How a 3D printer actually works: follow the filament

A spool of plastic string goes in. A solid object comes out. But what actually happens in between? Let's open up the machine room, and follow the filament.

Why should you care? Because failed prints leave clues. A clicking sound. Thin, gappy lines. A print that lets go of the bed. Each clue points to a different part of the machine.

Meet this episode's model: a mini printer you can print yourself. The filament starts up here, on the spool. Then it travels down to the toolhead, where all the action happens.

Step one: the extruder. A motor turns a small gear, often two. They grip the filament, and push it down, a fraction of a millimeter at a time.

Step two: the hotend. A heater block melts the plastic, often around 190 to 220 degrees for PLA. Just above it, a heatsink and a fan keep the filament solid until the last moment. If it softens too early, it jams.

Step three: the nozzle. The filament going in is 1.75 millimeters thick. The hole at the tip is usually just 0.4.

Pause and guess. You push one millimeter of filament in. How much line comes out? A: about one millimeter. B: about five. Or C: about 30?

It's C. About 30 millimeters. Nothing disappears. The same plastic gets squeezed into a much thinner shape, like toothpaste from a tiny tube.

Here's the math. The filament's cross-section is about 2.4 square millimeters. A typical line, 0.42 wide and 0.2 tall, is about 30 times smaller. Same volume, so it comes out about 30 times longer.

That's why the extruder looks so lazy. When the nozzle draws at 100 millimeters per second, the filament only moves about 3.

Step four: motion. On this type of printer, the toolhead slides left and right: that's X. The bed slides back and forth: that's Y. And the gantry steps up, one layer at a time: that's Z.

Other printer types split these moves differently, but they all need X, Y and Z. Each axis is driven by at least one stepper motor that turns in tiny, exact steps. Most printers don't measure where the nozzle is. They count steps.

Step five: the bed. The first layer is pressed onto it, slightly squished, so it grips. Most beds are heated, often around 60 degrees for PLA. Start too high, and the print lets go.

But who tells all these motors what to do? Your slicer. It cuts your model into thin layers. Even the tiny cube on this model's bed is 60 layers at 0.2 millimeters. More layers means more time, as we worked out in Print Math.

Then it writes every move as a line of G-code. Go here. Now draw ten millimeters, and push 0.31 millimeters of filament. That's the squeeze again, in numbers.

Your slicer settings talk to these parts too. Nozzle temperature goes to the hotend. Bed temperature goes to the bed. Layer height tells Z how far to step up. Names differ per slicer.

Now those clues make sense. Clicking? The extruder can't push the filament through. Often the nozzle is clogged, too cold, or too close to the bed. Thin, gappy lines? Not enough plastic is getting through. A print that lets go? Check the first layer and the bed.

In short: the extruder pushes, the hotend melts, and the nozzle squeezes it into a line about 30 times longer. And the slicer turns your model into the moves.

The mini printer is free to download, link in the description: 7 small parts, no supports needed. So tell me in the comments: which part of your printer has given you the most trouble? Next time: bed slinger versus CoreXY, two ways to move X and Y. Subscribe so you catch it. I'm Spoolbert, see you in the next one.