You cannot print on air
Almost everything odd about 3D printing follows from one constraint: each LAYER has to land on something. A PRINTER squeezes melted PLASTIC through a HEATED NOZZLE and draws a flat shape on the BED, then draws the next one a fraction of a millimetre higher, and so on. Where the OBJECT overhangs — an outstretched arm, the underside of an arch — there is nothing beneath the plastic, so software adds a scaffold of thin SUPPORT that you snap off afterwards.
The same constraint explains why printed parts break the way they do. Each layer bonds to the one below while it is still hot, and that bond is never quite as strong as the plastic itself, so a print is noticeably weaker along the direction it was stacked. Anyone who has printed a hook that snapped clean along a horizontal line has met the rule the hard way: the way a part is oriented on the BED matters as much as the DESIGN.
There are two common methods, and they trade against each other. A filament machine pushes solid PLASTIC through an EXTRUDER that melts it on the way down — cheap, forgiving, and it leaves faint ridges you can feel with a thumbnail. A RESIN printer instead hardens liquid with light, one thin cross-section at a time, and reaches detail fine enough for dental work and tabletop miniatures, at the cost of messier handling and a slower BUILD.
A patent expired and the price collapsed
3D printing is older than most people assume. Chuck Hull patented the first process in 1986, hardening liquid RESIN with a laser one cross-section at a time. For the next two decades the machines were industrial equipment, guarded by patents and priced at tens of thousands of pounds.
Two things changed that. In 2005 Adrian Bowyer at the University of Bath started RepRap, a deliberately open project to build a PRINTER that could FABRICATE most of its own parts — a machine designed to be copied rather than sold. Then, around 2009, the key patents on the plastic-extrusion method ran out. Within a few years a hobbyist could buy a working printer for a few hundred pounds, and the technology stopped belonging to factories.
Emailing an object
The thing being shared is a file, not a thing, and that is the genuinely strange part. You draw a MODEL in CAD software — a digital BLUEPRINT built from a MESH of triangles — and a second program will SLICE it into hundreds of horizontal cross-sections with instructions for each. Send that anywhere in the world and an identical OBJECT can be built from a SPOOL of FILAMENT at the other end.
In December 2014 astronauts on the International Space Station needed a ratchet spanner they did not have. Rather than wait months for a resupply rocket, an engineer on the ground designed one, and the file was transmitted up and printed in orbit. That is the clearest demonstration of what ADDITIVE manufacturing changes: not just less waste than cutting a shape out of a solid block, but the ability to move the design instead of the cargo. It is the same reason a PROTOTYPE that once took three weeks now takes a night.