were available on the development of sugar cubes and curry cubes using SLS that adds convenience to cooking (Nachal et al. 2019). SLS technique has a great control over the internal microstructure and porosity that imparts novel brittle texture. Sometimes the printed objects show a surface deformation and shrinkage due to the impact of laser irradiation of materials. Further, SLS is limited by its lower sintering speed and its suitability only for those materials that have low melting points such as sugars, lipids, and starches. The SLS technique of 3D printing is well established in the non‐food sectors particularly in tissue engineering and pharmaceutical formulations (Awad et al. 2020). However, the technology must be studied in detail for broader applications of SLS in the delivery of nutraceuticals and functional bioactive compounds in 3D printed foods.
Figure 2.10 3D printed sugar constructs using sintering process (a) 2D patterning, and (b) 3D printed complex sugar geometry by Windell H. Oskay retrieved from http://www.evilmadscientist.com.
Source: Reprinted with permission from CandyFab (2014).
2.5.2.2 Selective Hot Air Sintering and Melting
Another variant of selective sintering is the use of hot air instead of a laser as a thermal source. The process remains the same as the hot air is used to fuse the powdered material supply. Researchers had successfully printed sugar‐based 3D constructs using hot air as the heating medium for the fusion of printed layers. Powder density and compressibility are the critical factors in SHASAM that significantly affect the flowability and have an impact on the carnation of 3D design patterns (Schmid et al. 2013). In general, free‐flowing powder without any solid clumps is suitable for the sintering process. Further, the powdered material supply must be adhesive enough and possess the tendency to agglomerate to get adhere to the contact points of adjacent molecules. It was reported that the layer thickness was inversely related to the mechanical strength of the 3D constructs (Amorim et al. 2014). As a result, thin layers possess higher mechanical strength with decreased porosity of the fabricated structures. Also, the particle size of the powders greatly influences the finishing quality of 3D printed samples. Diaz et al. (2018) described the method for the fabrication of multi‐material 3D construct using the sintering process with a greater degree of printing precision and resolution. The material supply is comprised of structural elements and binder components. Thus, the structural element remains the base that provides bulk and scaffold function to the 3D construct and is fused by the particle‐particle sintering of the binder component. It was reported that the use of a combination of two or more binders with different glass transition and melting temperatures exhibits better printing performance (Liu and Zhang 2019). In general, the glass transition and melting temperature of the binder component should range between 10 and 200 °C. So, the phase transition of the binder material can occur in less than 5 seconds while the structural element remains unaffected at temperatures below 200 °C (Diaz et al. 2018). Similar approaches have yet to be studied in detail for the optimization and characterization of structural and binder materials for improved printing performance.
2.6 Inkjet Printing
Another 3D printing technology that has been in practice for the past decade is inkjet printing. The inkjet printing technology is mainly used for surface decorations on cakes and pastries in 2D forms (Sun et al. 2015). Later, the same principle is applied for the fabrication of 3D designs over food materials. The inkjet printing involves the controlled accumulation of liquid‐like slurry material supply in the form of droplets (Nachal et al. 2019). As a variant of powder‐based technology, the layers of solid particles are bonded together by the printed liquid droplets ejected from the printhead. Hence the technology is also referred to as material jetting. The major advantage of this technology is the precise use of material without wastage based on a DoD manner. The print head uses two types of actuation process to spray the droplets namely thermal based actuation and piezoelectric based actuation (Vithani et al. 2019). In order to achieve the desired flowability, it is a general practice to charge the material by the addition of conductive agents into the material supply (Liu et al. 2017). Since the material supply used for inkjet printing is liquid‐based, the mechanical strength to hold the 3D structures is poor (Godoi et al. 2016). Hence this technology is widely adopted for the printing of images in 2D forms. The success of printing the 2D and 3D design patterns using inkjet technology depends on the compatibility of food ink and the substrate (Holland et al. 2019). Factors such as rheological properties of food ink, printing temperature, and printing speed also significantly impact the final quality of design and printer performance.
2.6.1 Working Principle, System Components, and Process Variables
The process of ink‐jetting involves the deposition of a liquid droplet on the solid material substrate using a controlled dispensing unit for the fabrication of 3D constructs (Figure 2.11). Different components of a 3D ink‐jetting system include inkjet printhead, levelling roller, binder feeders, powder bed, build piston, and powder feed piston (Galeta et al. 2016). Two variations of the actuation mechanism used to initiate the flow of jet droplets are thermal and piezoelectric types. In a thermal‐based inkjet system, a resistor is equipped with the printhead which generates heat resulting in the sudden creation of vapour bubbles in the feed material reservoir (Daly et al. 2015). Due to this, a small volume of a material droplet (ink) is ejected from the nozzle tip. On the other hand, the piezoelectric printhead system employs a crystal or ceramic‐based piezoelectric element that transforms electrical energy into mechanical energy. The associated deformation process results in the built‐up of the required pressure to push the liquid in the form of droplets through the nozzle (Lee et al. 2012). The presence of the acoustic system in piezoelectric head assist in splitting the liquid into droplets. The change in the shape of liquid droplets can be evident when an electrical voltage is applied to the piezoelectric element that develops the required pressure as needed for the ejection of liquid droplets from the nozzle (Alomari et al. 2015). The less volatile liquid materials are inkjet printed at room temperature using a piezoelectric printhead. Although heat is applied to a small contact area for a short span of time, the application of thermal energy can potentially increase the local temperature of the material reservoir (Vithani et al. 2019). This sudden temperature fluctuation may affect the thermo‐labile bioactive components of the food system. Hence, piezoelectric print heads are more common for pharmaceutical applications.
Figure 2.11 Schematic diagram of inkjet printing.
The most significant material properties that affect the print fidelity of the inkjet printing are the rheological and thermal behaviour of the food ink. In general, the inkjet printing system uses a low viscosity material for easy ejection of liquid through tiny orifice channels of the printhead. Material viscosity is a crucial factor that ensures the flowability of material. So, the desired material viscosity ranges between 2.8 and 6 mPas. While the viscosity above 10 mPas causes cavitation inside the printhead during the printing process and the viscosity below 2 mPas is not stable enough to form a droplet (Liu and Zhang 2019). The surface filling and decoration involve the jetting of food inks dispensing through the micro‐sized channels in the range from 20 to 50 μm. About 1‐pl (picolitre) of ink is dispensed during ejection of each droplets that typically range about 13 μm across (Xaar 2018). The principle of dispersion of ink involves the breaking up of a stream of droplets of the same volume with reduced surface area. The underlying process of ink‐jetting technology is based on the Rayleigh–Plateau
