What Is FDM 3D Printing?

FDM stands for fused deposition modelling. You will also see it called FFF — fused filament fabrication. They are the same process; the difference is a trademark, not a technology. It is the process behind almost every plastic 3D printer you have seen, and it is the process we use.

How it works

A spool of plastic filament — a 1.75 mm strand, essentially a very long piece of plastic string — is pulled into a heated nozzle. The nozzle melts it and squeezes it out as a thin bead, roughly the width of the nozzle hole. The nozzle moves in the X and Y axes, drawing a flat slice of your part on a heated bed. When the slice is done, the bed drops (or the nozzle rises) a fraction of a millimetre and the next slice is drawn on top of the last, welding to it as it goes.

Do that a few hundred times and you have a part. That is genuinely all there is to it.

Three details follow from that description, and they explain nearly everything about FDM:

1. The part is made of beads, not solid plastic. Where beads meet, there can be tiny voids. That is why watertightness is a design question — covered in is PETG waterproof.
2. Layers are welds. A part is stronger along its layers than across them. See print orientation and strength.
3. Nothing can be printed in mid-air. Plastic needs something underneath it. Hence supports and overhangs.

The bits that matter

• Nozzle diameter — usually 0.4 mm. Bigger prints faster and stronger but coarser; smaller resolves finer detail and takes longer.
• Layer height — how thick each slice is, typically 0.1 to 0.3 mm. This is the main detail-versus-time dial, explained in choosing layer height.
• Walls (perimeters) — the solid outer shells. They carry most of the load.
• Infill — the internal lattice. Parts are hollow inside by design, which is why they are light and quick. See what infill should I choose.
• The heated bed — stops the first layer lifting as it cools.

What FDM is genuinely good at

• One-offs and small runs. There is no tooling to pay for, so printing one part costs roughly a proportion of printing ten. That is the opposite of moulding — see 3D printing vs injection moulding.
• Awkward geometry. Internal channels, organic curves and hollow shapes cost nothing extra. A machinist has to reach every surface with a tool; a printer does not.
• Speed of iteration. Change the model, print again the same day.
• Functional plastic parts. Brackets, housings, spacers, clips, jigs, guards, replacement bits of things. This is the bread and butter.

Where FDM is the wrong process

Being straight about this matters more than the sales pitch:

• You need real precision. FDM lives around ±0.2 mm on small features and drifts on larger ones. Bearing fits and precision threads are a machining job. Details in tolerances and fit.
• You need a mirror surface straight off the machine. Layer lines are visible. They can be sanded and painted — see painting and finishing — but that is labour.
• You need very high volumes of an identical part. Past a certain quantity moulding wins on unit cost, comfortably.
• You need metal, or transparent optics, or a pressure vessel, or a safety-critical load path. Not this process. Not any plastic printing process, in most of those cases.
• You want a tiny, ultra-fine miniature. Resin does that better. We do not offer resin, and we will tell you so rather than sell you a compromise.

Where to go next

If you have a file, upload it and we will tell you what we would do with it. If you have a broken part and no file, get an estimate and describe it — that route is normal. Pricing is driven by material, print time, size and finish, and how pricing works sets out the levers.