Explain the classification of additive manufacturing systems?

Additive manufacturing (AM) systems are classified based on the physical state of the starting material and the method used to fuse it together. The most widely accepted classification system comes from the ASTM International F42 committee on additive manufacturing technologies, which defines seven distinct process categories. This standardization helps to clearly define the different types of 3D printing and the technologies that fall under each category.

Here are the seven classifications of additive manufacturing systems:

1. Vat Photopolymerization: This category includes processes that use a vat of liquid photopolymer resin, which is selectively cured by a light source. The most well-known technologies in this group are:

   • Stereolithography (SLA): A laser beam traces the cross-section of each layer on the surface of the liquid resin, causing it to harden.

   • Digital Light Processing (DLP): A projector flashes an entire layer's image at once, which makes it faster than SLA for many applications.

   • Continuous Liquid Interface Production (CLIP): A process that pulls the object out of the resin vat while a continuous projection of light cures the resin, creating a very fast, continuous printing process.

2. Material Extrusion: This is one of the most common and accessible types of AM. It involves a nozzle that extrudes a continuous filament of material, typically a thermoplastic, onto a build platform.

   • Fused Deposition Modeling (FDM) / Fused Filament Fabrication (FFF): A spool of thermoplastic filament is heated and extruded through a nozzle, building the part layer by layer.

3. Material Jetting: This process is similar to a 2D inkjet printer but in three dimensions. It uses a printhead to deposit droplets of material, such as a photopolymer or wax, which are then cured or hardened, often with UV light.

   • This technology is known for its ability to produce parts with high resolution, fine details, and multi-material or multi-color capabilities.

4. Binder Jetting: In this process, a liquid binding agent is selectively jetted onto a powder bed to join the material particles. The powder bed itself provides support for the object during printing, eliminating the need for separate support structures.

   • This method is versatile and can be used with a range of materials, including metal, sand, and ceramics. Post-processing, such as sintering, is often required to achieve final part strength.

5. Powder Bed Fusion (PBF): This is a category of processes that use a thermal energy source (laser or electron beam) to selectively fuse or sinter regions of a powder bed.

   • Selective Laser Sintering (SLS): A laser sinters (fuses without fully melting) polymer powder particles.

   • Selective Laser Melting (SLM) / Direct Metal Laser Sintering (DMLS): A laser fully melts metal powder particles.

   • Electron Beam Melting (EBM): An electron beam, typically in a vacuum, melts metal powder.

6. Directed Energy Deposition (DED): DED systems use a focused energy source, like a laser or electron beam, to melt and fuse material as it's being deposited. The material can be in the form of a powder or a wire.

   • This method is often used to repair existing parts, add material to a component, or create large-scale structures.

7. Sheet Lamination: This process involves bonding sheets of material together to form an object. The material sheets can be metal, paper, or plastic.

   • Laminated Object Manufacturing (LOM): Layers of adhesive-coated paper or plastic are cut to shape with a laser or knife and then bonded together.

   • Ultrasonic Additive Manufacturing (UAM): Metal foil sheets are bonded together using ultrasonic welding. After each layer is bonded, a CNC machine cuts the profile of the part.