FDM stands for Fused Deposition Modeling — the most common and affordable type of 3D printing, especially for home machines. (You'll also see it called FFF, or Fused Filament Fabrication; the terms are basically synonyms, with FDM being a trademark of Stratasys.) The technology was invented back in the 1980s by Scott Crump, and today it's the go-to choice for printing functional parts and prototypes.

Here's how it works: a spool of plastic filament — typically 1.75 mm in diameter — is fed into a heated extruder, melted, and pushed out through a small nozzle. The printer then moves the nozzle around (and the build platform up and down) to lay down the melted plastic in thin layers, one on top of another. Each layer cools and bonds to the previous one before the next is deposited, so the part is built up additively, layer by layer. It's essentially the opposite of CNC machining, which cuts material away.

Before printing, a 3D model is run through slicing software that converts it into G-code — the instructions that tell the printer exactly where to move and how much plastic to extrude. One thing to know: overhanging features need support material, which acts like scaffolding and is broken off after printing.

The big advantages of FDM are low cost and ease of use — filament is safe and simple to handle compared to liquid resins or powders. The main downside is that the layer lines are usually visible to the naked eye, so surfaces aren't perfectly smooth.

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