What standards about AM are available globally?
Two main institutions, the International Standardization Organization (ISO) and the American Society for Testing and Materials (ASTM), globally prepare, publish, and develop standards regarding AM.
ISO has a technical committee, ISO/TC 261, working on AM. The scope of the committee is to standardize the field of AM concerning its processes, terms and definitions, process chains, test procedures, quality parameters, supply agreements and other fundamentals. (http://www.iso.org/iso/iso_technical_committee?commid=629086).
Standards developed under ISO/TC 261 are as follows:
- ISO 17296-3:2014
Additive manufacturing -- General principles -- Part 3: Main characteristics and corresponding test methods.
- ISO 17296-4:2014
Additive manufacturing -- General principles -- Part 4: Overview of data processing.
- ISO/ASTM 52915:2013
Standard specification for additive manufacturing file format (AMF) Version 1.1.
- ISO/ASTM 52921:2013
Standard terminology for additive manufacturing -- Coordinate systems and test methodologies.
American Society for Testing and Materials (ASTM) Standards
ASTM International Technical Committee F42 on AM Technologies is a non-profit organization working on AM. The scope of the committee is to promote knowledge, stimulate research and implement technology through the development of standards for additive manufacturing technologies. Standards developed by F42 are:
- F2792 Standard Terminology for Additive Manufacturing Technologies.
- F2915 Standard Specification for Additive Manufacturing File Format (AMF).
- F2921 Standard Terminology for Additive Manufacturing--Coordinate Systems and Nomenclature.
- F2924 Standard Specification for Additive Manufacturing Titanium-6 Aluminum-4 Vanadium with Powder Bed Fusion.
A proposed new ASTM International standard will serve as a guide to determine specific mechanical properties of materials made with an AM process. WK43112, Guide for Evaluating Mechanical Properties of Materials Made via Additive Manufacturing Processes, is being developed by Subcommittee F42.01 on Test Methods, part of ASTM International Committee F42 on AM Technologies.
In addition to WK43112, F42.01 is currently developing two other proposed standards:
- WK30107, Practice for Reporting Results of Testing of Specimens Prepared by Additive Manufacturing.
- WK40419, Test Methods for Performance Evaluation of Additive Manufacturing Systems Through Measurement of a Manufactured Test Piece.
AMF as new format for Additive Manufacturing.
Up to now, the STL file format has been in use for the last three decades for transferring information between the design software and the Additive Manufacturing equipment. Originally created by 3D Systems Company to be used with their Stereo Lithography equipment, the files store just graphical information of the object in the form of simple triangles, most of the time in plain ASCII text (there is a compressed/binary version of the file format too).
For several years, most of the tridimensional file formats was proprietary of some 3D design software company, such is the case of 3D StudioMax, Blender or SolidWork among others. Which mean that it was not possible to easily reconstruct the 3D shapes from those files in third party software as the ones included in Additive Manufacturing equipment such as 3D Printers. Those file's format was usually compressed and they store not just graphical information but also objects properties as also tools and steps made to create the object in the original software.
For those reasons, added to the fact that 3D Systems encourage the use of the STL format to gain a position on the Additive Manufacturing industry, there is a push for several new manufacturers of 3D Printers to easily adopt the STL format since it was easy to read and process in their own software.
But that simplicity also came with STL limitations, since it only describes the shapes in a form of simple flat triangles. There are no object properties stored (textures, colors, etc.), no tool or material information among other properties that are commonly used in several AM machines. Also, flat triangles can make bad approximations to the original shape and complex shapes can will be turned into large, heavy and cumbersome files.
Here is a simple triangle facet in STL format (note that just the vertex data for the triangle is stored):
facet normal 0 0 -1
vertex 0 0 0
vertex 0 2 0
vertex 2 2 0
For that reason in 2011 the “Additive Manufacturing File Format” or AMF was officially announced. Originally developed by Hod Lipson (and his colleagues at Cornell), the AMF is a XML-based Open standard to describe objects meant for additive manufacturing processes and to address the STL limitations .
- Describes the shape and composition
- Contains support for color, materials, texture and sub-structures (lattices, and constellations).
The way in which the AMF format works, is by starting each file with the XML declaration line specifying the XML version and encoding, for example :
<?xml version=91.09 encoding=9UTF-89?>
The rest of the file remains enclosed between the <amf> tag element. All XML standard tags can be used as :
<amf unit=9millimeter9 version=91.09 xml:lang=9en9>
Inside the AMF brackets, there are five top level elements:
- <object>: Defines volume(s) of material, each is associated with a material identification (ID) for printing. At least one object element shall be present in the file .
- <material>: Defines material(s) for printing with an associated material ID. A single default material is assumed if no materials IDs are included .
- <texture>: Defines optionally file image(s) or textures for color or texture mapping, each with a texture ID .
- <constellation>: Hierarchically combines objects and other constellations into a relative pattern for printing (optionally). Each object will be imported with no relative position data if no constellation elements are specified .
- <metadata>: Specifies additional information about the object(s) and elements contained in the file .
Each Geometry Specification inside an AMF files contains two child elements: <vertices> and <volume>. Starting at zero, in the order in which it was declared, each vertex is implicitly assigned to a number. The <coordinates> element is required and gives the position of the point in the three-dimensional space using the <x>, <y>, and <z> elements (v1, v2, v3 elements respectively) . At least one <volume> element shall be included after the vertex data. The volume encapsulates the object and multiple volumes can be specified for each object. Finally, the <triangle> child element is used to specify triangles that tessellate the surface of the volume, each of them will list three vertices from the set of indices of the previously defined vertices .
Regarding to Smooth Geometry, the AMF format assumes that, by default, all triangles and flat triangle edges are straight lines connecting their two vertices as the STL format, but also curved triangles and curved edges can be specified if needed. This improves fidelity and reduces the number of mesh elements required to describe a curved surface. To accomplish that, each curved triangle patch is recursively subdivided into four triangles by the parsing program, generating a temporary set of flat triangles at any desired resolution for manufacturing or display .
A simple sample AMF file takes the following shape:
<?xml version="1.0" encoding="ISO-8859-1"?>
Figure 1 show us the basic principle behind the triangle sub-division in the AMF format:
Figure 1: Notation used for sub-division of a curve in AMF (a). Triangles should be divided recursively to the desire resolution (b).
In the upcoming years, more and more AM software (desktop and on-machine), will include support for AMF files as the industry standardizes different elements of the process. It is not expected for us to deal with all the internals of the AMF format, but is important to understand the current limitations of the STL file format and how the new format approach will give us much more freedom and customization once it becomes mainstream.