It was during a night out with good Australian wine, when Mariana, Telesilla and I (Faidra), all attending the IASS conference in Melbourne, came with the idea of combining our expertise to explore 3d-knitted molds for freeform bent glass applications in architecture. Despite the vast forming potential of bent glass, currently its applications in architecture are typically confined to the repetition of a single moduli, mainly due to current limitations in mold technologies and associated manufacturing costs. 3D-knitted molds could be a promising solution, allowing for a labor-free, cost-effective, material-saving and equally importantly, customized solution for freeform bent glasses, enabling in turn for a new architectural language. Many questions immediately arose: what material could we use to withstand the high temperatures of glass slumping? What were the geometrical limitations of this mold solution? Were coatings needed? Could the molds be reused? Anna took up all these challenges, not only successfully providing the answers, but also highlighting the great potential of this fabrication approach for freeform glass curved façades.
Mentors: Dr. Faidra Oikonomopoulou (Assistant Professor, Structural Glass, Faculty of Architecture), Dr. Mariana A. Popescu (Assistant Professor, Digital Fabrication, Faculty of Civil Engineering and Geosciences)
Author: Dipl. Arch. Eng., Ir. Anna Konstantopoulou
Architectural design is continually evolving, pushing the limits of materials and aesthetics to create innovative façades that redefine our urban landscapes, with glass playing a crucial role in this transformation. However, the full potential of using glass in architecture has not yet been realized, and recent experiments point to new prospects.
Glass is a durable material of high compressive strength, a quality which makes it a great choice for architectural and structural applications in buildings. Currently, only float glass is used for structural applications, which due to its planarity leads to mainly 2-dimensional façades and building envelopes1. In the past couple of decades there has been a clear tendency towards fluid freeform architecture. There are several built examples of curved glass surfaces that try to facilitate such 3-dimensional façades.
The vast majority of curved glass applications make use of curved float glass. Flat float glass may be formed into the desired curved form using several methods, the selection of which depends on various parameters such as the final strength required, the curvature and geometry to be achieved or the cost of fabrication. Especially when heating glass, there is great potential to what can be achieved geometrically.
To curve float glass panels with any method, the use of moulds is required. Because of the usage of moulds and the significant expense of producing many different ones, curved glass is often applied by standardizing the components in a repeating manner. This is often the case with corrugated glass façades as seen in examples such as Casa da Musica in Porto, MAAS museum in Antwerp or the Nordstrom flagship store in New York. As a result, little room is left for freeform geometric possibilities.
In other materials, such as concrete, research and practice have shown that the use of flexible moulds can facilitate the creation of complex geometries in an easy way2. The recent development of knitted textile moulds has pushed the boundaries of flexibility in achieved geometry and shows significant potential by minimizing material use and manual labor, which lead to a more sustainable lightweight structure with decreased cost for manufacturing3.
Currently, there is no known manufacturing technique for freeform non-standardized glass panels that can achieve extreme geometries while being simple in mould fabrication, hence not raising the process cost significantly. However, there lies great potential in the combination of flexible moulds with glass. Using knitted textile moulds to curve glass opens up significant possibilities to architectural design with glass. CNC knitting is a method that can result in freeform lightweight and easily customizable moulds. This allows for high customization in façades using cost-efficient and easy to fabricate moulds that result in little waste.
CNC-knitted textile mould made of basalt. Mould after firing process in the glass oven.
Since this method has not been tested before, a series of physical experiments was designed and performed at the lab facilities of the Glass & Transparency Research Group at TU Delf to realize the potential and limitations of this novel fabrication technique. The starting point of the experiments was to select a yarn that can withstand the process of hot bending glass by slumping in the oven. Continuous basalt fiber was chosen as a promising material for the knitted moulds, since it can tolerate the maximum temperatures used for glass slumping at continuous exposure and it is also considered as the most environmentally friendly high temperature technical textile fiber to manufacture and recycle4.
The main objectives of the experiments were geometry, surface and redundancy. For geometry, the creation of several single and double-curved surfaces was attempted; for surface, the finishing surface quality and texture on the glass were examined, and finally, for redundancy, the possibility of having multiple glasses of perfectly aligned curvature for future lamination was tested.
Experimental set-up for a curved glass panel using knitted basalt mould: (a)basalt mould before and after firing; (b)mould and glass in the oven after slumping; (c)resulting double curved glass.
Regarding geometry, both single and double-curved surfaces were achieved, as well as surfaces of multiple curvatures. Both hand-woven and CNC-knitted moulds were produced and tested leading to different results in geometry due to the different structure of the textiles. Hand-woven moulds prove to be more rigid in their shape, while with CNC-knitting, the mould is able to stretch and deform according to its pattern. It was observed that in the case of knitted moulds, the maximum deformation of glass varies with glass thickness and knit pattern of the moulds, while hand-woven moulds lead to curvatures that follow the initial state of the mould.
Additionally, some of the experiments attempted to introduce multiple curvatures in the glass surface either by coating the basalt mould to create a rigidified shape, or by producing a textile combining multiple knitting patterns. Both experiments were successful. Changes in the knit pattern were observed to result in different curvatures in the glass, however, transitioning from a negative to a positive curvature is not possible solely with textile draping; it requires an additional supporting system. The use of a heat-resistant cement coating to produce the rigid mould proved to shape successfully the glass in a complex geometry, however, not achieving the extreme geometries possible with concrete on knitted moulds.
Replicating a double-curved surface was also successfully achieved. Experiments showed that the repeatability of an experiment is possible giving almost exactly similar results in geometry, even with a mould that has already been used once.
Double curved glass shaped with textile mould rigidified with cement coating.
Regarding the glass surface, experiments were conducted with direct contact of basalt and glass or by using coatings to achieve a better result. In terms of finishing surface quality, the use of no coating showcased easy de-moulding of the glass; however, crystallization occurred every time, producing a “foggy” surface. With a coating, the glass surface turned out almost transparent but still not perfectly so, achieving the best results with rigidifying coatings such as cement. Nevertheless, in an experiment conducted without a mould, crystallization was still evident on the glass surface, leading to the conclusion that this is an issue that can be tackled by adjusting the firing schedule of the process.
An interesting finding of the experiments regarding the surface was the texture imprinted by the mould. In cases of direct contact of basalt and glass, texture imprints vary based on the density of the mould and the thickness of the glass, leaving a mark showing the mould’s support points. Having textured glass might add an aesthetic quality, sought after in specific façade design. Furthermore, a closer look using the microscope revealed that there are basalt inclusions in the glass surface giving it hints of color, however, not being threatful for post-breakage of the glass. With the use of coatings, there is no texture from the mould itself, but a light texture from the coating layer. Rigidifying coatings, such as cement, leave scarce but strong imprints and inclusions of material, which can lead to potential microcracks, thus, concluding that the use of coatings is not an ideal solution.
Finally, achieving future redundancy of the glass panel proved to be feasible. Simultaneous slumping of glass pieces on a single mould showcased a perfect curvature alignment, both for single and double-curved surfaced, therefore, leading to the possibility of lamination to achieve the required strength of a façade panel.
Ultimately, while this method is still at an early stage of development and there is plenty future research to be done in order to make it applicable in the façade industry, using knitted moulds to curve glass in freeform non-standardized shapes shows great potential. Using such easily customizable moulds opens up the possibilities of what can be achieved without an excessive cost of manufacturing, leading to a new and enriched vocabulary of architectural design with glass.
Proposed fabrication process for a façade panel.
Vision for a freeform façade application.