Note: All schematics are courtesy of Steffen Ritter. Much of the following was excerpted from Wohlers Report 2022.
Additive manufacturing (AM), also known as 3D printing, creates a part by adding material layer upon layer. Read What is Additive Manufacturing? for more details on the process. The ISO/ASTM 52900 terminology standard categorizes commercially available AM systems into seven distinct processes by the way layers of material are created. Most AM systems fit into one of the seven categories. One exception is cold spray. Future AM processes could develop that do not fit into one of the categories, which could require an update to the ISO/ASTM 52900 standard.
The following provides details on the seven AM processes, presented in order of popularity based on unit sales. Detailed information on each process, including example use cases, can be found in Wohlers Report 2022.
Material extrusion (MEX)
This is a 3D printing process in which material is selectively dispensed through a nozzle or orifice. The moving nozzle, also called an extruder, deposits a layer of materials and then either the extruder or build platform raise or lower, respectively, and the process is repeated. The part’s shape is determined by a pre-loaded file created by a software called a “slicer.”
MEX is capable of printing different materials, yet the most popular are thermoplastics in filament form (e.g., ABS, nylon, PEEK, PLA, etc.). Composite and other filled materials, such as carbon-filled and glass-filled filaments, are also becoming more common. MEX can print paste-like materials, such as concrete, ceramics, or foods such as chocolate or dough. This process requires sacrificial support material for overhanging features.
Schematic of filament (left), paste (center), and pellets (right) MEX
Vat photopolymerization (VPP)
This is a 3D printing process in which liquid photopolymer in a vat is selectively cured by light-activated polymerization. The printing process begins with the resin in a vat (i.e., tank or build volume). The two common types of VPP use either a laser or light-emitting diodes (LEDs) coupled with digital light processing (DLP) as the energy source to cure the resin. Laser-based VPP systems typically cure one layer before the build volume is lowered and a new layer of liquid photopolymer is spread across the build area. Typically, DLP systems project light from below the vat and cure the photosensitive resin through an optical window.
VPP systems are desirable for high-resolution parts at a reasonable cost. The process requires a secondary curing and washing step for post-processing. VPP requires sacrificial support material for overhanging features.
Schematic of VPP (left) and DLP-based VPP (right)
Powder bed fusion (PBF)
This is a process in which thermal energy selectively fuses regions of a powder bed. Thermal energy from a laser or electron beam melts a portion or all of the powder that the beam contacts. The area adheres to the previous layer and becomes solid as the material cools. Once the layer has been fused, a new layer of powder is added.
A wide range of polymers and metals are suitable for PBF. Typically, polymers are semi-crystalline thermoplastics. For polymers, the unfused, loose powder surrounding the part serves as a support material. For metal PBF, support structures are required to anchor parts and features to the build plate. This process can lead to significant thermal stresses and heat treatment is typically required for metal PBF.
Schematic of laser (left) and electron beam (right) PBF
Binder jetting (BJT)
This is an AM process in which a liquid bonding agent is selectively deposited to join powder materials. The process starts with a layer of powder, which can be a polymer, metal, ceramic, or sand. A print head deposits droplets of a binding agent onto the material, fusing the particles together in a pre-determined pattern. Once a layer is complete, the print platform moves downward, and a new layer of powder is spread onto the build area.
Parts made using BJT typically require post-processing to improve their mechanical properties. This can involve adding an additional adhesive substance or placing the part in an oven to sinter the particles.
Schematic of BJT process
Material jetting (MJT)
In this process, droplets of feedstock material are selectively deposited using inkjet print heads. The material, which is typically photopolymers or wax-like substances, are cured and solidified with UV light. Once a layer is cured, the nozzles in the print head will deposit new material on top of it, one layer at a time.
MJT parts require supports, which are often printed with a dissolvable material and removed during post-processing. This process can print graded material combinations, producing different material properties or colors throughout the part.
Schematic of the MJT process
Directed energy deposition (DED)
The DED is defined as a process in which focused thermal energy is used to fuse materials by melting as they are being deposited. The feedstock used for DED is either a metal powder or wire. This technology involves a nozzle mounted on a multi-axis arm and is sometimes combined with CNC milling. The process produces near-net-shape parts and usually requires machining to achieve required tolerances.
DED offers unique capabilities, including depositing more than one material simultaneously. In addition to printing new parts, DED can repair damaged parts by depositing material directly onto them.
Schematic of powder-fed (left) and wire-fed (right) DED system
Sheet lamination (SHL)
This is a process in which sheets of material are bonded to form a part. This process can use a variety of materials, including metal, paper, polymers, or composites. SHL involves a layer of material being adhered to another using an adhesive or welding process. Layer contours are typically generated by a machining process either before or after a layer or material is deposited.
Parts produced with SHL require different design parameters as internal cavities can be challenging, if not impossible, to remove material from. Due to the limited head required for this process, electronics and other low-melting-point materials can be embedded into the part.
Schematic of SHL process