Data Discussion - The Ins and Outs of an STL File

PrintForm - Data Discussion - The Ins and Outs of an STL File


What is an STL file? An STL file stores information about 3D models and is required to manufacture parts in nearly all 3D printing processes. This format contains only the surface geometry data of a 3-dimensional object without any representation of color, texture or other common model attributes. It contains no solid bodies, only surfaces.

These files are usually generated by a computer-aided design (CAD) program, as a final file type (“terminal file”) of the 3D modeling process. “.STL” (“Dot 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 program, it allows a computer to communicate with 3D printer hardware.


File resolution scale



The letters S, T & L themselves are initials for Standard Tessellation Language. It’s often assumed to be an acronym for Stereolithography since it’s the file type associated with the process of SLA, rather it has become the standard for all 3D printing processes. The file is actually a series of triangles that, when summed, create a data file that is readable and (generally) printable by nearly every industrial 3D printer.  The process of tessellation or simply triangulation can be seen on a tiled floor or wall, which is one simple everyday example. The brick siding of a home or office building is another.

Since the late 80’s, the STL file format has been adopted and supported by most CAD software packages, and today is widely used for rapid prototyping, 3D printing, and computer-aided manufacturing. The genius lies in the fact that a 3D design can go directly from CAD to manufacturing without the historically cumbersome necessity of building a mold of the negative (usually through CNC machining), and filling the cavity with material to create the end part.


The basic inspiration of an STL model was to tessellate the two dimensional outer surfaces of 3D models using several triangles (also called “facets”) and store data about the facets in a file. When an STL file is described as low res or low resolution, it simply means it has fewer triangle faces. The term “faceted” gets its origin from this concept. There is an ideal middle ground in most 3D printing applications, that is, resolution which is neither too low nor too high. Files that are unnecessarily dialed up to produce several tens of thousands of tiny triangles tend to be oversized (25MB+) and are difficult for most software programs and 3D printers to successfully process, let alone build.

For a visual example of a real life shape that’s been tessellated, imagine a disco ball. From a distance it is round, but upon closer inspection, it’s actually many tiny flat surfaces that come together at the edges and corners to create a shape.

The two formats of an STL file are Binary & ASCII (sometimes pronounced “Asky” for short). The latter typically generates files that are far too large, so when choosing between the two at the time of file export, generally Binary is preferred.



Only a 3D design that’s specifically suitable for 3D printing, and correctly exported, is printable. The STL file is simply the vehicle for the data and not a guarantee that the part will build. Unshared edges, multiple bodies and missing faces can each be a symptom of designs improperly saved or exported and often only require a small tweak within the native design platform to correct, but can cause major headaches down the line once a part is printing. Files that are correctly designed and exported are known as water tight or air tight models, referring to the close knit triangles with points that meet and are free of voids or gaps.



Universal & Wide spread use: The STL format is universally supported by nearly all industrial 3D printers. This cannot be said for the others that were specifically created for individual printing platforms in mind (i.e. OBJ, VRML, AMF and 3MF). These other file types may be great for a niche printer model or manufacturer but lack the cross platform ease of the STL.

Mature & established, industry wide recognition: Many depositories of buildable 3D models you can find on the internet support the STL file format. The ubiquity of this file type within this ecosystem, combined with STL-based software upgrades by many 3D printer manufacturers, means a large user-base that is heavily invested in the STL file format. This means there are plenty of third party software packages that allow export of STL files.

About the Author: Bill Artley is the Vice President of Operations at PrintForm and has written many articles & white papers on product development, sourcing, additive manufacturing, and materials. He is a contributing author and peer reviewer of The 3D Handbook (, a top 10 Amazon Best Seller in its category.

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