Selecting the most suitable Additive Manufacturing (AM) process for a particular application can be difficult. The very large range of available 3D printing technologies and materials often means that several of them may be viable, but each offers variations in dimensional accuracy, surface finish and post processing requirements.
The goal of this article is to categorize and summarize the differences between each of the Additive Manufacturing technologies. We identified the most popular 3D printing processes and the most common applications and materials for each of them.
Photopolymerization occurs when a photopolymer resin is exposed to light of a specific wavelength and undergoes a chemical reaction to become solid. More details about the photopolymerazation mechanism can be found here. A number of additive technologies utilize this phenomena to build up a solid part one layer at a time.
SLA uses a build platform submerged into a translucent tank filled with liquid photopolymer resin. Once the build platform is submerged, a single point laser located inside the machine maps a cross sectional area (layer) of a design through the bottom of the tank solidifying the material. After the layer has been mapped and solidified by the laser, the platform lifts up and lets a new layer of resin flow beneath the part. This process is repeated layer by layer to produce a solid part. Parts are typically then post-cured by UV light to improve their mechanical properties.
DLP follows a near identical method of producing parts when compared to SLA. The main difference is that DLP uses a digital light projector screen to flash a single image of each layer all at once. Because the projector is a digital screen, the image of each layer is composed of square pixels, resulting in a layer formed from small rectangular bricks called voxels. DLP can achieve faster print times compared to SLA for some parts, as each entire layer is exposed all at once, rather than tracing the cross sectional area with a laser.
Continuous Direct Light Processing (CDLP) (also know as Continuous Liquid Interface Production or CLIP) produces parts in exactly the same way as DLP. However it relies on continuous motion of the build plate in the Z direction (upwards). This allows for faster build times as the printer is not required to stop and separate the part from the build plate after each layer is produced.
Vat polymerization processes are excellent at producing parts with fine details and gives a smooth surface finish. This makes them ideal for jewelry, low-run injection molding and many dental and medical applications. The main limitations of vat polymerization is the brittleness of the produced parts.
|SLA||Formlabs, 3D Systems, DWS||Standard, tough, flexible, transparent, & castable resins|
|DLP||B9 Creator, MoonRay||Standard & castable resins|
|CDLP||Carbon3D, EnvisionTEC||Standard, tough, flexible, transparent, & castable resins|
Powder Bed Fusion
Powder Bed Fusion (PBF) technologies produce a solid part using a thermal source that induces fusion (sintering or melting) between the particles of a plastic or metal powder one layer at a time.
Most PBF technologies employ mechanisms for spreading and smoothing thin layers of powder as a part is constructed, resulting in the final component being encapsulated in powder after the built is complete.
The main variations in PBF technologies come from the differing energy sources (for example lasers or electron beams) and the powders used in the process (plastics or metals).
SLS produces solid plastic parts using a laser to sinter thin layers of powdered material one layer at a time. The process begins by spreading an initial layer of powder over the build platform. The cross section of the part is scanned and sintered by the laser, solidifying it. The build platform then drops down one layer thickness and a new layer of powder is applied. The process repeats until a solid part is produced. The result of this process is a component completely encased in unsintered powder. The part is removed from the powder, cleaned and then it is ready for use or further post-processing.
Click here for a full guide on designing parts for SLS.
Both Selective Laser Melting (SLM) and Direct Metal Laser Sintering (DMLS) produce parts via the similar method to SLS. The main difference is that SLM and DMLS are used in the production of metal parts. SLM achieves a full melt of the powder, while DMLS heats the powder to near melting temperatures until they chemically fuse together. DMLS only works with alloys (nickel alloys, Ti64 etc.) while SLM can use single component metals, such as aluminum. Unlike SLS, SLM and DMLS require support structures to compensate for the high residual stresses generated during the build process. This helps to limit the likelihood of warping and distortion. DMLS is the most well established metal AM process with the largest installed base.
A full guide on designing parts for SLM anf DMLS can be found here.
EBM uses a high energy beam rather than a laser to induce fusion between the particles of a metal powder. A focused electron beam scans across a thin layer of powder causing localized melting and solidification over a specific cross sectional area. Electron beam systems produce less residual stresses in parts, resulting in less distortion and less need for anchors and support structures. Moreover, EBM uses less energy and can produce layers at a faster rate than SLM and DMLS, but the minimum feature size, powder particle size, layer thickness and surface finish are typically of lower quality. EBM also requires the parts to be produced in a vacuum and the process can only be used with conductive materials.
MJF is essentially a combination of the SLS and Material Jetting technologies. A carriage with inkjet nozzles (similar to the nozzles used in desktop 2D printers) passes over the print area, depositing fusing agent on the a thin layer of plastic powder. At the same time a detailing agent that inhibits sintering is printed near the edge of the part. A high-power IR energy source then passes over the build bed and sinters the areas where the fusing agent was dispensed, while leaving the rest of the powder untouched. The process repeats until all parts are complete.
An article comparing the capabilities of MJF with SLS can be found here.
Polymer-based PBF technologies offer a lot of design freedom, as there is no need for support, allowing the fabrication of complex geometries.
Both metal and plastic PBF parts typically have very high strength and stiffness and mechanical properties that are comparable (or sometimes even better) than the bulk material. There is a large range of post processing methods available, meaning that PBF parts can have a very smooth finish and, for this reason, they are often used to manufacture end products.
The limitations of PBF often center around surface roughness and internal porosity of the as-pritned parts, shrinkage or distortion during processing and the challenges associated with powder handling and disposal.
|SLS||EOS, Stratasys||Nylon, alumide, carbon-fiber filled nylon, PEEK, TPU|
|SLM/DMLS||EOS, 3D Systems, Sinterit||Aluminum, titanium, stainless steel, nickel alloys, cobalt-chrome|
Similar to how toothpaste is squeezed out of a tube, material extrusion technologies extrude a material through a nozzle and onto a build plate. The nozzle follows a predetermined path building layer-by-layer.
FDM (sometime also referred to as Fused Filament Fabrication or FFF) is the most widely used 3D printing technology. FDM builds parts using strings of solid thermoplastic material, which comes a filament form. The filament is pushed through a heated nozzle where it is melted. The printer continuously moves the nozzle around, laying down melted material at precise locations following a pre-determined path. When the material cools it solidifies, building the part layer-by-layer.
Material extrusion is a quick and cost-effective way of producing plastic prototypes. Industrial FDM systems can also produce functional prototypes from engineering materials. FDM has some dimensional accuracy limitations and is very anisotropic.
|FDM||Stratasys, Ultimaker, MakerBot, Markforged||ABS, PLA, Nylon, PC, fiber-reinforced Nylon, ULTEM, exotic filaments (wood-filled, metal-filled etc)|
Material jetting is often compared to the 2D ink jetting process. Photopolymers, metals or wax that cure or harden when exposed to UV light or elevated temperatures can be used to build parts one layer at a time. The nature of the material jetting process allows for multi-material printing. This ability is often used to print support from different (soluble) material during the build phase.
Material Jetting dispenses a photopolymer from hundreds of tiny nozzles in a printhead to build a part layer-by-layer. This allows material jetting operations to deposit build material in a rapid, line wise fashion compared to other point-wise deposition technologies that follow a path to complete the cross-sectional area of a layer. As the droplets are deposited to the build platform they are cured and solidified using UV light. Material jetting processes require support and this is often printed simultaneously during the build from a dissolvable material that is easily removed during post processing.
Click here for an introduction to Material Jetting.
Nano particle jetting (NPJ) uses a liquid, which contains metal nanoparticles or support nanoparticles, loaded into the printer as a cartridge and jetted onto the build tray in extremely thin layers of droplets. High temperatures inside the build envelope cause the liquid to evaporate leaving behind metal parts.
DOD material jetting printers have 2 print jets: one to deposit the build materials (typically a wax-like liquid) and another for dissolvable support material. Similar to traditional AM techniques, DOD printers follow a pre-determined path and deposit material in a pointwise fashion to build the cross-sectional area of a component. These machines also employ a fly-cutter that skims the build area after each layer to ensure a perfectly flat surface before printing the next layer. DOD technology is typically used to produce “wax-like” patterns for lost-wax casting/investment casting and mold making applications.
Material jetting is ideal for realistic prototypes, providing excellent details, high accuracy and smooth surface finish. Material jetting allows a designer to print in multiple colors and multiple materials in a single print. The main drawbacks of material jetting technologies are the high cost and the brittle mechanical properties of the UV activated photopolymers.
|Material jetting||Stratasys (Polyjet), 3D Systems (MultiJet)||Rigid, transparent, multi-color, rubber-like, ABS-like.
Multi-material and multi-color printing available
|NPJ||Xjet||Stainless steel, ceramics|
Binder jetting is the process of dispensing a binding agent onto a powder bed to build a part one layer at a time. These layers bind to one another to form a solid component.
Binder Jetting deposits a binding adhesive agent onto thin layers of powder material. The powder materials are either ceramic-based (for example glass or gypsum) or metal (for example stainless steel). The print head moves over the build platform depositing binder droplets, printing each layer in a similar way 2D printers print ink on paper. When a layer is complete, the powder bed moves downwards and a new layer of powder is spread onto the build area. The process repeats until all parts are complete. After printing, the parts are in a green state and require additional post processing before they are ready to use. Often an infiltrant is added to improve the mechanical properties of the parts. The infiltrant is usually a cyanoacrylate adhesive (in case of ceramics) or bronze (in the case of metals).
Ceramic-based Binder Jetting is ideally suited for applications that showcase aesthetics and form: architectural models, packaging, ergonomic verification etc. It is not suitable though for functional prototypes, as the parts are very brittle. Ceramic-based Binder Jetting can also be used to create molds for sand casting.
Metal binder jetting parts can be used as functional components and are more cost-effective than SLM or DMLS metal parts, but have poorer mechanical properties.
|Binder jetting||3D Systems, Voxeljet||Silica sand, PMMA particle material, gypsum|
|ExOne||Stainless steel, ceramics, cobalt-chrome, tungsten-carbide|
Direct Energy Deposition
Direct Energy Deposition (DED) creates parts by melting powder material as it is deposited. It is predominantly used with metal powders or wire and is often referred to as metal deposition.
LENS utilizes a deposition head, which consists of a laser head, powder dispensing nozzles and inert gas tubing, to melt powder as it is ejected from the powder dispensing nozzles to build a solid part layer-by-layer. The laser creates a melt pool on the build area and powder is sprayed into the pool, where it is melted and then solidified. The substrate is typically a flat metal plate or an existing part that material is added on (for example for repair).
EBAM is used to create metal parts using metal powder or wire, welded together using an electron beam as the heat source. Producing parts in a similar fashion to LENS, electron beams are more efficient than lasers and operate under a vacuum with the technology originally being designed for use in space.
DED technologies are used exclusively in metal additive manufacturing. The nature of the process means they are ideally suited for repairing or adding material to existing components (such as turbine blades). The reliance on dense support structures make DED not ideally suited for producing parts from scratch.
|LENS||Optomec||Titanium, stainless steel, aluminum, copper, tool steel|
|EBAM||Sciaky Inc||Titanium, stainless steel, aluminum, copper nickel, 4340 steel|
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