Mold making tutorial  
Step by step making of a simple silicone mold. An illustrated tutorial.

 

Preparation
	Kimberley has made a wooden heart and wishes to make multiple copies in a variety of materials (Plaster, resin , cement...) She needs to make a reusable mold and decides that a silicone rubber mold is what she needs. She starts by gluing her heart on a flat board with a little bit 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"
	When the mold is finished and turned upside down, the original wooden heart will be removed and a casting materials can be poured inside the empty silicone skin to make an exact copy, faithful to the original in all aspects but the material.
We start by building a clay box around the wooden heart. We will then pour the liquid silicone inside that box to cover the wooden heart. Without a clay box to retain it, the silicone would run and spread itself over the workbench.
	Kim rolls a lump of clay on a wet board....
...flattening it first with her hands...
	...and then using a rolling pin (in this case, an empty wine bottle.) until a thickness of about 1/3 inch (1cm) is achieved.
She then cuts stripes about 2 inches (2.5cm)  wide which is more than twice the thickness of her heart.
	These stripes are used to build a retaining wall around the wooden heart, leaving a space of about 1/2 inch.
See how the clay wall is attached against the board. Make sure that this join is airtight. Silicone is very runny and could escape through tiny gaps.
	Our finished clay box surrounding the wooden heart.
Pouring the silicone skin
We can now mix some silicone to pour over the heart and fill our clay box. 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 (sometimes called tin based ) silicone offers a good compromsie between ease of use, ease of release, durability and price.	Follow the manufacturer mixing ratio instructions. Here we only mix 1/4 oz (100 grams)
	We will first mix a very small batch of 1/4 oz (about 100grams) This is because we will apply a first thin layer of silicone from which any air bubbles will easily escape. Once this first thin layer has cured, we will mix some more silicone and fill the box.
The firts layer is painted on.
	and left to cure...
After about two hours the silicone is not fully cured but almost dry to the touch. We mix some more silicone and simply pour it to fill the clay box. If we wait too long before completing the mold and the first layer has completely cured, it is possible that the second layer will not bind properly to the first one.
	After 8 hours the silicone has fully cured (overnight) and we can remove the clay wall.
We trim any  excess.
Making 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.  We firts build a strong clay wall around the silicone heart, leaving a gap of about 1 inch (3 cm)
A batch of plaster is mixed by sprinkling plaster in a bucket of water until a small island forms at the surface. Let the plaster soak for about 5 minutes.
	While still liquid, the plaster is poured inside our clay walls to cover the silicone skin.
The plaster will set. It will have cured in about 2 hours. Plaster will become very hot when curing and when it cools down again, you know it has cured.
Opening the finished mold.
	Remove the clay walls.
Turn the mold over and remove the silicone skin from its plaster shell.
	Remove the original wooden heart from the silicone mold.
Reassemble the mold by setting the silicone mold inside its plaster mother mold. Done! The mold is ready for use.
Another way to make the mother mold. Because of the simple shape of our heart, The silicone mold  will be easy to remove from the  plaster shell.	However It is generally advisable to make the plaster shell in two or more parts, making it easy to open the shell and release the silicone mold from its shell.
	In our example we will make a two parts mother mold made of two half plaster shells The clay box is constructed to delimit only half of the silicone mold. The parting wall, separating the two half shell, is made in an irregular line so that the two half shell will interlock in place.
In addition, a number of "keys" are pushed in the parting clay wall. These keys will help to reassemble the two half plaster shells in the correct position.
	A batch of paster is mixed and pour in the clay box.
Let it set and cure.
	Once cured, remove the clay walls. Notice our positive "keys" now protruding from the plaster wall.
Construct the clay box for the second half shell.
	Paint a thick layer of "release agent" on the face of the plaster shell that will be against the second half. This "release agent" will prevent the two plaster half shell to bind together. Petroleum jelly. works well.
Mix a batch of plaster and pour it in. Try to make the two parts to the same level.
	When the second part of the shell has set, lift the mold off the board and turn it over..
Open the two half shells. It is sometimes necessary to gently pry them apart with a kitchen knife. If we did not use a release agent, it is very likely that the two plaster shells would have bound together and opening them apart would be difficult without damaging them.
	The two half shells are reassembled and the silicone skin put in place. A rubber band helps to keep the two half shells together. The mold is ready to use.
Here is our mold with the original wooden heart and a cement copy that has been cast from the mold.
Making the silicone mold without a mother mold. The reason why we only made a thin skin of silicone is because of the price of the product. By making only a thin skin we saved  a lot of money. However, this thin skin is veru flexible and must be supported by a rigid plaster shell, the mother mold.	We could have build the mold with a thick layer of silicone. The bulk of the mold would have provided sufficient rigidity and we would not have needed to build a plaster mother mold, thus saving a lot of effort. We would have simply build a high clay wall around the original, leaving a wide gap around it and filled it with silicone. This is a much faster but vastly more expensive method.
	Mold making tutorial   Step by step making of a multi part 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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