Preparing the file for metal printing: STL, 3MF, and why mesh matters more than it seems

An LPBF machine doesn't print your CAD model. It prints a mesh of triangles that someone has generated from that model, and then slices it. There's a conversion between the solid in the CAD file and the triangles that arrive at the machine, and that conversion can go well or badly without anyone noticing until the part is manufactured.

It is the most boring step in the whole process and the one that takes the most shop floor hours when it fails.

STL: the format everyone uses and no one defends

The STL file stores a list of triangles with their normal shape. That's all. It doesn't know units, colors, materials, or which face is which. It's from 1987 and remains the de facto standard because it reads absolutely everything.

Their two usual problems:

It doesn't have units. A 20-unit cube can be 20 mm or 20 inches, and the file doesn't specify. The person opening it decides. This is the number one cause of parts being manufactured to the wrong scale, and it's so silly it's hard to believe until it happens.

The curve no longer exists. When exporting, the CAD program approximates each curved surface with flat triangles. This approximation has a tolerance, and the default tolerance of almost all CAD programs is designed for visualization, not manufacturing. A Ø10 cylinder exported with the default settings can appear as a 30-sided polygon: it looks round on screen, but you can feel the roundness when you touch the part.

3MF: What you should be using

The 3MF solves almost all of that. It uses explicit units, supports multiple objects in the same file with their positions, allows you to include metadata—material, who made it, what version it is—and compresses, so it weighs a fraction of the equivalent STL file.

If your CAD software exports it and your supplier accepts it, send it as a 3MF file. If you have any doubts, send both: a 3MF file and an STL file, and mention it in the email.

And if you're working with modern parametric CAD, ideally you should also send the native solid or a STEP file. The mesh is for manufacturing; the STEP file allows the manufacturer to measure actual dimensions, understand which face is functional, and discuss tolerances with you without guesswork.

Export settings that do change the part

When you export the mesh, there are two numbers that determine the result:

Chord height (chord height, sag, tessellation tolerance). This is how much the triangle is allowed to deviate from the actual surface. For LPBF, a reasonable value is around 0.01–0.02 mm. With a layer thickness of 30–50 microns, a mesh with a 0.1 mm deviation is introducing a larger error than the process itself, which is absurd.

Angular Deviation. Controls how many facets are used on tight curves. Between 1° and 5° is usually sufficient. Lowering it to 0.1° does not improve the part and increases the file size by twenty.

The goal isn't "the finest possible mesh." It's that the mesh error is clearly smaller than the process error. Beyond that point, you're just creating giant files that take forever to open.

The four flaws that appear time and time again

Mesh is not watertight. Gaps between triangles. The cutter doesn't know what's inside and what's outside, so it either fills in the gaps or skips the layer altogether. This is a classic defect in meshes that come from scanning or batch conversions.

Reversed Normals. Triangles facing inwards. The part is cut upside down in loose areas: missing walls appear, or material where there should be a gap.

Degenerate or overlapping triangles. Faces with zero area, duplicate vertices, self-intersections. These usually result from poorly resolved Boolean operations in the original CAD software.

Shell with no thickness. Someone exported surfaces, not a solid. There's nothing to print, even though it looks perfect on screen.

Almost all of them can be fixed with a mesh repair tool in minutes. The problem isn't fixing them: it's detecting them before the part is manufactured.

How to check it yourself in two minutes

Before sending anything:

Open the file in a different viewer than the one that generated it. If it looks the same, that's a good sign. Many errors only appear when reading with a different engine.
Check the bounding box dimensions. If you were expecting 84 × 30 × 12 mm and you get 3.3 × 1.2 × 0.47 mm, you have a problem with the dimensions.
Check the volume. A well-closed solid has volume. If the viewer shows zero, negative, or something absurd, the mesh is not watertight.
Rotate the part and look for large facets in the curved areas that have a function. If the planes are visible, the tessellation is too coarse.

Steps 2 and 3 are performed by the website's quote calculator as soon as you upload the file: it tells you the volume, dimensions, and number of layers. And it does this right in your browser, without uploading anything anywhere, precisely because this is the stage of the process where people still don't want to show their geometry to anyone.

What a perfect file can't fix

A flawless STL file doesn't transform a poorly designed part into a manufacturable one. Overhangs still require supports, thin walls still have a minimum, and orientation remains part of the specification even if it's not explicitly stated in the file.

The file is a necessary condition. The design rules are sufficient.

In summary

Export in 3MF format if possible, and in STL format if not, with string deviation around 0.01–0.02 mm and units specified in the email. Send the STEP file along with the mesh whenever possible. And check the bounding box and volume before sending: it only takes ten seconds and will save you from an awkward conversation two weeks from now.

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What to do with the part once the file is ready, in how to order a metal 3D printed part without making a mistake. The geometry rules you should keep in mind while designing, in design rules for 3D metal printing. And if you want the volume, layers, and approximate cost of your part without sending us the file, just upload it to the quote calculator: it doesn't leave your computer.