FDM versus Resin Printing

How each one builds a part

FDM (fused deposition modeling, also called FFF) pushes a thermoplastic filament through a heated nozzle and draws each layer as a path of melted lines, a bit like a very precise hot glue gun. When a layer is done, the nozzle moves up (or the bed moves down) by one layer height and the next layer is drawn on top.

Resin printing is stereolithography in the broad sense: a light source cures a liquid photopolymer resin one thin layer at a time. The light can come from a laser (classic SLA), a projector (DLP), or LEDs shining through an LCD screen that masks each layer (MSLA or LCD printers, which is what most hobby resin machines are). The part is usually built upside down, peeled off a film at the bottom of the vat after each layer.

Side by side

Question	FDM (filament)	Resin (SLA, DLP, MSLA)
Fine detail	Limited by nozzle width. Formlabs cites minimum walls around 0.8 mm Rule of thumb.	Much finer. Formlabs cites walls down to 0.2 mm and embossed detail to 0.1 mm on its own printer Maker spec.
Surface	Visible layer lines; sanding needed for a smooth finish.	Smooth, with faint or no visible layer lines; small marks where supports touched.
Strength direction	Anisotropic: layers bond less strongly than the lines inside a layer, so parts are weakest across the layers.	Close to isotropic: each layer bonds chemically to the one before.
Materials	PLA, PETG, ABS, ASA, TPU, nylon and many blends; spools are easy to swap.	Standard, tough, flexible, castable and specialty resins; changing resin means swapping or cleaning the vat.
Build size	Formlabs surveys desktop machines up to 300 x 300 x 600 mm Rule of thumb.	Formlabs surveys desktop machines up to 353 x 196 x 350 mm Rule of thumb. Check the specific machine.
After printing	Remove supports, optionally sand. The part is ready to handle.	Wash off uncured resin, remove supports, post-cure with light (and sometimes heat), then sand support marks.
Running costs	Filament, the occasional nozzle and build sheet.	Resin, wash solvent, gloves, paper towels, vat film replacements and waste handling.
Safety and space	Hot parts and fumes; Formlabs recommends ventilation for desktop FDM.	Liquid resin is a skin and eye irritant and sensitizer; needs gloves, eye protection, ventilation and a dedicated mess-tolerant workspace.

Treat the numbers above as maker figures for specific machines, not limits for every printer. Your own printer's spec sheet is the number that matters.

What FDM is good at

• Functional parts. Brackets, jigs, enclosures, hooks and organizers, where size and toughness matter more than surface finish.
• Large, simple shapes. A filament printer prints a big box faster and cheaper than a resin printer prints a big solid block.
• Material choice. Flexible TPU, heat tolerant ASA, stiff carbon-filled blends and easy PLA all run on the same machine.
• Low-mess workflow. The finished part comes off the plate ready to use. No solvent, no curing station.

The weak spots are fine detail, smooth curved surfaces and strength across the layers. Many FDM problems trace back to the first layer, temperature and moisture; see First Layer Won't Stick and Stringing and Oozing.

What resin is good at

• Small detailed models. Miniatures, jewelry masters, figurines, dental and engineering models with crisp edges.
• Smooth organic surfaces. Curves and gentle slopes show far less stair-stepping than on FDM.
• Fine text and texture. Embossed lettering and surface texture survive at sizes where an FDM nozzle would smear them.

The trade-offs are the wet workflow, the safety precautions and the cost of consumables. Resin mechanical properties vary widely by formulation, so check the resin's technical data sheet before using it for a part that will flex or take impact.

The resin workflow, start to finish

1. Orient the model and add supports in the resin slicer. Supports matter more than in FDM, because each layer is pulled off the film.
2. Print. The vat stays full of liquid resin throughout.
3. Remove the part wearing gloves. It is still coated in uncured resin.
4. Wash in the solvent the resin maker specifies (often isopropyl alcohol, sometimes water for water-washable resins).
5. Remove supports, then post-cure with UV light as the resin maker directs. Some engineering resins need heat as well.
6. Dispose of resin-contaminated solvent, wipes and gloves according to the SDS and local rules.

Space and setup

An FDM printer needs a stable table, a power outlet and some airflow; Formlabs recommends ventilation for desktop FDM machines. A resin printer needs more: a surface you do not mind getting resin on, ventilation, somewhere to keep gloves, wash containers and a curing light, and a plan for waste. Resin makers also state a working temperature range for their resins; check it before putting the printer in an unheated room.

How to choose

• Mostly useful household or workshop parts? Start with FDM.
• Mostly small, detailed display pieces? Resin, if you can give it a ventilated space and commit to the safety routine.
• Large decorative pieces, such as relief panels? FDM handles the size more cheaply; resin gives finer surface detail on smaller panels. See Printing a CNC Relief STL.
• Sharing a living space with children or pets? FDM is usually the easier fit, because resin needs gloves, eye protection and a ventilated space for every print. Any printer still needs the ventilation its material calls for.
• Want both? Many people own one of each and route each job to the process that suits it.

Whichever you choose, layer height is the first setting that trades detail against time on both. See Choosing a Layer Height.