The short answer: FDM (filament) and SLA (resin) printers demand opposite design choices. FDM builds in horizontal layers from a moving nozzle — walls, supports, and orientation matter enormously. SLA builds by curing liquid resin layer by layer — fine detail is free, but you pay for it in support placement and post-processing. Designing the same model for both technologies usually means designing two models.
How they actually print, in one sentence each
FDM (Fused Deposition Modeling): a heated nozzle extrudes melted plastic filament in horizontal lines, one layer at a time, until the part is built up from the bed.
SLA (Stereolithography) / MSLA / DLP: a UV light source cures liquid resin in a vat, one layer at a time, while the build plate slowly lifts the cured part out of the resin pool.
The mechanics determine almost every design constraint that follows.
FDM design rules
1. Layer lines run horizontally, so loads should run vertically
FDM parts are weakest at the layer boundaries — they're literally glued together by partial melt. A part that needs to bear load along its long axis should be printed standing up, not flat. A bracket that holds weight from above is stronger printed in the orientation where the load goes through the layers, not along them.
2. Overhangs above 45° need supports
Most FDM printers can bridge gaps and do mild overhangs (up to 45° from vertical) without supports. Past that, the nozzle is depositing plastic on air, and the layer collapses. Either redesign the geometry to avoid overhangs, or accept that supports will leave a rougher surface where they were.
3. Holes are slightly oval, not round
A 5mm-diameter horizontal hole printed in FDM comes out around 4.7–4.9mm at the bottom, because the first layer of the hole is bridging across the previous layer. If you need precision, design holes 0.2–0.3mm oversized, or print holes vertically (perpendicular to the bed) so they print as a stack of circles.
4. Walls should be multiples of nozzle width
A typical FDM nozzle is 0.4mm. Walls thinner than 0.8mm (two perimeters) tend to fail. Walls of 1.2mm, 1.6mm, 2.0mm print reliably. If you're using a 0.6mm nozzle, scale these up accordingly.
5. Avoid thin features that aren't supported
A 1mm-tall pin standing alone on a flat base will probably print. A 10mm-tall pin without surrounding geometry will probably wobble and fail mid-print. If a feature is tall and thin, design a fillet at its base.
SLA design rules
1. Orientation is half the engineering
SLA prints upside-down: the build plate rises out of the resin vat, pulling the print up with it. Each layer is created with a peel force as the cured layer separates from the vat film. Heavy flat surfaces parallel to the build plate cause huge peel forces and often fail. Most successful SLA prints are angled at 30–45° to minimize peel area on any given layer.
2. Supports are surgical, not casual
Unlike FDM where supports are a cost paid for any overhang, SLA supports leave small "scars" on the surface and need to be carefully placed where they won't ruin the finish. Resin slicers (Lychee, ChiTuBox) generate supports automatically, but for any production-grade print you'll edit them manually — keeping supports off visible faces, off fine details, and off areas that need a smooth finish.
3. Suction cups will ruin a print
Hollow models with no drain holes create vacuum pockets when lifted out of the resin vat. The print fails when the suction force exceeds the layer adhesion. Always add 2–3 small drain holes (3–5mm diameter) to hollow SLA models, positioned so liquid resin can flow out as the model lifts.
4. Resolution is free, but small features cost time
SLA can print details an FDM nozzle can't reproduce — text down to 0.5mm tall, jewelry details, microscale geometry. Use that. The constraint isn't whether you can print fine detail; it's whether the layer-time and post-curing can keep up. Smaller layer heights produce better results but multiply print time.
5. Post-processing is part of the design
SLA prints come out of the printer covered in uncured resin. They need an isopropyl alcohol wash, then a UV cure. Then the supports are clipped off, the support marks are sanded. Design with this in mind: leave a 1–2mm sacrificial nub where supports attach, so when you sand, you're sanding the nub, not the part.
The translation table
| Design choice | FDM way | SLA way |
|---|---|---|
| Wall thickness | 1.2–2mm minimum | 0.8–1.2mm acceptable |
| Detail size | ≥0.5mm features | ≥0.1mm features |
| Print orientation | Flat-down for stability | Angled 30–45° for peel |
| Hollow models | Use slicer infill (15–20%) | Hollow shell + drain holes |
| Supports | Auto-generated, accept the scars | Manually placed, hide them |
| Surface finish | Visible layer lines | Smooth, near-injection-mold quality |
| Strength | Anisotropic (depends on direction) | Brittle, isotropic |
| Best for | Functional parts, prototypes, tools | Detail models, jewelry, miniatures |
When to use each
FDM is the answer for functional parts, prototypes, large objects (over 100mm in any dimension), brackets, jigs, organizers, and anything that needs to flex or bear load. The material catalogue is wide (PLA, PETG, ABS, ASA, nylon, TPU, carbon-fiber composites), the running cost is low, and the printer can run unattended.
SLA is the answer for detail-heavy work: tabletop miniatures, jewelry models, dental and medical applications, hyper-precise mechanical prototypes, and anything where layer lines aren't acceptable. The materials are mostly resin variants (standard, tough, flexible, castable), the running cost is higher (resin is more expensive per gram than filament), and the workflow involves chemicals.
The professional path uses both. FDM for the frame and structural parts. SLA for the detail components that bolt onto the frame.
DDDIMO accepts both FDM and SLA listings — sellers tag the technology so buyers know what they're getting. Get early access.
