Free-form architectural envelopes: Digital processes opportunities of industrial production at a reasonable price

2.1Subtractive procedures

Subtractive procedure involves the separation of particles of a raw material using different techniques (electric, chemically or mechanically-reductive). CNC subtractive methods comprise punch, lasering, water-jet, plasma-arc, hot-wire, milling two to six axis, robotic lasering (multi-axial) (Hauschild & Karzel, 2011). Usually subtractive processes are based on two-dimensional fabrication. The production strategies used for two-dimensional fabrication include contouring, triangulation (or polygonal tessellation), use of ruled, developable surfaces, and unfolding (Kolarevic, 2003). They all involve the extraction of two-dimensional, planar components from geometrically complex surfaces or solids comprising the building’s form. Nevertheless the milling of three-and-more axes has the ability to raise or lower the drill-bit along the Z-axis. Undercuts cannot be milled with three-axis machines. To achieve manufacturing undercuts, four or six axes machines are used. The CNC milling has been used to produce moulds for free-form panels, and parts of large proportions.

In Gehry’s Zollhoff Towers (Dusseldorf, Germany) (Fig. 3), the undulating reinforced concrete walls were made with moulds of Styrofoam (polystyrene foam). 335 different moulds were CNC milled that became the forms for the casting of the concrete. ‘The Bubble’, is the BMW pavilion, designed by Berhard Franken, for which free-form acrylic glass panels were formed, using polystyrene moulds, as well as created, using multi-axis milling.

NIO Architecten designed the curvy Bus Stop in Hoofddorp, Netherlands (Fig. 4). The structure was manufactured from polystyrene foam pieces with hot-wire procedure. The total measures of the construction are 50 × 10 × 5 m, and a layer of GRP coating protects the surface. The construction costs were € 1 million, and it would have been twice, if the structure had been traditionally built with concrete and steel. Using large scale hot-wire cutting is very well suited to concrete formwork and the manufacture of light, voluminous construction parts, as we can see in these constructions.

Subtractive processes are commonly the procedures more used in construction, due to the manufacturing industry, which has used them to mass-produce elements of conventional orthogonal geometries. These techniques have large relevance within free-form panels fabrication, based on their ability to create personalized architectural forms starting from typical construction materials (Naboni & Paoletti, 2014), overall producing mock-ups and moulds. However, the question of material economy continually arises due to waste of material with these subtractive procedures. It does not appear sensible to mill a specific individual component from a block of solid material if there is no decisive added value in doing so (Hauschild & Karzel, 2011).

2.2Forming procedures

Forming processes are particular manufacturing methods, which make use of mechanical forces, restricting forms, heat or steam to cause plastic deformation of a material to produce required shapes. Some Forming processes include CNC punching, CNC folding, CNC bending, Hydroforming, Linear flow splitting, Flexible roll forming and bending, Thermoforming, Injection moulding, Welding and Multi-point forming.

Sheet metal (aluminum, steel) have been traditionally formed by hand or in stamping moulds. If CNC processes could be used to form a metal sheet their main problem would be solved: cost. This is due to the fact that skilled labour and runtime are reduced. Three promising processes for manufacturing free-form panels on metal forming are under development: flexible roll bending, incremental forming and multipoint forming. Incremental sheet forming (ISF) (Fig. 5) is an umbrella term for a range of processes in which a sheet is formed incrementally by a progression of localized deformation (Alonso et al., 2014).

Mass production technologies for making aerodynamic automobiles, airplanes and ships using sheet metal exist, but are not feasible for irregularly shaped buildings because the requirement for panels of different shapes. Thus, the high price of mould fabrication cannot be justified. For this reason, existing buildings with curved shapes have rarely employed true double-curved panels, and have instead used tricks (Lee & Kim, 2012).

Double-curved panels can be approximated by arrays of adjustable height and numerically controlled pins (Fig. 6), which could be used for the production of moulded glass and plastic sheets, and for curved stamped metal (Alonso et al., 2013). For this and previous case studies, we can affirm it is technically possible to fabricate panels of complex shapes with sheet metal processing methods. However the costs of the existing methods normally exceeded the project budget due to the high price of the machines that manufacture such panels and the material itself.

2.3Additive procedures

In contrast to the subtractive processes, the Additive procedure is characterized by adding layers of material to produce a part, without the need of tools or preforms (tool-less), and it allows great freedom of geometry. The main advantage comes from the possibility to produce unique components, which would not be economically sustainable to produce with traditional manufacturing techniques (Hauschild & Karzel, 2011). This feature, together with the high freedom enabled in realizing complex forms, makes the additive manufacturing process particularly relevant within the perspective of advanced customization, and explains the increasing involvement of architects in the development of techniques and application in this field (Naboni & Paoletti, 2014).

The Additive process is frequently cited as being layered manufacturing, solid free-form fabrication, rapid prototyping or rapid manufacturing. The technologies associated to this method are Stereolithography (SLA), Fused Deposition Modeling (FDM), Laser Sintering (LS), Digital Light Processing (DLP), 3D Printing (3DP), PolyJett^TM methods, Direct Metal Fabrication (DMF), Direct Metal Deposition (DMD), Contour Crafting and D-Shape.

The principle of the Additive procedure is the same for all of the different methods: special computer software breaks the CAD 3D model down into layers. These layers form horizontal layers/building plans/foot prints of the model. The breaking down process is called slicing. The Additive manufacturing output device processes each layer of the model consecutively, whereby the contours and fillings of the part are cured. Depending on the method, this is done by exposition, heating, or bonding in a process chamber, which typically is confined to certain dimensions. Different technical strategies are used to bond each new layer with the previous one. Thus, layer-by-layer, the physical representation of the virtual CAD model is generated (Strauß, 2013).

The Additive Manufacturing Process was considered limited in building production, because of the reduced size of the objects that it could produce. Additionally they are used mainly for the fabrication of mock-ups with complex geometries like desktop printers. Currently, those assessments are changing, since the 3D printing industry is improving its machines. A start-up in Germany has created a fused deposition printer called BigRep One, which has a capacity of 1.15 × 1.00 × 1.19 m, and prints material such as PLA, ABS, PVA, HDPE, PP, Nylon, TPE, Laywood and Laybrick, with a dual extrusion head. The company 3DP as well has a large printer (1.00 × 1.00 × 0.5 m), which can extrude a variety of materials as BigRep One, and guarantee the possibility of working in series.

The Oak Ridge National Laboratory created BAAM (Big Area Additive Manufacturing) (Fig. 7), which 3D printed the Shelby Cobra car in 24 hours (Fig. 8). The BAAM can manufacture strong, lightweight composite (ABS reinforced by carbon fiber), the print area is 2.00 × 4.00 × 0.87 m and can produce components 500 to 1000 times faster than today’s industrial additive machines. The BigRep One and the BAAM are demonstrating the potential of large-scale additive manufacturing as an innovative and viable manufacturing technology.

In the last few years large-scale processes for construction applications have been developed based on Additive Manufacturing principles. Pegna considers a layer deposition method suitable for traditional construction materials (Pegna, 1997). These processes offer construction automation, the promise of design freedom (Buswell et al., 2007).

Enrico Dini developed in 2004 the world’s biggest 3D printer, which is capable of building structurally sandstone constructions automatically with virtually no human intervention. The D-Shape processes (Fig. 9) uses a powder deposition method, which is selectively hardened, using an inorganic binder. Dr. Richard Buswell at Loughborough University has focused on using traditional construction materials (Fig. 10). The machine uses a gantry and can produce components in a built volume of up to 2.00  × 2.50 × 5.00 m. Contour Crafting (CC) is a parallel process created by prof. Behrokh Khoshnevis of the University of Southern California, which follows the same principle of the generative build-up of other Rapid Prototyping methods. Khoshnevis’s goal is the construction of entire buildings on the earth and on the moon. These processes and researches can be extrapolated for building envelopes facade panels with no moulds.

In China the printing techniques that have been developed seem to be more advanced than Contour Crafting (CC), D-Shape and Buswell’s researches. WinSun Decoration Design Engineering Co. has been capable to print entire family houses made from recycled rubble, fiberglass, steel, cement and binder, in contrast to the other large 3D printers that have just produced a few building elements. The concept proposed by WinSun is similar to CC. Both techniques propose hollow printed walls inside, apart from corrugated filler, a design that saves material without sacrificing strength. However, whereas Winsun prints the building in parts off-site, CC has fully assembled its technology so that it can build directly on the construction site. This localization will be key to bringing the costs down. Khoshnevis observes: “Compared with prefabricated construction, 3D printing is not much cheaper, unless it can occur locally, onsite, eliminating costs of transporting materials and labour” (Ver Bruggen, 2014).

An obvious divergence exists between Additive Manufacturing and current building processes. The size, timelines and production terms are the main discussion topics. Nevertheless, in Friedberg, Voxeljet is manufacturing VX4000, a large-format 3D printing system of 8.00 m³ (4.00 × 2.00 × 1.00 m), which printed a small shelter, designed by Peter Ebner. It is possible to use this technology for printing facade panels, because of the size of the machine that is suitable for the proportions of architectural elements. Contrary to the opinion of other 3D printing companies it is not technically impossible to increase the size of the system, even if it is true that some material and technology parameters cannot simply be scaled up (Strauß, 2013).

Questions such as how much the elimination of labour compensates for the technical skills and time required and how well this balances against the cost of equipment, materials and maintenance can be answered aptly by stating that rapid prototyping or manufacturing does not allude only to ‘cost’ but also to ‘added value’ (Buswell et al., 2005).

There is an extensive range of materials available for rapid prototyping, but limited materials of varying properties and application are available for Additive Manufacture at large scale, despite that it is possible to print colours and maybe textures using for example the 3D printer. The conservation industry, as dr. Cooper of The Conservation Centre, Liverpool, states, still prefers to use traditional methods to finish objects that have been re-formed using Additive Manufacturing or machining, in order to achieve a high level of reality or ‘reproduction fidelity’.

If we translate concepts from the automotive industry, for example, (‘concept cars’, ‘just-in-time management’, new collections from standard components) into facade technology, we can expect an increase in precision and implementation quality by using digital and high-tech tools without compromising individuality. Let us use the ‘file-to-factory’ concept for the most diverse applications by combining a few standard components with highly specialized details and shaped pieces (Knaack, et al., 2008).

There are no tooling requirements for Additive processes and so customization of every ‘build’ comes at no extra cost. The cost-per-part of components manufactured using Rapid Manufacturing are constant, opposed to the high initial set-up costs for tooling used in conventional processes (Hopkinson & Dickens, 2003). The main component for Freeform Construction is in scaling-up these Additive Manufacturing processes, from manufacturing ‘desktop’ sized items to something the size of a wall (Buswell et al., 2008).