The Visitors Center of the U.S. Capitol building houses a 14-foot tall polyurethane model of the Dome that features the recognizable exterior and interior architectural details. The public can view, photograph, and touch the exterior section. 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 accurate renditions of the busts and statues displayed in the room.
The research and CAD process took more than 1,000 hours, and was handled by Midwest Model Makers, Inc., Indianapolis. The material used was IE-3075 polyurethane because it met the exacting specifications for the project.
The polyurethane had to reproduce detail and match color throughout the cast material. Good flexibility would reduce damage from tourists trying to carve or break off sections of the model. 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, it has a Shore hardness of 80D, flexural modulus of 420,000 psi and tensile strength of 10,500 psi. The material 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.
Five SLA machines were used to build the patterns and 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.”
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