Bubble Formation in Additive Manufacturing of Borosilicate Glass

Entrapped bubbles are an important problem in conventional glass manufacturing. It is also a significant factor determining the quality of glass products produced using additive manufacturing (AM). This paper reports on the bubble formation and entrapment in filament-fed AM printing of borosilicate glass. This process involves maintaining a local molten region using a CO2 laser. A 2 mm filament is fed continuously into the molten region while CNC stages position the workpiece relative to the laser and filament feed. Two different bubble regimes are identified in experiments with borosilicate glass. These regimes can be related to different physical phenomena, specifically, bubble entrapment at defects in the filaments and bubble formation due to reboil. These can be overcome by selecting defect free filaments and by minimizing the temperature inside the molten region to prevent breaking down the glass. Understanding these mechanisms allows bubble-free glass to be printed. Finally, residual stress in the deposited glass pieces is removed using post-deposition annealing and validated using a polariscope. Introduction Additive manufacturing has been widely studied for structural parts using numerous materials, such as metals, polymers, and ceramics [1]. Several studies has considered AM of transparent optical elements. Most of these studies have focused on transparent polymer materials, including fused deposition modeling [2-4], ink-jet printing [5,6], selective laser melting (SLS) with post index matched resin infiltration [7], and multiphoton stereolithography [8]. Though these techniques are able to print transparent parts, their applications are limited since the poor material properties compared to glasses and other inorganic materials. Glasses are widely used in high quality and high power optics due to higher transmissivity, lower coefficient of thermal expansion, and more stable refractive indices [9]. However, there is limited published research on AM transparent glasses. Conventional, AM techniques have been studied for glass printing, including selective laser melting/ sintering (SLM/SLS) [10-14], and extrusion techniques [15]. However, only non-transparent structural parts have been printed using these techniques. Recent studies have demonstrated the production of 2D and 3D solid structures using transparent soda lime glass and even quartz feedstock [14-20]. Compared to soda lime glass, borosilicate glasses have a lower Coefficient of Thermal Expansion (CTE) (1/3 of soda lime glass [9]). This leads to a higher thermal shock resistance that makes it a better material for applications ranging from optics to glassware. Two issues that confront additive manufacturing of glasses (including both soda lime and borosilicate) are bubble entrapment/formation [20] and residual stresses due to rapid cooling of the glass during printing. Bubble formation and entrapment is a significant issue for conventional glass industry. Bubbles cause optical scatter in glass which limits the optical performance. For example, six bubbles per ton of glass leads to a 10% rejection rate in television panel industry [21]. In addition to optical considerations, bubbles significantly weaken the glass mechanically. There are multiple sources of bubbles in conventional glass manufacturing, including air trapping, decomposition of the individual components that make up glass, galvanic oxidation reduction reactions, and “reboil”, which is the precipitation of super saturated glass when heated to a high temperature [22]. In our previous study of bubble formation in soda lime glass, three different forms of bubbles 998 Solid Freeform Fabrication 2016: Proceedings of the 26th Annual International Solid Freeform Fabrication Symposium – An Additive Manufacturing Conference 7 were identified in printed soda lime glass; periodic bubbles, sporadic bubbles and foam layers [20]. The foam layers were likely a result of reboil due to over heating the glass with the laser, while the periodic and sporadic bubbles could be attributed to bubble entrapment at the lower interface of the printed track and bubble generation around defects or contamination in the filament, respectively. In this paper, bubble formation in additive manufactured borosilicate glass is investigated. Two types of bubbles are identified for this glass system; filament defect/interface and reboil/volumetric defects. The physical phenomena associated with the two classes of bubbles are studied empirically. This helps understand the effects of the process parameters and leads to bubble-free borosilicate glass pieces. Finally, residual thermal stresses generated during the printing process are observed using a polariscope. These are removed using a post deposition annealing step without modifying the printed topography.