3D printing file format cage match: AMF vs. 3MF

With 3D printers, applications, and distributed manufacturing speeding ahead, our beloved .STL file may soon be considered an honorable mention. While it will always have a place in our toolbox, it lacks the information package required to keep pace with the evolutionary trajectory of 3D printing.

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Who will take the crown as the de facto file standard for additive manufacturing?

The rapidly evolving world of additive manufacturing is approaching a nexus in terms of interoperability. Your design process may involve multiple file formats as the product originates from napkin concept to 3D printed model. While the .STL file has been an industry standard for over 25 years, it only contains surface mesh information – just enough to print in a single material or homogeneous shape. It lacks any data accrued during the design loop such as information on materials, model orientation and position, textures, colors, sub-structures, and multi-material geometries.

During the design-to-print pipeline, the passing of file information through different applications (CAD, slicers, etc.) may also cause a loss of data or incompatible geometries in the model during translation. Intermediate file formats do not support all of the capabilities that each printer may offer. Other popular formats such as .IGES, .STEP, .NURBS, .OBJ, and .VRML provide varying degrees of capability, information, and accuracy.

This has fueled the call for a single, backward compatible, interoperable, and unified file format that would act as the model’s complete DNA; containing the entire genetic code of its design, from color to geometry that could operate across multiple platforms and applications. The intended result is to not only streamline manufacturing, but also provide a scalable and easy-to-understand format that could be interpreted by any system. It would need to optimize the ability for 3D printers or other systems to easily interpret the complexities of a model’s geometry, arrays, resolution, and internal features with improved accuracy.

While .STL reigns king, the .AMF (Additive Manufacturing Format) and .3MF (3D Manufacturing Format) file formats are jockeying for position:

AMF (ISO/ASTM 52915:2013)

In 2009, the American Society for Testing and Materials tasked ASTM Committee F42 on Additive Manufacturing Technologies to address the need for a replacement to the .STL file. At the time it was dubbed “STL 2.0” and a design subcommittee led the development effort. Their goal was to create a file format analogous to a .PDF file for documents, containing as much information possible to describe an object in the same way. This would provide more data prior to conversion into a format for fabrication and would also allow developers to create OEM software that could pick and choose whatever information was necessary for their 3D printing hardware to operate.

The subcommittee developed the .AMF format for technology independence, simplicity, scalability, performance, and both future and backward compatibility. Its first iteration was developed in 2011 and subsequently approved in 2013 in conjunction with the International Standards Organization (ISO).

AMF is an XML-based open format that provides complete information through a hierarchy of five main elements: object, material, texture, constellation, and metadata. This provides the basis for information on a model’s shape, composition, color, materials, and geometry. It also introduces the concept of print constellations, allowing information on multiple objects to be positioned and arranged together. This would result in increased packing efficiency or for use with large arrays of identical objects that would be 3D printed on a single build tray. Among .AMF’s additional characteristics:

  • XML-based for easy reading, writing, and processing with as much information as possible to describe an object, its materials, and other manufacturing features.
  • Incorporates triangular meshes to describe surfaces, allowing all vertices to be defined as opposed to single vertices in .STL files.
  • AMF triangles can describe curved surfaces and/or non-planar edges on 3D surfaces more accurately than .STL files. They are not limited to straight-edged, planar triangles.
  • Objects can be defined in CAD programs more easily, allowing 3D printers and other hardware to pick and choose information necessary to perform its operation. For example, if a printer is only capable of working with one color, multi-color information is simply ignored. This capability works for all elements including material, texture, and composition.
  • Incorporates extensive metadata including name, author, company, description, volume, tolerances, and more.
  • Geometry support for mixed and graded materials, sub-structures, microstructures, porous, and stochastic materials.
  • Defines color specification using RGBA values.
  • Includes support for graded colors, texture mapping, and transparency backgrounds.
  • Supports texture data for 2D and 3D maps.
  • Can be stored in plain text for or can be compressed into .ZIP file format.

While starting off slow, acclimation to .AMF has grown since its introduction. CAD vendors originally took a “wait and see” approach to the matter, preferring to integrate .AMF according to customer demand. Development has been ongoing since and eventually gained a foothold with larger application such as SOLIDWORKS and Autodesk’s suite of products. Stratasys also signaled its cooperation, foreseeing the potential benefits in .AMF and continue to support it today. 3D printing service providers such as Shapeways and Materialise added .AMF to their lineup of accepted formats while also contributing resources to its development. Due to the myriad of interdependent technology requirements these companies require, .AMF was ready to work with 3D printers on the market today. In addition, conversion and creation tools have been developed to help users migrate older .STL files to .AMF.

3MF

Parallel with application development for Windows 8.1 and subsequent work with the upcoming Windows 10 OS, the .3MF file was developed by Microsoft with the goal of creating a seamless, high quality, 3D printing experience for consumers or manufacturers. After concluding that current formats would be incompatible with their print pipeline, the .3MF format was developed as a standardized framework for Microsoft hardware and software ecosystems, easily passing data from apps to their devices while retaining detailed model information. .3MF would help define a feature set for 3D printers on the market while also supporting subtractive manufacturing devices such as laser cutters and CNC mills.

This year, Microsoft announced the 3MF Consortium, a managed program by big name companies to help develop a “full-fidelity” 3D file that could work on a variety of printers, devices, applications, services, and platforms. Companies such as NetFabb, Shapeways, Materialise, Stratasys, 3D Systems, SIEMENS, HP, Autodesk, and Dassault Systemes soon became members, seeing the benefit of developing a format that would solve classic interoperability issues. The result would help teams across multiple industries focus on innovation rather than the grunge work of 3D model integrity.

3MF is also XML-based and features geometry representation similar to .AMF, but in a more compact and size-friendly format. The file defines all standard, optional, and mandatory parts, with complete model information contained in a single archive. It centers on the concept of a “3D payload”, a collection of interdependent parts and relationships that reside in one standard package. The payload consists of a 3D model(s), core document properties, digital signatures, 3D print settings known as “PrintTicket”, thumbnail images of all models, and 3D texture information.

The goal is for 3D printing in Windows to be the same as any document: select a printer from list, choose options, and print. The application converts the model to .3MF and encapsulates it in an OpenXPS package. It’s then extracted by the print driver, converted into a readable format, and sent to the 3D printer. The .3MF file not only solves Microsoft’s own print pipeline, but also provides the following advantages for everyone across the board:

  • Complete model description designed around the principle of extensibility - allowing additional functionality and information updates in the future.
  • Seamless interoperability for Windows users and developers, offering a feature set designed with 3D printing capabilities in mind.
  • Human readable, XML based-data format with definitions relevant to 3D manufacturing.
  • Scalable and usable across multiple platforms.
  • Accommodates larger amounts of data in a smaller compressed file size.
  • Multi-material and multi-color support.
  • Support for custom metadata by third parties.
  • Support for various subtractive manufacturing devices.
  • Supports transforms, object references, and multiple objects within a single archive.
  • Single objects can be referenced or moved without affecting the mesh.
  • Support for older .STL files.

The complete specification document for the .3MF file is available through the 3MF Consortium or can be found here.

While still in its infancy, .3MF is gradually becoming acclimated with the 3D printing industry. Support has yet to be confirmed for additional features such as scripting and curved triangles. In addition, concern has also been raised on how .3MF handles hardware resource issues when working with larger meshes. Doubts also arose after its debut. Would 3MF become a proprietary Trojan horse for larger companies to monopolize and grind out competition or would it be released as open source? Would this open the door to DRM issues, resulting in .3MF derivatives that would be subject to lawsuits? Was it simply a matter of benevolently advancing the concept of “plug & play” for 3D printing?

Regardless of the dubious buzz, Microsoft chose wisely, donating code to Github and helping to form a consortium of companies that would help manage the process. You can also request the 3D Printing SDK and .3MF format specifications by contacting ask3dprint@microsoft.com. While doubts remain in terms of patent legalese and source code rights backed by a consortium of conglomerates, the move was seen overall as a positive step for the 3D printing industry.

Let the battle rage on!

Both formats are creating gradually creating their own path and plenty of development still lies ahead. Some argue that since the development of the .AMF file, there’s no need for .3MF. Technical differences aside, both accomplish what they were set out to do. The real difference is the backing and publicity behind .3MF as well as buzz about Microsoft’s modus operandi. Customer demand and acceptance for is also a factor that remains to be seen. But from a design and engineering standpoint, a unified file format is a welcome relief in a sea of crisscrossing compatibilities.