How 3D Printers Work

Building by adding

Almost everything humans have made for thousands of years was made by *taking away*: carving wood, cutting stone, drilling metal. A 3D printer does the opposite. It builds an object by adding material a little at a time, which is why engineers call it additive manufacturing. Nothing is wasted on the floor, and shapes that no drill could reach — a hollow ball with a smaller ball loose inside — are no harder than a cube.

Layer by layer

The printers in most schools are FDM printers: fused deposition modeling. A spool of plastic filament, about the width of a spaghetti noodle, feeds into a heated nozzle that melts it and squeezes it out as a thin thread. The nozzle traces one flat slice of the shape onto the build plate, the plate drops down by a hair, and the next slice goes on top of the first. A thousand slices later there is an object. Each slice is a layer, and the thickness you choose — the layer height, usually between a tenth and a third of a millimeter — decides both how long the print takes and how smooth the curves come out. Thin layers: smooth and slow. Thick layers: fast and stepped.

The slicer

The printer never sees your model. It sees instructions from a slicer, a program that takes the STL, cuts it into layers, and writes out the exact path the nozzle will travel on each one. The slicer also decides what is inside the part. A solid block of plastic would take forever, so the slicer fills the inside with a pattern called infill — a honeycomb or grid that is mostly air. Ten to twenty percent infill is normal; a part that must be strong gets more.

The 45-degree rule

Hot plastic has to land on something. A layer that sticks out over empty air — an overhang — droops and strings. The rule of thumb is forty-five degrees: a wall that leans out less than that holds itself up, and anything steeper needs support, scaffolding that the slicer adds automatically and you snap off afterward. Supports work, but they cost time, plastic and a rough surface where they touched. Good designers plan around them: put the flat side down, split the part in two, or lean the overhang in under 45°.

Orientation decides strength

A printed part is strong along a layer and weak between layers, because the layers are only melted together at their surfaces. A hook printed standing up can snap where the layers meet; the same hook printed lying flat holds far more, because the load runs along the layers instead of prying them apart. Choosing which face goes down on the plate — the print's orientation — is one of the most important decisions you make, and it costs nothing.

Materials, and what else is out there

PLA is the school standard: made from corn starch, low smell, prints easily, but softens in a hot car. PETG is tougher and a little harder to print. ABS is the plastic of Lego bricks and needs an enclosed, warm printer. Beyond filament, resin printers cure liquid with light for tiny, detailed parts, and industrial machines fuse nylon or metal powder with a laser to make parts for airplanes and hips. Different machines, same idea: one thin layer at a time.

A 3D printer is only as smart as the slicer in front of it and the person who chose the orientation. The machine adds plastic; you add judgment.

A student designed a wall hook with a long flat base and a curved arm sticking out. Help get it from model to a strong, clean print.

1. 1

   Put the printing workflow in order.

   💡 The slicer sits between the model and the machine.

   Tap the cards in the order they happen — first one first.

   Open it in the slicer and choose the orientationSnap off supports and check the fitSet layer height, infill and supportsPrint, layer by layerExport the model as an STL
2. 2

   Which of these features will need supports if printed as drawn? Choose every one.
3. 3

   Sort each process: additive (adding material) or subtractive (taking it away)?
4. 4

   The hook keeps snapping where the arm meets the base. Choose every change that would help.

✅ Check: How 3D Printers Work