The Format Wars
As the additive manufacturing industry has evolved, it has done so by piggybacking on the design software of related manufacturing industries. As a result, the process of design-to-manufacture utilizes many file types and modification programs throughout the cycle. This paper will examine the evolution of file formats used to define digital designs.
Programs of Use
Though there is not one industry standard, 3D models are traditionally stored using the stl format, the obj format, or most recently the amf/3mf format. Most design programs can open any of the three.
A number of design programs utilize their own unique file format, such as Autodesk Fusion 360 which, by default, creates an f3d file. These design programs are typically capable of opening and editing any of the traditional formats. Design programs are also typically capable of exporting the file in any of the traditional file formats.
File Formats
The formats below by no means capture the entire scope of the 3D modeling file community. Of these three the stl format is the most popular, the obj format is the most versatile, and the amf format is the most adept for additive manufacturing but has been succeeded by the open-source 3mf.
stl Format
This format is an acronym, but the community is not in 100% agreement about what the acronym means. There are multiple opinions, each as valid as the last. Standard Triangle Language refers to how solid objects are defined: as a mesh made of dozens, if not thousands of, two-dimensional triangles. Standard Tessellation Language refers to the same feature, as tessellation is the geometric method of describing a surface with multiple smaller shapes. Another going theory states that stl represents the important syllables of the word STereoLithography, which was the first method of additive manufacturing and utilized the file format.
The stl file is often the “go-to” file of the 3D printing community as a method of sharing designs. It is not uncommon for a user to share an image of their recent project to the community and be met with requests for “stl?” or “share STL?” in the comments. In fact, this ability to share files is an excellent feature of the 3D community and one which allows a high level of collaboration/file sharing.
obj Format
The obj format is native to the 3D modeling community as a geometric definition program. By defining vertices, vectors, and surface geometry, the language describes a three-dimensional object. A notable feature of the obj format is its ability to utilize companion formats defined in mtl files. This format is versatile and comes in as a second to the stl.
Materials Template Library files, or mtl files, can be incorporated into obj in a similar way that css is incorporated into an html file. These files provide more detail and texture as they relate the object to a light source and determine characteristics of reflection and shading.
amf and 3mf Format
As an abbreviation for Additive Manufacturing File, this format was created by the 3D printing community for the 3D printing community. As the community evolved, it needed a dedicated file type created for additive design.
Though the stl file provides accurate definition of a three-dimensional object within a digital workspace, it does little to define unique characteristics past the shape. The amf file format gives the model a boost by providing definition of color, material characteristics, metadata, and the tricky capability of defining curved triangles.
Recognizing the popularity of the stl file as the dominant method of transfer, the amf format was designed to optimize usage with six major considerations: technology independence, simplicity, scalability, performance, backward compatibility, and future compatibility. Of these six, the final two warrant deeper insight.
Backward compatibility allows the amf file to be easily transformed back into an stl for sharing around the community. This connection to the past is an excellent feature as it allows the stl-using community community to easily incorporate amf files into their work if they have limited software.
Future compatibility is dependent on the mindset that our technology, our needs, and the industry are ever changing as it seeks to pre-emptively avoid the problem which drove the creation of the amf format. Similar to provisions within the United States of America’s founding documents for creating amendments to the constitution, the amf format contains provisions to describe the process by which future updates are to be included.
This provision has already been implemented in the establishment of the 3mf format (3D manufacturing format), an open-source version of the amf format which the community has begun to accept. The program is coded in XML rather than binary, making it readable to the skilled programmer.
Relation to 3D Printing
It should be noted that any one of these files by itself is not adequate for production. These design files, once completed, are run through a slicer program which prepares the model for additive manufacture. This program will “slice” the three-dimensional object into a series of two-dimensional patterns which, when stacked upon each other, create the desired model. The file produced by the slicer is called a gcode.
Conclusion
Stl is by far the most widely accepted format while obj succeeds at storing “extra” data such as color or light. Amf takes the best of both worlds but is not widely accepted. 3mf is the open-source update which the community recognizes as the new up-and-comer. Always check file type before beginning an export or transfer and coordinate with the receiver.
The 3D printing community is scattered and diverse. By identifying areas of diversity which have not yet been standardized and doing so, we can push the industry forward just a little bit. Flexibility and consistent review will allow us to stay ahead of the industry by changing to meet the market. as it evolves (which it does, fast). Understanding the differences in these formats allows us to better design the future.
