How to prepare an STL file for 3D printing

Before a part comes out of a printer, there is a file. And print quality starts there: a clean file prints without surprises, a flawed one forces back-and-forth, sometimes a failed print. The good news: preparing a correct STL comes down to a few simple rules.

This guide is for those who model their own parts. If you have no file, don't worry — we explain what happens in that case too.

What exactly is an STL file

An STL describes the surface of an object as a mesh of triangles. It carries no colour, no material, and no guaranteed absolute scale — just geometry. It is the historic 3D printing format, simple and universal, but it needs a few precautions to stay faithful to your intent.

The formats we accept

You are not limited to STL. We also accept:

• STEP (.step / .stp): our preferred format for technical parts. It keeps the exact geometry (not a triangle approximation), which lets us reorient or adjust without loss.
• 3MF: more modern than STL, it embeds scale and units, which avoids many errors.
• OBJ: useful for organic or textured objects.

When in doubt, send the STEP if you have it, the STL otherwise.

Export resolution: neither too much nor too little

When your CAD software exports an STL, it converts curved surfaces into triangles. Two symmetrical pitfalls:

• Too faceted: too coarse an export makes facets visible on cylinders and curves. The part looks roughly hewn.
• Too heavy: conversely, an excessive resolution produces a file of several hundred MB, slow to process, with no visible benefit.

The right setting aims for a chord deviation (tolerance) of around 0.01 to 0.05 mm — fine enough that the eye sees no facets, without bloating the file.

A watertight mesh is the foundation

This is the point that causes the most failures. An STL must be closed and consistent (a "manifold" mesh):

• no holes in the surface;
• no inverted faces (normals pointing outward);
• no stray edges or overlapping triangles.

Most CAD software has a tool to check and repair the mesh. Run it before exporting. A non-watertight file is like a plan missing a wall: there is no telling where the inside is.

Scale and units: everything in millimetres

STL does not always store the unit. A part drawn in inches or centimetres can arrive ten or twenty-five times too small or too large. Export in millimetres and tell us the expected size of the part. This simple detail prevents most misunderstandings.

Think about thickness and clearances

A file can be numerically perfect and physically impossible to print:

• Minimum wall thickness: below a certain threshold, a wall becomes fragile or will not print. Count on at least 1 mm in FDM for a wall that must hold.
• Small details: an engraved text or a relief that is too fine disappears. The smaller the detail, the more resin is suited — see our article on FDM or resin.
• Functional clearances: two parts that must fit together need a gap (often 0.2 to 0.4 mm). Without it, they will not assemble — or will no longer come apart.

What we check before printing

Receiving your file does not mean sending it to the machine as-is. Before each print, we review:

• the watertightness and consistency of the mesh;
• the scale and proportions;
• the optimal orientation of the part (it changes strength, supports, finish);
• fragile or overhanging areas.

If something is off, we flag it before launching anything. That is our principle: we deliver nothing we would not keep ourselves.

And if you have no file?

It is common, and it is not a problem. Many projects start from a sketch, a drawing, a photo or an idea. We take over: our CAD design service models the part for you, with its printing in mind from the start. You do not need to master CAD to get a part.

In short

An STL ready to print means: the right format (STEP if possible), a balanced resolution, a watertight mesh, a scale in millimetres, and realistic thicknesses. Tick those boxes and your part starts off right.

Have a file on hand? Start an online estimate or send it to us: we review it and come back with a quote and, if needed, our recommendations.

Frequently asked questions

Which file format should I send?

STEP if you have it: it keeps the exact geometry and lets us adjust without loss. Otherwise, an STL exported at a reasonable resolution works very well. 3MF is also a good choice as it embeds the scale.

My file is very heavy, is that a problem?

A file of several hundred MB slows processing with no quality gain. If your export is far larger than necessary, lower the export resolution. When in doubt, send it anyway: we will tell you if it needs trimming.

I don't know if my mesh is watertight. How do I check?

Most CAD software has a mesh analysis and repair function. Run it before export. And in any case, we check watertightness on our side before printing.

Can you model from just a drawing?

Yes. With a drawing, a sketch or photos and a few dimensions, we design the file for you, optimised for printing.