3D PrintingConcise history and state-of-the-artPrinting in other industriesVarious terms are used in literature to describe the realm of 3D Printing and related methods. The term Additive Manufacturing (AM) stresses the fact that we are essentially adding materials (and not subtracting as in CNC Cutting, or forming as in bending, or combining as in assembling). The term Layered Manufacturing (LM) articulates the fact that we are building 3D parts from 2D layers. This can be achieved through either semi-liquid (e.g. extruding ceramics) or solid (e.g. adding layers of paper) methods. The term Direct Manufacturing (DM) stands for the production of end products (in a factory or on location). Printing (3DP) of either buildings or building parts is a member of AM and LM and DM, that uses semi-liquid material extrusion and deposition. Printing as a technology has been around since a first patent was filed in 1967 for a dual-light resin system (Swainson). In 1982, Hull conducted experiments with Stereo-lithography, with patents filed in 1984. Printing as an application has been around since in 1986 the 3D Systems corporation was established, and others followed. They made Rapid Prototyping a reality. Layered Manufacturing was first used for cast patterns. Since approximately 1995 it has also produced tools. And starting with the new millennium LM has been used to fabricate products (Reeves 2012a). AM, LM, RP, DM and 3DP technologies come in industry level, consumer level and everything in between. Industry level technology is typically advanced and expensive and capable of building high quality parts, while consumer technologies are - not surprisingly - simple and cheap, limited in capabilities and qualities of the build. For building production, the challenge is to find a method that can do both: cheap where it can and must be, and high-quality where this is needed or desired. Industry level print methods are Selective Laser Sintering (SLS), Selective Laser Melting (SLM), Metal Laser Melting (MLM), Polymer Laser Sintering (PLS) and Electron Beam Melting (EBM); Direct Metal Deposition (DMD); Stereo-lithography (SL/SLA); Multi Jet Modeling (MJM) and Polyjet printing. SLS, SLM, MLM, PLS and EBM are all sintering/melting techniques working a volume of powder with a laser or electron beam. Some usable materials are: nylon, polystyrene, polyamides, elastomeres, steel, titanium, alloys, sand, ceramics, glass. Direct Metal Deposition melts metal powders with a laser, yet it differs from MLM in that there is no voluminous box of powder, the material is deposited where it is needed. Suitable metals are iron alleys, stainless steel, nickel alloys, copper alloys and carbide. Stereo-lithography involves a liquid resin (epoxy, acrylate) bath and beams of UV-light to cure the material. These technologies are too slow and expensive and fragile and can therefor not one-on-one be applied for construction (except maybe for detailing). Jet-technologies are professional, but slightly less "industrial". Multi Jet Modeling, like SLA, also cures photo polymer material with UV, but in this case the photo polymer material is printed. The Stratasys Polyjet allows selection of various liquid photo polymer materials (flexible, rigid, opaque, transparent) along with a gel-like support material to uphold overhangs and scaffold complicated geometries. This can be considered the most advanced "office" application (Oxman 2011a). Low quality affordable consumer printing typically employs Fused Deposition Modeling (FDM) (sometimes called Fused Filament Fabrication). Thermoplastic materials are heated to a semi-liquid state and then extruded though a deposition nozzle, layer-by-layer, along with temporary support materials for scaffolding. Suitable materials are plastics (ABS, PLA, polycarbonate, polyamides, polystyrene, polyphenylsulphone, acrylate, lignin and many others) or metals. Thermojetting is a similar technique that prints a product/mold in wax with great level of smoothness and precision. This leaves as odd ducks two methods that do not fit well in the above categorization. Selective Inhibition Sintering (SIS) in a way does the opposite of selective sintering: it treats the disposable portions of powder layers (metal, polymer) with an anti-sintering agent or sintering inhibitor, then exposes the treated layer to heat (Khoshnevis 2003). Laminated Object Manufacturing (LOM) is a combined cutting and adding technique, where thin layers of paper (or plastic, ceramics, aluminum, pvc) are cut in shape and adhered to the layer beneath, as true example of Layered Manufacturing. Today, 3D Printing is applied in the aerospace industry (lightweight parts), in medical and dental environments (surgery devices, implants, prosthetics), automotive (motor blocks, coaches), micro- electronics (chips, circuits), creative industries (design products, fashion, jewelry, art), prototyping (mock-ups) and the gifts and toys market (merchandise, gaming avatars) (DMRC 2011). It has also reached the developing world (that seems to skip the steps of industrialization and move straight into the 21st century of mobile phones, internet and "Factory 2.0") where it is used to build spare parts, water tanks, water filters, toilets, toys, keys, buttons, glasses, cups, teeth, braces, combs, badges, zips, covers and sprinklers (Reeves 2012). Printing in the construction industryContour CraftingThe above mentioned technologies concern mostly other industries. In 1997, Pegna was the first to consider printing of buildings in an article in Automation in Construction (Pegna 1997). A few years later, in 2001, Behrokh Khoshnevis of USC was the first to propose and craft an actual method: Contour Crafting (Khoshnevis 2002). CC prints the thin outlines of a massive wall or object in ceramics-clay, which then function both as surface of the element and a sacrificed mold-formwork for concrete. The process is carried out in thin layers, to keep pressure low. A side trowel controls and smoothens the angle and surface of the ceramics. Contour Crafting is first generation technology with its inherent limitations. One of them is that the method prints only concrete walls. Facades, interiors, finishings, infill and services are not included and should thus be produced by other or conventional methods. CC is also limited to vertical and more or less vertical parts of a structure, typically walls. It cannot span distances. Door and window openings are crossed by inserted traditional beams. Floors are assembled from conventional prefab elements. Arcs, vaults and domes have been suggested as a solution. There are also strategic problems. Contour Crafting has been sold with arguments of quantitative benefits: big-scale, low-cost, high-speed, mass-production. But it has no strong efficiency benefits, and misses out on all the qualitative chances. Also, CC has been claiming sustainability, staging reductions in waste. Yet it does not qualify as a sustainable method as it avoidably uses cement for almost everything and creates buildings on the basis of structural walls (instead of for example open frames and open floors) that are not flexible in use, nor technically easily adaptable (Khoshnevis 2006). Concrete PrintingIn 2003, Loughborough University set out to investigate a large scale printing test bed. The research was recoined in 2008 by Richard Buswell. He developed Concrete Printing (Buswell 2005). Other than CC, CP does not first print any outlines in clay, but prints directly where the material needs to be. The deposition head can be swapped with a cutting-tool during the build. LU has tested and developed various high-performance fibre-reinforced fine-aggregate concretes for pumpability, printability, buildability and open time (Le et al 2011, Le et al 2012). Concrete Printing is being commercialized by Freeform Construction Ltd. Concrete Printing has the same problems and limitations as Contour Crafting, but it avoids most of them as it is meant for printing relatively small series or singular components, such as an extruded curve based chair or double curved panels. Soar says that a mayor problem is that the method is far too expensive for upscaling: the printing material is typically sold for $100 per kg, offset to pay for the costs of the machine, while it should come in at $100 per tonne (Soar et al 2012). This is important criticism, but it also needs further inquiry: why would a concrete mix and such relatively simple machine need to be that expensive? Multi-Resolution DepositionBuswell's next step is printing complex parts, such as a facade panel, in one single operation, using a reduced number of materials. A facade could for example be built from five materials: the primary build material, glass, a framing material, an insulating material and an additive to the primary build material for moisture control. Glazing can be achieved using CNC technology (Buswell et al 2007). Buswell suggests that such parts can be manufactured by a new, future 3D technology that he calls Multi-Resolution Deposition. MRD is to build objects at comparable scales and speeds as Contour Crafting (not Concrete Printing), but it will be capable of combining several materials and produce fine detailing. The likely specification and process features of MRD will be:
Apart from the floor and roof and span support problems, this approach may in the end succeed to deliver a mature solution that prints almost all parts of a (residential) structure, at competitive speed and cost. One can wonder though, if printing should aim to produce the conventional buildings that are now produced with conventional methods, as seems to be the aim of both CC and MRD. New technology can propose new types of solutions. An important question is: what do people want? D-shapeIn 2007, Enrico Dini (Dini et al 2014) developed another printing process called D-shape. His method is to drop an inorganic binder on a thin stone powder aggregate bed. Combined they form a stone or marble-like material. This procedure is repeated one thin layer at a time. At the end loose powder is taken away, and the structure remains. D-shape is demonstrated fabricating sculptural designs (FASE 2013), such as the Radiolara (design by Shiro Studio). It has been announced that D-shape will also be used to produce a house with folded floors (design by Universal Architecture) in sections. The method is patented and commercialized by Monolite UK Ltd. Dini's system and products are artistic and aesthetic. This attracts the attention of artists, architects, designers and press (providing evidence for the hypothesis that people indeed want the "romantic"). But D-shape also has severe limitations. One of them is that the printer manufactures only the stone structure, not the other parts or functions of buildings. And it is slow due to the necessarily thin layers; it needs mountains of powder; and it is not suitable for on-site production. Last but not least, D-Shape is too expensive for the building sector, to the same extent as CP (Soar et al 2012). The most probable application for D-shape is therefor (architectural) sculpture. But, NASA also investigates the method for building on the moon or Mars, using local soils and binders. (Dini et al 2010). Contour Crafting has caught the attention of NASA as well (Khoshnevis 2005b). Solar SinteringIn 2010, inventor-artist-designer Marcus Kayser demonstrated his first solar powered CNC machine in the desert: the Sun Cutter. It uses optical lenses to cut plywood with concentrated sun beams. In 2011, he created the next machine: a printer with computer numerically controlled optical devices (lenses, mirrors) that focus the heat of the sun in/on one point on/of a bed of sand. A demonstration in the Sahara shows how Solar Sintering (Kayser 2011) successfully melts and solidifies the sand. When one layer is finished, the next is added. This procedure is repeated until the built is finished. Solar Sintering has successfully overcome the (presumed?) economical constraints as it builds solid 3D objects from those few things that are superfluously, freely available in the desert: sun and sand. The balance is different for locations where solar radiation is less intense a/o not always available, where the soil has more insecure properties, or where the soil is wet. But for arid tropical climates Solar Sintering has successfully proven to create (monolithic, free form) structures out of dry sand. Variable Property Design and FabricationNeri Oxman, director of MIT Lab Mediated Matter, has patented Variable Property Design (VPD) and Variable Property Fabrication (VPF) (Oxman 2011c). VPX is aimed at designing, simulating, fabricating material assemblies with varying properties that correspond with varied functioning (Oxman 2011a, 2012c). The lab works on the development of two first examples and demonstrations of Variable Property Manufacturing: a variable-density concrete system (Variable Density Digital Fabrication) and a variable-elasticity polymer system (Variable Elasticity Digital Fabrication). Both are manufactured by a new automated tool that is capable of dynamically mixing and varying the ratios of component materials to produce complex continuous gradients in monolithic structures (Oxman 2012c). MIT works on a proof-of-concept printer. The Polyjet technology by Stratasys, one of the most advanced in its kind, allows for dual printing, but single material gradient technology still needs to be developed. The Variable Property approach can be extended to mechanical, electrical, thermal, chemical, magnetic, optical, acoustical, environmental, sensorial and other properties (Oxman 2011a), or combinations of those. Integral Shells SolutionsAn upcoming MIT project called "Building scale Digital Constructing and 3D printing" investigates the concept of printing a fast curing material, which serves as both a mold for structural concrete walls and a thermal insulation layer. The method bares resemblance with Contour Crafting, but in this case the material is an insulator; the geometry is flexible; the robots are small, mobile and many; and the aim is to integrate utilities like wiring and plumbing (Oxman et al 2014). A parallel project, "Monocoque" ("single shell"), investigates the concept of a (free form) tessellated structural skin (using the Voronoi pattern), with a density that corresponds to loading conditions. The distribution of shear-stress lines and surface pressure is embodied in the allocation and relative thickness of vein-like elements that are built into the skin. Printing provides the ability to print parts and assemblies made of multiple materials within a single build, as well as to create fine composite materials (Oxman et al 2014). PrognosisNext generation 3D Printing may see some of the following features:
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