An STL Is Not a Save-As, It Is a Translation

Your CAD model is a set of mathematical surfaces. An STL is a bag of triangles that approximates those surfaces. When you export, the software decides how finely to chop curved faces into flats, and that decision is permanent: the printer never sees your original geometry again. Most failed prints that get blamed on the printer or the material actually trace back to a bad export made in under a second by clicking OK on a dialog nobody read.

A print service will usually catch the catastrophic errors, but the subtle ones pass straight through into a part you pay for and then throw away. At typical prototype pricing, a wasted mid-size FDM part is thirty to eighty dollars plus two or three days of schedule.

Wrong Export Resolution

Every CAD package exposes chord height (deviation) and angular tolerance when writing an STL. Leave them at the default coarse setting and cylinders come out as visible polygons, and a hole that should be 0.250 in (6.35 mm) round becomes a twelve-sided pocket that a dowel pin will not enter. Push the other direction and you get a 400 MB file that chokes the slicer and adds nothing, because the triangles are now far smaller than the printer can physically resolve.

Practical target: chord deviation around 0.0005 to 0.001 in (0.013 to 0.025 mm) and angular deviation near 5 degrees for FDM and SLS, tighter for SLA where the machine can actually reproduce it. If the resulting file is between 5 and 40 MB for a hand-sized part, you are in the right band. Resolution does not fix dimensional accuracy on its own; the tolerance side of the problem is covered in 3D printing tolerances.

Non-Manifold and Open Geometry

A printable mesh has to be watertight: every edge shared by exactly two triangles, no inverted normals, no self-intersections, no stray shells floating inside the body. Assemblies exported as a single STL are a common source of this. So are models built by stitching imported surfaces, and models that went through several file format conversions on the way to you.

The slicer's response to a broken mesh is unpredictable. Sometimes it auto-repairs invisibly and gives you a part with a filled-in slot. Sometimes it prints the interior of the model as solid. Free repair utilities will flag and often fix these, but understand what the repair did before you accept it. If you are working from a scanned part rather than native CAD, the whole cleanup workflow is different and is described in 3D scanning to CAD.

Units and Scale

STL carries no unit information at all. It is a list of numbers. If your CAD file is in inches and the slicer assumes millimeters, your 4 in bracket arrives as a 4 mm chip. The reverse error produces a part that will not fit the build plate and gets silently scaled by whoever opens it.

Export in millimeters as a default, state the units in the filename or the purchase order, and give the vendor one overall bounding box dimension to check against. This is one of several reasons manufacturers prefer a solid format for anything beyond a quick print, a tradeoff laid out in STL vs STEP.

Walls, Features, and Threads Below the Machine's Ability

CAD will happily model a 0.2 mm rib. An FDM nozzle 0.4 mm wide cannot print it, and the slicer will drop it without warning. The same applies to embossed text under about 0.5 mm stroke width on FDM, small fillets that vanish into layer stepping, and modeled threads finer than roughly M6 or 1/4-20.

  • Minimum wall. Two nozzle widths for FDM, so 0.8 mm at a 0.4 mm nozzle. SLA holds 0.5 mm, SLS about 0.7 mm.
  • Holes. Print undersize by 0.1 to 0.3 mm on FDM and plan to drill or ream anything that has to fit.
  • Threads. Model a plain hole and use a heat-set insert or a tapped hole instead of printed threads. The options and their pull-out strength are compared in threads and inserts in 3D printed parts.
  • Clearances. Allow 0.3 to 0.5 mm between mating printed parts on FDM, less on SLA.

These constraints belong in the model from the first sketch, not in a rescue pass at export time. The full rule set is in design for 3D printing guidelines.

Ignoring Print Orientation

The file itself does not set orientation, but the geometry decides which orientations are viable. A part with a thin cantilever, a long unsupported bridge, or a critical hole axis will behave completely differently depending on how it sits on the plate. Layer adhesion is typically 40 to 70 percent of in-plane strength, so a load path running across layers is the usual explanation for a part that snaps in hand. Tell your vendor which axis carries load, and read print orientation and part strength before you assume the default nesting is fine.

The Two-Minute Check Before You Upload

Open the STL in the slicer you or your vendor uses, and confirm five things: the bounding box matches the dimensions you expect, the mesh reports zero errors, curved faces look smooth in preview at print scale, no feature is thinner than two extrusion widths, and the estimated mass is close to the CAD mass. Any mismatch means the export lied to you. If you are choosing between machines or shops at the same time, 3D printing services for prototypes covers what each process will and will not hold, and the wider 3D printing hub collects the rest.

Get the File Right the First Time

Projects House prepares production-ready print files, repairs and re-exports models that fail on upload, and adjusts geometry so parts come off the machine usable rather than close. Send your CAD and the part's job through our contact form and we will tell you what needs to change before you spend on a build.