3D Printing for Energy Applications. Группа авторов

Figure I.2 Classification of commercially available additive manufacturing methods according to dimensional order, process, and material.

Stereolithography (SLA) is an additive manufacturing process based on the photo‐polymerization of resin material upon exposure to a laser or UV light source. As the name suggests, ‐graphy is a Greek work that means ‐writing, something that is reflected on the way the photocurable material solidifies. The light source passes through the necessary focus lenses for controlling and adjusting the output and is reflected from a movable mirror system onto the photosensitive material surface to cure and solidify the area between the build platform and the liquid surface. The laser beam movement controlled by the mirrors will write the designed pattern on the build platform. When the first layer is formed by the solidified material, the build platform will move down at a distance equal to the layer height, to create the area and form the next layer. Adjusting the laser beam focus and spot size will determine the resulting curing depth and pitch of the solidified material line both in the X/Y plane and the Z axis, offering high vertical and lateral resolution in the order of 10–25 μm [1, 2]. As a result, high surface finish can be achieved with excellent mechanical properties, of porous and dense structures. In terms of material, when using ceramics, photocurable pastes with or without solid loading can be used, normally >50 vol% and grain sizes that range from 0.5 to 5 μm. The high viscosity of the ceramic paste is beneficial for providing support on the structure, however, the nature and the configuration of the process can make it difficult to print multi‐material structures. The chemistry of the formulations consisting of several additives, such as UV absorbers/blockers, stabilizers, and pore formers [3], making recycling and reusing of uncured material challenging. Moreover, post‐processing such as cleaning is required after printing, which adds to the expected de‐binding and sintering that is common in manufacturing of ceramics.

Selective Laser Sintering (SLS) is a process based on the same principle, that a laser beam will be reflected and directed by a mirror system, to fuse together powder particles that are placed inside a powder chamber. The fused particles will form a layer at the end of the writing path and the build platform will be lowered at a distance equal to the selected layer thickness of the 3D model. At this point, a leveling roller will transfer powder from an adjacent chamber, to the main build chamber and provides the next powder layer to be fused. In this case, the resolution of the process is controlled by the synergic action of the build chamber and the roller that levels the new powder layer, normally a resolution between 80 and 100 μm [1, 4] can be achieved. The powder that has not been exposed to the laser path will be removed at the end of the process, providing this way support for the proceeding layers with overhang features. The high energy provided by the laser is responsible for the solid state diffusion of the powder particles and the resulting densification, with final parts characterized with good mechanical properties. However, when higher temperatures are required, depending on the material requirements, thermal stresses can be introduced and practices such as preheating of the powder bed can be applied [5]. Surface finish is dependent on the powder grain size that ranges from 0.3 to 10 μm [1] and normally no post processing is required. Overall, the process utilizes several components for defining and handling the material layer therefore the equipment is expensive with increased manufacturing time.

      Fused Deposition Modeling (FDM) uses thermal energy to melt the filament material that is feed through a heating element with the aid of a roller system and is extruded directly onto the build platform or a substrate. The extruded line of material will adhere to the adjacent and underlying material upon cooling and will form the layered structure. For the deposition of the proceeding layers the build platform will move down as specified by the layer thickness settings which vary between 50 and 200 μm [4], while for the X and Y plane parameters the extruder diameter and the molten material pitch can be adjusted by the process parameters. It is important to mention that parameter settings will affect the resulting quality and performance of the final part. The extrusion nature of the process gives rise to several print defects and makes small features challenging to print. Cooling related issues can be solved with a control temperature build platform, while the extrusion parameters are responsible for structural and geometrical issues [6]. The filament materials used are thermoplastics with solid particle loading above 40% and grain sizes ranging around 1–5 μm [1]. Post processing to remove organic components and sintering is required for densification to occur, however it is a cost effective solution with fairly simple equipment.

Inkjet printing can be divided to two process groups, however, both processes are based on the same principal that is widely known from conventional 2D ink printers, a jetting nozzle controls the amount of material or binder deposited on the substrate/material that is build layer by layer. Direct Inkjet Printing (DIP) utilizes a ceramic suspension to be used as ink, where droplets are deposited onto a substrate to form the material layer, along with the support material that is deposited when overhangs or cavities are printed. Depending on the ink formulation an appropriate drying step has to be introduced before commencing with the next layer, such as cooling or evaporation [5], while several suspensions contain additives to counteract clogging of the nozzle that is a common issue [1]. In this case the jetting parameters, travel speed, and the layer thickness will affect the resulting printing resolution that is higher compared to the previously mentioned processes, ranging from 1 to 10 μm [1]. Several systems have been developed for controlling both material deposition and the build platform movement in order to achieve such low resolutions, in the expense of printing time when 3D structures with high aspect ratios are required. Highly diluted and stable inks containing nanoparticles are used, with a solid concentration less than 5 vol% and grain sizes range from 10–50 nm [1].

      Indirect Inkjet Printing (IIP) as the name suggests is similar to DIP, with the difference that liquid binder the jetting material deposited on ceramic powder to form the material layer. It is a powder bed process that can offer several advantages such as structural support during printing, reuse of powder material, increasing shape complexity, and reducing printing time compared to DIP, however print resolution is in the order of 100 μm [1], similar to SLS. In this case the mechanical performance of the printed parts is a significant disadvantage and post‐process hardening is a way to counteract it.

I.2 3D Printing Hierarchical, Material and Functional Complexity

      All types of industry will benefit from the introduction of complex geometries generated by additive manufacturing but it will be even more interesting to take advantage of other unique capabilities