How to prepare your STL or STEP file for SLS printing

Most problems with an SLS part can already be seen in the file, for example a wall that is too thin, a closed cavity that will stay full of powder or a model exported in inches and read as millimetres. A well-prepared file gets you an accurate quote the first time and a part that looks the way you expected. The guide below starts with exporting and ends with the information to include in your request.

STL, STEP, 3MF or OBJ?

We accept all 4 formats, but they do not contain the same information.

• STEP keeps the exact geometry from CAD, so a cylinder stays a cylinder and is not approximated with polygons. The units are stored in the file. We can measure dimensions directly and generate the triangle mesh ourselves at a suitable resolution. If you have the model in a CAD program, send STEP.
• STL is a triangle mesh, with no units and no information about colours or bodies. It is the most widely used format, but its quality depends on the export settings.
• 3MF is also a triangle mesh, but it keeps the units and can hold several objects in the same file, and the file is compressed. It is a better choice than STL if your software offers it.
• OBJ is mostly used for scanned models and models made in artistic modelling software. It works, but check the units and the scale.

How to export an STL correctly

An STL approximates curved surfaces with flat triangles. If the triangles are too large, the facets show on the part, because the printer reproduces what it receives, and a 110 µm layer is much finer than the facets of a coarse export. If they are too small, the file grows to hundreds of megabytes with no visible gain.

1. Export in millimetres. If the software asks for a unit, choose mm, and if it does not ask, check the overall dimensions after export.
2. Choose a fine resolution, but not the maximum. In most CAD programs this means a deviation (chord height) in the order of hundredths of a millimetre and an angle of a few degrees. Look at the small holes. If a 3 mm hole has only a few visible sides, the resolution is too low.
3. Export each part as a separate file. Export an assembly as a single body only if you want its parts printed together.
4. Use the binary format, which holds the same geometry as an ASCII STL in a file several times smaller.

Check that the model is watertight

For printing, the model has to be a closed volume. The mesh must not have holes, intersecting faces, triangles with flipped normals or surfaces with no thickness. A model exported from a solid CAD program is usually correct. Problems tend to appear with scanned models, models built from surfaces and models converted several times between formats.

The preparation software we use, Formlabs PreForm, repairs small errors automatically. Automatic repair does not know what you intended, though, so it may close a hole you wanted open or fill a channel. If the model has major errors, it is safer to fix them in the program you created it in. For a quick check you can use the analysis tools in free slicers or in Blender.

Minimum feature sizes in SLS

The values below are the minimums published by Formlabs for PA12. For PA11, TPU 90A and polypropylene the minimums are slightly larger.

The full list, with explanations, is in the SLS design guide. These are the minimums at which a feature still forms. A 0.6 mm wall will print, but that does not mean it will withstand the load in your application.

Hollow parts and cavities

In SLS, a solid part costs more and cools less evenly than a hollowed one. If you hollow out the part, think about where the powder will get out. A cavity with no exit stays full of unsintered powder, and the part weighs almost as much as a solid one. Formlabs recommends at least 2 escape holes of at least 3.5 mm in diameter, and larger or additional holes make cleaning easier. In lattice structures, the openings have to be at least 8 mm and the faces have to be open.

Uniform wall thickness also reduces the risk of warping, because very thick sections next to thin ones cool at different rates. If you can, hollow out massive areas and keep the walls at similar thicknesses.

How to send assemblies

If the parts are printed separately and assembled afterwards, send a separate file for each part and give the quantity for each. Clear file names, for example “lid-v3.step” and “base-v3.step”, avoid confusion between versions.

If you want a mechanism printed fully assembled, send it as a single file with the clearance between the parts already modelled. Check in the model that the parts do not touch anywhere, because any contact area or any clearance below the minimum fuses them together during printing. For PA12 the minimum clearance is 0.3 mm on small features and 0.6 mm on large ones, and for PA11 and TPU 90A it is at least 1 mm.

What to include in your request

1. The material, or what the part does if you do not know which material to choose.
2. The quantity for each file and, where relevant, the estimated annual volume.
3. Critical dimensions and tolerances. A PDF drawing with the important dimensions marked is the clearest option. For PA12, Formlabs gives ±0.5% or ±0.3 mm in X and Y, whichever is greater.
4. The visible faces, so that we can orient them towards the side that gives the better surface.
5. Threads and inserts. Say where you want threaded inserts and what size they are, or whether the threads are modelled in the part.
6. The finish, either blasted only or dyed black.
7. The date by which you need the parts, and the delivery address.

File size

The quote form accepts files of up to 8 MB. A STEP file is usually much smaller than a fine STL of the same part, and 3MF is compressed. If your files still exceed the limit, upload them to a file storage service and put the link in the form.

Common mistakes

• The model is exported in inches and read in millimetres, and the part comes out 25.4 times smaller.
• The threads are only cosmetic. Many CAD programs show a thread only as a texture or an annotation, and on export it remains a smooth cylinder. If you want the thread printed, model it as geometry.
• The assembly is exported as a single body with overlapping parts, and the result is a single fused part.
• The model contains surfaces with no thickness, for example an enclosure modelled only as an outer surface.
• The text is under 3 mm high and comes out illegible.
• Closed cavities have no escape holes.

If you are not sure the model is ready, send it anyway and tell us what concerns you. It is easier to discuss a problem before printing than after.

To choose a material, see PA12, the starting material for most parts. When your file is ready, send it from the quote page.