How FDM 3D printing works

What does FDM actually stand for, and what happens inside the printer?

FDM stands for fused deposition modeling. A spool of solid plastic filament, typically 1.75 mm in diameter, is pulled into a hotend where a heater block brings it above its melting point. A motor pushes softened plastic through a small nozzle opening, and the printer's motion system moves that nozzle in the shape of one cross-section of the part before the build plate steps down (or the nozzle steps up) for the next layer.

Every part is therefore a stack of two-dimensional outlines fused together vertically. The printer never sees the whole part at once; it only ever executes one layer's worth of motion at a time, reading instructions from a sliced file generated before printing starts.

How is a print built up, layer by layer?

Each layer starts as molten plastic squeezed out of the nozzle onto either the build plate or the layer beneath it. While still hot, that fresh plastic partially remelts the top of the previous layer, and the two fuse together as they cool. This is why print quality depends heavily on temperature control: too cold and layers do not fuse well; too hot and the part sags or strings between features.

The strength of a finished part comes largely from how well this layer-to-layer fusion happened, not just from the raw strength of the plastic. A part printed from strong filament with poor layer bonding will still fail along its layer lines under load.

Why can a layer never be taller than the nozzle?

This is a physical limit, not a setting. The nozzle orifice defines the maximum thickness of plastic it can deposit and press down against the previous layer. If the commanded layer height exceeds the nozzle diameter, the extruded plastic cannot make contact with the surface below it, and the layer will not bond at all. That is why every slicer restricts layer height to something meaningfully smaller than nozzle diameter, never larger.

How is horizontal detail different from vertical smoothness?

These two qualities are set by two different numbers, and confusing them is one of the most common mistakes newer users make. Nozzle diameter sets the finest horizontal detail a printer can trace, since it is literally the width of the line the printer draws in each layer. Layer height sets vertical smoothness: how visible the stair-stepping is on a sloped or curved surface.

A smaller nozzle does not make layers thinner by itself, and a shorter layer height does not sharpen horizontal detail. A part with fine vertical text needs a small nozzle; a part with smooth curved surfaces needs a short layer height. Both can be true requirements on the same model.

What is volumetric flow, and why does it limit print speed?

Volumetric flow is layer height multiplied by line width multiplied by print speed, measured in cubic millimeters per second (mm3/s). It describes how much molten plastic a hotend has to melt and push out every second, and every hotend has a ceiling on that number no matter what speed is typed into the slicer.

A stock PTFE-lined hotend typically manages roughly 12 mm3/s, an all-metal hotend around 20 mm3/s, and a high-flow hotend design around 32 mm3/s. Past that ceiling, the plastic will not fully melt on the way through, and the print starts skipping steps, under-extruding, or grinding filament against the drive gear.

What materials does an FDM printer typically run, and how do they differ?

The common desktop materials are PLA, PETG, ABS, ASA and TPU, and each has its own density: PLA is about 1.24 g/cm3, PETG about 1.27 g/cm3, ABS about 1.04 g/cm3, ASA about 1.07 g/cm3, and TPU about 1.21 g/cm3. Density matters directly for cost, since filament is sold by weight: a denser material yields less usable length from the same 1 kg spool.

Materials also flow at different rates through the same hotend. PETG flows at roughly 75 percent of PLA's rate at a given temperature and pressure, and flexible TPU flows at only around 35 percent of PLA's rate, which is one reason TPU prints are set noticeably slower than PLA prints on the same machine.

What safety issues come with running an FDM printer?

PLA is the lowest-fume common filament and is reasonable to run in an open room. ABS and some other filaments release VOCs and ultrafine particles while printing, and putting the printer inside an enclosure without also adding extraction makes this worse, not better, since an enclosure traps and concentrates fumes rather than clearing them. Anyone printing ABS or ASA regularly indoors should pair an enclosure with real ventilation or a fume extractor, not one or the other.

Safety