Design for 3D Printing: A Practical DFM Checklist for FDM, SLA, and SLS

Most failed prints are decided in CAD, not on the build plate. By the time a file reaches the machine, the part is already going to succeed or already going to fail — the printer just confirms which.

That gap matters more when you can't physically walk up to the machine. In a remote lab, you won't hear the first layer peel off or see the unsupported overhang sag at 40% height. You'll see the result. Which means the file you upload has to be right.

This is the checklist we run every incoming part through before we green-light it. It's organized by what applies universally, then by process. Print it, fork it, ignore the parts that don't apply to you — just run through it before your next upload.

The universal rules

These apply whether you're printing FDM, SLA, SLS, or anything else. Most of them are boring. All of them matter.

Units and scale. Export in millimeters. Every additive process assumes millimeters. A part exported in inches will come back at 25.4× its intended size, which is both expensive and darkly funny.

Watertight geometry. Your mesh has to enclose a single solid volume. No inverted normals, no non-manifold edges, no holes. Most slicers will try to repair the file silently, and the repair is almost always worse than fixing the CAD. If your modeler has a "mesh integrity" or "STL check" feature, run it before you export.

Meaningful tolerances. Don't specify ±0.01 mm on a chamfer that will be sanded. Don't over-tolerance cosmetic surfaces. Call out only the features that functionally matter — mating surfaces, press-fits, bearing bores — and accept looser tolerances everywhere else. Over-tolerancing is the fastest way to inflate cost on a prototype that will be replaced in two weeks anyway.

Orientation is a design decision. The best orientation for dimensional accuracy is rarely the best for surface finish, strength, or support minimization. If one axis matters, say so. Otherwise we'll orient for what usually matters: minimizing supports on visible faces and keeping load paths aligned with the print direction.

Wall thickness is not a suggestion. Every process has a minimum wall thickness below which the part either fails to print or comes out translucent and fragile. We'll cover the specific numbers below, but the rule is: if your CAD has a 0.3 mm wall somewhere because a loft got weird, find it and thicken it.

FDM: what to watch for

FDM is the cheapest process and the most opinionated about geometry. It extrudes molten plastic in layers, which means it loves vertical walls and hates unsupported horizontal spans.

Wall thickness. Minimum 1.2 mm for structural walls, 0.8 mm for decorative shells. Under 0.8 mm, you're relying on a single perimeter, and single-perimeter walls are effectively gift wrap — pretty to look at, zero strength.

Overhangs. Anything past 45° from vertical needs support or it will droop. 45° is the conservative rule; most modern printers handle 50–55° cleanly. If you're designing something with a 30° overhang and you don't want support marks on it, rethink the orientation or add a chamfer to bridge down.

Bridges. FDM can bridge small horizontal gaps (under ~5 mm) without support. Anything longer will sag in the middle. If you absolutely need a horizontal span — a roof, a lintel — design it as two ramps meeting at a peak instead of a flat bridge.

Holes and bosses. Small holes (under 3 mm diameter) print undersize because of the way the extrusion curves. If you need a clean hole for a screw or pin, model it 0.2–0.3 mm oversize and expect to ream it. Better: model the hole oversize and add a chamfer so the fastener self-aligns.

Layer adhesion is anisotropic. A printed part is stronger along the layer plane than across it. If your part will see tensile load in a specific direction, orient the print so layers lie perpendicular to that load. This is the single most common strength mistake we see.

SLA / DLP: what to watch for

SLA cures liquid resin with UV light, one thin layer at a time. The parts come off the printer soaking wet, get washed in isopropanol, and get post-cured under UV. Every stage of that process wants to fight your geometry.

Drain holes. Any enclosed cavity needs at least one drain hole, ideally two (one to let resin out, one to let air in). Without drains, uncured resin pools inside, cures partially over time, and distorts the part. Minimum drain hole diameter: 3 mm. Bigger is better.

Supports are not optional. Unlike FDM, SLA needs supports on almost every overhang — including some vertical walls, if the peel force on early layers is high. Let your service provider generate supports; manually placing them in CAD almost never ends well.

Minimum features. SLA can hold detail down to ~0.3 mm, but you'll get the best surface quality on features above 0.5 mm. Sub-millimeter text, tiny logos, and fine surface textures all print — they just print better when they're slightly thicker than you think they need to be.

Hollow with caution. Hollowing an SLA part saves resin and cost, but a hollow part that isn't drained properly will trap resin, cure asymmetrically, and warp. If you hollow, drain. If you don't want to drain, don't hollow.

Expect some shrink. SLA resins shrink during cure. For precision parts, assume 0.1–0.3% linear shrinkage and compensate in CAD if the application is truly dimensional.

SLS: what to watch for

SLS sinters nylon powder with a laser. It's the process to use when you need strong, isotropic, functional plastic parts — and the one with the most forgiving geometry rules.

No supports needed. The unsintered powder surrounding each part acts as support. This means you can print geometries that are impossible in FDM or SLA: interlocking assemblies, lattices, fully enclosed mechanisms. Design accordingly.

Escape holes for powder. Enclosed cavities trap powder that can't be removed. Design escape holes (minimum 5 mm diameter) for any internal volume you don't want filled with sintered-or-loose powder. Larger cavities need larger or multiple holes.

Minimum wall thickness. 0.7 mm is the floor, 1.0 mm is the practical minimum for anything you want to handle confidently. Thin walls in SLS have a slightly grainy finish and can be brittle if they're both thin and long.

Nesting awareness. SLS parts are typically packed densely into the build volume to maximize machine time. Parts that are long and thin relative to the build box are fine. Parts with extreme aspect ratios (1 × 200 mm rods, e.g.) can warp from thermal gradients — add sacrificial ribs or print them oversize and machine down.

Surface finish. SLS parts come out with a uniform matte powder finish. If you need smooth surfaces, plan for post-processing (tumbling, vapor smoothing, bead blasting) and budget for it upfront.

Tolerances: what to actually expect

Here are the tolerances you should design around. These are honest working numbers — not marketing best-cases.

• FDM: ±0.3 mm or ±0.3%, whichever is greater, on well-oriented features.