Читать книгу 3D Printing of Foods - C. Anandharamakrishnan - Страница 50

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 instability phenomenon (Godoi et al. 2019). Another factor that affects droplet deposition is temperature. Variations in the temperature modify the rheological properties and influence the surface energy of the food inks. Application of lower temperature reduces the surface energy and spreading tendency of food inks (Willcocks et al. 2011). The ingredient composition is another criterion that must be optimized for achieving the desired flowability for fabricating 3D designs on food substrate. Only a limited range of food materials has been analyzed for inkjet printing, in which chocolate ink is the most commonly used ingredient (Lanaro et al. 2019). However, other food materials in form of emulsion, slurry, and suspensions have a great scope for 2D and 3D inkjet printing. Future studies on the analysis of different material supplies suitable for inkjet printing are required to broaden the food applications.