3D Printing of Foods. C. Anandharamakrishnan

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Figure 2.9 Schematic diagram of selective laser printing.

2.5.1 Working Principle, System Components, and Process Variables

      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.

2.5.2 Classification of Selective Sintering System

Based on the operation, the powder bed fusion technology has been categorized as selective laser sintering (SLS), selective laser melting (SLM), electron beam melting (EBM), and multi‐jet fusion (MJF) (Awad et al. 2020). Thermoplastics, polymers, metals, and alloy powders are common feed materials employed in the sintering process. The above‐mentioned printing technologies differ based on the type of feed materials used, based on the type and amount of light used for the transmission of energy during the phase change. For food printing applications, SLS is the most used sintering technique for the fabrication of 3D construct (Nachal et al. 2019). Food scientists of TNO used sugar and Nesquik powders for the fabrication of 3D structures with intricate internal designs (Gray 2010). A similar approach was used in CandyFab Project that successfully printed the complex 3D constructs from sugars (Figure 2.10) (CandyFab 2014). Another variant of SLS is the use of hot air instead of a laser and the technique is known as selective hot air sintering and melting (SHASAM) (Godoi et al. 2016). Both these technologies are well applied for foods that offer a greater degrees of freedom to fabricate complex structures in a short period without post‐curing step.

2.5.2.1 Selective Laser Sintering

A laser power source is applied to a powdered bed of feed material that causes the agglomeration of the particle through the sintering process. The SLS technique works well for the construction of multi‐layered 3D food constructs. The process involves the superficial melting and fusion of materials without the use of liquid binders to form a 3D construct. The researchers of the Netherlands Organization for Applied Scientific Research (TNO) used this technology for sugar‐based 3D constructs with tailored nutrition and flavour СКАЧАТЬ