FDM or resin: choosing the process

Before you pick a filament or a printer, you make a bigger choice that decides everything downstream: how the plastic gets turned solid in the first place. There are two processes a Kapy user realistically buys into, and they don't overlap much. One draws your part from a molten strand, bead by bead. The other grows it out of a tank of liquid that hardens under light. Same goal — a solid object from a digital model — but they reward completely different parts, and the geometry you should draw for each is different. Get the process wrong for the job and no amount of clever modelling saves you.

FDM: molten plastic, drawn in the air

FDM (fused deposition modelling) melts a plastic filament and lays it down in thin beads, one layer stacked on the last. It's the workhorse. The parts are tough, cheap per gram, and can be large — a whole printer bed of bracket. You can print in materials chosen for a job: stiff PLA, tough PETG, heat-resistant ABS. The catch is that a part built from stacked beads is anisotropic — strong along the layers, weaker across them — and its finish is a visible ladder of layer lines. FDM is what you want for anything that has to do something: mechanisms, brackets, enclosures, jigs, functional prototypes. If it bears a load, clips onto something, or has to survive being handled, it's an FDM part. Everything else in this topic, and the whole FDM design path, assumes you chose this box.

Resin (MSLA): liquid cured by light

Resin printing does the opposite. The part hangs upside-down from a plate that dips into a vat of light-sensitive liquid; a screen below flashes a UV image and cures one whole layer at once, then the plate lifts a fraction of a millimetre and does it again. Because a layer is defined by sharp pixels of light rather than a squashed bead, resin resolves exquisite fine detail and comes out glassy-smooth, with far less directional weakness. That's the win: crisp, tiny, beautiful. The costs are real. Cured resin is comparatively brittle. The workflow is messy and toxic — you handle sticky uncured resin that's a skin and eye irritant, so gloves and ventilation aren't optional, and every part needs washing in alcohol then curing under UV before it's done. Build volumes are smaller. Resin is the right call for miniatures, tabletop figures, jewellery masters, dental-style detail, anything where a hair-fine edge matters more than toughness.

Deciding for the part in front of you

The honest question is what the part needs most, because you rarely get both. A planetary gearbox, a phone stand, a wall bracket, a snap-together case — those want strength, size and low cost, so they're FDM every time, even though the surface won't be flawless. A 28 mm knight with chainmail you can actually read, a ring with a filigree band, a scale model with rivets — those want detail you simply cannot lay down with a 0.4 mm nozzle, so they're resin, and you accept the brittleness and the gloves. When a part genuinely wants both — a functional piece with fine cosmetic detail — you usually split the difference by choosing the property you can't fake: strength can't be added after printing, but you can live with slightly softer detail on FDM.

Most of what you model in Kapy — the mechanisms, the fits, the load-bearing brackets — lives on the FDM side, which is why most of the rest of this topic focuses on molten plastic, though resin has its own article. If you've settled on FDM, the next thing that shapes your part isn't the plastic at all but the machine's motion: read Printer shapes: bedslinger, CoreXY and delta.