Solar Reflector Dish Mold: Shaping the Plug

April 2004

Continuing with dish plug construction from last month, it's time to assemble our giant "cookie cutter" and begin shaping the plug, or dish mold.

The basic concept is to swing our plywood template in a 360 degree arc, shaping the sand, and finally the concrete lay-ups. To do this, we have to attach a vertical shaft to the plywood template, chuck the shaft into a bearing arbor, and firmly attach the arbor to a solid suspended cross beam (Figure 39a).

Next, mound up enough sand under the beam and swing the template in a complete circle to shape the sand into the base layer.

Move the arbor vertically about 1.5 inch to 2 inches, wet the sand, and continue building up the plug with mortar mix (concrete and sand).

Finish the plug by rotating the template and screeding a final layer of four parts concrete to two parts fine sand.

Remove the arbor and template assembly, then allow the plug to set up (about ten days). 

The plug will have a bit of roughness to it and some ridges left behind from the screeding process. In order to get a perfectly smooth surface, we need to use plaster or joint compound to fill in all of the imperfections. Plaster is easy to sand and produces an excellent surface if enough effort is put into the finishing process.

Once the plaster is finish sanded, blow off the dust and paint it with a dark latex color. Black is the best color for revealing imperfections in the finish, which can be corrected once the paint sets.

Figure 39h

Next month we will continue with the FRP lay-up process...

Ken Rieli