# Emerging Technologies in Transportation Casebook/3D Printing/What is 3D Printing? Software & Hardware

3D printing is a process that adds material layer by layer to create a physical object from a three-dimensional digital model. Because only enough material needed to make the shape is used, it has the potential for cost and material savings compared to other manufacturing methods. Other advantages compared to traditional manufacturing include:
* easier customization and production of unique objects, as objects are manufactured one at a time or in small batches
* easier production of certain complex shapes, especially lattice or honeycomb interior structures that give pieces strength with less weight
* onsite production, reducing the time required for shipping and shipping costs
* on-demand production
Disadvantages include:
* print times of several days or more for certain objects
* limited selection of materials
* in some cases, reduced strength compared to a traditionally manufactured object
* large quantities of objects cannot be produced quickly as traditional manufacturing

## The 3D Printing Process
### Preparing the Model

The process begins with the creation of a 3D model, usually using CAD (Computer Aided Design) software. Some common CAD software packages used to model objects for 3D printing include SOLIDWORKS, SketchUp, Autodesk Inventor (commercial), FreeCAD and BRL-CAD (open-source). Alternatively, an existing object can be scanned by a 3D scanner to create a 3D model. Once the CAD file has been created, it must be converted into a format that approximates its shape, including any curves, with x, y and z coordinates. The most commonly used format is an .STL file, but other filetypes, such as .3MF and .OBJ, are also used. The .STL file, or other file approximating the object’s shape, must then be prepared for printing with a process called slicing. Sometimes slicers are incorporated into software created by the manufacturer of a printer (e.g. MakerBot’s MakerWare). Other slicers are standalone programs, including Simplify3D (commercial), Repetier (freeware), Cura and KISSlicer (open-source). During slicing, the virtual object is oriented for printing and converted into layers. The output of this process is a .gcode file, which contains the instructions for where the machine should add or solidify material to produce the object.

### Printing Processes

Although there are many 3D printing processes, three are most commonly used. Whichever method is used, 3D printers perform every step needed to transform raw material into the final shape once the machine is set up with material and its .gcode instructions.

#### Fused Deposition Modeling (FDM)

Fused deposition modeling, the most widely used 3D printing process, begins by heating a filament of material (usually plastic or plastic composite) until soft and pliable. Then, the hot material is extruded through a computer-controlled nozzle and deposited onto a build plate into the desired shape for each layer. After each layer has hardened, another layer is deposited on top, and this is repeated until the entire object has been completed. After printing is finished, the object can be sanded and cleaned. Since overhanging parts of an FDM printed object cannot be created without a support structure, these supports must be removed after printing.

#### Stereolithography (SLA)

Stereolithography uses photosensitive liquid resins. Computer-controlled lasers are directed at the resin to solidify it into the desired shape of each layer. Once a layer is complete, either the vat of resin or the object itself is moved vertically so that the next layer can be added. After the object is finished being built, excess resin is removed using a solvent and the object must be cured. Although stereolithography can produce smoother objects with finer detail than FDM printing, it must use photosensitive materials and so it is more limited in terms of producing functional objects.

#### Selective Laser Sintering (SLS)

This method of 3D printing begins with a deep bed of powdered material, often metal. Guided by the .gcode file, lasers heat the surface of the powder in the desired shape for each layer. This sinters, or binds together, the particles of powder. The bed of powder is then moved downward and a new layer of powder is applied. The next layer of the object is sintered together on top of the first, and the process is repeated until the entire object has been formed. Like SLA printed objects, SLS printed objects must be cured before use.

## 3D Printers

Printers range greatly in size and capability – for example, the LulzBot Mini FDM printer has a build area of 6 inches by 6 inches (15.24 cm by 15.24 cm), while industrial size FDM printers such as the Big Area Additive Manufacturing (BAAM) machine can produce pieces the size of a car. As of April 2020, prices range from around $200 USD for small printers to over $1 million USD for industrial size machines.
* Implementation: current uses of 3D Printing >

## References

* ↑ "three-dimensional (3D) printing." In A Dictionary of Construction, Surveying and Civil Engineering, edited by Gorse, Christopher, David Johnston, and Martin Pritchard: Oxford University Press, 2020. https://www.oxfordreference.com/view/10.1093/acref/9780198832485.001.0001/acref-9780198832485-e-8676
* ↑ Noorani, Rafiq. “How Does 3D Printing Work?” In 3D Printing : Technology, Applications, and Selection. CRC Press, 2017. https://doi.org/10.1201/9781315155494.
* ↑ MANUFACTUR3D. “The 3D Printing Process: How Does It Work?” MANUFACTUR3D (blog), January 1, 2018. https://manufactur3dmag.com/3d-printing-process/.
* ↑ OpenLearn. “Fused Deposition Modelling (FDM).” Accessed April 28, 2020. https://www.open.edu/openlearn/science-maths-technology/engineering-technology/manupedia/fused-deposition-modelling-fdm.
* ↑ Escudier, Marcel, and Tony Atkins. “Stereolithography.” In A Dictionary of Mechanical Engineering. Oxford University Press, 2019. https://www.oxfordreference.com/view/10.1093/acref/9780198832102.001.0001/acref-9780198832102-e-6207.
* ↑ OpenLearn. “4.1 Polymer Materials.” Accessed April 28, 2020. https://www.open.edu/openlearn/science-maths-technology/additive-manufacturing/content-section-0.
* ↑ Escudier, Marcel, and Tony Atkins. “Selective Laser Sintering.” In A Dictionary of Mechanical Engineering. Oxford University Press, 2019. https://www.oxfordreference.com/view/10.1093/acref/9780198832102.001.0001/acref-9780198832102-e-9526.
* ↑ OpenLearn. “4.2 Metallic Materials.” Accessed April 28, 2020. https://www.open.edu/openlearn/science-maths-technology/additive-manufacturing/content-section-0.