Learning Objectives
By the end of this section, students will be able to:
- Define commonly used terms.
- Understand the building blocks of additive manufacturing.
- Understand the digital workflow required for 3d printing a part.
- Describe the STL file format
Additive manufacturing is often referred to as “3D printing,” which is an important component of the domain, but does not describe the entire process. 3D Printing is the act of making a 3-dimensional shape via a “printer,” while additive manufacturing is everything needed to make that shape into a “part.” Parts have requirements where form, fit and function are critical.
To make a shape, the 3D printer adds the material where it is needed, most likely in a thin, essentially 2-dimensional layer. The printer only concerns itself with a 2D layer at any point in time. It creates a stack of these layers, incorporating spaces and other details to build a 3-dimensional shape that can have complexity, internal details, and doesn’t typically require additional tooling to be considered complete. 3D printing is that additive approach, which is a major shift in innovation from legacy manufacturing. But as the box below demonstrates, 3D printing is only one aspect of additive manufacturing.
The Four Building Blocks of AM
Four building blocks are necessary for success in the world of AM: Machines, Materials, Digital, and People.
Machines are the easiest to focus on, specifically the 3D printers, but our definition expands beyond the printers to include the machinery required to meet the part requirements.
Materials
The building block of materials includes material science, metallurgy, polymers, ceramics, welding, powder, wire manufacture, finishing steps, and possibly heat treatment. How we manufacture the materials and the changes they will undergo both as a feedstock and subsequently as a printed object are very important.
Digital
Within the building block of Digital, we encompass design, simulation, sensing, and essentially any data input or output.
People
The management and organization of people is an essential skill to be successful in AM due to the multidisciplinary and complex nature of AM. Skilling to include AM thinking alongside traditional design for manufacture has to be recognized and permeating those skills throughout a company is a challenge.
Additive manufacturing has diverse process technologies. This diversity creates unique design spaces for shapes, sizes, materials, mechanical performance, etc. When we couple the mechanical operations with design, business, and qualification guidance, we need to seek a more overall comprehensive appreciation. For example, for a part to be qualified, we must first understand the requirements for that part. These requirements likely include both technical and commercial considerations. These requirements will eliminate some of the manufacturing processes as candidates. We then may need to consider what material is appropriate and the properties of that material in the manufacturing process. Now we get into the design of the part and the design of the build which will critically determine the cost of the part as well as the strength. And while design for manufacturing (DFM) is not a new topic, designing for additive manufacturing can be quite counterintuitive to those versed in legacy manufacturing. In this book, we weave together important topics to gain a strong, holistic understanding of additive manufacturing – which we’ll often abbreviate as AM – including how it is done and where it can be used.
Process Steps: Concept to Part
The best place to start thinking about using AM is at the concept stage. Often, this is not a luxury afforded to an engineer working on a part, because they need to replace a part that is already designed. However, the concept stage is the most fertile because we are still conceiving the product requirements and because AM enables complexity. The concept is the first opportunity to design to the requirements before we have a constraint of a specific manufacturing method.
The AM journey requires a 3D file and follows a similar digital workflow regardless of the AM process employed. The source for the future 3D object can originate from Computer Aided Design (CAD) or from a 3D scanning source such as structured light, Magnetic Resonance Imaging (MRI), or Computed Tomography (CT). Once available in a 3D file, the source data will be turned into an STL file.
The STL file format originally hails from stereolithography and can only describe the surface geometry of a three-dimensional object. It does not relay any other information, including color, material, or texture. The STL file describes an unstructured, triangulated surface by unit, and vertices of the triangles use a three-dimensional Cartesian coordinate system. STL files contain no scale information and have arbitrary units. There are many efforts afoot to replace the STL file because, while simple, it contains little real information making the task of tracking and documenting changes difficult. One approach is to stay in the native CAD format which has the advantage that, if changes are made to the file for printing, they could be traced back to the design file.
The build file is the next step in the workflow. The build file can be just a single part or part number but could also be multiple parts, part numbers, test coupons, etc. It is very important to remember that the original engineering design may or may not specify items like orientation during the build, but this a requirement for the build file. The build file is very important to ensure consistent results, expected results, economic results and compliance. The build file should be treated like a released engineering or manufacturing drawing. It is akin to a forging die that is employed to manufacture parts over and over.
From here our build file will be sliced into finite 2D layers. Recall that the computers in 3D printing typically only concern themselves with a layer at a time, so each slice represents a build layer. Once sliced, a tool path is generated, and the shape can be printed.
Post-Processing is the name given to a host of manufacturing processes employed after the 3D printing step. These processes serve to finish an AM part by improving the appearance or material properties after it has been printed. These processes could be, but are not limited to, material removal, thermal processing, and surface treatments. The printed shape can be dimensionally measured to ensure consistency to the original drawing or source file.
What Does STL Stand For?
Similar to JPEG or GIF, the STL file name (or extension) is regularly used without much consideration to what it means. In the case of STL, this is helpful because there are several accepted terms associated with it. The abbreviation initially stood for stereolithography, a common additive manufacturing technique. But since STL files are used for other processes within additive manufacturing, it has been re-associated with two other terms. “Standard triangle language” refers to the surface of an object being broken into a series of triangles. And “standard tesselation language” refers to the surface being made up of a series of shapes (or tiles) that do not overlap. None of these perfectly describe all of the potential use cases of the STL file, so most people in the field simply use the abbreviation – just like other file extensions.