between the Puller Rollers, or any belts and pulleys on the machine. Electrical Shock Warning -The Recyclebot uses both 12 V DC and mains voltage, ensure proper hookup and grounding procedures to avoid electrical shock. Static Shock Warning -The filament extruder and winding mechanism can create a static charge, although not inherently dangerous to operators, care must be taken to avoid accidental discharge on sensitive electrical equipment. Proper Ventilation Warning -Only use the Recyclebot in a well-ventilated area and on known polymers. Using some polymers (e.g. fluorinated ethylene propylene or FEP) could result in the release of toxic gasses. Only operate at temperatures recommended for extrusion of a particular polymer to avoid decomposition or burning.
If at any point, an emergency occurs, immediately hit the emergency stop button and unplug the machine. Never leave the machine unattended.

Operation
1. Plug in the machine and release the emergency stop to turn it on. The power supply should turn on, and the LCD screen should light up (Fig. 102). 2. Make sure all the knobs are off, and the heater is currently off, then flip the ''Entry Switch below the LCD screen, this will progress you through the initial menus, these menus are not functional yet, so click the rotary encoder into skip until you get to the fan cooling section. Set this to 0% to allow for warm up. Fig. 103 shows the cooling settings. 3. Navigate to the ''Manual" menu (Fig. 104). 4. Warm up extruder by turning on the PID Heater Controller with the switch below it (Fig. 105). 5. Once the heater reaches the desired temperature, wait an additional 12-15 min to allow for entire barrel and plastic to warm up. Then turn on the auger by going to Manual > Auger > Power > On (Fig. 106). 6. Feed in polymer into the hopper as shown in Fig. 107. 7. Once the auger is on, slowly start to turn the puller (P) knobs until the puller rollers start to turn. 8. Once the plastic starts to extrude, grab the plastic with a pair of tweezers or pliers, navigate the filament through the cooling section and place into the already powered puller. The filament should be pushed automatically through the diameter sensor, then guide the filament under the first tensioner, over the traverse tensioner and wind it onto the spool (Fig. 108). 9. Turn the traverse (T) motor on by turning the knob until it begins to spin, this also needs to be finely tuned to find the best speed for even winding. 10. Adjust speeds of both the puller and winder to find the perfect spot for making the desired diameter of filament.

Validation and characterization
The RepRapable Recyclebot (Recyclebot V5) shown operating in Fig. 109 was tested with various polymers and can handle any thermopolymer up to a maximum temperature of 250°C. It should be noted that the highest temperature achievable is 287°C, which is the maximum operating temperature for the Nichrome wire. However, 250°C is the maximum tested to provide a safety factor.
The RepRapable Recyclebot was validated with PLA the most common desktop FFF 3-D printing filament. They system can create a full kilogram spool of virgin PLA measured with a digital balance (±0.01 g) in 2.5 h (±10 min). The PLA extrusion rate is 0.4 kg/h after the initial heating of 30 min. is complete so in a 24-h period running continually the system can produce more than 9 spools (9.4 kg). The diameter was monitored in real time over the entire spool with the Mulier system and verified with a digital caliper (±0.001 mm). The diameter has a variation of ±4.6%. The energy used to manufacture the filament was recorded with a digital multimeter (±0.01 kWh). The machine used a total of 0.24 kWh for the whole process of making 1 kg of filament. This is notably more efficient than past Recyclebot designs [17,18,21]. Thus, filament can be manufactured for very low costs as well as with a reduced environmental footprint [19][20][21].
The cost per kilogram of Natureworks Ingeo Biopolymer PLA pellets is $5.50. Thus, for the cost of electricity of 10 cents per kWh, filament costs can be reduced to $5.52/kg or less than 22% of current commercial filaments. It should be noted there is a wide range in the costs of commercial filaments, with many suppliers well over $25/kg. Examples of both virgin filament (made from PLA pellets at 210°C) as well as recycled filament (from waste PLA at 200°C) are shown in Fig. 110. The RepRapable Recyclebot can also use ground up PLA from old or failed 3-D prints as well as PLA waste (e.g. cleaned and ground up food containers) as shown in Fig. 111 with comparable results to that of colored virgin PLA that was also made in the extruder and PLA that was purchased. Using waste plastic and only the costs of electricity (e.g. excluding labor costs and capital  equipment costs) the RepRapable Recyclebot can produce filament at less than 2.5 cents per kg, which is 1000X less than the currently available commercial filament. This makes the remarkable reduced costs of 3-D printing scientific equipment [24,[46][47][48][49][50][51] even more clear. In addition, as the RepRapable Recyclebot is largely printed this can decrease the cost of it by about $40 for the plastic to make itself.
Overall the system has been shown to be functional to be used in laboratories for fabricating filament for 3-D printing experiments. It is meant primarily as a materials preparation tool for the rapidly expanding FFF materials science and engineering community. However, the RepRapable Recyclebot can still be improved. The current limits of the design of the Recyclebot are that it only has manual operation where motor speeds must be adjusted until the desired diameter is reached. The design also has a limiting factor of how much filament can be made in a certain amount of time due to the small barrel, single heating zone and screw size. If the barrel was increased in diameter and length, the overall length of the recyclebot would need to increase because of the extra length of aluminum that would be needed to cool off the larger amount of filament running through at one time. Finally, the recyclebot requires that the inlet particles be approximately uniform to ensure consistent material feed into the auger, which will result in uniform material extrusion through the nozzle and thus a reliable  filament diameter. This can be accomplished by making and chipping filament, but each pass through the recycling process reduces the mechanical properties (yield and elongation for example) of the polymer through a reduction in molecular weight [25,26]. Reduction in mechanical properties of polymers also has the consequence of broadening the liquidus region for the given polymer, resulting in a less defined temperature where the polymer is completely liquid or solid resulting in less predictable rheology control during the filament production process.

Future work
The RepRapable Recyclebot is an open source hardware platform intended to be improved by the community. It can be further improved with i) software enabled feedback loops and quality control, ii) real time changes in extrusion settings based on monitoring (e.g. PID control of filament diameter to change pull speed), iii) alternative cooling mechanisms such as a liquid bath to extend the range of polymers that can be used, and iv) a library of presets for known recyclable thermopolymers [35]. In addition, further work could improve the design to minimize the amount of polymer (and thus cost and print time) by optimizing the percent infill based on the mechanical loading for each component. Additional work is needed to compare vertical as used in ref [21] and a design for a RepRapable version (Fig. 112) or angled systems to the horizontal recyclebot system described here.
Far more work is needed to use recyclebots to explore all of the potential recycling possibilities from the most common to the least common and to gauge the impacts of additives on filament. Recently, for example, Zander et al. [52] demonstrated how to make fused filament feedstock from the most readily available and largest quantity recycled plastic, polyethylene terephthalate (PET), on a commercial system that would be exceptionally valuable to replicate on a low-cost recyclebot system. On the other hand, Hart et al. [53] showed how products as highly specialized as MRE pouches could be processed into 3-D printing filament. Making filament from known feed stocks and additives could help the current lack of information about material ingredients in most plastic products until consumer rights laws are modernized [54].
In addition, the optimal parameters for the RepRapable Recyclebot need to be found and reported for each waste feedstock and the interactions between the various parameters internal to the Recyclebot (e.g. extruder temperature, speed and nozzle diameter) and external to it (e.g. room temperature and humidity) need to quantified and shared. In addition, work is needed to quantify the impact of size uniformity of waste polymer feedstock to filament quality as recycled waste and particularly composites can be made up a large size distribution of particles. For example, small particles can aggregate and stick to the auger as it warms by thermal conduction during extrusion potentially limiting feedstock from being moved to the hot zone at an even rate. Methods to solve this problem are needed. Far more work is needed on the impact of additives on filament properties. For example, here a 5% mixture by weight of pellets was used to die the filament red (shown in Figs. 109-111) using a master batch colorant. Further work could optimize the colorant needed for different colors and shades and gauge the impact of any type of additives to filament quality and materials characteristics.
Lastly, an open source pelletizer addition is needed for the RepRapable Recyclebot design. By fabricating pellets rather than filament from some feed stocks (e.g. wood waste polymer composites that need two passes to make uniform highquality filament [33] can be reduced to one for pellets) both the polymer mechanical properties can be preserved through multiple cycles as well as reducing the embodied energy, costs, and time needed for each processing recycle loop. Significant progress has been made by the maker/RepRap communities [55,56], the academic community [57,58] and open source hard- ware companies like re:3D [59] in making viable pellet-based 3-D printing systems. With the addition of an open source pelletizer the RepRapable Recyclebot will be able to recycle waste into high-quality filament for applications demanding tight tolerances and control as well produce pellets for large volume recycling and direct fabrication with large-format 3-D printers. This will thus help democratize not only filament manufacturing but also aid in the expansion of viable forms of distributed recycling at the small scale.