The seven categories of additive manufacturing technologies

Additive manufacturing (AM) has fundamentally changed how parts across many industries are designed and fabricated. As companies invent new AM techniques, they tend to create new terms even though the core techniques are similar. With this in mind, in this post, and as per the ISO ASTM 52900 which relates to additive manufacturing general principles and terminology, we’ve produced a helpful guide to the seven core technologies of additive manufacturing, including their advantages and disadvantages. 

1. Binder Jetting – BJT

Binder Jetting is a process in which a liquid bonding agent is selectively deposited to join powder materials

Binder Jetting (BJT) produces parts by selectively depositing a binding agent over a powder bed. BJT uses the same powder-spreading methods as powder bed fusion (PBF). However, unlike PBF, a liquid binding agent is used to bond parts instead of a laser/beam.  A roller spreads a layer of powder over the build platform then the print head moves horizontally and sprays the binder agent onto selective areas of the powder layer. The printer creates the object by bonding the powder wherever the print-head has deposited the binder, layer by layer. After the build process, the product undergoes post-processing. This process is different to most other AM technologies because it doesn’t use heat to fuse the material.

Advantages of Binder Jetting (BJT)

• Can produce complex, high-precision parts – high resolution equal to PBF
• Fast and affordable
• Different mechanical properties achievable
• Multi-colour parts possible (non-metallic parts)
• A wide range of powdered materials is available
• Minimal material wastage
• Relatively large build area
• Can be integrated with most traditional foundry processes
• The powder isn’t melted so few/no issues related to residual stress

Disadvantages of Binder Jetting (BJT)

• Needs to be infiltrated and sintered which causes shrinkage
• Before post-processing, parts are fragile and can crumble easily
• Even after post-processing, the mechanical properties don’t meet the quality of traditionally manufactured parts or PBF prints

Applications of Binder Jetting

• Often used for sand casting moulds
• Ideal for full-colour/high fidelity prototyping
• Complex, high-precision metal and ceramic parts

2. Directed Energy Deposition – DED

The process of Directed Energy Deposition (DED) fuses materials (usually metal) by melting them as they’re being deposited.

With DED, powder or wire is pushed through a nozzle and melted by an intensely focused energy source (usually a laser) at the point of deposition. The molten material is incorporated into the energy flow, melting and depositing at the same time. The nozzle can move in multiple directions (five-axis) which gives it freedom of movement not seen in most other AM technologies. This process can deposit the molten material onto a build platform or alternatively onto a component needing to be repaired/modified. DED is similar to the traditional welding process but can produce fine detail which welding cannot. 

Types of this category (proprietary)

The Direct Energy Disposition process can be subdivided into categories based on the energy source for making the material molten, but they are all based on similar principles:

1. Laser Engineered Net Shaping (LENS),
2. Electron Beam Additive Manufacturing (EBAM)
3. Wire Arc Additive Manufacturing (WAAM) – which is plasma or electric-based

Other names used are Directed Light Fabrication (DLF), Direct Metal Deposition (DMD) or 3D Laser Cladding.

Advantages of Directed Energy Deposition (DED)

• Can add material to existing parts because it can deposit material from many angles
• Wide selection of print material that is inexpensive – also used for welding
• Large build area
• Creates parts that are dense and strong, and have great mechanical properties
• Fast build times and minimal material wastage
• Can utilise multiple materials

Disadvantages of Directed Energy Deposition (DED)

• Expensive equipment so costly initial outlay compared to most other AM processes
• Support structures are difficult because the large liquid melt pool at the point of deposition doesn’t allow for overhang
• The energy required to maintain the melting point creates thermal gradients that can cause residual stress
• Parts may require post-processing due to poor process resolution

Applications of Direct Energy Deposition (DED)

• Repair, maintenance and feature additions of functional or structural parts especially in heavy industry and machinery that is very expensive to replace
• Useful for applications in space as doesn’t require gravity
• Near-net-shape part production
• Parts produced using specialist hybrid manufacturing machines

3. Material Extrusion – MEX

Material extrusion is a process in which material is selectively dispensed through a nozzle or orifice

Material Extrusion (MEX) is the least expensive method of additive manufacturing and can only use plastic polymer as the raw material. It is most often called Fused Deposition Modelling (FDM) because FDM is a famous registered trademark. MEX/FDM varies from other AM processes because the material is added through a heated extrusion nozzle under constant pressure and in a continuous stream.  With MEX/FDM, the material feeds into the printer from a coil/spool and the machine produces parts by layering extrusions of molten thermoplastic filament. The molten plastic is continuously deposited at very specific locations, where it then becomes cool and solidifies. The fusion of the layers is achieved by precise temperature control or with chemical bonding agents.

Types under this category (proprietary technologies)

1. Fused Deposition Modelling (FDM) – This process is registered as a trademark in the name
2. Fused Filament Fabrication (FFF) is another term used but is very similar to FDM

Advantages of Material Extrusion (MEX)

• Desktop printers can be priced for entry-level
• Generally speaking, the equipment is small, user-friendly and affordable to run
• Low-temperature process
• Printing times are fast for single/small parts
• Wide selection of inexpensive print material available
• The raw material is easy to handle
• Parts don’t need much post-processing

Disadvantages of Material Extrusion (MEX)

• The heating method uses low temperatures so the process is limited to plastic polymers
• There are no economies of scale so high volume runs are very slow
• Low accuracy and often the layer lines are visible
• The resolution relates to the filament which will never be thin enough for fine detail
• The component strength is weak in the vertical direction
• Susceptible to warping and shrinkage
• Parts are anisotropic
• Some materials can be toxic

Applications of Material Extrusion (MEX)

• Prototyping especially proof of concept and rapid prototyping
• Buildings can be made with concrete extrusion machines
• Human tissues and organs
• Small-scale production of functional parts and prototypes
• Jigs and fixturing

4. Material Jetting – MJT

Material Jetting is a process in which droplets of feedstock material are selectively deposited

Material Jetting (MJ) is one of the fastest and most accurate additive technologies. Although MJ is relatively new, it’s considered to be full of a lot of possibilities. The MJ process begins by melting a photo-reactive resin into a liquid photopolymer, then a print head moves horizontally across the print bed and sprays many micro-droplets of the liquid polymer very precisely onto the build platform, either continuously or on-demand. They are then cured usually by UV light to become solid. One printhead can house jets for multiple materials, which means multi-material printing, full-colour printing and disposable support structures in another material, such as wax is possible.

Types under this category (proprietary technologies)

The Material Jetting category can be subdivided into several distinct printing technologies. Components produced by NPJ and DOD process have better mechanical properties than Poly jet components, however, the basic principles for all of them are the same:

1. Poly Jet – In this process, the printer selectively jets a micro-thin layer of photopolymer material onto the platform and the layer is immediately cured by the UV light. Polyjet is by far the most popular MJ process
2. Nanoparticle jetting (NPJ) – This process is unique in that it uses cartridges containing solid metal nanoparticles suspended in a liquid. Two print-heads jet fine droplets of both build and support materials simultaneously to form a layer. The print bed has a high temperature of around 300º C and immediately evaporates the liquid, leaving behind the solid Nanoparticles. The parts are cured once all the layers are deposited.
3. Drop on Demand (DOD) – This process is similar to Polyjet but only deposits dots as required (on demand) and not continuously. The printhead is unique in that it can print curves in high resolution.

Advantages of Material Jetting (MJ)

• The resolution and accuracy is the best offered by any AM process
• Can print incredibly realistic objects with intricate shapes and sharp edges
• Ability to layer multiple materials so multicolour possible 
• Very smooth surfaces
• Parts have very high tolerances, although less than what PBF can achieve
• Large build area

Disadvantages of Material Jetting (MJ)

• Expensive equipment and slow build time because it builds one droplet at a time
• Support structures are solid which causes significant material wastage
• Limited materials available that are expensive – generally use wax-like materials, which can be fragile
• Parts have poor mechanical properties and so are not generally suitable for functional use – generally brittle and cannot take much load

Applications of Material Jetting (MJ)

• Highly realistic prototypes
• Injection mould manufacturing
• Patterns for investment castings, specifically in the jewellery industry
• Medical devices, surgical tools and anatomical models

5. Powder Bed Fusion – PBF

Powder Bed Fusion is a process in which thermal energy selectively fuses regions of a powder bed

Powder Bed Fusion (PBF) uses an energy force to selectively melt or sinter powdered material.  A large bed of powdered material is heated to just below its melting point and spread over the build platform in a fine layer. The energy source (usually a laser or electron beam) is then directed across the powder’s surface which bonds the powder together, forming a thin layer of combined material. Most PBF processes happen in a near-vacuum chamber with an inert gas that protects the object from corroding /oxidation. EBM utilises a total vacuum instead of an inert atmosphere. PBF is the most common type of technology for additive manufacturing of metallic parts for aerospace and biomedical applications.

Types under this category (proprietary technologies)

1. Selective Laser Sintering (SLS) produces parts using a laser to selectively sinter particles together. It differs from the other PBF processes in the material powder used (thermoplastics like nylon and Alumide).
2. MJF (by HP) is very similar to SLS.
3. Selective laser melting (SLM) is a similar process to SLS but the laser doesn’t sinter, it actually melts particles together which takes more heat.
4. Direct Metal Laser Sintering (DMLS) In broader terms, DMLS is quite similar to SLS. However, DMLS is optimised for 3D printing metals and alloys.
5. Electron Beam Melting (EBM) uses electron beams which are more powerful than a laser beam, which makes the process faster. However, parts processed by EBM have a higher surface roughness than parts produced using a laser beam.
6. Selective Heat Sintering (SHS) is similar to SLS, however, SHS uses a thermal print head to sinter powder rather than a laser. SHS was developed by a team of doctors at the University of Texas (UT) in the 1980s and is not used much today.
7. High-speed sintering (HSS) combines the advantages of two AM technologies: SLS and binder jetting (BJT). In this process, the whole building platform is printed in one go. A layer of powder is deposited, an inkjet printhead then deposits a radiation absorbing material (RAM) onto the desired area, and an infrared lamp then irradiates the entire surface.

Advantages of Powder Bed Fusion (PBF)

• Parts have tolerances equal to VPP but PBF parts are generally stronger
• Can produce parts with complex shapes
• Mechanical properties of metal parts are comparable to machining and casting
• Capable of functional plastic parts
• A wide range of materials is available
• Multiple materials in one build is possible
• More than one part can be produced simultaneously
• No minimum support structures are needed for the build (plastic)
• The used powder can generally be recycled

Disadvantages of Powder Bed Fusion (PBF)

• Long print time and costly post-processing also needed
• Expensive material and lots of electricity used, especially when the raw material is metal
• Since it requires a bed of powder, it’s not suitable for an office environment
• Parts have varied surface texture quality
• Parts can have thermal distortion – primarily with polymer parts

Applications of Powder Bed Fusion (PBF)

• Low volumes of functional parts across all industries, especially medical and, defence-related and aerospace
• Common applications include one-off industrial hardware such as
  1. machine parts,
  2. jigs, grips, and fixtures
  3. low-volume production runs of customized plastic components
• Rapid prototyping of complex parts
• Heavily used for production parts.
• Architecture models (SLS)

6. Sheet Lamination – SHL

Sheet lamination is a process in which sheets of material are bonded to form a part

Sheet Lamination (SHL) is a simple method of AM. It’s mainly used to produce coloured objects in a high resolution. SHL is essentially a process of stacking very thin sheets of material and laminating them together through ultrasonic welding, bonding, or brazing. Thin layered materials like aluminium foil or paper-based filaments are bonded and then cut into shaped layers by lasers or a sharp blade. The most common SHL technology is Ultrasonic Consolidation (UC) or Ultrasonic Additive Manufacturing (UAM). In this technique, room temperature metal sheets are bonded together by the application of ultrasonic waves and mechanical pressure. The final object is made by laser/knife cutting/CNC machining. 

Types under this category (proprietary technologies)

There are several different proprietary technologies that have different materials, lamination and cutting methods. In most cases, the process is a simple variation of Paper Laminated Object Manufacturing (LOM). Ultrasonic consolidation (UC) is the only technology that is majorly different to LOM because it uses ultrasonic welding rather than a bonding agent.

1. Laminated Object Manufacturing (LOM) – This process laminates sheets together with a bonding adhesive and then subtracts features layer by layer.
2. Selective Lamination Composite Object Manufacturing (SLCOM)
3. Plastic Sheet Lamination (PSL)
4. Computer-Aided Manufacturing of Laminated Engineering Materials (CAM-LEM)
5. Selective Deposition Lamination (SDL)
6. Composite Based Additive Manufacturing (CBAM)
7. Ultrasonic Consolidation (UC) / Ultrasonic Additive Manufacturing (UAM) – UC/UAM follows the same process at LOM, except the lamination is achieved through ultrasonic vibrations as a form of friction welding.

 composite fibre (SLCOM)

Advantages of Sheet Lamination (SHL)

• Little to no pre-production, fast print time, relatively low cost
• Easy material handling
• No support structures needed
• Large build area
• Ability to layer multiple materials so multicolour possible 
• Minimal energy consumption
• Ability to integrate into a hybrid manufacturing system
• In some instances, cut material can be recycled easily

Disadvantages of Sheet Lamination (SHL)

• Parts with complex shapes aren’t possible
• Parts with internal voids and cavities are difficult to produce
• Adhesive bonds deteriorate over time so the process is not ideal for parts that will be used long-term
• A lot of material waste if the part is smaller than the sheet size or build area
• Requires time-consuming post-processing
• Limited to materials that can come in sheets
• Layer height can’t be changed without changing the sheet thickness
• The finish achieved is sometimes variable

Applications of Sheet Lamination (SHL)

• Building models for architectural projects
• Coloured objects for proof-of-concept and look-and-feel prototyping.
• Rapid prototyping for parts without complex geometries

7. Vat Photopolymerisation – VPP

Vat Photopolymerisation is a process in which liquid photopolymer in a vat is selectively cured by light-activated polymerisation

Vat photopolymerisation (VPP) was the very first industrial additive manufacturing technology invented and is generally considered one of the fastest AM technologies. It can create incredibly detailed parts: Micro-stereolithography for instance, precisely prints parts on the nanometric scale. Some resins used in VPP are biocompatible so it’s one of the best technologies to print medical implants. Photopolymerisation is essentially a process in which photopolymer liquid resins in a vat are selectively exposed to ultraviolet (UV). When exposed, these materials undergo a chemical reaction and become solid. Conventionally, this light is a laser beam that draws the shape of each layer into the resin-filled vat. Once the part is complete, it is post-processed in a chamber with additional UV light. Vat Photopolymerisation is similar to Material Jetting (MJ) except the material rests in a vat rather than being jetted.

Types under this category (proprietary technologies)

There are three categories of Vat Photopolymerisation which differ in terms of the light source and how light is directed. Stereolithography is the most popular.

1. Stereolithography – SLA – uses a single-point laser to trace a thin line along the surface of the resin, filling in the shape of the cross-sectional layer to be cured
2. Digital light processing – DLP – uses a digital light projector to flash a single image of an entire layer all at once.
3. Continuous digital light processing – CLIP – the same as DLP except the build platform moves in a continuous motion

Advantages of VAT Photopolymerisation (VPP)

• One of the fastest AM technologies
• Parts can have incredibly small complex features with a super realistic finish
• Has the property of being watertight and/or airtight once cured
• Parts have a smooth surface finish
• Consistent repeatability
• Large build area
• Resin drained after the build can be used again
• Biocompatible materials available

Disadvantages of Vat Photopolymerisation (VPP)

• Relatively expensive resin and limited materials are available
• Long post-processing time
• Can become brittle especially when exposed to prolonged sunlight

Applications of VAT Photopolymerisation (VPP)

• Jewellery – short runs or prototyping
• Dental applications
• Medical applications
• Aerospace – short runs or prototyping
• Automotive – short runs or prototyping