# The Additive Manufacturing of Glass: A Critical Review
## 2 Glass AM Techniques
### Binder-Based Additive Manufacturing  
Binder-based glass AM is enabled by leveraging existing material fabrication techniques, by introducing glass feedstock into more readily 'printable' materials, such as photopolymers and colloid inks. The AM methods applied in this context vary rather greatly and are summarised in Table 2. Generated models form a green object, similar to those in ceramic manufacturing. Subsequent treatment processes remove the binder from the model and join the remaining feedstock into a solid glass object. Notable in this subset of processes is the significant shrinkage of objects after treatment, due to the complete removal of binding material, and the near-complete consolidation of the glass feedstock. The most explored binder-based additive manufacturing methods to date have been direct ink writing and light-based methods. Direct ink writing (Figure 13a) notionally involves the use of fine ground glass powder mixed into a slurry that may hold its shape after deposition. Computer numerical control guides a syringe which extrudes the glass slurry into complex shapes, with the resulting object ultimately sintered (at high temperature) to produce a glass solid [51]. The light-based methods (Figure 13b) use a glass feedstock or precursor in combination with a photopolymer binder. Controlled exposure to light (such as through existing stereolithography or two-photon polymerisation methods) cures the photopolymer in a desired shape, ready for high-temperature processing, where the binder is removed and the remaining glass is sintered into a solid object [52]. Limited research has been conducted on a similar EAM method, where a thermoplastic polymer binder is combined with glass powder [53].  
Binder-based glass AM techniques can yield objects with excellent transparency and fine detail. The development of the glass + binder material and subsequent treatment processes has enabled completely solid glass objects, without residual organic materials or porosity [54]. The significant shrinkage of the printed object also enables higher-resolution features than typically possible with other material on the same machines.  
Limitations in model size are expected with these processes due to the cost and complexity of the feedstock material and the need for the treatment of the green models. Exceeding a certain size, treatment procedures through temperature or chemical means become rather cost-prohibitive or may introduce problems such as uneven shrinkage or variance from strict tolerance. There have been investigations into improving these treatment processes [55; 56], lowering the time in which and temperature at which the  
Figure 12: Photographs of various samples produced by EAM with chalcogenide glass. (**a,b**) Samples intentionally fractured for observation of 100% infill density. (**c**) A sample with an infill pattern. (**d**) A sample with text [49].  
consolidation process takes place, but this research still only explores the smaller scales available in glass AM (\(\sim\)10 mm and below). Initial research has also been conducted regarding a hybrid technique, which may lessen the limitations of size in this process [57]. This involves a direct ink writing method depositing a layer of green glass feedstock. This method differs by following deposition with a laser melting process (as the energy input) that fuses the deposited glass in situ. Such a development may be applied to eliminate post-treatment procedures, though it should be noted that further research is needed to characterise the mechanical properties of objects prepared by such hybrid material approaches.