# The Additive Manufacturing of Glass: A Critical Review
## 3 A Focus on Semi-Solid Processing for Glass AM
### High-Viscosity Glass Feedstock Methods  
Solid glass rods (i.e., filament) or fibres also serve as a viable feedstock for the semi-solid processing of glass. With the use of a heating system (called a hot end), glass filament  
Figure 16: AM glass parts using low-viscosity glass feedstock methods: (**a**) low side table [91]; (**b**) lighting structure [92]. Figure 15: Low-viscosity feedstock system showing (1) the crucible, (2) heating elements, (3) the nozzle, (4) the thermocouple, and (5) the removable feed access lid [10]. may be heated and extruded through a nozzle. Induction heating [88], joule heating [89], and direct flame methods [93] have all been used in hot end systems. These methods allow a filament to reach the temperatures and low viscosities required to achieve semi-solid processing. In the case of using glass fibre, [PERSON] [93] uses a flame to heat the fibre as it exits the nozzle. This allows the glass to be rapidly heated to over 1000 \({}^{\circ}\)C. [PERSON] explored two methods of actuation using a robotic arm. One employs the arm to actuate the nozzle in all directions. The other uses the arm to actuate and roll the printing platform, while glass flows from a vittrigraph kiln [94] mounted above. Both methods are suited for achieving a unique drizzle effect with thin beads of glass, as depicted in Figure 17. The nature of the process does not enable detailed geometric accuracy, as the molten fibre is presented at some height above the built object. The fibre partially cools during deposition, causing it to buckle and coil as it reaches the object. Moreover, the use of glass filament enables the use of retraction and extrusion movements, allowing material deposition to start and stop with a rapid response time. This approach is typified by the procedures of Micron3 DP and Swarovski [88], and Maple Glass Printing [89]. There is little information available regarding the process employed by Micron3 DP. The examples of glass models produced with the Micron3 DP process showcase semi-solid processing's ability to achieve a part resolution of ~0.1 mm (Figure 18). The process begins by extruding glass filament through an induction-heated nozzle, while the print platform actuates in all three directions within a heated chamber. Borosilicate and soda-lime glass have been processed. A similar example of well-resolved but undocumented glass AM using semi-solid processing is provided in Figure 19, revealing unique designs produced (at scale and in volume) by Swarovski for the Venice Exhibition in 2022. The Maple Glass Printing (MGP) process employs a comparable EAM technique to the Micron3 DP process, in which glass filament is extruded through a nozzle as the feedstock material. In the case of the MGP process, the hot end utilises a joule heater to reach temperatures suitable for semi-solid glass processing (to ~150 \({}^{\circ}\)C). The method of actuation employs a platform that moves in a Cartesian coordinate system, within a heated chamber, with a standard build volume of ~170 \(\times\) 200 \(\times\) 300 mm (as seen in Figure 20a). For soda-lime glass materials such Bullseye(r) glass (a piece prepared from such glass is shown in Figure 20b) or bottle glass, printing temperatures for the nozzle are between 900 and 1000 \({}^{\circ}\)C, enabling a typical flow rate of 21 mm\({}^{3}\)/s. The heated chamber allows the ability to anneal glass, the simultaneous production and annealing of the printed  
Figure 17: (**a**) A robotic arm used to actuate a nozzle, with a direct flame preparing the glass fibre for deposition. (**b**) Magnified image of glass exiting nozzle and being deposited onto printed component [93]. models, or programmed annealing (if the application or workpiece requires it) following the production of a model. Many glass materials have been explored by the MGP process, including soda-line glass, waste container glass (i.e., recycled cullet), crystal lead glass, glasssblowing waste (i.e., soda-line'studio glass' of various colours), and borosilicate.