Stereolithography (Vat photopolymerization)
Stereolithography (SL) is considered the first 3D printing process and it was also the first to be commercialized. SL is a process based on the interaction of a laser beam with photopolymer resins which harden and solidify to form a solid in a very precise manner and is widely used for the production of detailed parts. It is a complex process, where the photopolymer resin is held in a transparent container with a mobile platform inside. A laser beam is directed in the X-Y axes along the surface of the resin according to the 3D data provided to the printer from the digital file, causing the resin to harden exactly where the laser hits the surface. Once the layer is complete, the platform inside the tank is lowered by a fraction (in the Z-axis) and the next layer is detected and created by the laser. This process continues until the entire object is completed and the platform can be lifted out of the tank to be removed.
Upon completion of the SL process, support structures are required to stabilize some parts, which are manually removed. In addition, the printed objects must be cleaned (with special solvents) and followed by their final solidification hardening. The final stage involves exposing the part to intense UV light which helps to fully cure the resin and display its properties.
Stereolithography is probably one of the most precise 3D printing processes that yield an excellent surface finish. However, the stability of materials changes over time, and printed objects may become more fragile.
Material extrusion (FFF/FDM)
It is made by melting a filament of plastic that is deposited onto a surface through a heated extruder. It forms, one layer at a time, by imprinting in space the data of a 3D digital design, which is used by the user and is probably the most common way of printing. Each layer hardens and cools as it is deposited and bonded to the previous layer. Complex geometries require support structures that are carried through during printing and removed once printing is complete. For the supporting structures, either a water-soluble material is usually used that dissolves in water, easily washing off the printed piece or the same material that the object is made of and removed manually after the printing is finished. One of the biggest problems of this type of printing is the adhesion between the layers which can be used for annealing and achieving a relative improvement in the strength of the final object.
Selective sintering (SLS)/laser melting (SLM)
Selective sintering (SLS)/laser melting (SLM) refers to a 3D printing process that uses materials in powder form. In the SLS process, the main objective is to fabricate a layer of predetermined geometry by fusing powders using a laser beam. The process follows the following sequence: (1) a substrate is lowered to a depth equal to the thickness of the layer, (2) a layer of powder is spread over the substrate, (3) the laser beam scans the area of the layer indicated by the digital model to merge the dust grains in the specified area. Sequence (1), (2), and (3) is repeated until the desired construction is completed. In the SLM process instead of fusion, we have integration by melting the material and it is used both in metals, polymer plastics, and ceramic materials. A major advantage of this method is that no supports are required in complex geometries. The operating conditions are stricter and more demanding than other 3D printing methods. An important problem is the porosity of the final part. Also, the printed surface requires post-processing to normalize it.
Binder jetting
Inkjet 3D printing (BJ3DP) is a non-fused powder bed process. In this process, a polymer in the form of powder deposited by a roller on the printing surface is used as a basic construction material as well as a binding material in the form of ink sprayed from the print head which causes the material grains to join. The advantage of this method is that it does not need support material, it can be used in different materials (polymers, ceramics, food, etc.) and we can also have the addition of color. The final part needs further processing such as (1) removal of binder, (2) solid-state sintering, so as not to change the structure of the part, and (3) infiltration, to enhance density and strength.
Material Jetting (MJ)
The 3D printing process in which building materials are in a liquid state and are selectively fired through multiple firing heads. The materials are liquid photopolymers, which are cured using UV light as each layer is deposited. This process allows for the simultaneous deposition of a range of materials, which means that a single part can be produced from multiple materials with different characteristics and properties. The material injection is a very precise method of 3D printing, which produces precise parts with a very smooth finish, the possibility of multiple colors as well as the possibility of different mechanical or optical properties in the same part.
