Читать книгу 3D Printing of Foods - C. Anandharamakrishnan - Страница 54

The process of binder jetting follows an even distribution of the powder layer across the printing platform and a liquid binder is sprayed onto the powdered bed to bind the two consecutive powder layers. In general, 3D printing using binder jetting technology starts by spraying a mist of water in order to stabilize the powdered material and to reduce the effects of distortion caused by spraying the liquid binder (Sun et al. 2018a). The system components of binder jetting are the same as that of a 3D ink‐jetting system instead of a material reservoir, a liquid binder reservoir is used. So, the components are inkjet printhead, levelling and spreading roller, built envelop, binder feeders, powder bed, build piston, and powder feed piston (Trenfield et al. 2018). The first layer is formed by spreading powdered material of appropriate thickness onto the build plate and the subsequent ejection of the liquid binder. It is followed by lowering the powder feed piston in order to ensure the adequate gap for the formation of the next layer that gets fused with the previous layer (Ziaee and Crane 2019). The process is repeated until the pre‐determined 3D object is produced based on a 3D data file. The customized 3D shapes were printed out of sugars and starch mixtures using the Z Corporation powder binder 3D printer in the edible 3D printing project (Southerland et al. 2011). Similar to inkjet printing, binder jetting offers fast fabrication at reduced operational cost. However, the 3D printing machine costs higher, and also the end quality of the fabricated 3D structure possess a rough surface. The commercial firm, 3D Systems utilizes colour jet printing technology for the fabrication of sugar‐based complex 3D constructs with different flavoured binders (3D Systems 2013). This approach is well‐known for the fabrication of complex sculptural cakes for special occasions and events. The developed system demonstrated a safe printing process that meets all the food safety standards.

Common process variables that influence the printing precision and accuracy are head type, droplet size and shape, droplet spacing, printing speed, velocity, and frequency of droplets (Shirazi et al. 2015). All these process variables are interrelated with the material properties. The compatibility of the liquid binder with powder bed substrate is adequate for achieving good finishing quality and resolution of the printed image. The surface tension of liquid binder and substrate material determines the interaction and fusion of printed layers onto substrate base (Liu et al. 2017). Sometimes, the compatibility of the substrate surface is improved by coating the surface with a binder or compatibility‐enhancing film before the start of the printing process. Considering this aspect, binder shellac (poly 1‐vinyl‐2‐pyrrolidone) was added into edible food ink for increasing surface compatibility (Willcocks et al. 2016). It was also reported that water‐based glazing along with gums or other surfactants such as polysorbates and polyglycerol oleates were used for modifying the chocolate ink for achieving high‐resolution images (Willcocks et al. 2011). Thus, the application of multilayered surfactants over the substrate surface had proved to significantly improve the compatibility yields a good precision and higher resolution of the printed images. The material’s surface tension is mostly preferred to be lower than 35 dyne cm⁻¹ for good compatibility (Shastry et al. 2009). Also, it was reported that a contact angle of less than 50° is desired for achieving good printability. Low polar materials such as carnauba wax were used as a coating material over the hard‐panned sugar confectioneries that showed adverse effects on the accuracy and precision of printed images. Hence, hydrophilic materials are used for coating of substrate’s surface in order to improve the compatibility of the water‐based ink with the substrate (Liu and Zhang 2019). Thus, the material viscosity, ink density, and surface tension are significant properties that must be optimized for preventing the overflow and spreading of droplets from the nozzle. A successful 3D printed construct using binder jetting must possess adequate mechanical strength, minimal shrinkage and minimal bleeding or oozing out of liquid binder (Von et al. 2015a).

The binder concentration greatly influences the dimensional stability and precision of the printed sample. Other factors such as flowability and wettability also influence binder jetting (Godoi et al. 2016). Generally, materials with free‐flowing nature are more suitable for binder jetting that possess good spread ability and packing properties. Typically, the angle of repose of material should be low enough generally smaller than 30° (Diaz et al. 2017). Adequate wettability must be preferred for binder jetting as low wettability of materials affects the powder alignment that subsequently declines printability. On the other hand, higher wettability interferes with the binding of the layers that significantly affects the end quality of printed construct. The particle size and distribution affect the binding behaviour of the water‐based binder (Liu et al. 2017). As a common practice, the coarse particles are mixed with the fine powders for achieving an edible construct with compact packing. Materials with a moisture content of less than 6% are more suitable for binder jetting (Von et al. 2015b).