There are two kinds of desktop 3D printer, and they're good at different things. A filament printer, often called FDM, melts a plastic strand and draws each layer with a hot nozzle. A resin printer cures liquid resin with ultraviolet light, one thin layer at a time. Filament makes strong, practical parts with almost no mess. Resin makes tiny, crisp detail and asks for a proper workshop routine, and buying it used means judging a screen with a limited life.
How each one works
A filament printer pushes plastic from a spool through a heated nozzle and lays it down in lines. Most here print on a removable spring-steel sheet, and the printer moves either the bed, in a bed slinger, or the head, in a CoreXY. Layer lines are visible up close, and fine detail is limited by the nozzle, which is usually 0.4 mm.
The resin printers here are mono-LCD printers. A UV light shines up through a monochrome screen that masks the shape of each layer, curing it against a release film in the bottom of a vat of resin. The build plate lifts the print out a layer at a time, upside down. The screen sets the detail. The Anycubic Photon Mono M5s resolves 19 µm in X and Y and the Elegoo Mars 4 Ultra 18 µm, far finer than any nozzle.
What each is best at
Choose filament for functional parts and anything big. Brackets, clips, enclosures, replacement parts, cosplay armor, planters and toys all print well in PLA and PETG, and parts can be large. The Neptune 4 Max prints 420 x 420 x 480 mm in one go. You can print tough materials like ABS and ASA in an enclosed printer, flexible TPU on most direct-drive printers, and carbon-fiber blends on a hardened nozzle like the X1 Carbon's.
Choose resin for small, detailed work. Tabletop miniatures are the classic use, along with jewelry models and small display pieces where you'd see filament's layer lines. Build plates are smaller. The Saturn 4 Ultra, one of the bigger ones here, prints about 219 x 123 x 220 mm.
What you'll need besides the printer
A filament printer needs very little else. Buy filament, keep the spools dry, keep a spare nozzle, and wipe the build sheet with isopropyl alcohol so prints stick. It sits happily on a desk.
Resin needs a routine. Elegoo's beginner guides say to wear nitrile gloves, a mask, goggles and an apron, and to work somewhere with good ventilation. Don't fill the vat past its MAX line. Every print gets rinsed in isopropyl alcohol, Elegoo suggests about two to three minutes, and cured under UV, either with a lamp or a wash-and-cure station like Prusa's CW1S. Never pour used alcohol down the drain. Let it evaporate and cure the sludge with UV before throwing it out. Uncured resin shouldn't touch skin, so plan a corner of the house for it, away from the kitchen.
What wears out
On a filament printer, the wear parts are the nozzle, the build sheet and the belts, plus rods, rails or wheels on some printers. All are easy to replace and widely sold. Bambu Lab, Prusa and Creality publish guides for swapping hotends, belts and sheets on their printers.
On a resin printer, two wear parts matter most. The screen is a consumable. Anycubic, Elegoo and Prusa all rate their mono screens at about 2,000 hours, and Elegoo's replacement screens carry a three-month warranty. The release film in the vat dents and clouds with use and gets replaced, too. Film types differ by printer. The Photon Mono 2 uses FEP, the M5s uses ACF film, and the Saturn 4 Ultra ships with PFA. Both screens and films are sold as spares for most of the resin printers here, so budget for them as running costs.
Buying a used filament printer
Ask for a recent print with the first layer facing up, a close photo of the nozzle, and photos of the build sheet. Ask the seller to pluck the belts and push the bed or head by hand. If the printer keeps statistics, like Prusa's printers and Bambu's X1 series do, ask for that screen. Our guide to checking a used 3D printer covers the full list.
Buying a used resin printer
Ask roughly how many hours it has printed, because the screen's 2,000-hour rating is the clock that matters. The Prusa SL1S records its display exposure time under Support, then System Information, which makes it unusually easy to judge. For the others, ask the seller. Then ask for a photo of the screen with the vat removed, looking for cracks and resin under the edges, and a photo of the film held up to a light, looking for dents and cloudiness. Check that a replacement screen is sold for that model. Anycubic lists one for the Photon Mono M5s and Mono 2, and Elegoo for the Mars 4 Ultra and Saturn 4 Ultra. We couldn't confirm one for the Photon Mono X 6Ks, so its screen condition matters more. If the seller used a wash-and-cure station, ask whether it comes with the printer, since you'll need one anyway.
Shipping is where resin printers get hurt. The vat must be emptied and cleaned, and the plate removed. Prusa's guide for the SL1S lays thin cardboard over the screen and packs the platform inside the tank. Resin leaking in the box or a cracked screen is what to check for on arrival.
Which to buy
If you want one printer for everything, buy a filament printer. A self-levelling bed slinger like the Bambu Lab A1 mini or Prusa MK4S will print most things you can think of, with almost no setup. If you paint miniatures or want jewelry-level detail, a resin printer will do things no filament printer can, as long as you're happy with gloves, alcohol and a ventilated corner. Plenty of people end up with one of each, and buying both used makes that easier.
