Mold making tutorial  
Step by step making of a multi part silicone rubber mold. An illustrated tutorial.

 

Preparation
	I have made this small sculpture which I want to cast in an edition of 12 copies in bronze. I must make a mold that will allow me to make many wax copies. This is a fairly complex shape and my plan is to make two half thin skins of  silicone encased in a multi parts palster shell. I start by welding the feet on two lumps of wax and welding them solidly on a slab of wax.
The mold will be made of a thin and flexible silicone skin which will be encased in a rigid plaster shell, the "mother mold" Because of the complex shape of the sculpture, we will make the silicone skin in two halves. Likewise the mother molds must be made of several interlocking parts.
	I roll some clay on a wet board, to a thickness of about  a thickness of about 1/3 inch (1cm)
I cover the back of the sculpture with a layer of clay overlapping the "parting line" by a 1/3 inch (1cm). I also form a clay retaining wall under the sculpture drip line. The glass jar serves as a support. I except that later on in the process, the combined weight of clay, silicone and plaster may bend or even break the thin wax arm of my sculpture.
	I also press small round depressions in the surface of the clay. These will serve as keys and help the two skins of silicone to join together properly.
This is another view of the clay defining the area to be covered of silicone.
Making the silicone skin
Follow the manufacturer mixing ratio instruction. Here we only mix 1/4 oz (100 grams)	We can now mix some silicone to paint over this first half of the sculpture. We choose a pourable RTV (Room Temperature Vulcanisation) condensation cured. We could also have used an addition cured silicone or a polyurethane rubber, but a RTV condensation cured  silicone (sometimes called Tin based ) offers a good compromise between ease of use, ease of release, durability and price.
I paint a thin layer of silicone all over the sculpture and the face of the clay wall. This thin layer will slowly run leaving only a very thin film in place. This will allow all air bubbles to escape. Because Tin based silicone does not stick to anything, there is no need to apply any "release agent" on the surface of the sculpture.
	After this first layer, we must build up thickness. I mix a second batch and make it thicker by adding a thixotropic additive. This additive will make the silicone creamier and less runny. It becomes "brushable" and will not run and fall off undercuts as easily. 3 layers have been necessary to build up a thickness of about 1/3 inch (1cm).
Once cured, (about 4 hours) I trim the edges with a sharp knife.
	The next step would normally be the removal of the clay and application of the second half of the silicone skin at the back of the sculpture. In this case, because of the shape of the piece, I want to start by building at least one part of the plaster mother mold. This will serve as support for the rest of the process.. I start by delimiting the area with a thick clay wall.
Each part of the plaster mother mold must be constructed so that once it will easily be removed from the silicone mold. We must find the best "parting line" by looking at it and make sure that all lines are visible.  We also make a number of small depressions into the clay. These will later  act as "keys" and help each part of the plaster mother mold to lock themselves in place.
	The clay walls are ready and are forming a box in which we can now pour liquid plaster.
A batch of plaster is missed by sprinkling plaster in a bucket of water until a small island forms in the middle. Let the plaster soak for 5 minutes
	It can then be painted or poured inside the clay cavity.
Once the whole cavity is filled, add a small blob of plaster somewhere in the middle. This protruding blob will later be used as an anchor for strings or rubber bands to hold the mold together.
	Before the plaster sets, clean the edges with a wet sponge. When cured (3 hours), remove the clay walls.
The rest of the clay can be removed. The sculpture is carefully cleaned form any clay marks.
	We can now paint the second half of the silicone skin. We start by delimiting the area with some clay.
We start by painting a first thin layer as we did for the firts half. Let it cure until touch dry and apply more silicone to the desired thickness of about 1/3 inch (1cm).
	We let this second half of the skin to fully cure (4 hours) before removing all the clay walls.
Using a sharp knife, jagged edges are trimmed for a clean join. We are now ready to build the rest of the mother mold.
Finishing the mother mold
	To save money we have only build a thin skin of silicone. It is too flexible to support its own weight, let alone the weight of a casting material. It must be encased in a rigid shell, the mother mold. Plaster is the cheapest material to make a mother mold. We could also use resin or more specialised, harder gypsum based plasters. Because of the complex shape of this sculpture, we will build many interlocking parts. Aftre having visualised each part, we construct a clay box around the firts one. Not forgetting to press small holes in the clay walls.
We paint the existing plaster walls with a "release agent" such as petroleum jelly. This will prevent the two plaster parts to bind together.
	Then paint or fill the box with liquid plaster.
Make two or three small protruding blobs of plaster...
	...and clean the edges with a wet sponge. Let the plaster cure for a bout 2 hours before remove the clay walls.
Repeat the same operation for the third part...
	..the fourth, the fifth part...
..and the sixth and last part of our plaster mother mold.
	Once the last part of the mold has cured our mold is ready to be opened to free the original inside. I normally let the plaster cure overnight for maximum strenght.
Opening the finished mold.
	Some of the plaster parts will come off easily...
...some others will need to be gently pried apart by sliding the tip of a kitchen knife in the joint.
	Here are the 6 parts separated and the silicone mold freed. The two openings are visible at the base of the silicone skin. This is where
The silicone skins can be gently pulled off the sculpture to free the wax original.
	All done!  The mold can now be used but we must first reassemble it before it can be filled with liquid wax.
The two silicone skins are put together again. You find them wobbly and not keeping together perfectly. Lay them in place inside the plaster shells and reconstruct the mold piece by pieces.
	Here is our mold fully assembled and ready to be used. I have used rubber bands cut out of old inner tubes to hold it together firmly. The plaster blobs become very handy to secure the rubber bands.
I have filled the mold with molten wax at 212F (100C) and let the wax cool and solidify again. Here is my firts wax replicate coming out of the mold
What can go wrong?
There is really only two mistakes commonly made when constructing such a mold. 1- The silicone skin is too thin. This will make it difficult to join them together again and to lay them properly in place inside the plaster backing shell. make sure you silicone skin are 1/3 inch (1cm).	2- The plaster shell are tightly interlocked and once cast with a solid material, the silicone mold cannot be removed from the plaster shell. It is encased inside the plaster shell. This is due to a poor choice of parting line for each part. Experience will help you to choose proper parting lines but he diagrams below illustrate the principle.
	This is a round wax ball set a on wax slab for molding
We will make a plaster mold encasing the whole ball. The plaster shell must obviously be made in two parts so that it can open to take the wax ball out. This will obviously be two hemispheres.
	We will construct the two half shell so that they separate on the ball's equator.
That way, the two half plaster shells can easily be removed from the wax ball.
	If the parting line between the two half shell does not follow the ball's equator, we have the following problem...
... only one half shell can easily come off. The second half partially encased the ball which remains trapped inside...
	Mold making tutorial   Step by step making of a simple silicone mold. An illustrated tutorial.

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Mold Making Manual An illustrated tutorial for artists, sculptors and hobbyists demonstrating the principles of mold making  using a variety of methods and materials. Coming Soon Contact me with a expression of interest.

 

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