Effects of slurry mixing methods and solid loading on 3D printed silica glass parts based on DLP stereolithography

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Abstract

In this work, digital light processing (DLP) 3D printing technology was used to prepare silica glass parts. The effects of slurry mixing methods and solid loading on the rheological properties of the silica slurry were investigated. The viscosity of the multi-step mixing method was less than the one-step mixing method at different solid loadings. When the solid loading of the slurry was 60 wt%, the viscosity by the multi-step mixing method and the one-step mixing method was 2059.7 mPa s and 5461.2 mPa s respectively at a shear rate of 0.1 sec−1, with a difference of 3401.5 mPa s. The results of heat treatment experiments showed that transparent glass samples could be obtained when the solid loading was ≥40 wt%. The density of the sintered sample remained substantially constant as the solid loading increased, but the linear shrinkage gradually decreased. The heat treatment time could be reduced to less than 16 h by optimizing the debinding and sintering process using a simultaneous thermal analyzer. Glass samples with the relative density of more than 99% and a transmittance of more than 90% in the range of 300–1800 nm were acquired, and the microstructure and physical properties of the sintered glass were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and digital micro-Vickers hardness tester.

Introduction

Silica glass is widely applied in optical fiber, electric light source, laser, chemical, nuclear industry and other high-tech fields owing to its high temperature resistance, low thermal expansion coefficient, stable chemical properties and excellent performance for light penetration [[1], [2], [3], [4], [5], [6]]. However, it is difficult for silica glass to be thermoformed and machined because of its high melting temperature and the characteristics of hard brittle materials [[3], [4], [5], [6], [7]]. The glass devices with complex structure cannot be obtained by the conventional preparation techniques, such as high temperature melting method [8], chemical synthesis method [2] and sol-gel method [9], and generally require hazardous hydrofluoric acid etching [10] or laser processing [11], which limits the use of silica glass due to high cost, low efficiency and environmental pollution.

3D printing is an additive manufacturing technology based on the principle of discrete/stacking forming, which can realize the rapid manufacturing of complex structural parts such as hollow and thin wall, and has broad application prospects in microfluidic chip, micro machinery, aerospace, automobile, biology, medicine and other fields [[12], [13], [14]]. At present, the mainstream materials of 3D printing are metals, resins and ceramics. Although several methods of glass 3D printing have been reported, such as fused deposition modeling [15], laser melting of glass fiber [4], direct laser melting silica paste [5], direct ink writing [16] and stereolithography [6,17,18]. There is still a certain gap between the quality of printed glass and commercial glass, and achieving high quality glass 3D printing remains a major problem.

Compared with other 3D printing technologies, stereolithography has the advantages of high forming accuracy, good surface quality and fast forming speed [12,13,19]. It is also the earliest 3D printing technology applied. Kotz et al. [17] proposed for the first time to achieve 3D printing of glass by stereolithography and prepared a nano-silica resin mixture with a solid loading of 37.5 vol%. After 3D printing and heat treatment, glass devices with complex shapes were obtained, but the high viscosity of the mixture limited the increase of the solid loading. In addition, the sintered samples were prone to lamellar cracking, which affected the use of 3D printing glass. Liu Chang et al. [6] also prepared silica slurry with a solid loading of 53.7 wt%. In order to overcome the influence of slurry viscosity on 3D printing, a top-down laser scanning SLA (Stereo Lithography Appearance) printing method was adopted to print the glass. Compared to top-down SLA technology, bottom-up DLP (digital light processing) technology uses mask projection instead of laser spot scanning, which greatly improves the forming efficiency, and the surface quality and forming accuracy of printed parts are higher because it is not affected by the spot diameter [13,19]. Moreover, DLP uses less slurry per printing, which reduces research and development costs and improves material utilization. However, DLP technology has a higher demand for slurry fluidity. Too high viscosity makes it difficult to level the slurry because there is no scraper system in common DLP equipment. On the other hand, it also causes excessive adhesion between the cured layer and the bottom of the resin tank, which easily leads to printing failure [6,13,19]. To reduce the viscosity of the glass slurry, Cooperstein et al. [18] used a sol-gel method to prepare precursor slurry which could be printed by a commercial DLP equipment to replace the nano-silica resin mixture. Transparent glass devices were also obtained through drying and sintering, but because of the low solid loading, it caused a linear shrinkage of 33–56% and exhibited non-isotropic shrinkage, and the entire process took up to 7 days.

The nano-silica resin mixture has higher solid loading than the precursor slurry, lower shrinkage after sintering and shows isotropic shrinkage, and the whole process can be completed in two to three days with higher production efficiency [6,17,18]. However, the excessive viscosity at high solid loading is not conducive to 3D printing, which is the main problem that restricts the development of nano-silica resin mixtures [6,17,18]. In this work, the viscosity of the mixed slurry was reduced by changing the mixing methods and the solid loading to make it suitable for general commercial DLP equipment. The rheological properties (viscosity, viscoelasticity and thixotropy) of the glass slurry prepared by different mixing methods were compared by rheometer. The debinding and sintering process was optimized according to the simultaneous thermal analyzer, and the effects of different solid loadings on the sintered samples were investigated. The microstructure and physical properties of the sintered glass were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and digital micro-Vickers hardness tester.

Section snippets

Materials

Hydrophilic fumed silica (Aerosil OX50, Evonik, Germany, Specific surface area SBET = 50 ± 15 m2/g) was utilized as the raw material of glass. TEM results of OX50 indicate that the particles are spherical and the particle size is less than 100 nm, as shown in Fig. 1. 4-hydroxybutyl acrylate (4-HBA, Osaka Organic Chemical, Japan), polyethylene glycol (200) diacrylate (PEG(200)DA, Macklin, China) and trimethylolpropane ethoxylate triacrylate (TMP(EO)3TA, Macklin, China) were used as

Results and discussion

The preparation of slurry with high solid loading, low viscosity and good dispersion is the key issue for stereolithography based 3D printing of ceramics [20,21]. Generally, dense ceramic parts can be obtained with solid loading above 40 vol%, and slurry viscosity below 3000 mPa s to meet the requirements of ordinary stereolithography equipment [20]. The viscosity reflects the fluidity of the slurry and affects the slurry leveling time in the resin tank. The shorter the time is, the higher the

Conclusions

In this work, the effects of slurry mixing methods and solid loading on 3D printed silica glass parts based on DLP stereolithography was investigated experimentally. The viscosity of glass slurry increased with the increase of solid loading, but the viscosity by the multi-step mixing method was lower than that by the one-step mixing method. In order to further illustrate the effect of mixing method on the slurry, rheological tests were performed on the slurry with a solid loading of 60 wt%. The

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was financially supported by the National Key Research and Development Program of China (2017YFB1104500); Department of Science and Technology of Guangdong Province (2018B030323017); Key-Area Research and Development Program of Guangdong province (2020B090922006); Young Innovative Talents Project in Universities of Guangdong Province(2018KQNCX057); Young Scholar Foundation of South China Normal University (19KJ13).

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