FDM — Fused Deposition Modelling — is by far the most common form of 3D printing for functional plastic parts. If you've ever seen a 3D printer, it was almost certainly an FDM machine. This guide explains how it works, what it produces, and when it's the right choice.
How FDM works
An FDM printer melts a thermoplastic filament (plastic wire on a spool) and extrudes it through a nozzle — essentially a very precise hot glue gun. The nozzle traces a pattern across a build plate, depositing plastic in a thin line. When one layer is complete, the printer moves up by one layer height and deposits the next layer on top, bonding to the layer below.
Repeat this thousands of times and you build up a solid three-dimensional object, layer by layer, from the bottom up.
The nozzle and layers
The nozzle diameter on most printers is 0.4 mm — this determines the minimum feature width. The layer height is adjustable, typically between 0.1 mm and 0.3 mm — this determines the vertical resolution and surface finish.
- Thinner layers (0.12 mm) — smoother surface, finer detail, longer print time
- Thicker layers (0.30 mm) — faster printing, visible layer lines, same mechanical bulk
What FDM can produce
FDM is excellent for:
- Solid functional parts — brackets, housings, mounts, fixtures
- Parts with internal structure (infill patterns rather than solid material)
- Large parts up to the build volume (256 × 256 × 256 mm on our machines)
- Engineering thermoplastics including PETG, which are usable for real-world applications
What FDM can't do well
- Very fine surface detail — features smaller than about 0.4 mm (one nozzle width) don't print reliably. SLA (resin) printing handles this better.
- Extremely smooth surfaces — layer lines are always visible to some degree, especially on curved surfaces parallel to the Z axis.
- Extreme overhangs without support — anything beyond about 45° from vertical needs support structure, which leaves marks on the surface.
- Uniform strength in all directions — Z-axis strength is lower than X-Y strength. Good design can minimise this, but it's a fundamental characteristic of the process.
FDM vs other 3D printing processes
| Property | FDM | SLA (Resin) | SLS |
|---|---|---|---|
| Surface finish | Moderate | Excellent | Good |
| Functional strength | Good | Moderate (brittle) | Good |
| Part size | Large | Small | Moderate |
| Cost (low volume) | Low | Moderate | High |
| Engineering materials | Yes (PETG, ABS, etc) | Limited | Yes |
For functional parts in engineering thermoplastics at accessible prices, FDM is the right choice for most applications. It's what we do at 3D Guru, using PETG on Bambu Lab machines.
Common questions
What does FDM stand for?
Fused Deposition Modelling. A nozzle melts plastic filament and draws each layer of the part onto the one below, building it up from the bottom.
How accurate is FDM printing?
Expect around 0.3 mm on a typical functional part, better on small features and worse across a large flat span. It is a fitting tolerance, not a machining one.
Is FDM strong enough for working parts?
Yes, within limits. Parts are strong along layers and weaker between them, so a well oriented FDM part carries real load while a badly oriented one splits.
What is the difference between FDM and resin printing?
Resin gives a smoother surface and finer detail but the parts are more brittle and degrade in sunlight. FDM parts are tougher, which is what a working part needs.
How large can an FDM part be?
Our build area handles parts up to roughly 250 mm in each direction. Anything larger we split and join, which is often invisible if the seam is placed well.
Does the part have visible layer lines?
Yes, always, though they are fine and tidy at our standard setting. On a hidden part nobody notices; on a visible one you will see the texture close up.
Published 9 September 2026 · last reviewed 5 October 2026.
Ready to print?
Upload your STL and get an instant quote based on your file's actual geometry. No account needed.