As 3D printing / additive manufacturing becomes a more accepted every day industrial tool, the focus is turning to best practices that help smooth out the kinks in preparing parts for the additive build.
By Leslie Langnau, Managing Editor
–Don’t think about designing for manufacturability; simply create the design and address manufacturing later.
–Let your creativity free.
These are common bits of advice when it comes to developing a design for additive manufacturing. The implication is that design engineers are not taking full advantage of the benefits of additive manufacturing.
The other side of the story is that many service bureaus and additive support builders comment on how often they must “fix” an engineer’s design to ensure a proper additive build. Certain features, like closed volumes, overhangs, undercuts, or minimum thicknesses for example, must be dealt with before the actual additive build.
Designing specifically for additive is not as simple as many tell you it is. And because the process is not all that simple, it leads to several questions:
• Is there a missing piece between the CAD-STL-Additive build process? If so, what is it?
• Do CAD programs lack data or features to help engineers ensure that their designs can be made in an additive process?
• Do engineers have insufficient experience and knowledge to take advantage of additive’s design flexibility?
• Is the STL program insufficient for today’s machines?
• Is the problem with the additive machine itself?
The answer to all of these questions is a qualified yes.
Part of the issue is the assumption that CAD tools will automatically, accurately and smoothly move critical design data to each successive tool used in the product development process. But CAD vendors have built their software using proprietary algorithms and formulas. When CAD drawings go to the next step, some design data, such as designer intent, do not go with them. And neither STL nor AMF can fill in the data gaps from CAD files.
To address part of this issue, many AM vendors have developed a type of 3DP driver or middleware that takes the data from the STL file and fills in some of the information needed for printing. These parts include support structures and build topology, but this middleware can still miss some of the engineer’s design intent. Noted Anthony Graves, product manager CAM for Autodesk, “Middleware for CNC is CAM. The middleware for 3DP is what each vendor has developed. This middleware is a practical state of the union for the process workflow, and gets parts prepped for a machine quickly and easily. But STL is not the right tool to check the design to ensure it can be printed. And in some cases, this middleware may not support the STL model.”
Additive technology is so new as a manufacturing process (within the last 3 to 5 years) that no one has yet established best practices for CAD design or design for AM in general. Such a list is evolving. But, as Mark Barfoot, Managing Director at Hyphen Services, noted, “What’s needed is a ‘button’ on the CAD program that analyzes whether the design can be additively made, similar to what is available in most CAD programs for CNC machining and molding.”
Hod Lipson, a developer of the Additive Manufacturing File (AMF) program (a replacement for STL) would agree. He noted in a recent presentation that additive machines can print just about any shape, but it’s not quite as easy to design every shape in CAD. Thus, some of the issue of “designing for additive manufacturing” rests with the CAD programs.
For example, some of the issues relate to the design being modeled as a surface body in a CAD program rather than as a solid body. CAD programs do not automatically convert 2D surfaces to 3D objects.
Here’s another example, this one from a Geomagic tutorial. When processing a CAD model for printing, many 3D printers project a series of cross section curves through the model to determine what to fill and what not to fill. Any gap in the cross section curves, i.e., not water tight, will result in print errors.
Another common issue is designing volumes that intersect each other. 3D printing requires one surface. Intersecting volumes in a CAD file become separate meshes in the STL file, rather than a mesh around the entire part. The result will be a failed 3D print build.
Some CAD developers take a different view. Noted Graves, such issues are more with CAM than CAD.
The newness of using additive for manufacturing, as opposed to using it for prototyping, is also a factor in the limited experience many engineers have with this technology.
The newer 3DP/AM machines’ ability to print/build in color, multiple materials, and textures taxes the ability of the STL program to deliver all the data to these machines. That is why Lipson and his team were given the task of developing a newer CAD to AM translation program, known as AMF. See the sidebar Building a better CAD to 3D print translator for more on this issue.
Finally, part of the design for additive manufacturing issue rests with the machines themselves. The process of 3D printing is not standard. There are at least seven technologies you can use to additively build a part, and more are being developed as innovators experiment with new ideas. (Just watch a crowd-funding resource like Kickstarter to see what may come.) Each machine executes the build process a bit differently. Each machine has nuances to its operation.
As Fred Fisher, Director of Materials and Applications Product Management, Stratasys Ltd., noted, “The best chances of success are when engineers know how to optimize the design for use on an AM process to produce their part.”
For example, some AM machines are not good at horizontal cylindrical holes. But a powder based AM machine handles these types of holes just fine. Thus, understanding how each AM technology works will help engineers create a design that can be efficiently and effectively additively made.
Future 3DP/AM software should be able to deliver better slicing algorithms, better support structuring and development, and preview functions. In the mean time, software is available to check whether a part is ready for 3D printing. Many 3D printing service and prototype providers use some of the following programs to make any needed changes to a CAD design.
Magics, from Materialise, is viewed by many as one of the best tools around for this need. It is a data preparation software and STL Editor. The STL Editor portion lets you fix STL files without having to return to the original CAD file.
File fixing includes repair of flipped triangles, bad edges, unwanted holes, and the program can check for and fix collisions and other defects in the STL file. It also lets you apply textures, perform Boolean operations and advanced cuts, and duplicate parts or orient them in the best way for printing. It will even let you create no-build zones. Magics will also let you view slices and detect collisions.
Materialise recently announced 3-matic STL. This software enables design modification, remeshing, and the creation of 3D textures, lightweight models, and conformal structures all on STL. This software is used to quickly prepare CAD files from topology optimization, as well as to clean them up for 3D printing. Design modifications may be made directly on the STL, the scanned model or the CAD data. Rough STL files can be smoothed, reconstructed and simplified, creating a cleaner part. Making changes at this level ensures that the file is immediately ready for further finite element analysis or 3D printing. Additionally, 3-matic STL can add lattice structure to the design, saving even more material.
Netfabb is a cloud-based tool, available in several versions. The free version lets you analyze, test and repair STL files. You can split and cut them into parts, close holes, repair meshes, and control part placement and orientation on a build platform.
The Netfabb Studio Professional version is a full-range-mesh edit, repair, analysis, and slicing software for a number of input and output formats. It includes Boolean operations and file size reduction in its feature offerings.
Another popular mention is Rhinoceros 3D, or Rhino. It lets you use NURBS to create complex and unique 3D shapes. This software offers mesh repair tools, like CAP Holes and Remove Duplicates, which can make 3D printing preparation work easier.
A few 3D scanner programs offer some STL repair tools. Geomagic, from 3D Systems, for example, helps repair meshes made from scans.
Building a better CAD to 3D print translator
Chuck Hull invented the first stereolithography 3D printer, and he is also credited with inventing a program that can take a CAD file and convert it into code that would enable the 3D printer to build the design. That program is known as STL. Today, every 3DP/AM system is compatible with it.
STL uses the triangle—the most basic form for a surface that can be described from point data only—to convert CAD data into 3D printable data. Any three points can be chosen to approximate a portion of a surface. The issue is that the triangle is, by definition, flat. Thus, if the surface in the CAD file is curved, than the STL file delivers an approximation of that CAD data, resulting in a measurement deviation. One way to reduce the deviation is to make the triangle smaller than the surface it’s describing. But you can only go so far with this before the STL file becomes over large.
But this issue, while inconvenient, is not the main concern for many users. The recent 3DP and AM machine developments of printing in colors, textures and multiple materials have exceeded the ability of STL to contain all the needed data.
Other issues users are reporting are unit of measure inconsistencies, it doesn’t scale well with complex geometries like lattices and high resolution curved surfaces, it doesn’t handle parts with high volumetric complexity well, the file tends to have data redundancies, in addition to the previously mentioned inability to accommodate the newer features of AM machines, such as color and multiple materials.
For many users, a critical issue is the inability to alter an STL file and make corrections. For example, it would be nice to go into the file and change any meshes with normals pointing in the wrong direction.
However, because it is so well known, STL is still the dominant program used to send CAD data to any 3D printer.
But there is another option—the Additive Manufacturing File format (AMF). AMF (ISO/ASTM 52915 AMF specification) is supposed to be the solution to the problem of frequent corrections to 3DP/AM part design. The software does the job of delivering buildable parts; the problem is that it is not yet a part of every CAD software program or every 3D printer or AM machine. Vendors claim they are working to incorporate AMF into their CAD programs and printing machines, but they also claim they are waiting for stronger user demand for AMF.
AMF is an XML-based open standard for additive manufacturing. It divides code into “markup” and “content” tags. Similar to STL, objects are described using a mesh of triangles, but AMF lets you describe more details of your design, including the color of the surface and the volumetric structure of the interior. For example, you would use meta-materials to describe the internal structure inside a mesh surface. A meta-material is a combination of primary materials and voids in various combinations.
AMF handles the increasing part complexity and size of today’s parts, as well as the need for greater resolution and accuracy. It can handle large arrays of identical objects, complex repeated internal features (such as meshes), smooth curved surfaces that require fine resolution, and multiple components arranged for efficient printing.
In addition, the developers of AMF made it so that existing STL files can be converted into a valid AMF file without any loss of information. You can also take an AMF file and convert it to STL.