This article is not intended to be a comprehensive guide, but rather a primer that will cover most of the unique molding and casting situations that model rocketeers will encounter. Although the RTV rubbers and resins are neither corrosive nor highly toxic, it is advisable to use them only in a well-ventilated area away from open flame or sparks. Before using the materials, read and understand the Material Safety Data Sheets and the manufacturer's safety instructions that are included with the RTV rubbers and resins.
To make a nose cone master, for example, a hardwood nose cone can be turned on a lathe or a drill press, then sanded and sealed. The nose should be sprayed with a wax-based release agent (Synair's Synlube 531, for example) and allowed to dry. The dowel at the nose's rear can be pressed into a smooth bed of sulfur-free "plasticene" modeling clay (other clays can inhibit curing in the RTV rubber). The dowel is pressed into the clay until the back of the tenon is flush with the clay surface. A superior master can be made by turning polycarbonate, lucite, lexan, nylon, or aluminum rod stock on a lathe. Masters made of these materials require neither sanding & sealing nor a release agent. A transition or a boat-tail would be produced in a similar fashion. Consult Peter Alway's "The Art of Scale Model Rocketry" (Saturn Press, 1994) for more information on turning parts.
If you have access to a vacuum chamber, it is best to de-air the rubber before pouring it into the mold. De-airing prevents air bubbles from forming in the mold. If a bubble is on the surface of the master, every casting will have a raised "bump" at that spot. If you de-air the rubber, it should be in a container that is three to five times as high as the level of the rubber. The rubber will foam up to this height, then settle back down as the air bubbles escape.
If you don't have a vacuum chamber, pour the rubber slowly in a thin stream into the mold box. Pour it as far from the master as possible and let it rise up over the master of its own accord. This is advisable even if the rubber has been vacuum de-aired. The low durometer rubbers usually have low pouring viscosities, and some of them (Ace Resin RTV, Alumilite "Quick-Set" RTV, and Dow Corning HS-III & 3110) do not require vacuum de-airing. Some RTV rubbers, such as Alumilite "Quick-Set," will cure in as little as four hours, but for a long mold life it is best to let the rubber cure for at least twenty-four hours before attempting to use the mold. After the rubber has cured, carefully remove the clay from the master and then carefully remove the master from the mold.
The resin is very easy to measure and mix. Most polyurethane resins (Ace Resin, Alumilite, Por-A-Kast, etc.) are mixed 1:1 by volume. Equal volumes of the two parts are poured into separate containers. The "A" (or thinner) side can be poured into the "B" (or thicker) side and mixed, or both sides can be poured into a third container and mixed. Stir vigorously for 10-15 seconds or until a uniform color is achieved. Slowly pour the resin into the mold. The resin should cure within 2-3 minutes. The curing resin will get quite hot (up to 215 degrees F for Alumilite), so let the casting cool to room temperature before attempting to remove it from the mold. The casting is still pliable while it is warm, so it could be distorted if it is removed from the mold before it cools to room temperature.
Two masters are required for the nose cone (with tenon) and for the nose cone base--an outer shape and an inner "plug" for each piece. In the case of the nose cone, the outer shape would be turned on a lathe or a drill press. The plug would be turned in the same way, and it would have the same shape as the outer shape. The plug would not have a tenon, but rather a cylindrical section on the back that would have the same length as the outer shape's tenon. The plug's radius should be at least 3/32" less than the radius of the outer shape, and the plug's length would be proportionally less than that of the outer shape. The alignment pins (either turned with the piece or a dowel cemented into the blank before turning) on the plug and the outer shape will serve to center the two pieces in the next step.
Take the outer shape and press its alignment pin into a smooth bed of plasticene modeling clay. Press it in until the back of the tenon is flush with the clay's surface. Use a dowel or other object to make several depressions in the clay around the master, and build a mold box around these. The rubber will flow into these holes and create keys which will align the two mold halves. Mix and pour an RTV rubber as described above and set it aside to cure. After the rubber has cured, carefully remove the clay from the master and the mold, but leave the master in the mold. Next, either brush a rubber-to-rubber mold release ONLY on the exposed upper mold surface, or spray a wax-based mold release on the upper mold surface and the exposed part of the master. After it has dried, build a dam around the edges of the mold (sheet styrene taped around the edges is sufficient). Mix another batch of RTV and pour it onto the mold surface, then set it aside to cure. After the rubber has cured, separate the two halves of the mold. Remove the outer shape from the upper mold half.
Before casting a nose cone, insert the plug's alignment pin into the upper mold half and push it all the way in. Spray the plug with a wax-based release agent and allow it to dry. Mix the resin and pour it into the mold cavity, then slowly press the upper mold half into place. Allow the casting to cool to room temperature before removing it from the mold.
The plug must be sprayed with the release agent before casting each nose cone. If desired, a second all-RTV upper mold half can be cast. To do this, leave the hollow cast resin nose cone in the mold cavity. Spray the exposed upper mold surface with a wax-based release agent and let it dry. Build a dam around the mold edges as before. Mix a batch of RTV rubber and pour it onto the upper mold surface. Pour it away from the hollow nose cone and let the rubber slowly run down into the nose cone. After covering the upper mold surface (1/4" to 1/2" deep should be enough) with the rubber, set it aside to cure. The new upper mold half will have an integral plug. The nose cone base mold would be produced in the same way, and transition and boat-tail molds would use similar techniques.
Because this casting method requires that the mold be held and rotated, it must be protected from being squeezed during casting. There are two ways to do this. The mold can be cast into a rigid cylinder (a length of plastic pipe, for example) or into a rigid box. Another method is to cast the mold in a temporary mold box, then brush on a thixotropic (a thixotrope is a gel that will not run when applied to a surface) mold support shell resin. Synair makes a mold support shell resin called Mother Mold, and other formulations are available from other casting suppliers. If you go this route, leave the master in the mold while brushing on the mold shell support resin. The mold itself will be a two-piece mold, and the piece covering the truncated cone will be the smaller of the two. Brush the mold shell support resin onto the entire mold except the top of the smaller piece. The cured mold support shell will be a rigid cylinder or box, open at the top so that the upper mold half can be removed.
Mix a batch of RTV rubber and pour it in a thin stream into the mold box. Pour it away from the master and let the rubber slowly rise up over the master. Set aside to cure. After the rubber has cured, carefully remove the clay from the master and the mold, but leave the master in the mold cavity. Brush a rubber-to-rubber mold release ONLY on the exposed upper mold surface, or spray a wax-based release agent on the upper mold surface and the exposed part of the master.
Build a dam around the upper edges of the mold (if you're not casting it in a rigid cylinder or a rigid box). Mix a batch of RTV rubber and pour it over the mold surface. Pour enough rubber that it rises almost to the top of the alignment pin on the truncated cone, then set it aside to cure. After the rubber has cured, separate the mold halves and carefully remove the master from the mold. The opening in the upper mold half left by the alignment pin is the channel into which the casting resin will be poured.
To cast a nose cone, assemble the mold halves and insert the mold into its cylinder, box, or shell. Cut a piece of pressure-sensitive adhesive paper (or a pressure-sensitive mailing label) large enough to cover the opening in the upper mold half. Cut or tear off a piece of masking tape. The tape will be used to keep the upper mold half from falling out of the box or shell during casting. Insert a funnel into the opening in the upper mold half. Mix the casting resin and pour it into the funnel, then quickly remove the funnel and press the pressure-sensitive adhesive paper over the opening. Using the masking tape, tape the upper mold half into place. Gently rotate the mold in all directions so that the resin coats the entire surface of the mold cavity. Rotate the mold until you can no longer hear the resin flowing inside, then set it aside to cure. The casting will be rather thin, so make sure it has cooled to room temperature before attempting to remove it from the mold. If desired, leave the casting in the mold and repeat the casting process to build up the casting's thickness. Hollow transitions and boat-tails can be cast in the same way.