Creating 3D models on a computer is becoming increasingly popular among students and in architectural offices. Today, architects have an extensive range of technology available for the visualization of buildings. In addition to photo-realistic rendering and animation, these include models to which access is provided in virtual form. Professional model-builders and universities now offer rapid manufacturing (RM) as a service to architects. The costs of a 1:500 model are usually no more than a few hundred euros, which makes this system competitive alongside conventional model construction. One condition for this, however, is that the 3D data supplied by the planners can be converted to the STL format.

The use of CNC moulding cutters depends on various factors such as the volumetric capacity, robustness, speed, precision and ease of operation of the machinery, and “intelligent” control software. Virtually all solid materials – wood, metal, plastics, etc. – can be accurately worked. CNC cutting machines can also incise lines (e.g. facade divisions in perspex) if they are equipped with the right software. 3D milling takes a lot of time, but even rearcut hollow forms can be created with a triaxial system. In many cases, though, this complex process will not be necessary; for example, a double-curved wall can be built up in a series of layers (see ill. 2: the stands). If two-dimensional forms are sufficient, laser cutting apparatus offers an interesting alternative (ill. 3). In addition to hard substances, soft materials like paper, foils, leather and fabrics can also be worked in this way. If one wishes to cut metals, the investment costs are disproportionately high. Laser cutting plant costs from about ™30,000 upwards.

Since the early 1980s, research scientists and companies have been developing processes to produce objects by fully automatic means based on CAD data. A number of new technologies have emerged under the heading of “rapid manufacturing” (RM). These are capable of building up three-dimensional forms layer by layer. RM plant is used today in vehicle, machine and shoe production, for example, but none of the equipment in existence at present was developed to make architectural models. We shall concentrate, therefore, on technology that can be used to create conceptual and presentation models.

Equipment for laminated object manufacturing (LOM) is among the most economical on the market. Layers of film are automatically built up with adhesive and cut to the required shape by a computer-operated knife or by laser. This technique is not suitable for filigree scale models, however. Stereolithography (STL) is the best-known and oldest RM technique. In a thermochemical process, a UV laser hardens layers of liquid synthetic resin. Models of this kind have a high degree of accuracy (± 0.1 mm). The material itself is pale yellow and translucent (ill. 2). The equipment is relatively expensive.

Multi-jet modelling (MJM), like STL, is based on the hardening of a photopolymer. In this case, the liquid plastic is polymerized by means of a UV lamp. To generate rear-cut, hollow forms, a wax-like supporting structure is created, which can be melted or washed out at the end. This technology ensures a fine quality and accurate details, but the costs of the materials and equipment are high, and the working speed is relatively slow.

With fused deposition manufacturing (FDM), model layers are created with a thermoplastic melted thread squeezed through a hot nozzle. Supporting structures are made with a similar material and have to be manually removed or dissolved and washed out with a warm alkaline liquid. This technology allows the creation of precise details, and the surface finish is of high quality, despite the step-like effect. Costing from ™30,000 upwards, the equipment is among the most economical on the market.

The basic materials used in selective laser sintering (SLS) are in powder form. They are applied in very thin layers and fused together by laser at those points where the model shape is to be created. After construction, the model stands in a powder-filled cube. A supporting structure is not necessary. Using this process, models can be generated in metal, ceramic and plastic. The machinery is among the most expensive available (more than ™100,000).

Three-dimensional printing is a further modelbuilding technique. Here, too, powder materials are used. The form is built up layer by layer on gypsum-based materials with a bonding agent. The finished model again stands in a bed of powder. In this case, however, it is a fragile, porous intermediate product that has to be impregnated with a hardening substance such as synthetic resin. The negative aspects of this technique include the many materials required, the thin layers produced by the machine (with a danger of breakage) and the cost of the apparatus (starting at around ™40,000).

Nearly all RM machines involve a layered form of construction with steps between the strata. The steps can be removed subsequently if the geometry permits. The surface can also be refined by painting, galvanizing, chromiumplating and powder-coating, depending on the material. Generally speaking, the more complex a structure is, the more the use of RM processes pays off. As a rule, the import format is STL, in which the surface geometry is described in the form of triangles. Few architectural CAD programs have a functioning STL interface that can accurately describe complicated geometry, however.

Architectural data are usually much more complex and inhomogeneous than those in mechanical engineering. Nor should one overlook the fact that a skilled model-builder sometimes needs only a simple sketch to create an adequate conventional model of a building. Model construction in architecture is usually to a small scale. With RM machines that cost less than ™50,000 and are capable of creating very slender elements, a volumetric capacity of 20–30 cm per spatial axis will rarely be sufficient for 1:100 models. With a CNC milling machine, in comparison, window bars can easily be cut to an accuracy of 0.5 mm.

The term “rapid manufacturing” is misleading, since construction times of 10–20 machine operating hours should be allowed for architectural models 10 ˜ 15 ˜ 30 cm in size. The construction process often runs overnight. Today, rapid-manufacturing technologies can create models from aluminium, steel or titanium powder, and computer-based processes hold a great potential for the future in the field of architectural model-building.