The Smithsonian Museum in Washington DC, USA, is currently showing an exhibition called ‘Within These Walls’. The centre-piece is a 200 year old house that was moved from Ipswich, Massachusetts, to the second floor of the museum, and the exhibit also includes scale models of the house. These are embedded with Braille for use by sight-impaired people and suggest how some of the rooms might have been used in the 1760s, 1840s and 1940s. Demanding absolute accuracy, the museum turned to rapid prototyping materials and stereolithography to produce the models.
What is Stereolithography?
Stereo lithography is a layer-additive rapid prototyping process based on the use of photopolymer liquid resins that solidify when exposed to UV light. Software transfers a 3D computer aided design (CAD) model into an electronic file for the stereolithography machine, splitting the information into thin cross-sections. A laser then traces each layer onto the surface of a vat of the resin, building the part in repeated layers until a solid replica of the original CAD model is completed.
Production of Models From Sketches and Photographs
The Smithsonian work was undertaken by protoCAD, a specialist engineering firm based in LaPlata, Maryland, who produced the models in four stages. First, the Smithsonian Museum provided protoCAD with a set of hand sketches and photographs. Then, using these, protoCAD recreated the architectural details using Pro/Engineer (Pro/E) software. ‘There was a significant degree of difficulty at this level of the process,’ said Brian Edwards, Design Director for protoCAD. ‘We were faced with the challenge of replicating extensive details at a very small scale. Also, we were committed to the task of being true to historic accuracy, both in cosmetic and functional specifics of the building. It was necessary to accommodate diverse geometries in modelling not only the main structure but also the intricacies of doors, windows and other subcomponents of the house.’
The third stage saw approved 3D CAD files sent to a stereolithography machine to create the actual models - a process that required only a couple of hours. To ensure that proper physical attributes were achieved, protoCAD selected DSM Somos 7100 ProtoFunctional (R) resin as its modelling material. According to Brian Edwards, ‘Somos ProtoFunctional materials are a good choice for rapid prototyping applications because they replicate the performance parameters of production materials. For our purpose in rapid modelling, this particular Somos grade was ideal because of its strength. We knew it would be ideal in its ability to act as a master pattern for tooling.’ Finally, the scale models were used in a rubber tooling process to create multiple copies.
Success of the Project
Brian Edwards summarised the project saying, `We were very pleased with the results of this effort. There is nothing that compares with providing museum visitors a realistic visual and hands-on perspective of living history. The use of rapid modelling and the reliability of the materials used to create these models helped to ensure the preservation of historic accuracy in meaningful 3D forms. In addition, new materials and processing technology enabled us to complete our portion of the project within a very short turnaround time.’
Production of Scale Models From Original Parts
A second project, also coordinated by protoCAD on behalf of the Smithsonian institute featured the enlargement of an eighteenth century antislavery medallion, so that the public could more easily see its detail. ‘The detail of the medallion is difficult to see locked away behind glass’, said Edwards. An enlarged replica needed to be made in order to emphasise its features and for it to be handled by the public. The medallion was scanned with 3D digitising equipment creating surfaces that could be exported into Pro/E. The surfaces were converted to a solid in Pro/E and further edited before being scaled up.’
In this application, as with the miniature house models, a replica was produced using a stereolithography machine. DSM Somos 7100 ProtoFunctional resin was again used because of its ability to achieve rigid, undistorted, high-accuracy parts.