Mold Making & Casting for the Viaduct

Above are a couple of views of the stone viaduct castings being dry fit into place.  These will be fit together tightly and epoxied to each other, and to the plywood track base at the top.  The thin Masonite board at the bottom is temporary.

Silicone molds were made to produce the plaster castings.  I started with designing and having a mold master made for an arch face and a liner half, shown here.  These could be hand carved from a block of plaster, clay, or other material.  In this case, they were modeled on CAD, and "grown" on a rapid prototyping machine.  There are many companies that offer this service as it becomes more available and less expensive to do.

After these masters were grown, I hand carved the rock detail with a Dremel tool.

Next, I built wood and clay formers to hold the silicone, placed the masters face up, and mixed and poured the silicone into each box.

 

Once cured, I broke away the formers, and what's left are the silicone molds.  One is purple because of using a different batch of  hardener - just happened to be what they had at Hobby Lobby.

 Hydrocal, a very hard casting plaster that picks up fine detail, is then mixed and poured into the molds, and after the plaster cures, about 90 minutes, the silicone easily peels away, leaving an identical copy of the master.  The molds are very resilient, and can be used again and again.  I mold a rock or two with any leftover Hydrocal.