Parts of an FDM Printer

The path the filament takes

Follow the plastic from the spool to the part and you meet most of the printer in order: spool holder, extruder, hotend, nozzle, print, bed. Names differ between makers; the jobs are the same. The definitions below follow Prusa's glossaries, which are clear and typical, but check your own printer's manual for the names it uses.

Extruder

The extruder grips the filament and pushes it into the hotend. Prusa's glossary describes it as a drive gear, an idler that keeps the filament pressed against the gear, and the extruder motor. The idler's pressure is adjustable on many machines.

What goes wrong: filament dust builds up in the grooves of the drive gear, and the gear slips instead of pushing. Prusa's maintenance guide lists debris in the gear as a cause of reduced extrusion and says to clear it with compressed air, a brass brush or a toothpick. Symptoms are thin, gappy lines and a clicking or grinding sound; see Under-extrusion and Clicking Extruder. Whether the extruder sits on the print head or on the frame with a tube to the hotend is the direct drive versus Bowden choice: Direct Drive versus Bowden.

Hotend

The hotend is the assembly of metal parts that melts the filament. Prusa lists its parts as the nozzle, heat break, heatsink, heater block, heater cartridge and thermistor (some designs add a PTFE tube and collet clip). Prusa's glossary sums up the split: the heater block and nozzle are heated to melt the filament, while the heat break and heatsink control the melt zone.

Part	What it does	Typical trouble
Heater block	Holds the nozzle, the heater and the thermistor.	Old plastic baked onto it. Prusa cleans it hot with a small brass brush, kept away from the cables.
Heater cartridge	Heats the block and nozzle.	A heating fault is a maker or service repair, not a tinkering job.
Thermistor	Temperature sensor. Prusa notes there is one on the hotend and one on the bed.	Temperature reading faults are a safety matter; follow the maker's diagnosis steps.
Heat break	Sits between the hot block and the heatsink and, with the heatsink, controls where the filament melts.	Jams that start above the melt zone; see Heat Creep.
Heatsink and its fan	Draws heat away from the upper part of the hotend. The heatsink fan cools it.	Dust and filament threads in the fan; Prusa's routine includes cleaning it.
Nozzle	Shapes the melted plastic into a line. Prusa calls 0.4 mm the desktop standard Rule of thumb; check what yours has.	Clogs, and plastic stuck to the outside.

Nozzle sizes, materials and swaps are covered in Hotends and Nozzles and Hardened Nozzles and Abrasive Filaments. For a blocked nozzle, see Clogged Nozzle.

Cooling fans

There are usually two. The hotend or heatsink fan keeps the top of the hotend cool so the filament only melts where it should. The print fan (Prusa's MK4S glossary calls it a high-performance turbine for cooling the printed object) blows through a shroud onto the fresh plastic. How much print cooling to use is in Temperature and Cooling.

What goes wrong: dust and stray filament threads. Prusa recommends compressed air and tweezers, and warns never to blow compressed air on a running fan and never to bypass the fan speed check.

Bed and print sheet

The heatbed is the heated surface the part is built on. On many printers a removable print sheet sits on top, often a steel sheet held by magnets in the bed. A bed probe measures the bed so the firmware can compensate for small height differences; Prusa's older glossary describes its probe as an induction sensor for auto-leveling.

What goes wrong: grease and residue on the sheet stop the first layer sticking. Prusa says all sheets should be cleaned regularly to keep adhesion, and the right cleaner depends on the sheet type: Prusa allows acetone only on its smooth sheet and warns against it on textured and satin sheets. Follow your sheet maker's cleaning guide. Sheets are compared in Build Plates and Surfaces, probing in Bed Tramming, Probing and Mesh Compensation, and adhesion problems in First Layer Won't Stick.

Motion system

Stepper motors move the print head and bed. On Prusa's MK4S, for example, the X axis is two smooth rods, a motor and a belt carrying the head left and right, and the Z axis is threaded and smooth rods moving it up and down. Bearings ride on the smooth rods; belts and pulleys turn motor rotation into straight-line travel. Other layouts are compared in Cartesian, CoreXY and Delta Printers.

What goes wrong: slack belts, debris around pulleys, a loose pulley set screw and dry rods. Prusa's guide says belts should be tight enough to sound like a low bass note when plucked, to clean and lightly lubricate the smooth rods with the maker's lubricant, and to check that the pulley set screw sits tight on the flat of the motor shaft. It also warns not to lubricate the Z lead screws on that model, so follow the guide for your exact printer before oiling anything. Loose belts and pulleys show up as shifted layers; see Layer Shifting.

Electronics and controls

The mainboard runs the firmware, drives the motors and heaters, and reads the sensors. The power supply feeds it. The screen and knob (or touchscreen) control the printer, and a USB or network port loads print files. See Marlin, Klipper and Proprietary Firmware.

What goes wrong: connectors work loose over time. Prusa's guide says to check that connectors are seated only with the printer switched off and unplugged, and to press the latch before pulling any latched connector. Anything inside the power supply or mains wiring is for qualified service.

How often to check what

Prusa's schedule for its MK4-family printers is a reasonable model Rule of thumb; your maker may differ:

When	What Prusa says to check
Before every print	Printer clean, hotend free of old plastic.
About every 200 print hours	Axes, rods, pulleys and belts.
Every few hundred hours	Fans.
About every 600 to 800 hours	Electronic connectors, with the printer unplugged.

A full routine is in Maintenance Schedule.