Читать книгу 3D Printing of Foods - C. Anandharamakrishnan - Страница 45

As stated earlier, selective sintering is based on the powder bed fusion principle that refers to the selective consolidation of solid powdered particles in a specific area according to 3D design into a finished 3D printed construct using a thermal source. The use of a light source and the subsequent increase in printing temperature allows fabricating 3D constructs through melting and fusion of the powdered particles in a layer‐by‐layer manner (Vithani et al. 2019). One major advantage of this technology is the localized phase transition of powder particles that allows the reuse of the unfused feedstock materials. The basic system components of a selective sintering system include a build platform, a thermal power source, Galvano mirrors, powdered feedstock reservoir, mechanical roller, and powdered material vat (Ma et al. 2018). The printing process starts with the rising of the build platform to its uppermost position where a fresh layer of the powdered feed material is spread across the platform and flattened by the roller for uniform dispersion (Liu et al. 2017). Here the print head consists of an inbuilt thermal power source that scans across the powder and sinters it by following a pattern of 3D design. After the formation of each layer, the build platform is lowered to provide enough space for the formation of the next layer. Meanwhile, the reservoir platform ascends and spreads the next layer of material to form a new layer. Thus, the above process continues until the completion of whole 3D structure. After completion of 3D printing process, the system is allowed to cool that assists in the removal of excess un‐sintered material from the fabricated 3D object. The surface finishing of the 3D printed construct can be improved by appropriate post‐processing such as coating and polishing (Awad et al. 2020). These post‐processing methods not only improve the appearance but also it enhances the mechanical properties like hardness and tensile strength.

Various process variables that must be considered during the sintering process are power source type (laser or thermal heaters), beam diameter, beam power, and scanning speed (Liu and Zhang 2019). The material phase change is associated with the complex interaction of powdered feed material with the light beam. Here the strength of interaction greatly depends on the energy density of the power source (Gu et al. 2012). Hence, the optimization of these process variables is crucial for attaining higher precision and resolution of 3D constructs. The energy density of the laser beam can be adjusted by varying the scan speed and laser power. A higher laser energy density can be obtained by a longer interaction time that results in the fabrication of denser 3D constructs with higher mechanical stability. On the other hand, a lesser interaction time produces a laser beam with a lower energy density that results in a porous brittle 3D construct (Amorim et al. 2014). Other process variables such as printing temperature, powdered bed thickness, space/gap between the print platform and laser head, and laser spot diameter also impact the stability of the printed structures. Apart from process variables, material properties such as particle size, bulk density, wettability, crystallinity, flowability, compressibility, glass transition temperature, melting, and solidification behaviour influence the printability of powdered materials (Yang et al. 2017). The powder bed fusion technology is useful in the fabrication of multi‐material structures that requires a more detailed understanding of the chemical transitions and interactions at the molecular level of feed material and binder component. This opens an array of research opportunities in fabrication of 3D constructs using sintering technology especially in system design, material science, and processing.