# The Additive Manufacturing of Glass: A Critical Review
## 2 Glass AM Techniques
### Direct Methods
#### 2.1.1 Directed Energy Deposition (DED)  
Directed energy deposition (DED) utilises a focused energy source to fuse a separately fed feedstock material to a constructed object. Such a method has been explored for glass AM, mainly using a laser for fusing a feedstock material of solid glass filament. Depending on the level of detail required, filaments may be drawn by hand or by some mechanically assisted process such as those required for lab glassware or optic fibres. CO\({}_{2}\) lasers are typically used, as they generate the wavelength of radiation that is most conducive to the melting of glass, whereby the glass feed material is opaque and has minimal reflections [22; 23]. The ability to melt glass using a laser, along with the associated efficiency, is an area that requires focused future work. In recent reports, preliminary investigations have also been undertaken, exploring the use of a flame for the fusing (melting) step [24; 25]. Depicted in Figure 7a, the DED process proceeds by focusing the energy source to generate a pool of molten glass, to which additional feedstock can be added. Via computer numerical control, the melt pool is scanned across a user-defined path with the continuous addition of feedstock, depositing a pattern of fused glass. The currently explored DED techniques are mainly '2.5D' in nature, where the heat involved in deposition fuses single layers of material to previous layers. Some non-planar deposition techniques have been explored [26; 27] but are still in their infancy at the time of review. With the appropriate process optimisation, DED is capable of excellent fusion between layers, with transparencies nearing those of cast glass [29; 30; 31; 32]; however, a thorough treatise regarding the mechanical properties of these objects is yet to be written. Variables for process optimisation typically include laser power (W), the material feed rate (\(x\)/min), and laser scan speed (mm/s). These process parameters are varied so that the melting of glass feedstock is balanced in terms of sufficient material fusion to avoid the overheating of the molten region. Overheating may cause the vaporisation of the glass material, creating bubbles within the printed object [12; 33; 34; 35; 31; 36; 37]. Further process adjustments, such as focusing the laser beneath rather than above the feedstock, are also being explored to improve melting, with a reduction in material vaporisation [13]. The glasses explored to date using the DED method include those elaborated in Table 1.  
The excellent fusion in the DED process comes with a trade-off for the precision and resolution of printed features. In the currently explored processes, interactions between the melt pool and feedstock are seen to create inconsistent deposition and deformation of existing glass material. As illustrated in Figure 7b, presently explored DED-manufactured objects exhibit a fluctuating outer surface with a tendency of the liquid melt pool to smooth  
Figure 7: Diagram (**a**) and photograph (**b**) of the typical filament-fed DED process [28]. over the finer details of the object, with such characteristics also being a detriment to overhanging features. In 2.5D processing, overhangs require a stepover of material, where deposition occurs partially unsupported by the previous layer, relying on the quick cooling and high viscosity of material to remain in place. These unsupported steps will likely see a similar smoothing and sagging of fine detail, limiting the possible overhang angle. The possibility exists for improvements to this process, such as enhanced object resolution with tighter control of the melt pool [26; 34; 36]. This is at odds with the feasible scale of the process, as a smaller melt pool limits the rate of material deposition. Print times become infeasible for larger objects, as the DED machine deposits less material for each unit of movement. Solid 3D objects would require additional scanning of the melt pool upon each layer, causing individual layer times to increase rapidly.