Many different types of 3D printing. But, they all progress layer by layer. FDM, Fused Deposition Modeling, uses a plastic extruder with a spooled plastic filament feed, like a glue gun, to lay down layer upon layer of plastic 'noodles' usually about .4mm diameter one upon the other. One layer is laid down, then another is laid down on top, melting into the previous layer slightly. On and on till the top. The insides are usually a honeycomb of plastic (defined by the printing program), however, for strength, most of our parts are solid inside.
Next method we use - SLS - Selective Laser Sintering. Same idea, but a very thin layer of powdered plastic is laid down. Then a laser traces out the shape of the layer and melts the plastic only where you want solidity. Another thin layer of powder, on and on. At the end, your parts are inside a 'snowdrift' of plastic powder which must be sifted to get them. The parts are then blown with an air gun to clean them. We use this method to make the tiny nylon gears inside planetary gear drives for the Autohelm 4000 and ST4000 wheel pilots. Very accurate and detailed parts are possible. Parts are solid - no honeycombing inside as it would just be filled with powder if you did that. If you try to print a hollow object, you have to make sure you have a vent hole to get the powder out.
Next method we use - DMP - Direct Metal Printing. Same as SLS only uses any type of powdered metal - gold, titanium, aluminum, you name it. In our case - stainless steel 316L. Laser melts it just the same. Part tolerances are in the 5 thou range - so super accurate. We use this method to make the eccentric cam assembly for the Autohelm 4000 - all in one piece! The original part is milled as a cam and a shaft and pinned together - and the pins always shear. But, with 3D printing, we just make it as one solid object - no pins - no breakage.
Many other methods, most quite similar. Can print in almost any material you can imagine - clay, food, glass, you name it.
