Greenboy3D V1 Pellet Extruder Conversion for the Original Prusa i3 MK3S

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This page describes how we converted an Original Prusa i3 MK3S to print from shredded recycled plastic and pellets with the GreenBoy3D Pellet Extruder V1. It covers the hardware conversion and the checks to make before printing. Firmware, calibration and tuning are covered in the GB3DPE firmware repository.

Only the MK3S has been tested.

• Before the first heat-up, check that the hotend reads room temperature. While heating, the temperature must rise steadily; if it does not, switch off immediately and check the wiring.
• The firmware stops the heater if the hotend reads above 305 °C or below 10 °C, which also catches a disconnected or shorted thermistor, and if the temperature does not follow the heater (thermal runaway). Prusa's model-based thermal check is turned off, because its model is fitted to the stock 40 W hotend.
• The 305 °C cutoff is not a safe working temperature. GreenBoy3D rates the extruder to 330 °C, and to about 300 °C with PLA-printed parts. The material of your printed parts sets the limit. This 300 °C claim is unverified by us, we have printed at up to about 230 °C, with our printed parts being ABS.
• The 70 W heater draws about 2.9 A at 24 V, against 1.7 A for the stock 40 W heater. Prusa supports a 60 W hotend (2.5 A) on the same board. We have not verified the board, connector or wiring ratings at 70 W.
• Switch off and unplug the printer before mechanical work. The heater block stays hot for several minutes after power is off.
• Do not leave the printer heating unattended.

• Extruder barrel
• Extruder screw (auger)
• Heater block
• Coupling
• Geared stepper motor
• 70 W 24 V heater cartridge
• Thermistor
• M6 0.4 mm brass nozzle
• 1 m conveyor tube
• Screws, nuts and washers
• Two 24 V fans (not used)

• STLs of all parts: https://osf.io/3e6rf/overview

All from the design files below. The parts shown were printed in Spectrum Filaments Smart ABS (Lion Orange). PLA and PETG can also be used, as in the original design, but check for softening or warping during longer or hotter prints.

• Hex keys, 1.5–3 mm, and a 7 mm wrench
• Wire cutters, wire strippers, and a crimp tool for Molex SL contacts
• A hacksaw or angle grinder, if the adapter screws need shortening
• A thermometer or thermocouple reading to at least 250 °C
• A scale reading to 0.01 g, for the extrusion check
• A computer with PrusaSlicer, and Pronterface or another serial terminal

• CAD: Pellet Extruder - FAST Modified, version v3 (Onshape). It contains GreenBoy3D's printed parts, unchanged, and our two:
  • Pellet-Extruder-Fan-Duct > MK3 PINDA Back Right Mount holds the PINDA on the fan duct, close to its stock position. GreenBoy3D's probe adapters place the probe outside the MK3S's travel.
  • Prusa MK3 > Motor Faceplate Shim sets the height of the motor on the adapter. It is 1.5 mm; to change it, copy the part studio and edit its height variable.
• Firmware: GB3DPE firmware v1.0, with its setup procedure and firmware reference.
• Wiring: Prusa's Einsy RAMBo wiring guide.
• Bulk hopper (optional): feedstock-hopper, our printable bulk hopper. The extruder's own hopper holds 20–30 g, and GreenBoy3D's storage hoppers are not compatible with this setup.

Steps 2, 3, 11, 13 and 16 differ from a standard GreenBoy3D build.

Take the MK3S printhead apart until only the X-carriage and the extruder back plate remain. Keep the PINDA and its cable.

Fix the Prusa-MK3S Adapter to the MK3S extruder back plate with 3 × M3 × 25 mm screws and 3 M3 nuts.

The screws must not touch the coupling or the auger. If they are too long, remove them, cut them with a hacksaw or angle grinder, file the ends, and refit them. Do not cut screws on the printer.

• 
  Figure 1a. The adapter on the MK3S back plate, nuts on the inside.
• 
  Figure 1b. The three M3 × 25 mm screws, from behind.

Place the Motor Faceplate Shim on the adapter and the motor on the shim. Fix the motor with the kit's M3 × 10 mm screws and washers, at least two and ideally four.

The shim sets the height so the barrel's mounting screws line up with the adapter in step 8.

• 
  Figure 2. The shim between the motor and the adapter.

Slide the Internal Sliding Pit into the adapter along the two lower guide rails, fully to the back.

Screw the heater block onto the barrel, then the nozzle into the barrel.

Put the auger into the barrel. Start the two M3 × 3 mm grub screws in the heater block. Insert the thermistor (small black cable) into the smaller hole and the heater cartridge (large white cable) into the larger hole, push both fully in, and tighten the grub screws (1.5 mm hex key).

Put the coupling on top of the auger, as close to the motor shaft as it goes. Leave it loose.

Slide the barrel assembly into the adapter, with the coupling onto the motor shaft. Insert the two M4 × 50 mm screws from the right side of the adapter. Move the barrel up or down until the screws line up.

Tighten the back M4 screw, then the front one. When the front screw comes through the barrel, fit the M4 nut and tighten against it (7 mm wrench, 3 mm hex key).

Push the coupling fully onto the auger and tighten its two screws (2.5 mm hex key).

• 
  Figure 3. The coupling on the auger, seen with the hopper cap off.

Fix the MK3 PINDA Back Right Mount to the pair of holes on the fan duct with M3 × 10 mm screws and nuts, and fit the PINDA in it. Set the PINDA height as on a stock MK3S.

The firmware assumes the probe is X 2.3 mm, Y 0.86 mm from the nozzle. This is the mount's design value; it has not been measured. A different mount needs a different offset (see the setup procedure).

• 
  Figure 4a. The PINDA in its mount on the fan duct.
• 
  Figure 4b. The M3 × 10 mm mounting screws, nuts underneath.
• 
  Figure 4c. The PINDA from behind.

Fix the fan duct to the two front holes of the adapter with two M3 × 10 mm screws.

Fit the 5 V blowers, not the kit's fans:

• Right side: the part-cooling fan, with two M3 × 18 mm screws.
• Left side: the hotend fan, with two M3 × 25 mm screws.

Slide the External Sliding Pit into the adapter along its two guide rails.

Slide the hopper fully down the front guide rails, then slide the cap onto it.

Tape each fan-to-duct joint with electrical tape, so the air goes through the duct.

• 
  Figure 5a. Left side: hotend fan and duct, taped.
• 
  Figure 5b. Right side: part-cooling fan and duct, taped.

Screw the conveyor tube into the thread on the adapter. Twist the tube about 90° clockwise, then push the other end onto the bulk hopper and screw it on counterclockwise, so the tube ends up straight.

Only the toolhead wiring changes; the PINDA keeps its stock connector and cable. Re-terminate these with the Molex SL parts above and plug each into the Einsy connector used by its stock counterpart (Prusa's wiring guide):

• Extruder motor
• Heater cartridge
• Thermistor
• Hotend fan (left)
• Part-cooling fan (right), to the print-fan output

The connector assignment, fan polarity and motor coil order for this conversion have not yet been written down (issue #8). Until they are, check each against Prusa's wiring guide, and check every connection for continuity and shorts before powering on. Push each crimp fully into its housing, keep wires clear of moving parts and the heater block, and give each cable strain relief.

Flash firmware v1.0, then follow the setup procedure in order. It covers erasing the EEPROM, turning off the checks a pellet head cannot pass, the thermistor check, PID tuning, extrusion direction, priming, the extrusion check by weight, and bed calibration.

Do not run Calibrate XYZ: the nozzle cannot reach its calibration points.

Check the hardware in this order before printing. Pronterface at 115200 baud sends the commands.

1. Before power: wiring checked, nothing loose, the head moves freely by hand.
2. Thermistor: M105 reports the hotend at room temperature.
3. Part-cooling fan: M106 S255 runs it, M107 stops it. The hotend fan starts when the hotend passes 50 °C.
4. Motion: X, Y and Z move the right way from the LCD.
5. PINDA: LCD Support > Sensor info shows it change state with a steel tool under it.
6. Homing: G28 completes.
7. Heater: heat to 180 °C while watching the temperature, and compare with a thermometer.
8. Extrusion: heat to 215 °C and extrude from the LCD. Melt must come out of the nozzle. The firmware does not extrude below 175 °C.

We printed PLA at about 215 °C and PETG at about 230 °C with a 0.4 mm nozzle, at 20–40 mm/s to start. Prime before each print: hold at temperature for a few minutes, then extrude until the flow is steady. Slicer settings are in the setup procedure.

Plastic left in the barrel can set around the auger. Do not force it out cold. This is the procedure we used:

1. Empty the hopper.
2. Heat to the last material's printing temperature and hold for a few minutes.
3. Switch off and unplug the printer.
4. Wearing heat-resistant gloves, remove the heater block screws and the coupling.
5. Pull the auger out of the heater block.
6. Clean the auger and the barrel with a lint-free cloth or a non-metal tool.
7. Let the parts cool, inspect the auger threads, and reassemble.

• 
  Figure 6a. Heater block screws and coupling removed.
• 
  Figure 6b. The auger out of the heater block.

• Pellet extruder and its printed parts: GreenBoy3D. No licence is stated; copyright stays with GreenBoy3D.
• Firmware: based on Prusa-Firmware by Prusa Research, GPL-3.0.
• MK3S conversion, PINDA mount, shim and this page: the FAST research group, Western University. Page started by HaddenC in 2026.
• This page: CC BY-SA 4.0.

We are not affiliated with GreenBoy3D or Prusa Research. Questions about this conversion: use the Discussion tab of this page.

Items still being verified are tracked in issue #8.