Washington, D.C. Visitors Center Selects Material for Scale Model Sculpture of Capitol Dome

“Touchable” 14-foot Tall Model Cast from Ultra-durable PUR
To Withstand Examination by 3 Million Visitors Annually
 

The celebrated new Visitors Center that is now the main entrance to the U.S. Capitol building is being hailed as a destination site of its own in addition to the primary role as a waiting area and information center for tours. One of the key exhibits drawing the attention of visitors is a high-accuracy scale model of the Capitol Dome. The 14-foot tall polyurethane model features exterior and interior architectural details for which the landmark structure is recognized throughout the world. The exterior section is fully accessible to the public to view, photograph and touch. The lighted interior model of The Rotunda is displayed behind glass to protect the exacting miniature replicas of elaborately carved dome complete with the canopy fresco, printed reproductions of historical art, and fully accurate renditions of the busts and statues displayed in the room.

The unique architectural-quality structure is the product of a three-year project undertaken by Midwest Model Makers, Inc., Indianapolis, Indiana. The company began with an intensive research effort to locate available architectural information and document the myriad of other details needed to develop accurate electronic designs of each section of the iconic building.

“We estimate that the research and computer aided design processes alone represented nearly 1,000 hours of work,” reports Ed Watson, president of Midwest Model Makers. While the CAD portion of the project was being completed, Watson and his staff began working with Innovative Polymers, Inc., Saint Johns, Michigan, to select a polyurethane system that could perform to the exacting specifications for the project.

Among the requirements for the polyurethane were outstanding reproduction of detail, consistent and solid matched color throughout the cast material, and good flexibility to minimize the potential for tourists to carve or break off sections of the model. Ultimately, Midwest Model Makers chose to use Innovative Polymers’ IE-3075 polyurethane. The translucent material has a low 300 cPs viscosity, a convenient 1:1 mix ratio by volume and gel time of 8 minutes at room temperature. Once cured, the durable polyurethane has a Shore hardness of 80D, flexural modulus of 420,000 psi and tensile strength of 10,500 psi. In addition, because Innovative Polymers specializes in custom formulating and color matching to specific requirements, the product was formulated to the desired off-white tone to satisfy the need to maintain a solid color even if a portion of the model was scratched with a key or cut with a knife.

Building the Model

The first step in preparing to build the model was manipulating the CAD data to produce sections of the building small enough to create via stereolithography in the limited size of the 3D Systems equipment vat. The model was built in 18 segments, including symmetrical interior and exterior sections. Each of these sections was replicated every 12 degrees to form semi-circular exterior and interior sections of the dome.

Chris Young, stereolithography manager at Metro Rapid Prototyping, Noblesville, Indiana, that produced the model sections, explains, “We used five SLA machines to build the patterns and then hand-cast silicone rubber molds. The pattern- and moldmaking process took five months of nearly continuous work. Small parts were typically formed in less than one day while more intricate pieces took as long as a week to build the patterns and cast the molds.”

After more than 20 silicone tools were completed from the SL masters, several hundred IE-3075 polyurethane parts were cast for final assembly. The colored polyurethane model pieces required neither painting nor protective coating after they cured.

Completed model sections were cut and fit to a substructure of steel and plywood, using various steel hardware components for attachment. The seams between cast pieces were filled by hand using IE-3075 polyurethane as the adhesive.

“We were honored to have been part of this impressive and important legacy project that Midwest Model Makers completed using our artistic skills and modeling expertise combined with the best of today’s advanced materials and technologies,” Watson says.

 

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