Additive Manufacturing Processes
Although all additive manufacturing (AM) processes feature layer-by-layer fabrication of three-dimensional objects, production techniques vary. In 2010, the American Society for Testing and Materials (ASTM) grouped AM processes into seven categories in a new standard - “ASTM F42 - Additive Manufacturing.” Every year, members of ASTM Committee F42 meet to consider new or updated standards.
The current categories of additive manufacturing processes are:
- Powder Bed Fusion
- Vat Photopolymerization
- Binder Jetting
- Material Extrusion
- Directed Energy Deposition
- Material Jetting
- Sheet Lamination
These seven additive manufacturing processes include notable variations on the layered 3D printing concept. Material state (powder, liquid, filament), heat or light sources (laser, thermal, electron beam, plasma arc), number of print axes, feed systems and build chamber characteristics all vary. Also, some additive manufacturing techniques require additional post-production processing while others do not.
Additive manufacturing processes continue to make inroads across broad areas of manufacturing. Netscribes estimates that the global 3D printing market will grow at a CAGR (compound annual growth rate) of more than 25 percent, generating $8.7 billion in annual revenue by 2020.
Powder Bed Fusion
Powder bed fusion (PBF) is a process common to a variety of popular additive printing techniques -- direct metal laser melting (DMLM), electron beam melting (EBM), directed metal laser sintering (DMLS), selective laser melting (SLM), selective laser sintering (SLS) and selective heat sintering (SHS).
As the name suggests, PBF involves melting powder to a sufficient degree for the particles to fuse together. Particles are either “sintered” (partially melted) or fully melted in various PBF processes. Thermal energy in the form of a laser, beams of electrons or a heated print head partially or fully melt plastic or metal powder.
An ultrathin layer of material is spread by a roller or blade over the preceding layer. The powder is fed from a reservoir beneath or next to a build platform that lowers to accommodate each successive layer of powder. At the conclusion of the additive process, the unfused powder is blown or blasted away.
A large powder bed DMLM machine being developed by GE Additive’s Concept Laser allows build volumes of up to 1.1 x 1.1 x 0.3 meters.
In general, EBM is a faster additive manufacturing method than SLM and DMLM, although the layers are thicker, and the surface is rougher. This is actually an advantage in the production of orthopedic titanium implants because the rough outer surface facilitates bone growth. The EBM process also produces parts with less residual stress and distortion, another advantage with implants, jet engine parts, and more. EBM works with a wide range of metals, including titanium, stainless steel, copper and cobalt chrome.
Powder bed fusion is ideal for almost all types of end manufacturing, allowing for the easy design and build of complex geometries. Parts typically possess high strength and stiffness with a large range of post-processing methods available.
Vat photopolymerization differs from many additive manufacturing processes in that it begins with the use of a liquid rather than a powder or a filament. Additive printing techniques vary although they all use photopolymer resins -- often tough, transparent and castable materials.
Stereolithography (SLA) is a vat photopolymerization method that dates back to the 1980s. It is one of the earliest 3D printing processes actively used today. SLA makes use of a build platform in a tank of liquid polymer. It is a light-activated, not a heat-activated process.
Since laser light comes from beneath the object, it is the reverse of other additive manufacturing processes that feature a heat source directed from above. Resin solidifies as the laser maps each layer. After the interaction between laser light and photopolymer creates an object layer, the platform rises, and more liquid resin gathers immediately below the object. UV light cures each layer.
Direct Light Processing (DLP) is very similar to SLA. However, it creates each layer of an object by projecting laser light on tiny mirrors, resulting in the projection of square pixels, layer-by-layer. The idea of using lasers and tiny mirrors is also used in digital projectors in movie theaters. As an additive manufacturing process, DLP is often faster than SLA because each layer is fully projected in a single operation.
A DLP variation is continuous direct light processing (CDLP), which relies on the constant motion of the build platform to yield higher build speeds.
Vat photopolymerization is excellent at producing parts with fine detail and smooth surfaces. Ideal for jewelry, medical applications and low-run injection molds.
Directed Energy Deposition
Directed energy deposition (DED), (also called direct metal deposition or metal deposition), utilizes highly focused thermal energy delivered via laser, electron beam or plasma arc to melt and fuse material jetted into the heated chamber from either powdered metal or wire filament. The additive process is most commonly used with metal, although some DED systems can be used ceramic powder or polymers.
The system usually features metal deposition along four or five axes. This makes it one of the few additive manufacturing technologies that can be used to repair worn tools and parts in the aerospace, defense and automotive industries.
Laser engineered net shape (LENS) technology builds solid parts as the powder is dispensed from nozzles and selectively melted by a laser. This creates a melt pool on the build platform that solidifies layer upon layer.
Thanks to its multi-axis printing capabilities, LENS is also used to selectively build up worn or damaged parts in an additive process. Since the system requires support structures, it is actually better suited to repairing components rather than creating them from scratch.
With electron beam additive melting (EBAM), metal melting occurs via an electron beam firing in a vacuum chamber. Either metal powder or wire filament is fully melted in layers as thin as 20 microns each.
Finally, with rapid plasma deposition (RPD), a plasma arc melts a wire filament in an argon gas environment to produce parts that require little or no post-production machining. Like other additive manufacturing processes, RPD is often more cost-effective than traditional forging.
Exclusively used in metal additive manufacturing, this process is ideal for repairing or adding material to existing components.
Material jetting is an additive manufacturing process that uses drop-on-demand (DOD) technology. Like a 2D inkjet printer, tiny nozzles dispense tiny droplets of a waxy photopolymer, layer by layer. UV light cures and hardens the droplets before the next layer is created. Since this additive technology relies heavily on support structures, a second series of nozzles dispenses a dissolvable polymer that supports the object as it is printed. When printing is complete, the support material is dissolved away. Material jetting produces patterns used in lost-wax casting, investment casting and mold making.
Nanoparticle jetting (NPJ) is a sub-category of material jetting that uses liquids infused with metal particles. As each layer of droplets is deposited onto the print bed, the high temperatures in the heated build chamber cause the liquid to evaporate, leaving a layer of metal.
To better understand the material jetting process, it is worthwhile to compare 2D inkjet printers and 3D-printed objects. The piece of paper in a 2D inkjet printer is very much like the print bed in a 3D printer.
Although it appears flat, the 2D image has an observable vertical dimension. To illustrate this, Finnish researchers combined three techniques -- X-ray microtomography, optical profilometry and laser ablation -- to create a 3D image of the ink on the paper. Although the image area was highly contoured, the average thickness observed was 2.5 microns, about 1/40 the thickness of the paper upon which it was deposited.
Now, imagine the print head applying layer upon layer of ink as the print bed descended in tiny increments. As long as each layer hardened before the deposition of the next, the process would generate a 3D object. Material jetting is one of the more precise additive printing processes. The technology can print layers as thin as 15-16 microns, only six times the thickness of a 2D inkjet image.
Ideal for realistic prototypes with high detail, high accuracy and a smooth finish. Material jetting allows for multiple colors and materials in a single printout.
The binder jetting process is similar to material jetting, although it employs powdered material and a binding agent. Nozzles on these 3D printers deposit tiny droplets of a binder on an ultrafine layer of powdered metal, ceramic or glass. Multiple layers result from the powder bed moving downward after each layer is created.
The resulting object is in a green state, so post-processing is required. For example, bronze may be used to infiltrate a metal object. This improves its mechanical properties enough to make it a functional component. A cyanoacrylate adhesive is a common infiltrant when the object is ceramic. However, ceramic objects produced by binder jetting are still fairly brittle, so they are primarily used as architectural models or models for sand casting.
Ideal for aesthetic applications like architectural and furniture design models. It is generally not used in functional applications due to its brittle nature.
Fused deposition modeling (FDM), or fused filament fabrication (FFF), is perhaps the most well-known additive manufacturing process. When the general public hears “3D printing,” this is the process they often think of. A thermoplastic filament is extruded through a heated nozzle and onto the build platform. The material solidifies as it cools, although not until it fuses to adjacent layers. FDM uses a wide variety of thermoplastic filaments, including ABS, PLA, nylon, PC , ULTEM and more complex filaments like those that are metal-filled and wood-filled.
Because the process is fast and inexpensive, it is often used to produce prototypes. Although dimensional accuracy was a concern in the past, some modern industrial FDM machines produce functional prototypes.
FDM is directionally dependent, or anisotropic. Since the material is deposited along the horizontal x- and y-axes, strength is an issue along the vertical z-axis. Research continues into post-processing methods which improve z-axis strength. One approach involves printing the object with voids across layers. In post-processing, these voids are filled with a hardening resin to improve z-strength.
Quick and cost effective, FDM is often the go-to method for producing non-functional prototypes or rapid prototyping where several iterations are needed.
Sheet lamination is an additive manufacturing process in which ultra-thin layers of solid material are bonded by alternating layers of adhesive. It is possible to use a variety of materials in this additive process called laminated object manufacturing (LOM).
For example, paper lamination technology (PLT), combines sheets of paper and layers of adhesive into a layered object resembling plywood. At the conclusion of the process, a laser, metal knife or tungsten carbide blade directed by a CAD file cuts through the many layers to create the final 3D object.
Ultrasonic additive manufacturing (UAM) uses metal sheets, ribbons or foils to build objects a single layer at a time. A variety of metals are used in UAM, including titanium, stainless steel, copper and aluminum. Metal layers are typically conjoined through ultrasonic welding and compression via a rolling sonotrode, a device that generates the ultrasonic vibrations.
The UAM process does not require melting, and it uses less energy than most additive manufacturing processes. CNC machining may be used to further refine the surface of the object and remove excess, non-bonded metal.
Best for non-functional models, benefits include speed, low cost, ease of material handling