Introduction and Basic Principles of Additive Manufacturing

Defining Additive Manufacturing and Rapid Prototyping

• Additive Manufacturing (AM) is the formalized term for what was previously known as Rapid Prototyping (RP) and is popularly called 3D Printing.
• Rapid Prototyping is a process used to rapidly create a system or part representation before final release or commercialization. The emphasis is on building a prototype or basis model quickly for derivation into further models or the final product.
• Management consultants and software engineers use the term Rapid Prototyping to describe a piecewise development of business or software solutions, allowing stakeholders to test ideas and provide feedback.
• The transition from the term "Rapid Prototyping" to "Additive Manufacturing" occurred because improvements in quality have allowed the technology to produce final products rather than just prototypes.
• The Technical Committee within ASTM International agreed on the new terminology, which is now used in most consensus standards and standards bodies worldwide.
• AM simplifies the production of complex 3D objects directly from 3D Computer-Aided Design (CAD) data without the need for extensive process planning, which is required in other manufacturing processes to determine tool ordering, fixtures, and geometric analysis.

Principles of Additive Manufacturing

• The fundamental principle of AM is adding material in layers. Each layer is a thin cross-section derived from the original CAD data.
• In the physical world, layers must have a finite thickness. This means the resulting part is an approximation of the original data.
• The relationship between layer thickness and accuracy is direct: the thinner each layer is, the closer the final part will be to the original digital model.
• Major differences between commercial AM machines include:
  • The types of materials used.
  • The method of layer creation.
  • The method of bonding layers to each other.
• These differences influence accuracy, material properties, mechanical properties, speed of fabrication, post-processing requirements, machine size, and overall cost.

Applications and the "3 Fs"

• Visualization Models: Initially, AM was primarily used to create visualization models to help designers present conceptual designs. Models provide better understanding of design intent than drawings or renderings and are necessary for final validation.
• The 3 Fs of product assessment:
  • Form: Models are used to appreciate the shape and general purpose of a design.
  • Fit: Improved accuracy allows components to be built to tolerances required for assembly testing.
  • Function: Improved material properties allow parts to be handled and assessed based on how they will eventually work in operation.
• Beyond modeling, AM is used in process chains to shorten development times and costs. It has evolved for use in producing end-use parts.

The Generic Eight-Step AM Process

1. Step 1: CAD: All parts must start as a software model describing external geometry. This requires professional CAD solid modeling software or data from reverse engineering equipment like laser/optical scanners.
2. Step 2: Conversion to STL: The 3D model is converted to the STL file format, which is the de facto standard for AM. It describes the external closed surfaces of the CAD model using triangles and forms the basis for slicing.
3. Step 3: Transfer to AM Machine and STL File Manipulation: The file is moved to the AM machine where it may be manipulated for correct size, position, and orientation.
4. Step 4: Machine Setup: Build parameters are configured, including material constraints, energy source settings, layer thickness, and timings.
5. Step 5: Build: A largely automated process where the machine builds the part. Only superficial monitoring is needed to check for errors like running out of material or software glitches.
6. Step 6: Removal: Parts are removed once the build is complete. This may require waiting for safety interlocks, low operating temperatures, or the stopping of moving parts.
7. Step 7: Post-Processing: Parts may require cleaning, removal of support structures, priming, painting, or heat treatment. This step can be costly and laborious if finishing requirements are high.
8. Step 8: Application: The part is ready for use, which may include assembly with mechanical or electronic components.

Maintenance and Material Handling

• AM machines often use fragile laser or printer technology that requires careful monitoring. They should ideally be kept in environments that are not dirty or noisy.
• Regular maintenance checks are essential, though machines are designed to operate unattended. Maintenance standards are developed by groups like the ASTM F42 Technical Committee on Additive Manufacturing Technologies.
• Some raw materials have a limited shelf life and must be protected from chemical reactions caused by moisture, excess light, or contaminants.
• While materials can often be reused, repeated recycling can degrade properties, requiring procedures to maintain material quality.

Alternative Terminology for Additive Manufacturing

• Automated Fabrication (Autofab): Popularized by Marshall Burns in the early 1990s. It emphasizes automation and computer control of actuators to simplify manufacturing by removing manual tasks.
• Freeform Fabrication / Solid Freeform Fabrication: Emphasizes the ability to create complex shapes without the geometric constraints of conventional processes. The concept of "complexity for free" suggests that building a complex object takes similar effort to building a simple one if the volume is the same.
• Layer-Based Manufacturing / Additive Fabrication: Focuses on the layer-wise addition of material. Terry Wohlers popularized the term Additive Fabrication.
• 3D Printing (3DP): Originally used for MIT's inkjet-based technology. It alludes to extending 2D printing into the third dimension and is currently the most popular term used by the general public.
• Stereolithography (SL): A term specifically used by the company 3D Systems for their specific machine technology.

Benefits of Additive Manufacturing

• Speed: Speed advantages go beyond building time to the whole development process. Because it uses 3D CAD and is relatively seamless, it is considered "What You See Is What You Build" (WYSIWYB).
• Reduction in Process Steps: AM can typically build complex parts in a single step, whereas conventional manufacturing requires multiple iterative stages and is sensitive to design changes.
• Streamlined Workshops: AM can replace multiple construction methods (hand carving, molding, CNC) with a cleaner, more versatile setup.

Comparison: AM vs. Conventional Manufacturing (CNC)

• Material: CNC is ideal for hard, brittle materials like steels and metal alloys, producing homogeneous and predictable parts. AM was initially for polymers and waxes but now includes metals and ceramics; however, AM parts may have voids or anisotropy.
• Speed: CNC can remove material faster by volume, but AM is often faster for the total process because it requires less setup and process planning for complex parts. CNC may take weeks for a first part due to custom jigs and fixtures, while AM takes hours or days.
• Complexity: AM excels at undercuts, internal features, and enclosures (e.g., a ship inside a bottle) that are impossible or extremely difficult for CNC without breaking the part into pieces.
• Accuracy: AM resolution is typically a few tens of microns, with the vertical axis resolution determined by layer thickness and horizontal resolution by the build mechanism (e.g., laser beam diameter). CNC accuracy is determined by positioning resolution and tool diameter.
• Geometry: AM breaks 3D problems into 2D cross-sections. CNC must handle complex 3D paths, making freeform surfaces extremely difficult.
• Programming: CNC programming is highly complex, involving tool selection and angles. Incorrect CNC programming can damage the machine or cause injury. AM programming is simpler and relies on pre-configured parameters.

Related Technologies and Secondary Processes

• Reverse Engineering (RE): The process of capturing geometric data from an object.
  • Initial data is a "point cloud" (unconnected points).
  • Software like Geomagic is used for hole-filling and smoothing.
  • 3D scanning can now be done via smartphones or high-end laser systems.
  • Computerized Tomography (CT) uses high-energy X-rays to scan internal features with a resolution of approximately $1\,\mu\text{m}$.
  • Capture Geometry Inside is a destructive 2D imaging approach where a part is machined away layer-by-layer.
• Computer-Aided Engineering (CAE): Used for simulation (forces, stress, heat flow) using methods like the Finite Element Method (FEM). Newer AM simulation tools predict distortion, porosity, and build failures.
• Haptic-Based CAD: Uses devices like the "Phantom" to provide force feedback, allowing a designer to interact with "Virtual Clay." This is more organic and freeform than traditional engineering CAD.
• Computer-Aided Manufacturing (CAM): Uses computers to control machine tools. It evolved from Numerical Control (NC) using punch cards to Computer Numerical Control (CNC).

Example AM Parts

• Stereolithography: Fuselage for an unmanned aerial vehicle (UAV) with a conformal lattice skin.
• Material Jetting: Parts with multiple materials (e.g., clear and black) and working revolute joints created with dissolvable supports.
• Powder Bed Fusion (PBF): Metal facial implants (electron beam energy source) and polymer "brain gears" that rotate together without needing supports during building.
• Sheet Lamination: Multi-color parts using inkjet printing capability.
• Material Extrusion (MEX): Ratchet mechanisms and low-cost desktop prints.

Questions & Discussion

• 1. Find three other definitions for Rapid Prototyping other than that of Additive Manufacturing as covered by this book.
• 2. From the web, find different examples of applications of AM that illustrate their use for "Form," "Fit," and "Function."
• 3. What functions can be carried out on point cloud data using reverse engineering software? How do these tools differ from conventional 3D CAD software?
• 4. What is your favorite term (AM, Freeform Fabrication, RP, etc.) for describing this technology and why?
• 5. Create a web link list of videos showing operation of different AM technologies and representative process chains.
• 6. Make a list of different characteristics of AM technologies as a means to compare with CNC machining. Under what circumstances does AM have the advantage and under what would CNC?
• 7. How does the Phantom desktop haptic device work and why might it be more useful for creating freeform models than conventional 3D CAD?
• 8. With a basic understanding of Additive Manufacturing, what is an application you can think of where additive manufacturing could be used in your daily life? Explain how.
• 9. What are the differences between end-use parts and prototypes?
• 10. What was Additive Manufacturing initially used for, and how is it utilized today?
• 11. Why is the term "Rapid Prototyping" not suitable for additive manufacturing anymore?
• 12. What is "concurrent engineering"? How can Additive Manufacturing help with concurrent engineering?
• 13. What type of file is typically required to use as input to process a part in an AM machine?
• 14. List one academic institution with research activities in AM and some of their research projects that interest you.
• 15. List one company which produces Additive Manufacturing machines and associated products such as materials and also list one of their products.
• 16. A company manufactures missile casings with internal channels. Why might Additive Manufacturing be a better manufacturing process than CNC machining?
• 17. Why have 3D printers not been widely applied for household use today? What benefits do you think AM will bring to your daily life in the future?
• 18. Visualize a process in which small robots dispersed materials in various places to construct a component. Is this an additive manufacturing process?
• 19. Recently historical museums have started using additive manufacturing and reverse engineering techniques. What are museums using them for?
• 20. Why does NASA wish to create an AM lab in outer space?
• 21. What changes are additive manufacturing making to the mechanical engineering field?
• 22. Design a product that is very difficult to make using traditional manufacturing methods but that can be printed using AM.
• 23. Find one Additive Manufacturing application example not covered in this book. List the benefits and disadvantages of this application compared with its traditional manufacturing method.
• 24. Find the newest definition for Additive Manufacturing by the ASTM F42 Committee. Make a table of all the categories of AM technologies covered in the latest standard and list one typical technique for each category.
• 25. Discuss how AM can improve manufacturing environmental impact and sustainability.