# The Additive Manufacturing of Glass: A Critical Review
## 3 A Focus on Semi-Solid Processing for Glass AM  
### Process Overview  
The descriptor of'semi-solid processing' in glass AM encompasses methods that manipulate glass materials within a specific temperature range, typically between the glass transition temperature (T\({}_{\rm g}\)) and the melting temperature (T\({}_{\rm m}\))--which, in the case of glass, are associated with changes in viscosity (over orders of magnitude) that permit glass processability [90]. This temperature range enables glass material to flow through a nozzle for deposition, and then quickly fix in place upon cooling. Semi-solid processing methods can enable the production of intricate digital designs using a variety of glass compositions--including recycled materials--through processes that include EAM. Digital designs for semi-solid glass AM are typically defined using text files, known as 'G-code' files. With 'G' representing geometry, the G-code comprises a series of commands that translate to physical actions of the AM system. These commands autonomously manipulate and deposit glass to form desired geometries. Simple or complex G-code can be created directly using software including Rhinoceros 3D\({}^{\rm TM}\), FullControl G-code\({}^{\rm TM}\), and many other commercial and open-source software tools, offering intricate control over the manufacturing toolpath. Alternatively, 3D models (e.g., OBJ, STL,.STEP files) may be converted to G-code through what is known as slicing software (e.g., Simplify3D\({}^{\rm R}\) and PrusaSlicer\({}^{\rm R}\)). Slicing procedures facilitate the incorporation of geometric features such as infill density, infill pattern, multiple walls, and many other common AM parameters. Depending on the specific system employed, pausing and resuming printing may be enabled to print multiple parts and multi-contoured designs. Moreover, some semi-solid glass AM procedures support multi-coloured printing capabilities. Following developed EAM techniques, the semi-solid processing of glass begins with the deposition of glass onto a printing platform. The printing platform acts as the first point of contact for the printed glass, preventing the displacement of deposited glass during production. In most cases, the first layer of glass adheres to the printing platform. This may be achieved by heating the printing environment and reducing the first-layer printing speed (as is common in polymer EAM). Compressing the initial glass layer onto the platform will also encourage bonding. Printing platforms are commonly chosen to be flat, high-temperature/refractory materials, capable of withstanding direct contact with molten glass. Alternatively, sheet glass may also be utilised as a platform, if the detachment of the component is not required. A high-quality first layer must exhibit both precise dimensional accuracy and strong adhesion to the printing platform. This is crucial, especially for larger 3D-printed components, as each build layer establishes the foundation for the subsequent layers. Upon the completion of the first layer in semi-solid glass AM, the objective of subsequent AM fabrication layers is altered, since semi-solid glass is subsequently deposited onto existing glass. This allows for accelerated deposition speeds, further facilitated by the reduced energy requirements for interlayer bonding. Glass processing necessitates significantly higher temperatures than polymer EAM to achieve comparable (or even lower) viscosities. For soda-time glass, typical settings involve nozzle temperatures of ~950 \({}^{\circ}\)C, with surrounding heated chambers of ~500 \({}^{\circ}\)C. Electronics are not typically able to withstand such a high-temperature environment; thus, semi-solid glass AM cannot utilise the layer cooling fans common in polymer EAM. Such fans, conventionally positioned near the nozzle outlet, play a critical role in rapidly cooling the material once it has bonded to the preceding layer, ensuring structural integrity and geometric precision.