Using RTV Molds For Creating Scratch-Built Models

by Tony Segro

RTV (Room Temperature Vulcanized) Rubber molds are simple to make and can allow you to cast several identical detail castings for your scratchbuilt models. RTV Rubber is a two-part mixture made by Dow-Corning. It comes in various styles. I find 3110 the easiest to use because it does not require an expensive vacuum pump to decompress air bubbles.

Take the detail you wish to copy, called a MASTER, and glue it into a cardboard or styrene box with dimensions about one inch longer than the master by one inch wider than the master. When placing the master in the box, you will have 1/2″ on all sides. The height of the box should be a minimum 1/4″ higher than the master (the master should be 1/4″ below the top of the box.

To find out exactly how much RTV it will take to make the mold (I don’t like to waste any RTV because it is a bit expensive), I fill the box to the top with salt, or some other granulated substance. I then pour the salt into a clear plastic cup, and level it off. With a black marker, I mark a line around the cup at the height of the salt. I then return the salt to its container.

In the cup, I slowly pour the liquid rubber until it’s just about to the line. I then take some of the catalyst (it’s in a tube with the liquid rubber mixture) on a popcicle stick. Read the directions on the tube for how much to use. It should be a 10:1 ratio of rubber to catalyst.

Slowly stir the catalyst into the rubber until the two are blended well (about 2 minutes). Air bubbles will surface as you stir. Trickle the rubber mixture into the box slowly, trying to fill the corners first. Then, slowly fill the box with the rubber. The master should be totally covered by the rubber. THE SLOWER YOU POUR, THE FEWER AIR BUBBLES THERE WILL BE.

After pouring, the box should be just about full. Gently tap the box on a flat surface for 3-5 minutes to get rid of air bubbles. Another way to eliminate the bubbles would be to use the warm air from a blow dryer. Any trapped air bubbles will ruin your mold as they will destroy the details.

Once the air bubbles stop surfacing, let the box sit on a flat surface for 24 hours. To check to see if it hardened after 24 hours, take a toothpick and gently rub it over the smooth, rubber surface. If it’s still liquidy, let it sit another several hours, checking it from time to time.

Once the rubber is hardened, cut down the sides of the box and remove it slowly and carefully from the master detail, and powder it with baby powder.

The mold, if handled with care (cleaned with dish detergent, blow dried and powdered between each use), will last for an extremely long time without losing detail. You can use Alumilite (2 part liquid plastic) or a type of plaster or polyurethane to cast your copies.

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