What are the methods used in Additive Manufacturing?
Currently, the ASTM F2792 Standard defines Seven Families of AM Technologies plus one hybrid. The following shows those families along with descriptions, main strengths and materials used.
Each technology used in AM has different processing capabilities, advantages, and limitations. The factors that need to be considered to select the technology for AM are material, build volume, processing speed, part quality (mechanical performance, dimensional accuracy and surface finish), and the amount of post-processing required to improve the material properties, surface finish, and/or dimensional accuracy.
AM families apply similar physical and chemical phenomena to progressively add material in production processes as given in Figure 1.
Figure 1: Physical and chemical phenomena in material removal and material addition manufacturing
Another way to classify AM technologies is by the state of the material used (liquid, solid, powder, etc.), in this way, different AM manufacturing systems are grouped as (including some new ones):
- Layer Generation by Liquid-based Systems (Polymerisation of Resin - Stereolithography, Solid Ground Curing, Inkjet Deposition),
- Solid-based Systems (Fused Deposition Modeling, Laminated Object Manufacturing) and
- Powder-based Systems (sintering or melting of powder - selective laser sintering, electron beam melting).
Stereolithography is a method using focused UV light to transform liquid photopolymers into solid form. The process in stereolithography flows on a moveable platform above a reservoir of the photopolymer plastic. The platform plunges into the reservoir to create a thin layer of liquid. An ultraviolet laser flows on the liquid to create the first layer of the object. Each layer is attached to the previous one to build the object.
Melchels states that the sub-processes in stereolithography contain two distinct methods of irradiation. In the first method, an image is transferred to a liquid polymer by irradiating through a patterned mask. In the second method, a focused UV beam produces polymer structures in a direct writing process.
Figure 2: Stereolithography (SL) mechanism.
Selective Laser Sintering (SLS)
SLS produces parts by fusing or sintering together successive layers of powder material (Kruth et al. 1998: 535). SLS technology is presented as the lowest-cost 3D printing method by Royal Academy of Engineering Report. One of the strongest features of SLS is that it is able to process a very wide range of materials such as polymers, metals, ceramics, foundry sand, etc..
Figure 3: Selective Laser Sintering (SLS) mechanism.
According to Melchels et al., SLS is a technique using laser emitting infrared radiation to heat powder material just beyond its melting point. The laser works on giving the shape to each cross-section of the model, sintering powder in a thin layer. After the layer is hardened, the piston takes a new position and a new layer is built.
Hopkinson and Dickens reported that SLS as a manufacturing process was better suited to small parts due to machine speed. They also stated that small parts were more suitable for rapid manufacturing.
In laser generating, CMB (Controlled Metal Build-up), LENS (Laser Engineered Net Shape), SDM (Shape Deposition Modeling) and LAPSJ (Laser Aided Power Solidification with powder Jet) processes, the powder material is sprayed through a nozzle into the spot of a laser beam focused on the part as shown in Figure 4. “The advantage of laser cladding is that through melting of the powder permits the generative manufacture of a metallic part with a dense homogeneous structure” (Kruth et al. 1998: 536).
Figure 4: Laser cladding - Powder fed into laser spot in CMB.
Laminated Object Modeling (LOM)
In LOM, stacking thin sheets are built layer by layer on top of each other. Parts are cut according to the part’s cross section as shown in Figure 5.
Figure 5: Laminated Object Modeling (LOM) mechanism.
Fused Deposition Modeling (FDM)
FDM builds parts by depositing a stream of hot viscous material onto a base plate or previously deposited material.
FDM is a process in which thin thermoplastic filaments or granules are melted by heating and then guided by a robotic device. The material leaves the extruder in a liquid form and hardens immediately. The previously formed layer as a base for the next layer is kept at a temperature below the solidification point of the thermoplastic material. By doing so, good interlayer adhesion is maintained. Melted thermoplastics are squeezed through a nozzle to produce the designed product.
Figure 6: Fused Deposition Modeling (FDM) mechanism.
Powder Bed Fusion (PBF)
In this method, micro-particles of a binder material are deposited over the surface of a powder bed (structure material). Micro-particles are joined, and then the powder bed is lowered to build a new layer on the previous layer produced. The mechanism in which the layers are created is analogous to the one used by the SLS process. Usually PBF is more frequently used with polymers, but is also being used with metals to create "Green Parts" that later need post-process (infiltration, heat, pressure.), to produce final parts.
Laser metal deposition
Laser metal deposition (LMD) is an AM technology in which metal powder is melted using a laser beam to form a metallurgical bond. LMD systems can be installed at locations where repairs of high-value aerospace parts are expected (http://dupress.com/articles/additive-manufacturing-3d-opportunity-in-aerospace).
LMD is a technology containing some of the characteristics of Stereo-Lithography and Laser Cladding. It has a capability of processing complex metal components, using CAD files through 3D printers. LMD includes all possible general advantages of AM, such as shorter lead time and lower investment cost. The material addition mechanism is analogous to the one used by Laser Cladding.
Direct Metal Laser Sintering (DMLS)
Khaing et al. define DMLS as an AM method employing metal prototypes and tools directly from a CAD file. They state that DMLS is preferred by manufacturers using metal as a material when metal powder can be used to produce metal parts. The layer creation mechanism is analogous to the one used by SLS.
A comparison (Figure 7) done by Melchels et al helps determine the differences between AM methods. The materials, advantages, and disadvantages of the methods are briefly given in the table.
Figure 7: Comparison of AM methods