Definition
Instead of carving a shape out of a solid block, additive manufacturing builds a 3D object by adding thin layers of material, one on top of another, starting from the bottom. Using a digital 3D design file on a computer, it stacks plastic or metal precisely, layer by layer, only where the part needs it.
A carved statue vs. a layer cake
When you carve a statue out of stone with a chisel and hammer, all the stone dust that falls away is waste. One classic way factories have made parts is exactly this: machining, which means cutting a big block of metal or plastic down to shape. Depending on the shape, this can throw away more material than ends up in the finished part.
3D printing works more like building a layer cake. It stacks material from the bottom up, one thin layer at a time. The computer slices the design into thousands of thin cross-sections. Then the printer lays down a flat, thin layer of material, hardens it, and adds the next layer right on top.
Because material is added one layer at a time from the bottom, the technical name is additive manufacturing. Since it puts material only where it is needed, instead of cutting it away, it is getting attention as an eco-friendly way to cut raw material waste.
There is also no need for a mold, which has to be carved out in advance. Just edit the design file, and you can make a new shape right away. Unlike a mass-production factory, it is a great fit for quickly making and testing prototypes.
To be more precise: how does each layer harden?
To be more precise, 3D printers build layers in several different ways, depending on the material and the energy used to harden it. The most common method works like a glue gun. It heats a strand of plastic until it melts, squeezes it out of a nozzle in a thin line, and lets it harden.
Another, more precise method shines an ultraviolet laser on liquid plastic that hardens when it is hit by light (photopolymer resin), curing one very thin layer at a time. The surface comes out very smooth and exact, so this method is often used for models used to make dental aligners and for detailed figurines.
For parts that go into airplanes and cars, a thin layer of fine metal powder is spread out, and a powerful laser melts it in an instant so it fuses together. In this way, different kinds of energy join the material together, turning not just plastic but also metal and ceramics into strong solid objects.
Strength, precision, and speed vary a lot from one method to the next. So you need to pick the best process for what the object will be used for and what material it is made of.
Hollow, net-like insides can be printed in one go
With traditional cutting tools, it was almost impossible to dig deep into the inside of a part, or to machine a complex, tangled lattice interior as a single piece. But additive manufacturing builds up from the innermost bottom layer, so a shape that is closed on the outside and hollow on the inside is easy to make.
If you make an important part for an airplane or a satellite hollow inside with a strong lattice shell, much like a bird's bone, you can keep its strength and cut a lot of weight. A lighter part means rockets and planes burn less fuel, which is why the aerospace industry has adopted this technology so actively.
In medicine, it is used to make custom artificial joints that fit each patient's own bone shape. It is even reaching into bioprinting, which stacks living cells like ink, layer by layer, to make artificial skin and blood vessel tissue. That is how it keeps widening the reach of custom manufacturing.
It can also print several complex parts already joined together in one go, which means fewer parts and better durability. It opened up a new way of making things beyond the old limits of design.
Common misconceptions
Press a button on a 3D printer, and a perfect finished product pops out, ready to use right away.
Usually you still need post-processing: removing the temporary supports that held the print up, then sanding the surface smooth or heat-treating it.
Where you meet it
Titanium connecting parts (brackets) for passenger jets are made with metal 3D printing.
Based on a patient's detailed CT scan, a custom artificial jawbone is made to fit the shape of their facial bones.
On a space station, a needed repair tool is not shipped from Earth. Only the digital design file is sent, and the tool is printed on the spot.
In one sentence
Instead of carving away a solid block, additive manufacturing adds material one layer at a time from the bottom up, making complex, lightweight, custom objects.