href="#ulink_03d3ba00-4c8f-50dd-bf14-27fe1cb5cbff">Figure 2.1).
Material extrusion: A polymeric filament is melted using an electrical heat source and the melted liquid is deposited in a layered fashion using a 3D printer through an extrusion mechanism. The deposited material gets fused together upon cooling and attains the desired 3D shape (Shahrubudin et al. 2019). A filament is delivered using a Bowden mechanism for pushing the material filament through a long flexible Teflon tube to the hot end of the printing nozzle. From the nozzle tip, the melted polymer is deposited over the printing platform. Materials such as polylactic acid (PLA) and Acrylonitrile butadiene styrene (ABS) are the most commonly used polymer filaments. A temperature controlling unit is used for varying the temperature according to the melting point of the polymers used. Commercial 3D printers that are based on extrusion technology are referred to as fused deposition modelling (FDM) and fused filament fabrication (FFF). Extrusion‐based technology is comparatively cheaper than other AM processes (Vithani et al. 2019). Although the printing accuracy and precision are good, sometimes post‐polishing is required to enhance the surface smoothness of the 3D printed construct.
Powder bed fusion: In this AM process, a heat source of either hot air or a high‐energy laser beam is employed to scan the 3D CAD design over the powder bed (Godoi et al. 2019). Due to which the powdered material gets melted and fused according to the shape and design of the 3D CAD model. After completion of the scanning process, the printed material is taken out from the loosened powder of the bed.Figure 2.1 Classification of AM technologies.
Material jetting: Materials like wax and photo‐polymeric resins are the commonly used materials in this type of AM process. Here, the feed material is deposited in the form of a fine jet either in a continuous fashion or in a drop‐on‐demand (DoD) manner over the printing platform (Holland et al. 2019). The movement of printer arms can be operated based on the 3D model that controls the deposition of the printed layers. The melting point of the material can be precisely controlled through a temperature controller.
Binder jetting: In the case of binder jetting, the liquid binder is sprayed over the powdered bed. Here, the binder acts as an adhesive that binds the material together. Above which the second layer of powder bed is spread over the printing platform then the liquid binder is sprayed (Sun et al. 2015). Likewise, the process is repeated continuously until the entire 3D model is printed. Commercial 3D printers such as Chef Jet and Chef Jet Pro works based on a binder jetting mechanism.
Vat polymerization: In this process, a vat of liquid polymer‐based resin mixture is used for constructing a 3D object in a layer‐by‐layer manner. Here, a ultraviolet (UV) light source is used for curing the polymeric resin that results in the hardening of the layer of resin (Xu et al. 2020). After the formation of each layer, the platform moves the fabricated 3D object in a downward direction. Technologies such as SLA, digital light processing (DLP), and liquid crystal printers (LCD) are based on vat polymerization techniques.
Sheet lamination: In this AM process, a laser beam acts as a knife that cuts the materials like papers, plastics, metals, and ceramics that are suitable for thermal pressing to bind thin layers together into a 3D construct (Ngo et al. 2018). Here, the movement of the laser light is controlled by a computer that moves according to the 3D CAD design. Thus, the 3D construct can be fabricated through laser as a thermal source and the unwanted portions of the material can be easily removed after the completion of the printing process.
Direct energy deposition/laser metal deposition (LMD): This method employs a high‐energy laser source to melt the material over the processing surface. Thereafter the metal powder is injected into the melted material for the completion of the deposition process (Barro et al. 2020). The movement of the robotic arms is controlled with integration to a computer that moves according to the 2D directions that aids in the construction of a 3D object.
The above‐mentioned AM process differs based on the process of formation of the 3D construct and the way of assembling the finished structure. These AM technologies can be broadly grouped based on the type of chemical transition as controlled fusion, controlled deposition, controlled cutting with lamination, and combined technologies (Wegrzyn et al. 2012). Techniques based on controlled fusion are sintering and SLA where a heat or light source is directed to fuse the powdered materials. On the other hand, extrusion and inkjet technologies are based on the controlled deposition methods where a liquid or semi‐solid material is deposited in a controlled manner over the printing platform for the fabrication of a 3D object. Sheet lamination works based on the controlled cutting of thermal pressing materials followed by a lamination process to bind the layers together. Lamination‐based 3D printing technologies are referred to as layered object manufacturing (LOM) that are widely applied for tissue engineering for the fabrication of 3D scaffolds in the biomedical field. Sometimes, a combination of AM processes is adapted especially in bioprinting applications for the fabrication of soft scaffolds and tissues. However, not all these AM technologies are applied to the food layer manufacturing process. The 3D printing techniques that are adapted and applied for foods are discussed in the subsequent sections of this chapter.
2.3 3D Food Printing Technologies
3D food printing is a digitalized process of fabrication of edible 3D construct in a layered manner. The subsequent layers of materials are bonded together through appropriate chemical transitions that alter the state of the material upon deposition. In contrast to the robotic manufacturing process, 3D printing of foods not only allows for automation of process but also helps in customization and personalization of diet (Sun et al. 2018b). More complex food geometries with intricate designs can be fabricated using 3D printing with accurate precision that is nearly impossible with conventional food processing techniques. Interestingly, 3D printing as a single standalone technology can integrate multiple unit operations of conventional processing into a single step (Nachal et al. 2019). Food printing follows a sequence of steps that starts with the development of 3D CAD design or scanned 3D model. The 3D model in its STL format is fed to the slicing software for retrieving the design information in machine‐readable G and M codes. These computer codes are applied to the 3D food printers that assist in the printing process. A typical 3D food printer consists of the coordinate movement axes, printing platform, motor assembly, feed supply, print head, and microprocessor controller unit (Anukiruthika et al. 2020) (Figure 2.2).
In a broader sense, food printing technologies are categorized based on the nature and state of raw materials used (Feng et al. 2018). It includes 3D printing of flowable liquids, solid powders, semi‐solid paste, and cell cultures. Accordingly, the AM processes that are adapted for 3D printing of liquid‐based semi‐solid food materials include extrusion printing, material jetting, and binder jetting. While the powder materials are 3D printed based on the sintering process. As a variant of existing 3D printing techniques, bioprinting involves the culturing of live cell cultures and tissue scaffolds (Godoi et al. 2016). Extrusion‐based food printing is the most common technique used for a wide range of food materials such as starch (Theagarajan et al. 2020), proteins (Liu et al. 2019a), dietary fibre (Lille et al. 2018), and myofibrillar tissues (Dick et al. 2019b). This is because of its simplicity, flexibility, and ease of operations. However, these types of food materials require appropriate pre‐processing for enhancing printability and post‐processing for its edibility (Feng et al. 2020). Next to the extrusion, the sintering technique is widely used for the fabrication of sugar and starch‐based 3D constructs. In the process of sintering, the solid particulates get compacted into a solid mass of material upon external stimuli such as either heat or pressure. Materials such as plastics, ceramics, and metals are commonly used in the sintering process. Considering food printing, sintering is used to fabricate 3D constructs from low melting powders such as sugars, fat, cocoa powder, and Nesquik without melting the material to the point of liquefaction (Sun et al. 2018a). Another powder‐based technique is inkjet printing. It involves the controlled deposition and accumulation of droplets of food inks either in a continuous or DoD manner. The solid powder particles are bonded together with the printed liquid material. Commercial