FDM printer anatomy: hotend, extruder, heatbed, sensors and G-code explained
An FDM printer part by part: how the hotend melts filament, direct drive versus Bowden, the XYZ axes, what each sensor does, and how to read the first lines of G-code.
- The hotend is the melting assembly: nozzle, heater block (with heater and thermistor), heatbreak, and a heatsink that stops heat climbing upward.
- Direct drive puts the motor on the print head, a short filament path (about 30–50 mm), 0.5–2 mm retraction and it prints TPU. Bowden pushes through a long tube and needs 4–8 mm retraction.
- The slicer turns a model into G-code, one command per line: M104 sets a temperature, G28 homes the axes, G1 moves while extruding.
- Thermal runaway protection cuts the heaters when the thermistor misbehaves (for example a 15 °C drop lasting over 45 s). Never disable it.
The previous lesson helped you choose a first 3D printer. This one opens the box and names every part and what it does, because these words show up in every lesson from here on: in slicer menus, in manuals, and when you ask for help in a user group. Knowing the right name for a part makes finding the answer much faster.
The plastic’s path in one line: spool → extruder pushes → hotend melts → nozzle lays it on the heatbed, while the head moves along X, Y and Z as a G-code file instructs. Part by part:
The hotend: the melting assembly
The hotend is a metal column that is hot at the bottom and cool at the top. Filament must enter solid and melt only in the last stretch. From the bottom up:
- Nozzle: the plastic’s exit hole. The most common size is 0.4 mm; it sets line width and detail. Lesson 3 is entirely about nozzle sizes and materials.
- Heater block: the aluminium block that holds the nozzle, with the heater cartridge (for example a 24 V 40 W rod) and the thermistor (temperature sensor) embedded in it.
- Heatbreak: a thin-walled tube joining the heater block to the heatsink. Its job is to stop heat travelling upward; the solid-to-molten transition happens here. It is the part that snaps when someone changes a nozzle with too much force.
- Heatsink + hotend fan: the finned top section. This fan must run the whole time the hotend is hot. If it fails, or the enclosure gets too warm, heat climbs, filament softens above the nozzle and you get heat creep: a mid-print jam.
The other fan on the head is the part cooling fan. It blows on the print so each layer solidifies quickly: strong for PLA, low or off for ABS/ASA. A different job from the heatsink fan.
The extruder: direct drive or Bowden
The extruder is a stepper motor with gears that bite the filament and push it into the hotend. There are two layouts, named by where the motor sits.
| Topic | Direct drive | Bowden |
|---|---|---|
| Motor location | On the print head, above the hotend | On the frame, pushing through a PTFE tube |
| Filament path from gears to melt zone | about 30–50 mm | about 300–800 mm |
| Retraction (pulling back to stop stringing) | 0.5–2 mm | 4–8 mm |
| Flexible filament (TPU) | Works, down to about 85A | Hard: under 20 mm/s, and softer than 95A barely feeds |
| Head mass | Heavier (adds roughly 200–400 g) | Light, higher acceleration |
| Consumables | Gears need cleaning | PTFE tube degrades; replace every 6–12 months |
Most new printers have gone back to direct drive, because motors got small enough that the weight matters less. If yours is Bowden, remember two things: cut the PTFE tube square and seat it all the way down to the nozzle (a gap there is the number one cause of clogs), and use a longer retraction than a direct-drive machine would.
The heatbed and build plate
The heatbed is a plate with a heating element underneath and its own thermistor, separate from the hotend’s. Bed temperature makes the first layer stick and keeps the part from shrinking and lifting at the corners. Prusa Knowledge Base starting points: PLA 50–60 °C, PETG 85–90 °C, ABS 80–110 °C, ASA 105–110 °C.
On top sits a removable build plate: smooth PEI, textured (powder-coated) PEI or glass. Each grips materials differently; PETG bonds to smooth PEI so hard that removing a part can damage the sheet, so Prusa recommends a textured sheet for PETG.
The X, Y, Z axes and what moves them
- X is left-right, Y front-back, Z up-down. On a bed-slinger the bed travels in Y; on a CoreXY the bed moves only in Z.
- Belts drive X and Y. A loose belt means skewed parts, ripple marks (ringing) or shifted layers.
- A lead screw drives Z straight up: slow but precise, and it must be kept clean and lubricated.
- Stepper motors move every axis in steps with no position feedback, which is why the printer must “home” before every print (G28).
Sensors: what the printer knows
| Sensor | Where | Purpose |
|---|---|---|
| Hotend thermistor | In the heater block | Reads nozzle temperature so the firmware can control the heater |
| Bed thermistor | Under the heatbed | Reads bed temperature |
| Endstops | At the end of X and Y (Z on some) | Switches that mark the zero position during G28 |
| Bed probe | Beside the head, or the nozzle itself touching | Measures nozzle-to-bed distance at many points and compensates for a tilted bed (auto bed leveling) |
| Filament runout | At the extruder inlet | Pauses when the spool runs out instead of printing air |
Bed probes come in several kinds: a touch pin (BLTouch), inductive probes that work only on metal beds, load-cell probes where the nozzle itself taps the bed, and eddy-current probes. Lesson 9 goes deep on bed leveling.
The temperature sensors are the ones tied to safety. If the thermistor slips out of the heater block, the firmware reads too low and keeps pouring power into the heater. Thermal runaway protection watches for this. Prusa documents the trigger as a drop of more than 15 °C below target for over 45 seconds, after which the printer cuts the heaters and stops with an error. Usual causes: a thermistor not fully seated, a cable broken inside, or a fan blowing straight onto the heater block.
Never disable thermal runaway protection to “make the print go through”. It is the last line against a fire. If the error appears, look for the physical cause.
G-code: the language the printer reads
A slicer (PrusaSlicer, OrcaSlicer, Bambu Studio, Cura) takes a 3D model (STL/3MF), slices it into layers and writes a plain-text G-code file, one command per line, which the printer runs line by line. A typical start of a file (the Marlin command set, which most firmware shares):
M140 S60 ; set bed to 60 °C and carry on, do not wait
M104 S210 ; set nozzle to 210 °C and carry on, do not wait
G28 ; home all axes (hit the endstops)
M190 S60 ; wait until the bed reaches 60 °C
M109 S210 ; wait until the nozzle reaches 210 °C
G90 ; absolute coordinates
G1 Z0.2 F600 ; lift to first-layer height 0.2 mm at 600 mm/min
G1 X120 Y20 E3.74 F1500 ; move to X=120 Y=20 while pushing 3.74 mm of filament, at 1500 mm/minNote the pairs M104/M109 and M140/M190: the lower number means “set and continue”, the higher one “set and wait”. The order above heats bed and nozzle while homing, saving time and oozing.
Worked example: where the E number comes from
E3.74 is the length of filament to push. Work it back from a printed line 100 mm long, 0.2 mm layer height, 0.45 mm wide (0.4 mm nozzle), with 1.75 mm filament:
- Volume of plastic to lay down = 100 × 0.2 × 0.45 = 9.0 mm³
- Cross-section of 1.75 mm filament = π × (1.75 ÷ 2)² = π × 0.875² = 2.405 mm²
- Filament length to push = 9.0 ÷ 2.405 = 3.74 mm
The slicer does this for every line. If the filament really measures 1.72 mm, its cross-section is smaller and the same E value lays down about 3% less plastic. That is where the filament-diameter and flow settings in lesson 7 come from.
Safety
- The heater block and nozzle run at 190–260 °C, the heatbed at 60–110 °C. Keep hands away while printing; let it cool before removing parts or changing a nozzle.
- Heater and thermistor wires flex with every head move and break inside their insulation. If the temperature readout jumps as the head moves, stop and check the cables.
- The heaters run on 24 V, but the power supply takes mains voltage. Use an earthed socket, and keep the printer off carpet and away from curtains.
Common mistakes
- Mixing up extruder and hotend and ordering the wrong spare.
- A Bowden PTFE tube not seated down to the nozzle, leaving a small gap where plastic collects and clogs again and again.
- Switching off the heatsink fan because it is noisy, which produces heat creep and a mid-print jam.
- Part cooling fan at 100% on ABS: split layers and lifted corners.
- Disabling thermal runaway or ignoring the error instead of finding the loose thermistor.
- Direct-drive retraction values on a Bowden machine (or the reverse), then a part covered in strings.
The next lesson zooms in on the smallest part with the biggest effect on quality: the nozzle, its size and material.
Sources
- Heater block and Thermal runaway — Prusa Knowledge Base
- PLA, PETG, ABS, ASA — Prusa Knowledge Base
- Direct Extrusion vs. Bowden Extrusion — QIDI
- G28 Auto Home, G0/G1 Linear Move, M104, M140 — Marlin G-code docs
FAQ
What is the difference between the hotend and the extruder?
The extruder is the motor and gears that grip and push the filament. The hotend is the part that melts it and pushes it out of the nozzle. People lump them together as the print head, but when you order spares you must tell them apart.
Why does a heatsink fan have to run the whole time the hotend is hot?
So the filament stays solid up to the heatbreak and only melts in the last short stretch. If heat climbs upward, the filament softens before the nozzle, the gears cannot push it, and you get the jam called heat creep.
Do I have to write G-code myself?
No, the slicer generates all of it. Reading it still helps: open the file to check that the temperatures sent to the printer match what you set, or edit the start G-code so the bed heats before the nozzle.
Circuits and code were tested on sample boards. Boards differ in pin layout, so check your board's pinout before you power anything.