I got pretty into 3D printing in 2022. Here are some things I've learned. Be aware that the industry moves very quickly, and this information is likely to go out of data within a few years. You'll see the word "fusion" a lot, but despite my earnest prayers it never means nuclear fusion.
3D printing is incredible technology capable of shifting one's personal paradigm. When it all works out, you feel like a god, willing material and matter into configurations of your own desire. Other times, you go to sleep with a fourteen hour print queued up. Fifteen minutes later, the nascent print is uprooted from the print bed, and begins to move along with the extruder. Twenty minutes after that, the growing nest of crap rotating around the print bed reaches up to the nozzle like so much thermoplastic diarrhea, and the nozzle jams. Yet the print must go on, and the extruder continues to unspool your filament. Perhaps a Roomba will catch it, and they'll clash for the title of Most Infuriating Product.
3D printing methodologies
The printing techniques one is likely to see in personal use are almost all material extrusion (FDM) or vat polymerization (MSLA and DLP). There are numerous other methodologies (see this All3DP article), but these make up the vast majority of printers intended for the home. Sadly, all known methodologies are cursed.
- Fused Deposition Modeling printers push filament (usually thermoplastic) through a hot nozzle to melt it. It is then extruded, where it cools and solidifies. The nozzle is moved in the xy plane until a layer is completed, at which point it begins depositing the next layer. The printing proceeds from the bottom to the top of the model, with the bottom resting on the buildplate (which stays at the same height throughout, or moves down).
- Masked StereoLithogrAphy (sometimes just SLA) and Digital Light Processing printers shine light into a vat of photoactive resin (a photopolymer), using e.g. a laser or an array of LEDs. The original SLA traced each xy layer using a laser and mirrors. DLP likewise uses mirrors, but they form a mask allowing a plane to be drawn as a single unit. MSLA uses an LCD mask, eliminating the need for a mirror layer (the light is blocked wherever the mask is activated; where it gets through, it solidifies the resin). The light source is underneath the mask, which is underneath the vat. The printing proceeds from the bottom to the top of the model, with the bottom attached to the buildplate. This buildplate rises as printing continues, so the bottom of the model ends up the furthest away from the vat (and thus at the highest point of the print).
Note that in both methodologies, problems can arise when a layer occupies part of the xy plane unoccupied by the layer underneath it. In the FDM case, material extruded into such areas is likely to fall to a lower z coordinate. In the case of vat polymerization, if the area is similarly isolated in the xy plane (i.e. disconnected from all elements below it or at the same layer), the solidified material is unlikely to rise along with the rest of its layer, settling instead to the bottom of the vat. If the object cannot be redesigned, this is handled by printing supports.
Other methodologies include powder bed fusion (SLS, SLM, EBM, MJF), material jetting (including DOD), binder jetting, and direct energy deposition (LENS, EBAM). Some of these get pretty esoteric. Some of them are practically welding. LENS spot-fuses titanium alloy powder using a multi-kilowatt laser in an atmosphere of inert gas to build jet engine blades. It is not cheap even before one considers the logistics of ensuring a regular supply of bulk-tanked CGA-680 monatomic argon, and furthermore avoiding asphyxiation thereby.
Which one is right for me?
Honestly? None. No matter which type of printer you use, it's likely to be semi-regularly fucked, and anyone telling you otherwise is a goddamned liar. Accept that going in.
Your choices are primarily FDM or MSLA. Well-known FDM options include Prusa, Ultimaker, Creality, Anycubic, and about four thousand other operations, including the ultra-hip Voron (a kit-only design for which you spend a few weeks sourcing parts, then more-or-less assemble over the course of several increasingly unhappy days). MSLA has only more recently arrived on the mass market, with Elegoo, Phrozen, Formlabs, and some of the FDM folks leading the way. You can acquire an entry-level FDM or MSLA printer for a few hundred dollars (FDM printers are now sometimes available at the $100 mark), with significant improvements available near $1000. At the professional level, choices begin at several thousand dollars, and go up pretty much without bound. I don't know anything about that world.
Larger build volumes are pretty much always more expensive than smaller build volumes, and probably the single starkest distinguishing point between various models.
Bed alignment is critical for both FDM and MSLA printing. It is ideal for the printer to sit on as even a surface as possible.
You can get perfectly good FDM filaments more cheaply than MSLA resins, and you'll want a gas mask after working with resins exactly once. With that said, exotic filaments get expensive quickly.
Selecting an FDM
The gold standard consumer FDM printer is the Prusa i3 MK3S+, currently available in a kit at $800 or assembled for $1100. When properly assembled, it's said to be pretty robust. Whether your dumb ass can properly assemble it is another question entirely. Newer printers such as the Bambu X1 are bringing more advanced software solutions to bear, though their efficacy is not yet generally proven (I can state with personal experience that the motherfucking extruder still jams). The open source Voron design spares no expense in achieving the finest possible consumer FDM hardware solution...but millions of people print with cheap Creality Enders and Anycubic Cobras and are pleased with the results. Spending a few hundred dollars more can improve some quality-of-life issues, but it will not free you of the obligation of understanding how your printer works. You will have to perform some configuration and maintenance, you will have to diagnose some print failures, and you will suffer some printer downtime due to jams and upgrades.
Placing the extruded material in three dimensions requires moving along three axes. G-code is sent to the printer to specify this movement. Polar printers spin either the printbed or the extruder in a circle, using polar coordinates r, θ, and z. Cartesian printers use x, y, and z. In CoreXY, and Cartesian-XY printers the bed itself moves along the z axis. Cartesian-XZ printers ("bedslingers") are common, moving the print bed along the y axis. Delta printers use arms, but most rectilinear printers employ gantries to move the nozzle. Gantry implementations include v-slot extrusion, sliding rods, and linear rails. In a great simplification, linear rails moving a nozzle in the x and y dimensions while the print bed moves in the z dimension is pretty much optimal when done correctly, but e.g. gantry misalignment will ruin any system.
Some FDM printers come with an enclosure providing thermal isolation. This is pretty much necessary for reliably printing temperature-sensitive/odorous materials such as ABS, and can reduce noise. Printing your own enclosure is a non-trivial task (they're large), and printing one that looks and functions as well as the manufacturer's is generally difficult. Any printer worth buying can heat its bed to at least 100℃, but some provide a uniform bed temperature more reliably than others.
Assuming reasonable hotend movement, nozzle size defines your resolution. The nozzle is the most exterior part of the hotend. Hotends can be open source (e.g. the MK8 and E3D v6) or proprietary. You can assume you'll pay a heavy multiple for proprietary hotends, so open designs are desirable. The 0.4mm brass nozzle can be considered standard. Larger nozzles can print the same volume more quickly (assuming sufficient supply from the heating element), are less prone to jamming, and have fewer layers per unit height (possibly resulting in stronger prints). Smaller nozzles can achieve higher resolutions in all three dimensions. Nozzle material governs heat transfer and wear resistance, with the latter being particularly relevant for abrasive materials.
Within a hotend form factor, there are both all-metal and through-PTFE models, with the latter being more standard. PTFE (teflon) tubing provides a low-friction path for cold tubing through the extruder. Unfortunately, PTFE and its PEEK insulate doesn't like temperatures much above 240℃. If you're heating your material to the melting point of say Nylon-CF, it's going to deform your PTFE. An all-metal hotend terminates the PTFE above the heat sink, permitting higher temperatures in the hotend. Downsides include less filament bandwidth due to increased friction, and less retraction capability due to a larger temperature gradient between the heater and the heat sink. Furthermore, if heat is conducted past the PTFE coupler above the heat sink, the benefits of the metal hotend are lost. A thermistor colocated with the heating element facilitates temperature management of the hotend, and if it provides inaccurate data you're going to have a Bad Time.
There's (almost) always a bit of PTFE tube between the hot and cool ends of the extruder. In the cool end lives a BLDC and some gears, responsible for feeding filament from its spool into the hotend. If this BLDC stepper motor rides along with the hotend as an all-in-one extruder, it's known as direct drive. If the cold and hot ends are not conjoined, separated by a longer length of PTFE, it's a Bowden extruder. A direct drive setup requires less work to drive or retract the material, but adds weight and size to the moving printhead. Flexible filaments are less reliable in the longer Bowden tube.
FIXME print bed FIXME firmware FIXME automatic leveling
Basic filaments (PLA and ABS especially) are cheap and come in a wide range of colors, and can be effectively printed using brass nozzles in standard hotends. ABS has more aggressive temperature requirements, especially at the print bed, and deforms more when cooling, but is much tougher than PLA.
Selecting an MSLA
MSLA printers are great gifts for enemies: they fail almost as often as FDM printers, but can also poison you. They're a lot like cigarettes in that the worst part is the cancer. Just lifting the top off an SLA printer fills the room with the baleful stink of modern technical death, materials not known at Creation diffusing through your body, binding to various activation sites, inviting proteins into strange and exotic configurations, heralding a mephitic end that leaves your corpse too toxic to be burnt. Hah, just kidding, they're fine!