Schematic of fused filament fabrication. Credit: Paolo Cignoni, CC BY-SA 4.0, Wikimedia Commons.
How FDM Works
Fused deposition modelling (FDM), also referred to as fused filament fabrication (FFF), constructs objects by melting a thermoplastic filament and depositing it in successive horizontal layers onto a build platform. Each layer bonds to the one below through thermal fusion before the platform steps down — or the printhead steps up — by one layer height increment.
The process is controlled by G-code generated from a slicer application. The slicer converts a 3D mesh (usually in STL or 3MF format) into toolpaths, specifying extruder movement, extrusion volume, temperature, fan speed, and travel moves for each layer.
Printer Assembly and Initial Checks
Most desktop FDM printers sold as kits require partial assembly. Before the first print run, the following mechanical checks are worth completing:
- Verify all frame bolts are tightened evenly. Loose frame joints cause layer shift.
- Confirm the build plate is secured and does not flex when pressed at the corners.
- Check belt tension on X and Y axes. A correctly tensioned belt produces a low musical note when plucked.
- Inspect the extruder drive gear for debris and confirm the idler arm spring is correctly seated.
- Verify PTFE tubing continuity from the extruder to the hotend, with no gaps at the heatbreak transition.
Bed Levelling
Bed levelling determines the distance between the nozzle tip and the build surface at the first layer. An incorrect offset causes adhesion failure (too far) or nozzle dragging and clogs (too close).
Manual Levelling
Position the printhead at each corner and adjust the corner thumb screws until a standard piece of 80 gsm printer paper produces slight friction when slid beneath the nozzle. Repeat the circuit two or three times as each adjustment affects adjacent corners.
Mesh Bed Levelling
Printers equipped with a probe (BLTouch, CR Touch, or inductive probes) can map the build surface and compensate for warp in firmware. After running a mesh calibration routine, confirm the probe trigger height matches the saved Z-offset in EEPROM.
Live Z Adjustment
During the first layer of the calibration print, observe how filament bonds to the surface. A correctly offset first layer produces lines that slightly spread and overlap, forming a continuous flat surface. Lines that remain rounded and do not merge indicate the nozzle is too high.
Extruder Calibration (E-Steps)
E-steps (extruder steps per millimetre) determine how far the motor rotates to push a given length of filament. Incorrect calibration causes under-extrusion or over-extrusion regardless of other settings.
To calibrate, mark the filament 100 mm and 120 mm from the extruder entry point. Command a 100 mm extrusion via the printer menu. Measure the distance from the entry point to the remaining mark. If the printer extruded less than 100 mm, increase e-steps proportionally using:
new_e_steps = current_e_steps × (100 / actual_extruded_mm)
Apply the new value with M92 E[value] and save with M500.
Temperature Calibration
Each filament brand and colour variant has a specific melt window. Manufacturers typically list a temperature range on the spool label. The optimal print temperature within that range can be found by printing a temperature tower — a test object that steps through temperature increments every few millimetres.
Indicators of Correct Temperature
- Smooth layer surfaces with no visible gaps at seams
- Good layer-to-layer adhesion without stringing on travel moves
- No discolouration or burnt smell at the nozzle
Common Temperature Problems
| Problem | Likely Cause | Adjustment |
|---|---|---|
| Poor layer adhesion, weak parts | Temperature too low | Increase by 5–10°C increments |
| Stringing between features | Temperature too high | Decrease by 5°C; check retraction |
| Blobs and zits on surfaces | Overextrusion or pressure build-up | Tune retraction distance and speed |
| Warping at corners | Bed temperature too low, or draft | Increase bed temperature; add enclosure |
Flow Rate Calibration
After e-steps are set correctly, flow rate (also called extrusion multiplier) compensates for differences in actual filament diameter versus nominal diameter. Print a single-wall cube and measure the wall thickness with digital callipers. Adjust the flow multiplier:
new_flow = current_flow × (target_wall_width / measured_wall_width)
First-Layer Troubleshooting Reference
First-layer quality predicts overall print success more reliably than any other single variable. The table below covers the most common first-layer problems encountered with desktop FDM printers.
| Symptom | Probable Cause | Fix |
|---|---|---|
| Filament not sticking to bed | Nozzle too far, cold bed, dirty surface | Lower Z-offset; increase bed temp; clean with isopropyl alcohol |
| Nozzle scraping bed | Z-offset too low | Increase Z-offset by 0.05 mm increments |
| Uneven adhesion across bed | Warped or tilted build plate | Run mesh levelling; use brim |
| Gaps in first layer lines | Under-extrusion | Check e-steps; clean nozzle; verify filament diameter |