Constructing the Fiberglass Solar Reflector Dish Mold

May 2004

Fiberglass Reinforced Plastic or FRP is used in many industrial processes and products due to its ease of use, and superior strength -- when used properly.

Although FRP is a general term for the process, the components used vary greatly. There are many different fabrics used -- from fiberglass to Kevlar -- and a number of liquid resins, including polyester, vinyl ester, urethane and epoxy.

Polyester is a low-cost resin typically used in "bondo" to repair vehicles. Most larger products such as car bodies and satellite dishes are made using this plastic. While the cost is low, there are undesirable characteristics, such as shrinkage, that limit its use to medium precision work at best.

Epoxy resin, on the other hand, is much stronger and exhibits much lower shrink rates, but costs quite a bit more.

Since our dish will require between five and ten gallons of resin, it is best to buy it in bulk (5-10 gallon quantities), along with sufficient hardener.

In laying up our dish, we used a high strength K-glass, 10 ounce (lightweight) fabric and epoxy resin. The fiberglass cloth was cut into overlapping pie-shaped pieces, with enough material left on the outer edge to form the 3-inch perimeter stiffener ring. The pie-shaped fabric pieces must overlap each other on the radial edges by at least 1 inch -- preferably 2-3 inches.

Begin by waxing down the entire plug with several coats of parting wax, or simply a good quality floor wax. Another option is to use a parting compound -- typically sold at fiberglass supply houses.

After the wax is completely dry and buffed, "paint" the entire surface of the plug with a slow-cure gel coat or bonding resin.

Next, lay down one layer of the pie-shaped fiberglass wedges all the way around the plug -- taking care to overlap the edges. This should be done while the gel coat is still wet on the plug.

Continue by mixing, pouring, and screeding bonding or lay-up resin into the first fiberglass layer until it is completely saturated.

Follow with the next layer of fiberglass wedges (overlap edges), and saturate with more resin. The final lay-up should be about 3/16 inch thick, or 7-10 layers of 10 ounce fiberglass cloth.

Depending on the ambient temperature, the lay-up should require a couple of days to fully cure (at 72 degrees f). We let ours cure for about a week, after which we popped the dish from the mold using wooden wedge shims driven around the perimeter. It took about 8 or 10 wedges, equally spaced around the perimeter and successively driven, to pop the dish.

Next month we'll look at how to hang the dish on a steel mount.

Ken Rieli