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Preshaping clear glass at low temperatures

Post Time:Apr 09,2021Classify:Industry NewsView:937

Advances in available glass chemistries and glass processing methods have accompanied and enabled some of the biggest technology revolutions, from the development and mass production of light bulbs to low-loss fiber optics and durable smartphone touchscreens. An emerging generation of low-temperature processing technologies aims to continue this important trend and make a broader array of glass components mass producible. On page 182 of this issue, Mader et al. (1) describe one such innovation in glass processing—the use of low-temperature injection molding to preshape silica particle–filled composites that can later be transformed into transparent fused silica glass objects.

Traditionally, transparent glass objects are manufactured in high volume from molten or softened glass, which is floated, drawn, blown, cast, or blow-molded to a desired shape (see the figure, top). The glass composition and processing technique dictate the working temperature, which is usually quite high (near 1000°C) and often restricts the choice of compatible equipment or limits the choice of glass composition. Because geometry-specific capital investment is required for production, drastic or frequent component design changes or small batches may be cost prohibitive. Alternatively, transparent glass components can also be shaped at ambient temperature from solid glass by a series of subtractive processes, including cutting or multiple stages of grinding, followed by slower processing steps, such as polishing or etching. This approach is somewhat less amenable to mass production, and certain geometries containing tool-inaccessible regions cannot be fabricated in this way.

Several emerging glass-shaping technologies aim to reduce the required manufacturing temperatures and still provide access to a broader range of glass compositions and component geometries (see the figure). These approaches use a three-step process. First, a desired shape is preformed at low temperature from a glass-forming, organic-inorganic composite. Next, the preform is dried, and organic materials used to bind particles are removed. Finally, the preform is heated (sintered) well below the glass-melting temperature to densify to transparent glass. Although the second and third steps do occur at increased temperatures, only standard, geometry-agnostic driers and furnaces are required.

This strategy builds on the well-studied sol-gel approach to forming monolithic glass, where silica network–forming chemical solutions are poured into molds, slowly dried, and condensed into dense glass without melting (2). In a departure from the sol-gel process, these new technologies use solvents, cross-linkers, and polymers to formulate organic-inorganic composites tuned for compatibility with a particular shaping process, with formats ranging from photocurable liquids to shear-thickening pastes to solids. The composite inorganic loadings are also typically higher than those in the pure sol-gel approach, which drastically reduces shrinkage in comparison.

A host of glass-forming composites and low-temperature shaping technologies for the production of transparent glass have been introduced during the past 4 years, each offering its own benefit (see the figure, bottom). For example, a solid nanocomposite material, called Glassomer, can be shaped at ambient temperature by conventional machining tools like lathes and can also be nanotextured, molded, and roll-to-roll processed in sheets before being converted to fused silica glass (3). Further, an additive manufacturing method called direct ink writing has been used to extrude filaments of nanocomposite pastes into preform structures ranging from monoliths to meshes to open vessels (4). Multiple glass-forming compositions can be mixed inline to create spatial composition variation, such as gradient refractive index glass lenses (5). Stereolithography printing—a layer-by-layer additive manufacturing route by which photocurable nanocomposite liquid resins are selectively patterned by exposure to light—can produce transparent glass components that have complex and hierarchical structures with the potential for fine-scale individual features (6–9).

Most recently, Mader et al. have demonstrated the use of pelletized glass-forming composites that are compatible with conventional, low-temperature injection molding—a process used in the high-volume manufacture of polymeric components. Offering an important advance toward sustainable manufacturing, much of the debinding occurs in water rather than at increased temperatures, and the polymers used in the composite can be reclaimed and reused. Fused silica glass items, including tubes, beakers, and microlens arrays, were produced with this composite and existing injection molding equipment at speeds of up to 5 s per piece, without any postprocessing after sintering.

These approaches could make glass available in new formats as well as more cost-competitive with plastic for applications requiring thermal stability, environmental resistance, or improved light transmission. Distributed manufacturing may become a more viable option because processing is done at low temperatures with less-specialized equipment. Additive manufacturing allows changes in component shape through programming rather than retooling for small-volume manufacture and rapid prototyping of glass components. Immediate applications in high-volume optics, lighting, and packaging will drive continued development in low-temperature processing. The combination of methods that enable tunable glass composition and hierarchical microstructures could find use in microfluidics and catalysis.

Source: https://science.sciencemag.org/Author: shangyi

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