# The Additive Manufacturing of Glass: A Critical Review
## 4 Properties of AM Glass
### Thermal Properties  
For meaningful improvements in direct processing methods for glass AM, a clear need exists for investigations regarding a detailed understanding of thermal gradients. Discussed in Section 1.2, thermal gradients formed while printing may create stresses leading to fracture. The characterisation of such gradients can be applied to advise improvements in object scale and complexity, by augmenting the existing methods with more intelligent process control. For example, while a heated chamber has enabled the production of glass objects without fracture, there will ultimately be limitations in scale with this method. Exceeding a certain size of object, an enclosing heated chamber will become problematic due to cost, heating uniformity, and user safety. An alternative approach may be derived from detailed knowledge of cooling and the resulting temperature within a printed model. Thermal fracture may be avoided by intelligently accounting for thermal gradients and introducing self-annealing behaviour, possibly by controlling the rate and size of material deposition. Similarly, some methods which enable complex geometries in polymer AM (bridging and overhangs) may be introduced based on deeper knowledge of cooling behaviour. Glass AM already has the advantage of being able to exploit wide ranges of glass viscosity--even allowing some non-planar printing, as observed in Figure 29. Bridging and overhangs, however, are an issue for some methods as they require material deposition to occur in the absence of supporting material from previous layers. Precise knowledge of viscosity and the cooling rate is required to develop a bridging function which deposits material without sagging and with the capability of supporting subsequent layers. The study of [PERSON] et al. [10] contains some preliminary investigation into problem behaviours during printing, using thermal imaging to quantify a temperature gradient between layers. Iteration upon this known gradient is, however, ignored in favour of a heated chamber to prevent thermal fracture. Essential further research would build upon this in situ monitoring, manufacturing objects of a known, standardised geometry to compare the thermal behaviours of various materials and process parameters. For example, the cross-section size of deposition (influenced by temperature, feed rate, and layer height) may be linked to the rate of cooling per deposited material. Applied to more complex object geometry, knowledge of cross-section cooling may advise areas of printing with problematic thermal gradients and allow more suitable parameters to be chosen. For example, the size of material deposition may be increased in a certain area, retaining temperature for a longer period and reducing the severity of the thermal gradient. An alternative, but equally viable, approach to understand and optimise thermal behaviours is simulation. The modelling of the true thermal behaviours in glass is a complex  
Figure 29: Example of the non-planar printing of glass, using EAM, which exploits the wide variation in the viscosity of glass across its processing temperature range. subject. Glass materials are partially transparent, making them what is known as a participating medium. In cooling, thermal modelling must therefore account for the behaviour of conduction, coupled with radiation transfer throughout the material [104; 105; 106]. Such modelling has many implementations in modern computational fluid dynamics. ANSYS Fluent, for example, may be configured to apply a finite volume approach, approximating radiation with a discrete ordinates method (ANSYS documentation [107] cites [108; 109; 110] as the basis for modelling). There is vital work to be conducted in applying this software to scenarios specific to glass AM. The examination of isolated mechanisms, such as the deposition of a single layer upon another, will provide insight into the complex heat transfer and viscoelastic deformation in the process. [PERSON] et al.