The Guide to Creating .STL Files Perfect for 3D Models and 3D Printing

What is a .STL file?

STL is a standard format for 3D models. 3D geometry is described by a mesh of triangles and dimensional information is contained in each individual triangle. Unlike other CAD formats, the object appears to be formed only by the external surface or "skin". The information of where the inside of the component is located is saved in every single triangle by defining the normal, a line coming out of the triangle and perpendicular to it. It is important to underline that the STL format, unlike the CAD formats such as the step, is "ignorant" of how the object was modeled and constructed or how the surfaces that form it are mathematically defined. The STL file in fact saves, for each triangle, only the coordinates of the 3 vertices in space (coordinates x, y, z) and the direction of the normal. This leads to a series of problems, as we can see from the image shown here as an example, dictated to approximate curved contours with broken straight lines.

In this article we are going to analyze these problems.

What format to choose, binary or ASCII?

The STL format has two different saving standards: ASCII and binary.

These are just two different ways of saving and formatting information about the triangle mesh. The binary type file is the most convenient and used due to the best efficiency in terms of space occupied (it can reach a quarter of the corresponding file in ASCII format) and is recommended for applications related to additive manufacturing.

The ASCII format, on the other hand, has the advantage of being easy to understand and read even by an operator, which is useful only if you want to manually change the coordinates or information relating to the individual triangles.

What is an .STL file used for?

STL files are often used by programs related to the world of additive manufacturing, as an intermediate step between the perfectly described geometry of the CAD and the machine language of the printer. In fact, most job preparation and slicing software use this format to do their job.

It is therefore necessary to fully understand the advantages and limitations of this format in order to obtain the best possible results in the printing phase.

How to open a .STL file?

STL files are now recognized and imported correctly by most 3D modeling software due to their simplicity.

Integrated into Windows 10, there is a tool called 3DBuilder able of opening and viewing this and other types of 3D files.

Where to download free or paid STL files?

There are several sites for the diffusion of both free and paid 3D models. Below is an overview of the most used and widespread.

• Thingiverse: It is the site par excellence for sharing 3D models online. It has the largest community and all the templates you can find here are free. But pay attention to the quality of the downloaded files, which often have problems in the mesh.

• Myminifactory: This site has established itself as a platform with the highest standards and which verifies the objects that are uploaded. There are both free and paid files, the latter often with guides to obtain the best results when printing.

• Yeggi: Yeggi is a 3D-files search engine, to facilitate the search for 3D models on some of the most famous sites at the same time. It is a useful tool for carrying out fast and effective searches.

• Prusa printers: The famous Czechoslovakian printer brand has for some time now proposed its alternative to thingiverse, but with an interesting variation: being able to download files already processed and ready for printing on the prusa machines. All designs are free and integration with the Prusa ecosystem is a great advantage if you own one of their machines.

• Thangs: This site aims to be a thingiverse 2.0 with a number of new and interesting features. Some of these are the ability to collaborate with multiple people on a project and the ability to search the site using a 3D geometry/file.

• Cults3D
• Pinshape
• Grabcad: Unlike the other sites proposed here, Grabcad proposes itself as a platform voted not only for sharing stl files, but for real cad files in the most used formats.

• NASA: The American space exploration agency has organized a collection of space-themed material for 3D printing enthusiasts. The models are really excellent and cared for.
• CGTrader: This is an asset sharing marketplace not only for 3D printing but also architecture and game development.

How to export .STL files?

As an example for this guide we will use Autodesk Fusion360, but the considerations we will make are translatable to any other software that does the conversion both during import and export from other 3D formats to STL.

After pressing the right button on our object and selecting "Save as mesh", we will find ourselves in front of the following interface that allows us to define the key parameters for exporting and creating the triangulation.

As outlined above, we select the binary format, which is more efficient and allows you to create smaller files. We also select our unit of measurement, the millimeter.

The "mesh preview" box allows us to visualize the triangulation on the component in real time, but it is a very intense function for the PC, especially in the case of dense meshes with a high number of triangles.

In the section "refinement options" we have the 4 fundamental parameters that determine how the triangulation is created:

• Surface offset - defines the maximum distance between edges of facets and edges of surfaces - recommended value 0.005 mm.
• Perpendicular Offset - defines the maximum angle between the perpendicular vectors of the facets - recommended value 1.
• Maximum edge length - as the name suggests, this parameter limits the maximum length of the sides of the triangles - recommended value 10 mm.
• Aspect Ratio - indicates the maximum ratio that two sides of the same triangle can have. This parameter limits and avoids the creation of degenerate triangles, for example with a very small base (0.01mm) and a very long height (20mm) - recommended value 15.

By varying these 4 parameters we can adapt the creation of the mesh to our object and for example have a more sparse triangulation for components with flat surfaces or more dense in the case of curved freeform surfaces.

In other software, we can identify these parameters in the same way, perhaps with different names but which play the same role. In case of doubts about how to proceed with specific software, we suggest using the manual or technical support offered by the software-house or drawing from the huge library of online tutorials.