Explained
How a fdm 3d printer works.
Building with lines of plastic
A fused deposition modelling, or FDM, printer builds objects by laying down thin lines of molten plastic, one layer on top of another. The technique was patented by S. Scott Crump, whose company Stratasys owns the FDM trademark; hobbyists usually call it fused filament fabrication, or FFF. Almost every desktop 3D printer works this way, and the open-source RepRap project, started in 2005, helped make the machines cheap enough for homes and schools.
The plastic arrives as filament, a long strand wound on a spool. The extruder grips it with a toothed gear driven by a motor and pushes it into the hotend. Inside the hotend a heater block melts the plastic and a brass nozzle with a tiny opening squeezes it out as a thin, hot bead. The bead fuses to the layer underneath and cools in seconds. A heatsink and fan keep the upper part of the hotend cool so the filament only softens right at the nozzle, and a fan aimed at the print helps each line set before the next one arrives.
Moving in three axes
To draw each layer, the printer moves the nozzle relative to the bed in X and Y, then steps up in Z for the next layer. In the design shown here the print head runs left and right along the X gantry on a toothed belt, the heated bed slides forwards and backwards for Y, and two lead screws lift the whole gantry for Z. Other printers split the axes differently, for example by moving the bed down instead, but the principle is the same.
Every axis is driven by a stepper motor, which moves in small, equal steps rather than spinning freely. Because the controller knows how far one step moves each axis, it can position the nozzle precisely just by counting steps, without measuring where it is. Limit switches or sensors tell it where zero is when it starts up.
From model to print
Before printing, a program called a slicer cuts the 3D model into hundreds of horizontal slices and works out a path for the nozzle on each one: outlines first, then a lightweight infill pattern inside. It saves those instructions as G-code, a simple list of moves and temperatures that the printer follows line by line. Press Print in play mode to watch the model build a twisted vase one layer at a time.
Parts glossary
- Frame
- Aluminium extrusions bolted into a rigid frame hold everything square. Any wobble in the frame shows up as ripples on the surface of the print.
- X gantry
- The gantry is a pair of rails that carries the print head from side to side. In this design the whole gantry rises a fraction of a millimetre after each layer.
- Hotend
- A heater block melts the filament and a brass nozzle squeezes out a thin line of plastic. A finned heatsink and a fan keep the upper part cool so the plastic only melts where it should.
- Extruder
- A motor-driven gear grips the filament and pushes it into the hotend at a precise rate. Too little and the print is weak and gappy; too much and it blobs.
- Heated bed
- The print is built on a flat, heated plate that keeps the first layers warm so they stick and don't warp. In this design the bed slides forwards and backwards for the Y axis.
- Belts and pulleys
- Toothed belts, reinforced with fibreglass or steel cords, turn the motors' rotation into precise straight-line movement without slipping.
- Stepper motors
- Stepper motors turn in small fixed steps, commonly 200 per revolution, so the controller can track each axis's position by counting steps. Fun fact: Most hobby printers use NEMA 17 steppers; the name describes the 1.7-inch faceplate size, not the motor's power.
- Lead screws
- Threaded rods turned by the Z motors lift the gantry. Each turn raises it by a small, fixed distance, giving fine control over layer height.
