How to properly prepare a file for valuation and 3D printing – what is the .STL format?

3D printing is a set of manufacturing techniques that differ in the types of materials used and the methods of their selective bonding layer by layer. However, there is one thing that is the same for all methods: .STL – the file format in which 3D models are delivered to 3D printer software. In this article, we describe what this extension is, what its features are, and list some of the most important errors that appear during their generation.

Regardless of the type of 3D printing technology, the process of creating 3D prints always takes place in the same five steps. First, you need to create a 3D model of the printed part in CAD software. Flat drawings are not suitable for additive production or preparation of a valuation of the 3D printing service – each object that is to go to the 3D printer must have three XYZ dimensions. The model can be made in any 3D design software – paid or free.

When the model is ready, it should be exported to the .STL extension (although selected manufacturers of 3D printers also allow files saved in other formats: .STP, .OBJ, .3MF, and recently also native files of selected CAD programs). The third step is to prepare the 3D printing process in the 3D printer software to which we upload the .STL file. The fourth – the most important one is the 3D printing itself. The fifth – often omitted in various studies, is the so-called post-processing, i.e. cleaning 3D prints from material residues and supporting structures, as well as possible grinding, varnishing or staining.

The .STL format was created in the mid-1980s with the first additive technique in the world – stereolithography (3D printing from resins hardened with a laser beam). The abbreviation of stereolithography comes from the name (other extensions such as “Standard Triangle Language” or “Standard Tessellation Language” are also used occasionally). The extension was developed by 3D Systems and made available for free. Currently, it is supported by every CAD and 3D modeling software – after creating the model, just export it to .STL.

Each 3D model when exporting to .STL is converted into a mesh of triangles that make up its geometry. Simple geometric solids, such as a cube, consist of only a dozen or so triangles, but the curved surfaces must have much more generated in order to recreate all roundings with the help of triangles:

The more triangles, the more accurate the object will be, but the file will weigh a lot more and there may be a problem when trying to email it. At the same time, one of the most common mistakes when exporting .STL files is generating too few triangles, which results in very sharp (“angular”) edges. In other words, a 3D model in CAD software may be perfectly circular, but it will turn into a polygon when exported …

Based on the .STL file, the 3D printer software generates instructions for the device, which, depending on the type, performs certain operations with its instrumentation. The 3D printer does not know what it is printing – the 3D printer executes the instructions from the file. If the file is badly prepared – the 3D printer will print it incorrectly. The 3D printer does not know that it is to print the ball, but in which direction it should move the print head or the laser beam that is baking the powder.

The 3D printer software simulates the 3D printing by specifying, among others production time and amount of material used. In the 3D print preview, you can see all possible design flaws and react earlier. When using the services of a 3D printing house, professional companies will check it before starting the 3D printing and inform the customer about potential errors or threats. Nevertheless, in the end, the responsibility for the occurrence of this type of defects is always the responsibility of the designer / client – the 3D printing house may try to guess that a part of the project is defective, but it is not she who is the creator of the project?

3D printing is based on solids. One of the mistakes often made by designers is designing objects with planes (especially architectural mock-ups). For example, if a designer uses six planes to build a cube, as long as it looks correct in the program, after exporting to .STL format, it will simply be a set of six surfaces, the thickness of which has not been defined anywhere and will tend to infinity. Such a model is not suitable for 3D printing and the 3D printer software will not read it correctly (or not at all).

Other common design flaws include:

• too thin walls of the model, impossible to print correctly by a 3D printer (e.g. in SLS technology it is theoretically possible to achieve a wall thickness of 0.5 mm, but there is a high risk that it will deform during operation or damage during post-processing; safe limit is 0.8 mm)
• too small size of the 3D model or too much detail for the selected 3D printing technology
• wrong dimensions – during the export, the designer either set the scale incorrectly and the part is 10 millimeters instead of 10 centimeters, or he confused the units and generated a file in inches instead of centimeters; theoretically, models can be enlarged in the 3D printer software, but scaling up always results in a deterioration of quality – increasing the size, the segments connecting the vertices of the triangles increase, but no new ones are generated; for example, a sphere becomes angular after it is scaled.

The .STL format is an excellent interchange format, but it also has some shortcomings. As it was developed in the 1980s, i.e. at a time when most 3D printing techniques did not exist yet, it does not have some quite important information:

• color
• texture or surface quality
• dedicated material
• dimensioning (as described above – only after checking in the software you know what the dimensions are, but you don’t know if they are correct?).

The last important thing about the .STL format is that it is a “closed” format. 3D models cannot be freely edited or disassembled into component parts – they can only be cut or hollowed using simple geometries – cut in a plane or, for example, hollow a hole using a cylinder. They can also be combined or fused with other 3D models. Nevertheless, 3D models saved as .STL are very limited in terms of subsequent modifications – they are not assembly objects. Sure, there are programs for modeling (“sculpting”) .STL models, but they are expensive, specialized, and have more to do with manual modeling than with parametric CAD design.

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