I'm curious to know more about what RTV you use, its brand name, and availability. I'm using Dow Corning 3110 RTV. It is good quality, easy to use, requires no vacuuming, and has a long life. But it does cost a lot of money, about two hundred dollars for a nine pound kit. Am I paying too much? Maybe I should try something else.

The price is outrageous and even my local Dow Corning representative admits their twenty-two dollar per pound Silastic 3110 is a relative gutless wonder. Fortunately Dow Corning makes three others much better for molds. As for their cost and availability, contact K. R. Anderson Company, the Dow representative nearest to you in Bellevue, Washington. Ask them about their...

Silastic HS-2: Very soft (Shore A hardness of only 19), has a remarkable tear strength of 140 pounds per inch, nearly ten times what you are now using.

Silastic E: Soft (hardness 33), with seven times the tear strength (100 pounds per inch).
Silastic T: Soft (hardness 35), eight times the tear strength (125 pounds per inch).
All three come in one gallon cans.

The DC 3110 you now use is firmer (hardness of 45) but has very poor tear strength (only 15 pounds per inch). The representative told me Dow Corning's high strength Silastic HS-2 is best for your molds with their very fine detail and undercutting.

As for cost, I suppose I should be thankful for the price of the silicone mold material I use for my epoxy castings. At (only?) $7.22 per pound, it must be a bargain. As an alternate source I suggest you call Hastings Plastics in Santa Monica, California (310)829-3449. They give good advice and will ship any quantity anywhere.

Hastings takes somebody else's silicone resin and compounds it to their own specifications. I use their high strength silicone rubber, RTV 500. It is soft (hardness 26) and has a satisfactory, medium tear strength (45 pounds per inch). They also make a firmer, cheese-like version, RTV 100. It has properties nearly identical to the Silastic 3110 you mention but it still costs the same as their high-strength material. The RTV 500 I use costs $8.50 per pound in ten pound (one gallon) cans or $7.22 per pound in fifty pound cans.

Mr. Crow continues:

You talk about making your molds 1/3- to 1/2-inch thick. That still requires forming boxes. I hardly ever use boxes in my production casting; that thickness is more than enough to hold the shape. Perhaps your need for boxes is due to your mold material and the scale (or is that gauge?) you work in. I build in HO scale but some of my molds are large (6- x 12-inches) and need no reinforcement.

I make all of my rubber molds just thick enough to hold their shape and as thin as possible to save money. As you, I find it unnecessary to support the sides of the shallow molds I use for casting flat or curved slabs. A box must support rubber molds for large, complex, three-dimensional castings. Figure 2 shows a five-sided box holding the rubber shell for a cement casting of a 50 scale foot double track viaduct. The RTV (Room Temperature Vulcanizing) polyurethane rubber in that mold averages 3/8-inch thick and weighs 50 pounds. Silicone would weigh even more. A costly silicone RTV mold would be a dream to use but only justifiable for commercial production.

When I pour between 80 and 120 pounds of wet cement into a rubber shell, I need the box. Figure 3 shows the stiffening ribs I designed into the sides of the shell. They keep the shell sides from flopping into the box before I pour the cement.

For an arch of such size, I use a hoist and sling to grab the three day old semi-cured casting. The sling holds it in such a way as to keep it from breaking at the thinnest part of the arch. (It saves my back at the same time). By pulling out the semi-cured portland cement casting and setting it aside to cure fully, I am able to fill the mold every three or four days. Three separate castings-pier, arch, and tower-assemble into a European-type, double-track, stone viaduct 88 scale feet tall with a 50 scale foot span. Ultimately I will install catenary masts on the tower sections.