Page T1.2 . 02 August 2000                     
ArchitectureWeek - Tools Department



Scale Models from Thin Air


Rapid prototyping options

As many as 23 companies manufacture and sell machines for rapid prototyping, according to Terry Wohlers, whose consulting firm, Wohlers Associates in Fort Collins, Colorado, specializes in rapid prototyping and related technologies. "Fewer than half of these companies sell their products in the U.S., yet the U.S. dominates the application of the technology by a large margin," Wohlers says.

The oldest and most common technology is stereolithography, in which a laser beam moves through a vat of ultraviolet-sensitive liquid polymer, following the contours of the model's floor plan. Where the beam hits the liquid, a thin layer is solidified. Then the model is lowered slightly within the vat, and the laser produces the next layer. StereoLithography machines are manufactured by 3D Systems Inc. and are generally referred to as stereolithography apparatuses (SLAs). There are several models, which vary in size, speed, and cost.

Because the layers are built from the bottom up, temporary supports must be provided in the CAD model for roof overhangs and similar geometries. Breaking off the supports after construction is easy, but care must be taken not to damage the model. The model then requires light sanding to remove evidence of the support structures. Toxic fumes from the SLA make it unsuitable for an office environment. The process, however, affords great precision and strength in even delicately shaped objects.

The process called selective laser sintering (SLS), developed by DTM Corp. in the U.S. and EOS Gbmh in Germany, is similar except that the laser beam incrementally raises the temperature of a nylon-based powder to the point of fusing. The surrounding powder supports any overhanging geometry, so extra supports are unnecessary. Dust from the powder makes SLS also unsuitable for the office.

Fused deposition modeling, or FDM, sold by Stratasys, Inc., features an extrusion head that moves over a horizontal plane, carrying a heated nozzle that deposits melted thermoplastic material in specified locations. The material solidifies as it is deposited, layer by layer. The FDM process can produce several colors but any given piece is monochrome. Stratasys designs its equipment for use in office environments.

Laminated object manufacturing (LOM), from Helisys, Inc., involves the successive layering of thin sheets of special paper. Each sheet is automatically placed, bonded to the layer below with a heated roller, and cut with a laser. When complete, the model has the look and feel of wood. It can be sanded and finished with techniques already familiar to architectural model builders. LOM machines can produce larger models than most other rapid prototyping technologies. Because the surrounding cut paper supports any overhanging geometries, it is not necessary to construct extra supports. But this system's cooling and outside air venting requirements may make it unsuitable for an office environment.

In 3D Printing (3DP), invented at the Massachusetts Institute of Technology and commercialized by Z Corporation, a water-based liquid binder is deposited on a thin layer of starch/cellulose or plaster powder, using a standard ink-jet print head. 3DP is often called concept modeling because it is relatively fast and inexpensive but lacks the precision of SLA. These machines are designed for use in an office setting.

Overcoming obstacles

High cost is no longer a barrier, though Wohlers suspects most architects don't realize how dramatically prices have dropped for simple models. "Not long ago, you'd pay $1000 or more for a medium sized model," he says. "But now, you can produce a model that fits in a four-inch cube for under $150. It's difficult to do that by hand."

Even models with complex geometries are more affordable when done by rapid prototyping than by more traditional means. Complete 3D printing systems are now in the $40,000 to $50,000 price range, affordable for large firms that build a lot of models. These lower prices are also reflected in the charges of service bureaus, where architects can go to have the models made for them.

A more serious barrier is the investment of time and training in conventional software that architects have made. In most cases, a 3D model developed from 2D drawings will make a poor STL file. Even this, however, did not stymie Rafael Tapanes, whose Miami-based firm, Reelization, Inc., specializes in architectural visualization services for architects.

Tapanes recently experimented with generating physical models from AutoCAD base drawings. He created an AutoCAD model of a Key West prototype house, which features detailed woodworking for the front porch. By exporting the AutoCAD file to 3D Studio Max, intended primarily for rendering and animation, he was able to create an STL file.

The process, however, required some manual tweaking of the model's surfaces to make them come together into an actual solid. Nonetheless, even with this extra work, Tapanes believes he invested less time and money than if he had rebuilt the model from scratch in a solid modeler. Technicians at DTM processed Tapanes's file in an SLS machine. "The result was an amazingly accurate model," he reports. "The railings are 1/64 inch wide and still sturdy."

Tapanes hopes that architects will come to appreciate the advantages of this technology. "What better way to explore schematic design options than to produce a model within hours, even minutes, and hold it in your hands to examine it from any angle." Despite Tapanes's eventual success after starting with a conventional CAD file, most architects trying rapid prototyping use solid modelers, which more readily produce good STL files.

Multiple technologies

Architect Kevin Chaite Rotheroe of New York City, who recently completed doctoral research in computer-aided design and manufacturing at the Harvard Design School, chose to create models with Pro/ENGINEER. Rotheroe's research focuses on complex freeform metal structures, and he builds both scale models and full-size mockups.

His design for an organically shaped conservatory could not have been modeled with conventional architectural CAD software. The metal frame for this structure is composed of members resembling curving tree trunks that branch out into a delicate lattice supporting compound-curved glass. To create a physical model of the conservatory, Rotheroe made the base with LOM, an appropriate technology for large solid parts. The top component was made with stereolithography; no other method could have produced the necessary combination of precision, strength, and delicacy.

The branching columns were sufficiently complex to require rapid prototyping, but they were all identical, so he found cost savings in mass production. He created one column component with stereolithography and this served as a master pattern for making a silicon rubber mold. Ten identical urethane parts were then cast from this mold, and each casting cost 17 percent of the SLA pattern. Such mass production techniques would have been possible and financially beneficial even if the geometry of his conservatory were not so complex.

Rotheroe points out that the often necessary use of multiple technologies for different types of components argues against architects buying their own rapid prototyping equipment. It would be better to develop relationships with specialist service bureaus. This would also lessen the burden of having to learn, maintain, and keep up with changes in the various technologies.

Although most of them produce fairly small models, Rotheroe believes that rapid prototyping devices also offer an important opportunity to improve design development through full-size mockups of small-building components. He is experimenting with prototypes of a structural column with a complex internal structure.

"The use of rapid prototyping during design development can preempt difficulties and misunderstandings during the documentation and construction phases by clarifying a design solution, determining its viability, or highlighting its limitations," he says. "In such cases, a mock-up quickly justifies its cost by avoiding late changes and construction change orders."

For now, such full-size prototypes are limited to connections and other critical details in a structure. But suppose the size of rapid-prototyping machines grew while their cost went down. It's easy to imagine a day when the plastic model emerging from the vat of liquid is the size of a small building. Or when the model emerging from the LOM machine is an actual house.

B.J. Novitski is managing editor for ArchitectureWeek and author of Rendering Real and Imagined Buildings.

This article first appeared in Architectural Record, December, 1999.



ArchWeek Photo

In stereolithography, a laser beam moves through a vat of liquid polymer. Where the beam hits the liquid, a thin layer solidifies to create the model. Temporary supports are later removed.
Photo: Carl Dekker, Met-L-Flo, Inc.

ArchWeek Photo

In the LOM process, layers of paper are placed, bonded to the layer below, and cut with a laser. Before sanding, the model resembles wood.
Photo: Kevin Chaite Rotheroe

ArchWeek Photo

In 3D Printing, a standard ink-jet print head deposits liquid binder on successive layers of powder.
Photo: Kevin Chaite Rotheroe

ArchWeek Photo

Reelization, Inc. built the Key West model to demonstrate the feasibility of applying existing AutoCAD files into rapid prototyping systems.
Photo: Reelization, Inc.

ArchWeek Photo

After sanding and painting, the LOM-produced base for the conservatory.
Photo: Kevin Chaite Rotheroe

ArchWeek Photo

Rotheroe's intricate conservatory model would have been very difficult to build without rapid prototyping.
Photo: Kevin Chaite Rotheroe


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