The most important file formats in computer-aided design for 3D printing

What is an STL file?

In short, an STL file stores information about 3D models. This format describes only the surface geometry of a three-dimensional object without any representation of colour, texture or other common model attributes.

These files are usually generated by a computer-aided design (CAD) programme as the end product of the 3D modelling process. A 3D scan of a physical product can be created as a starting point. The file as “.STL” is the file extension of the STL file format.

The STL file format is the most commonly used file format for 3D printing. When used in conjunction with a 3D slicer, it allows communication between a computer and the 3D printer hardware.

Since its humble beginnings, the STL file format has been adopted and supported by many other CAD software packages. It is now used for rapid prototyping, 3D printing and CAD-aided manufacturing. Hobbyists and professionals alike use it.

How does a 3D model become an STL file?

The main purpose of the STL file format is to encode the surface geometry of a 3D object. It encodes this information using a simple concept called “tessellation”. Tessellation is the process of tiling a surface with one or more geometric shapes so that there are no overlaps or gaps. If you have ever seen a tiled floor or wall, this is a good example of tessellation in real life.

The basic idea is to tessellate the 2-dimensional outer surface of 3D models using tiny triangles (also called “facets”) and store information about the facets in a file.

How can an STL file be improved for printing?

The STL file format approximates the surface of a CAD model with triangles. The approximation is never perfect and the facets result in a coarsening of the 3D model in memory requirements.

The 3D printer prints the object with the same coarseness that is specified in the STL file via the resolution. Of course, as the triangles get smaller and smaller, the approximation can get better and better, resulting in higher quality prints. However, if you reduce the size of the triangle, the number of triangles needed to cover the surface also increases. This results in a gigantic STL file that cannot be processed by 3D printers. Furthermore, it is tedious to split or save such huge files.

It is therefore very important to find the right balance between file size and print quality. It makes no sense to reduce the size of the triangles to infinity, because at some point your eye will no longer be able to distinguish between the print qualities.

Most CAD programs offer a number of settings when exporting STL files. These settings control the size of the facets and thus the print quality and file size. Let’s dig into the most important settings and find out their optimal values.

OBJ

What is the OBJ format?

In short, the OBJ file format stores information about 3D models. It can encode the surface geometry of a 3D model and also store colour and texture information. An OBJ file is usually created by computer aided design (CAD) software as the end product of the 3D modelling process. The file extension corresponding to the OBJ file format is simply “.OBJ”. The format is manufacturer-independent and thus enables exchange between different CAD software. In recent years, it has become increasingly popular for multi-colour 3D printing, as the otherwise common 3D printing format STL does not support colour and texture information.

The use of OBJ in 3D printing

The most dominant format in the world of 3D printing is STL. However, STL is an old file format that, while popular, has not really kept up with the times.

The fidelity of 3D printing is rapidly approaching micron resolution and multi-colour models are becoming increasingly popular. The STL file format does not handle high resolutions very well, as higher resolution comes at the cost of increasing file size. The STL format is also not suitable for multi-colour 3D printing as it does not support colour and texture information.

In contrast, it can approximate the surface geometry as accurately as required without increasing the file size. This is possible using Bezier curves and a method called NURBS, which we will not cover later in this article. In addition, the OBJ file format provides native support for multiple colours and textures within the same model. Thus, the OBJ file format wins out over the STL format when you need accurate, multi-coloured models, and it will likely become a very popular 3D printing format in the future because of this.

On the other hand, the OBJ file format is not as universal as the STL format. Almost all 3D printers support the STL format. The same cannot be said about the format, although it also enjoys reasonable acceptance and support. So if you are 3D printing a single-colour model with a standard printer, the STL format is still preferable.

Both the OBJ and STL formats have a very mature ecosystem with a large number of users and a variety of third-party tools.

STEP or STP?

The name results from the task of the format – Standard for Exchange of Product Model Data. As the successor to the IGES format, it serves the purpose of recording the entire life cycle of a product. It is manufacturer-independent and standardised, which is why it is so important within the production chain. It is used for exchange between CAD, CAM and CAE systems and thus has a wide field of application in various industries. The automotive and aerospace industries, as well as architecture and construction, use the format.

What influence does the STP format have on 3D printing?

The format is primarily used for exchange, so that the formats can be brought to a common denominator in the most diverse software. For example, a component may have been designed in Catia and then processed in Fusion360 or Solidworks. Since all software manufacturers have their specific formats, files are exchanged in STP format.

It also saves the position of objects in relation to each other, geometric tolerances and material properties and types. For 3D printing, the format is then converted to STL or OBJ, as most printers work on the basis of these formats.

If you need 3D data for a product or component, please send us a request for digitisation via our spare parts request.