Figure 15
After the plaster shell is completely cured, it is time to demold. Using a putty knife or paint scraper, slip the tool under one edge of the mold. Be careful as the edges are thinner and could easily break. So before you add any upward pressure insert the scraper about two thirds of the way in -but taking care you do not push it into the model. Then slowly move it all around the outside diameter until the edge is mold is freed from its baseboard.
Figure 16
Once freed, remove the mold from the model to inspect it. Make certain you pull up the plastic wrap with the mold to prevent any clay to remain on the model. That will have to be thoroughly cleaned if it does.
Figure 17
This is where the legs you installed come in handy. Sit the mold on its legs and using the putty knife carefully dig out all of the clay. All the clay must be removed from the plaster sides. To remove the clay in any small crevices use an old toothbrush and dishwashing liquid. Just make certain the clay has been cleaned out thoroughly.
Figure 18
Even though all the clay has been removed, there will remain clay stains on the sides and bottom of the mold. That is acceptable. Now it is time to add mold release to the plaster mold. Petroleum jelly works well for this purpose. Apply will a rag or an inexpensive chip brush. Apply at least twice to assure complete coverage.
Figure 19
Next it is time to place the shell back over the model. Line up the registration line(s) on the shell to the registration line(s) on the baseboard. Then using four short screws (short in that they will not go out the backside of the board when screwed in) screw all four into the holes you previously drilled. This will hold the shell mold securely in place. Inspect the pour hole and the sprue holes to make certain they are open and free of any debris. Clear them if they are clogged.
Figure 20
Now you are ready to pour mold rubber In this example we will use MoldRite 25 silicone. This has a 10-to-1 mix ratio. Using a gram scale weigh out the amount needed of each component. Thoroughly mix and if you are able using a vacuum chamber to de-gas. Slow pour the de-gassed rubber into the sprue or pour hole until the rubber begins to fill the pour tube. Let the mold stand undisturbed to allow it to cure. With MoldRite 25 this will take three hours. Feel the rubber in the pour hole to make certain it is not tacky. If that is the case you cane unscrew the shell from the baseboard and carefully lifte it off the silicone rubber mold you created. You may need the putty knife to loosen the rubber mold in order to free it from the model. When you do, carefully reinsert the rubber mold into the plaster shell and stand it on its legs.
Figure 21
Once you have a completed the curing of the silicone mold you may use it to cast a number of materials including polyurethane resin as well as art stone such as CastRite. Whichever material you choose you can be assure you have created a detailed mold with the least amount of mold rubber in this technique.
