Saturday 8 August 2015

3D printing: the future of manufacturing- Part 1


3D printing was initially used for Rapid Prototyping (RP) purposes only. However, recent developments and the expiry of many patents have lowered the costs of 3D printing. Besides this the only process where the manufacturing cost is independent of the part complexity. In other words very complex parts can be produces as easily as a very simple one. The combination of these and a number of other factors have led to a widespread use of this techniques to manufacturing of actual part apart from RP. This article is a very humble attempt to acquaint the reader with the basics of this revolutionary manufacturing technique.


What is 3D printing?


3D printing, also known as additive manufacturing is a process of fabricating 3D objects layer by layer (cross-section by cross-section). For this purpose, a 3D model of the object has to be made first followed by slicing the model into a number of layers (cross-sections) [1]. The information of each layer is stored in a file, the most commonly used file format being STL [2], which is understood by the 3D printer. The printer then prints each layer, using different technologies, as per the information contained in the STL file. Depending on the technology used for the purpose of deposition, 3D printing can be classified into a number of branches.

Classification of 3D printing technologies


3D printing technologies can be classified into the following 7 categories:

  1. Material extrusion
  2. Powder bed fusion
  3. Photopolymerization
  4. Material jetting
  5. Binder jetting
  6. Sheet lamination
  7. Direct energy deposition

1. Material extrusion

This technology was developed by Scott Crump (founder of Stratasys Ltd.) in 1988 and trademarked as FDM (fused deposition modeling). In this process, build material (mostly thermoplastics) supplied in the form of filament, is melted and mechanically extruded on to a substrate layer by layer. The process is inexpensive and can be used even for multi-material printing. Hence is popular among the DIY crowd. However, material extrusion based 3D printing requires a relatively large deposition head (approx. 0.4 mm) in order to effectively process the molten polymer. This limits the possible feature size and hence the feature resolution.


2. Powder bed fusion

In this process an energy beam, usually LASER or electron beam, is used to selectively melt the build material supplied in the form a of fine powder bed. Once a layer is scanned/melted, the powder bed is moved downwards by an amount equal to the desired layer thickness and the scanning of the next layer begins. This process continues until the complete part is printed. Materials used include both polymer and metal powders. The parts produced by this process have high accuracy and detail. The fact that fully dense parts can be produced in this produced has lead to the use of the technology for the manufacture of high performance parts for the aerospace and the automotive industries. There are three popular powder bed fusion technology based 3D printing techniques:
  1. Direct Metal Laser Sintering (DMLS)
  2. Selective Laser Melting (SLM)
  3. Electron Beam Melting (EBM)


3. Photopolymerization

This technology was developed by Charles Hull of 3D systems in the year 1984. It involves the use of stereolithography in which an ultraviolet LASER selectively scans and polymerizes uv curable resins thereby creating a layer of solidified material. After the first layer is completed, the printer platform holding the liquid resin moves downwards, depending on the layer thickness, and the uv LASER scans the next layer. This process continues until the whole model is completed. The model is then cured in an ultraviolet oven and the support structures removed. The process is characterized by high building speed and high resolution. However the cost of materials and supplies is relatively high.



4. Material jetting

This process is similar to the popular inkjet technology wherein the printer head directly deposits the build material, which may be wax or photopolymer, layer by layer and the deposited material is then photocured. This technology can be used for multi-material printing (poly-jet) as well to create functionally graded materials (FGM).




5. Binder jetting
This process is similar to material jetting but instead of the material, a liquid polymer is selectively deposited on a bed of material powder. This polymer acts as a binder and penetrates into the powder bed and forms an agglomerate. After one layer is formed, the powder bed moves downward and new layer of material is spread by a roller mechanism. The binding agent is partially cured immediately after application. The printer head then sprays the next layer and the process is repeated. The final printed powder consists of bound powder and post processing is necessary to give sufficient strength to the part. First developed in MIT, researchers have used this technology to process a variety of metal, ceramic, foundry sand, and polymer materials.




6. Sheet lamination

This technology was first developed by Helisys (now Cubic technologies) in the year 1986 and trademarked as Laminated Object Manufacturing (LOM). In this process layers of the build material in the form of laminates are fused together by heat and pressure and then stacked and cut to shape. Products made by this process has high surface finish low cost of production. Plastics, metals as well as ceramics can be fabricated by this process.




7. Direct energy deposition

In this process build material in the form of metallic powder or wire is fed directly to the focal point of an energy source (LASER or electron beam) resulting in the formation of a molten metal pool which is then deposited on to a substrate in an inert atmosphere. This is the basis of Laser Engineered Net Shaping (LENS) technology developed by Sandia Labs and made commercially available by Optomech. This process can be used for multi-material printing of parts, repairing of damaged components, development of FGM, coatings, etc. 




N.B:

[1] The number of layers determines the resolution of the model. In general, higher the resolution, higher the product quality. 
[2] STL (Standard Tessellation Language) is a file format used natively for stereolithography as well as other additive manufacturing processes.

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