Categories of 3D printing technologies and processes

What is 3D printing?

3D printing is a fabrication technique used for building three-dimensional structures and solid objects. 3D printing is an additive manufacturing (AM) technique: the final object is created by adding layer of material on top of each others (by opposition to subtractive fabrication methods such as sculpture where you need to remove stone in order to from the final object). To create a solid object, the 3D printer deposits printing material on the print bed (also called build platform) following the design of a 3D file, often a STL format file. The material, typically melted plastic for FFF and FDM 3D printers, is deposited layer by layer. Each layer is very thin and quickly solidifies, thus forming a three-dimensional objects. Most desktop 3D printers use plastic filament spools as consumables.

The 3D printing technologies

There are many types of 3D printing technologies currently available commercially or at the early development stage. Each of these additive manufacturing techniques requires a specific type of 3D printing material: from plastic filaments (PLA, ABS…) to photosensitive resin to powdered material (metals, plastics etc). These 3D printing technologies have various advantages and can be used in specific applications and use cases.

There are three main categories of 3D printing technologies:

  • Extrusion (FFF and FDM): a plastic filament is melted and deposited on the build platform of the 3D printer to form the object layer by layer.
  • Resin (SLA and DLP): a liquid photosensitive resin is cured by a laser or a projector to form the object directly in the resin tank of the 3D printer. The most common 3D printing technology using photopolymerization (solidification of the photosensitive resin via a source of light) is called stereolithography (SLA).
  • Powder (SLS, SLM, DMLS…): a powdered material is sintered or melted by a laser, the grains of powder are bonded or melted together (sintered) to obtain a solid structure. The Selective Laser Sintering (SLS) technology is the most common among powder-based 3D printing technologies, although several derived processes exist.

Shedding many of the constraints of traditional fabrication techniques, 3D printers are a great tool for rapid prototyping, one of the most common uses. Advanced 3D printing systems are also used for direct manufacturing of end products, such as part in the Aerospace industry.

The rise of 3D printing is already greatly impacting manufacturing and design processes across many industries.

Infographic materials and 3D printing technologies mapping.

Infographic materials and 3D printing technologies mapping

Extrusion: FDM (Fused Deposition Modeling) and FFF (Fused Filament Fabrication)

FDM (Fused Deposition Modeling) and FFF (Fused Filament Fabrication)

 

Extrusion (also known as FDM for Fused Deposition Modeling or FFF for Fused Filament Fabrication) is the most common 3D printing technique, used by the majority of desktop 3D printers. Extrusion uses plastic filament (PLA or ABS) as the printing material. The filament is heated and melted in the printing head (extruder) of the 3D printer. The 3D printing head moves on two horizontal axes (X and Y axis) while the tray supporting the build platform object moves vertically on the Z axis.

The 3D printer deposits the melted filament by layer, each layer on top of the others, to build the object in 3D. When one layer is complete, the tray holding the object lowers very slightly and the extrusion process resumes, depositing a new layer of melted filament on top of the previous one. Deposited layers are fused together as the melted plastic quickly solidifies to form a solid three-dimensional object.

When one layer is complete, the tray holding the object lowers very slightly and the layering process resumes, depositing a new layer on melted filament on top of the previous one. Deposited layers are fused together as the melted plastic quickly solidifies to form a solid three-dimensional object.

The 3D printing extrusion process. Image credit: KUL3D.com.

The 3D printing extrusion process (source: KUL3D.com).

Stacked layers of material form the final 3D printed object. The precision and quality of the final result depend, among other factors, on the minimum layer thickness of the 3D printer (the thinner the layers, the higher the 3D print resolution).

3D printing materials compatible with extrusion 3D printers cover a wide range of plastic filaments (spools), mainly PLA filament or ABS filament. FFF 3D printers are also compatible with exotic plastic filaments containing a percentage of metal or wood for example. Most desktop 3D printers are based on the FFF 3D printing technology.

Direct Energy Deposition (DED)

Direct Energy Deposition (DED) is a quite advanced 3D printing technology used by only a couple of industrial 3D printer manufacturers. We chose to classify it as an “extrusion” technique because in DED, the printing material is slowly pushed towards a powerful energy source (like a laser or an electron beam) to be directly melted and fused to form an object.

A typical DED 3D printer consists of a nozzle mounted on a multi axis arm (up to 5 axis), which deposits melted material onto a surface, where it solidifies. The process is therefore quite similar to material extrusion, but the nozzle can move in multiple directions and is not fixed to a specific axis. The printing material, which can be deposited from any angle, is melted upon deposition with a laser or electron beam.

The direct energy deposition 3D printing technology can be used with polymers or ceramics but it is typically used with metal powders or wires. This 3D printing process is commonly used to repair or add additional material to existing components or parts.

Direct Energy Deposition

Direct energy deposition 3D printing process (source: Sciaky.com).

Photopolymerization and resin 3D printing: SLA and DLP

How do resin 3D printers work?

Resin 3D printers using the SLA or DLP technologies are based on the photopolymerization process: the resin contained in the 3D printer tank is cured (hardened) by a light source (laser or projector) to form the object, layer by layer. Resin 3D printers use liquid resin as the 3D printing material, versus plastic filament for extrusion 3D printers. Resins used as printing material in SLA or DLP 3D printers are light-sensitive photopolymers (photo resins) which solidify when exposed to specific light beams. During the SLA or DLP 3D printing process, a tray (build platform) is immersed inside a liquid resin tank, close to the surface. A projector or laser beam emits light, which will cure the liquid resin point by point (SLA) or layer by layer (DLP). Once a layer is complete, the tray is immersed slightly deeper into the tank and the process is repeated to create additional layers, fused to the previous ones to form the final object.

SLA or DLP 3D printers are recommended for situation when 3D printing objects with a a high level of details and requiring a smooth surface finish. Resin 3D printers are often used to make moulds for casting, typically in jewelry or dental applications.

Digital Light Processing (DLP) technology. Image credit: 3DPrintingIndustry.com.

Digital Light Processing (DLP) technology (source: 3DPrintingIndustry.com).

 

SLA vs DLP: resin 3D printing technologies comparison

The main difference between SLA and DLP is that in DLP, the resin is cured layer by layer, as the UV light is emitted by a projector whereas in SLA, the object if formed point by point by the light source of the laser. In DLP, the layers composing the final object are cured all at once since the projector projects a surface of light and not a single point as the laser. With SLA, since the UV light source is a laser, the photo-sensitive resin is cured dot by dot, to form solid lines, which will form layers. SLA is therefore more accurate but also potentially a slower 3D printing process. Laser are also more costly and difficult to maintain pieces of equipment, versus projectors which can be easily found and use replaceable lamps.

 

Note: To get a better understanding of differences between FFF and SLA, please read this comparison article between these two 3D printing technologies.

What is SLA and how does it work?

Stereolithography, know as SLA, is an additive manufacturing process used by SLA 3D printers. A UV (ultraviolet) light is projected via a laser beam to solidify a liquid resin contained in a tank and form the object layer by layer. In a SLA 3D printer, the resin tank (also called vat) is filled with a photo-curable liquid resin (photopolymer resin). The UV laser beam traces the shape of the 3D design in the resin tank and solidifies the curable resin to form the final object, point by point (high accuracy), and layer by layer.

DLP 3D printers (Digital Light Processing)

Digital Light Processing is a 3D printing technology know as DLP, used in DLP 3D printers. The object is formed by the solidification of a photo-reactive resin using a digital light projector as the UV light source. The projector used in a DLP 3D printer can be a regular video projector, its resolution will determine the 3D print resolution. DLP 3D printers are gaining traction, notably because of their superior print speed thanks to the light projector which hardens the resin layer by layer and not point by point, as lasers used in SLA 3D printers do.

SLA vs DLP infographie.

SLA vs DLP (source: Alibaba.com).

Powder bed fusion: SLS, SLM and EBM

Powder 3D printers use powder materials as consumables (metal powders for example). The main types of powder 3D printing technologies are SLS (Selective Laser Sintering) and SLM (Selective Laser Melting). Powder-based 3D printers are typically used for metal 3D printing in various categories of industrial applications.

SLS 3D printers (Selective Laser Sintering)

With the SLS 3D printing technology, a laser beam is used to sinter a powdered material: the energy of the laser bonds together the tiny grains of powdered material (plastic, ceramic, metal…) to form a solid structure. SLS 3D printers have a print bed full of powder material (a bit like a small sandbox). A laser, monitored by the 3D printer software, then traces the pattern of the 3D design to form the final object layer by layer. After each layer is complete, the print bed lowers on the Z axis and another layer is built on top of the previous one. You can learn more about selective laser sintering in this Wikipedia article.

SLM 3D printers (Selective Laser Melting)

Selective laser melting (SLM) uses a quite similar process as SLS. The difference between SLS and SLM is that in SLM, the powdered material is melted and not sintered. The high-power laser used in SLM 3D printers allows to fuse the particles of powder together to form the solid object. This additive manufacturing process is mainly used for direct manufacturing of end-use metal parts, in industrial applications for the aerospace or medical industry. Dental SLM 3D printers are also becoming more common for the production of metal dental appliances (crowns etc).

 

Selective Laser Sintering (SLS) 3D printing technology. Image credit: sculpteo.com.

Selective Laser Sintering (SLS) 3D printing technology (source: sculpteo.com).

 

 

EBM additive manufacturing (Electron Beam Melting)

The Electron Beam Melting (EBM) 3D printing process is based on the same principle as the SLM additive manufacturing technology: a powdered material (usually metal or alloys) is solidified into a 3D object by energy, in this case the energy generated by an electron beam. Electron Beam Melting 3D printers build solid objects by melting the powdered material (unlike SLS where the material in sintered).

Other powder 3D printing technologies (DMLS, SLM, SHS, LM…)

3D printers using powder materials are designed for industrial applications, such as rapid prototyping or direct manufacturing of parts. In addition to the powder-based 3D printing technologies listed above, you can also find: Laser Sintering (LS), Laser Melting (LM), Selective Heat Sintering (SHS), Direct Metal Laser Sintering (DMLS), or Plaster-based 3D Printing (PP).

The main difference between these advanced manufacturing technologies are the way the powder material is melted. EBM, EBAM and SLM entirely melt the material while SLS and LS rather fuse grains of powder together. The compatible 3D printing materials with powder SLS or SLM are Titanium alloys, thermoplastics, ceramic powders and metals, thus making powder 3D printers a fixture in industries such as Aerospace, where high resistance parts in metal are often required.

Material jetting (MJM, BJ, PJ...)

Material Jetting (Multijet Modeling or MJM)

Material Jetting is a 3D printing technology where inkjet print heads jet melted material on the build platform of the 3D printer, which then cool and solidify to form a 3D object layer by layer. Material jetting is also know as Multijet Modeling (MJM), Drop on demande (DOD), Thermojet or Inkjet printing. Material Jetting 3D printer produce high quality prints and surface finishes.

Photopolymer Jetting (PolyJet or PJ)

In Photopolymer Jetting 3D printing, the print heads jet a liquid photopolymer material (sensitive to light) on the print bed. The photopolymer material is directly cured by a UV lamp attached to the print head, to form the solid object. Stratasys notably uses this additive manufacturing technology with their PolyJet proprietary 3D print technology. Several materials can be jetted together to create multi-materials or multicolor objects.

Binder Jetting (BJ)

In the Binder jetting 3D printing technology, a liquid binding agent is deposited (jetted) onto a powdered material to bind powder particles together and form the final object. Both the Photopolymer Jetting (PJ) and Binder Jetting (BJ) can be categorized in the Material jetting category. Binder Jetting is also known as full color 3D printing or Inkjet Powder Printing: it is the additive manufacturing technology used in full-color 3D printers, ideal to produce detailed and color objects such as 3D printed figurines for example.

3D printing technology binder jetting. Image credits: 3dprintingindustry.com.

Binder jetting 3D printing technology (source: additively).

Sheet lamination

The lamination 3D printing technique uses thin layered materials (paper or aluminum foil for example) to produce highly detailed and full color 3D objects. The sheets are cut following the 3D design of the desired object, often by lasers or a very sharp blade. Layers are then coated with an adhesive and glued together layer by layer, similar to other additive manufacturing techniques.

The precision of the result depends mainly on the thickness of the layered material used (thickness of the paper sheet for instance). Paper is the most popular based material as it is affordable and easy to work with. 3D printed objects in paper are resistant and can be fully colored. The 3D printing technology developed by Mcor for its full color 3D printers is based on the sheet lamination process.

The lamination 3D printing technology is also known as Laminated Object Manufacturing (LOM) and Sheet Lamination (SL). The compatible 3D printing materials with the lamination technology are paper, metal foils and plastic film. However the only commercial 3D printers available on the market use paper (Mcor ARKe). use paper.

The 3D printing lamination process. Image credit: 3DPrintingIndustry.com

The 3D printing lamination process (source: 3DPrintingIndustry.com).

3D bioprinting

3D bioprinting is the process of generating spatially-controlled cell patterns using 3D printing, where cell function and viability are preserved within the printed construct. 3D bioprinting is a technology aiming to fabricate living tissues and functional human organs. With bioprinting, it is already possible to fabricate bones, cartilage and (almost!) functional 3D printed muscle and organs. That said, most bioprinters are not yet ready to 3D print ready-to-use replacement organs or limbs.

3D bioprinting is still at a mostly experimental stage, but this technology has gained traction in medical and pharmaceutical industries and applications include the fabrication of specific tissues for drug testing for example.

One of the first 3D bioprinter is called “Regenovo“ and was designed by researchers from Hangzhou Dianzi University in China. This 3D bioprinter has been used to successfully 3D print tissue samples, including functional livers and human ear cartilage. The fabrication of a functional tissue made of several cell types using 3D printing follows usually a 3-steps process:

  • Cells are sorted, multiplied and differentiated. They form what is called a bio ink.
  • Cells or cell aggregates are embedded in a 3D structure to enable subsequent hierarchical tissue building.
  • The obtained 3D structures are matured in a perfusion reactor to create a vasculature system. This is called the maturation phase.
  • The 3D printed tissue can be used in medical research
3D bioprinting technology infographic. Image credit: Broadsheet.ie.

3D bioprinting technology infographic (source: Broadsheet.ie).

 



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