Hardware: 3D printersBeginner14 min

How FDM works: filament to layer

Trace plastic through the hotend and build a layered part. Connect layer height, line width and flow.

From observation to a working decision

FDM builds a part by laying down softened thermoplastic, one path and one layer at a time. The extruder grips the solid filament; the hotend heats it; the nozzle shapes the deposited line. A slicer decides where those lines go.

Try the experiment

Reveal layers of a 4 mm coupon, then change the line dimensions and speed. Watch the flow demand change without pretending that speed alone predicts quality.

Your task

Reveal at least 10 layers and keep the requested flow at or below the example limit of 6 mm³/s. This limit belongs to this exercise, not to your printer.

Simplified teaching model. Dimensions and colours highlight the idea; they are not a printer calibration or measured prediction. Orbit changes your view, not the print orientation.

Inspect a numbered part

Filament spool

Solid feedstock before the drive gears. Keep the feed path free of tangles.

Layer height0.2 mm

Line width0.45 mm

Requested speed80 mm/s

Visible layers4

Requested flow

7.20 mm³/s

Layers for 4 mm

Keep experimenting: the target is not met yet.

A continuous material path

Spool → drive gears → heatbreak → heater block → nozzle → bead. The cold side must stay cool enough to guide solid filament. The part-cooling fan cools deposited material; the hotend fan protects the cold side. They have different jobs.

How a layer becomes a part

The first layer adheres to the plate. Later paths bond to earlier material while it is warm enough. Cooling, temperature, contact and orientation all matter. Walls form the shell, infill supports the interior, and top/bottom skins close it.

A useful approximation

For this rectangular-bead model, flow = line width × layer height × speed. A real bead has rounded edges and the slicer uses more detailed geometry. Halving layer height increases the layer count; it does not automatically halve the total print time.

Recognize it in the workshop

A direct drive extruder and print head on a CoreXY printer — A direct drive extruder: the motor and gears sit on the print head, right above the hotend.disinterpreter · CC BY-SA 2.0 · Wikimedia Commons

A nozzle laying the first layer on a blue-taped bed, with a cylindrical bed probe beside it — A real first layer going down. The metal cylinder beside the nozzle is an inductive bed probe: it measures the bed so the printer can level in software.Luke Jones · CC BY 2.0 · Wikimedia Commons

A spool of white filament with a loose strand held in a hand — Filament is a thin plastic strand wound on a spool, usually 1.75 mm wide. Keep it dry.SuperBlobMonster · CC BY-SA 3.0 · Wikimedia Commons

Terms to keep

Extruder: Gears that meter solid filament into the hotend.
Hotend: Heated assembly that softens filament before the nozzle.
Volumetric flow: Volume deposited per second, in mm³/s.

Use it on your next print

In your slicer, locate layer height, line width and maximum volumetric speed. Preview one layer and identify walls, infill and travel. Use a calibrated filament profile before trying a faster print.

Check yourself

Pick an answer for each question.

1. Which part meters solid filament?

2. Double speed at the same width and height. What happens to flow demand?

3. Why inspect the first layer?

Sources (5)

Figures such as temperatures and drying times are typical ranges. Always check the label on your own spool and your printer manufacturer's guidance.