Sir Skofis's Workshop: How to make a two part silicone rubber mold

Note: To read about Oyumary molds or Resin Casting look at this post:

http://sirskofisworkshop.blogspot.com.es/2013/12/how-to-make-molds-with-oyumaru-resin.html

First of all, you can probably find lots of tutorials like this on the Internet that are better explained as I'm not a native English speaker. Nonetheless, I pick up a few tricks with my experience that can be useful.

Another clarification before I started describing the process is that I try to use the least silicone that I can, as it is a quite expensive product and my economy it's what it is.

In this tutorial I gonna show how I obtained the mold shown in the picture below.

Materials needed:

Plasticine
Vaseline
Silicone+Cured Agent (I use a Spanish brand as I live in Spain Silastic 3483 de Ferroca)
Silicone rubber (already cured) for old molds or molds that weren't good enough (the cubes inside the yogurt container and the block under the brushes in the picture below)
A model or more that you want to copy

Tools needed:

Sculpting tools and a hobby knife to work the plasticine and left a good surface
An stick to mix the silicone and the cured agent
A couple of old brushes or really cheap ones
An scale to measure the weight with precision
A syringe to measure the cured agent
A plastic cup/a yogurt container/or another container to mix the silicone and the cured agent
Lego bricks or something similar
A tray or a base

Step 1: Mold size and plasticine side

First we place the parts of the model to copy in a way that uses the smaller space possible. Then we build a Lego wall around it.

If you are copying several models it is a good idea to make molds for the pieces of roughly the same size together (a mold for the bodies, a mold for the arms, etc.). Also bigger molds tend to give poorer results and you should probably limit your self to molds around 12 by 12 cm (5 by 5 inches).

Inside the Lego wall we are gonna put plasticine balls, bigger ones first and smaller ones latter. This will help separating the plasticine side from the first silicone rubber side and also leave a few bubbles of air trapped in the plasticine that will allow us to push the miniature inside it.

Once we have made a couple layers with plasticine balls, we take some more plasticine, flatten it with our fingers and place it on top of the balls. to have a roughly flat surface.

To joint the flat patches of plasticine together you can use a sculpting tool. It is important not to leave any hole or crack between the plasticine and the Lego walls or the silicone will leak through them.

Now is time to place the parts of the model in the plasticine. Before doing it you should thing how to do it. Silicone rubber is quite flexible but is better not to push its limits. Sometimes you will find that you have to add more plasticine to cover some small cracks (look at the horns).

Or also add plasticine so there is not an open space between the model and the plasticine surface; for instance, look at the left leg of the model, if we leave it like that the silicone rubber will end up all around it.

Once all the parts are well placed in the plasticine you should make a few conic or sphere marks on the plasticine, the silicone will enter in them and they will help to ensure that both sides of the mold are well placed when casting. You can make them with the rear part of a brush.

Now we are almost ready to cast the first part/side of the mold.

Step 2: Preparation for casting the first part of the mold

The preparations begin by applying Vaseline to the plasticine and only to the plasticine. If you apply it to the model you will loose details in the copies. You can also apply some Vaseline to the inside of the Lego wall.

If this is the first mold that you make or you are great mold casters and you haven't made any bad mold the preparation is finished. Also if you don't have any problems always using fresh silicone the preparation is finished. In the case you want to safe money by reusing cured silicone rubber from old molds keep reading.

Trick: The silicone has the property to stick/merge with silicone, even if that silicone is already cured. To reuse the cured silicone I made little cubes or stripes using scissors. I normally put the cubes inside the mix. The strips in the other hand I place on top of a layer of silicone and pour more silicone on top of them.

In this example I gonna make some strips. Enough of them to cover most of the space for the first mold part.

Step 3: Calculating the silicone needed for the first part of the mold (some maths)

I don't like making precise calculations to obtain the quantity of silicone needed. What I do is to estimate the quantity. To that purpose I count the number of "Lego points" inside the walls, in the case of this tutorial 6x6=36 points. Then I multiply that quantity by a roughly estimate of the number of Lego brick height that I have to cover and like that I obtain a volume, in my case something like 1.5 was needed, so the volume was 54. Experimentally I found that this value multiply by 0.7 gives you the grammes needed of silicone so I needed roughly 37.8 grammes of silicone for this part of the mold. If you are not certain you can multiply by 0.75 in your first molds. If you don't use grammes you can make the conversion of that value (0.7 grammes*X units of mass in a gramme).

As I planned to use the silicone rubber strips I weighted them obtaining a mass of 25 grammes so I needed around 13 grammes of "fresh" silicone".

The yogurt container in the picture above had a mass of 5 grammes but I set the scale to zero with the empty container on top of it.

Once we have the "fresh" silicone we have to add the cured agent. In the silicone are the specifications of how many cured agent you will need. If you add more the silicone will cure faster, if you add less the silicone will cure slower.

The silicone I use indicates that you have to add the cured agent in a proportion of 5 to 100, meaning that to cure 100 grammes of silicone we will need 5 grammes of cured agent. As it is quite difficult to measure with a normal scale something that is less than a gramme I make the hypothesis that my cured agent has a density similar to the water, so 1 gramme of cured agent is the same as 1 ml of cured agent and I measure the quantity of cured agent with a syringe.

Having in mind that I used 13 grammes of silicone I need 13/100*5=0,65 ml of cured agent. You can round the number to 0.6 ml or 0.7 ml so it is easier to measure, the silicone will cure in both cases.

I choose to use 0.6 ml to the "fresh" silicone. Then I mix thoroughly during 3 to 4 minutes. With my silicone I have an hour of work time so there is no rush and its better to spend sometime in order to ensure that is well mixed.

Step 4: Pouring the silicone for the first part of the mold

Trick: If you want to capture all of the details of your model instead of pouring directly the silicone is better to "paint" the part of the model with silicone using a brush. Like that you ensure that the silicone in contact with the model doesn't have any air bubbles, as the layer of silicone is thin and the bubbles could get to the surface quite easily.

It is also a good idea to fill the holes with silicone and ensure that there is no air trapped with a metal pin.

Now we started pouring the silicone from one of the corners of the Lego wall from a certain height so the least possible air is trapped, at least in theory. If you move from that corner it isn't the end of the world, so don't worry too much.
Trick: don't pour all the silicone at the same time. Pour just a part of it so if there are some air bubbles they have a shorter distance to the surface.

We let this first layer of silicone settle down for around 5 minutes and then we place on top of it the silicone strips that we had already prepared.

Once all the strips are placed we pour the rest of the silicone or another part of it.

Don't worry too much if the are some parts that seem that aren't cover, wait a few minutes, the silicone will probably flow to that region and cover it, I repeat, there is plenty of time to work with the silicone.

Trick: Is it probably a good idea to shake a little the mold to help the trapped air bubble in their way to the surface.

With my silicone and cured agent now I have to wait 24 hours so the cure process finish.

Note: I have a problem with the expiration date of the silicone and have to repeat the steps above

This is how it should look once cured, totally solid and not sticky.

Step 5: Separating the Plasticine from the first part of the mold

During this process try to avoid separating the model parts from the silicone. It has happen to me a couple of times and push hard I manage to settle the parts in its place but is better if the pieces stays with the first part of the mold.

You will see that there are a few places where there is still little bits of plasticine. The cause is that in those places the Vaseline didn't reach well. You have to make sure to remove all those bits. To that purpose you can use a hobby knife and/or some sculpting tools.

Also there is the possibility that the silicone has infiltrated around some part of the model in some cases even making a ring around it. If that happens with a very sharp hobby knife cut the silicone.

Step 6: Repeat steps 2 to 4

We apply Vaseline this time to the first part of the mold. Remember, as I told you silicone merge quite well with other silicone so if you don't apply Vaseline you will make a silicone tomb for your model.

Sometimes you will have to add an extra Lego row to your wall to ensure you can pour enough silicone to cover all the model.

I make the mistake of not multiplying the volume obtained by 0.7 (mistakes happen) and has to make a mold in extremis without having prepared it. This can happen, and I have an hour to make a new mold. I have a barrel that I wanted to copy and decided to make a sock mold. This is a one part mold that i personally don't like at all but now time was an issue.

I used a couple of pieces of plastic sprue to hold the barrel and a "resin through" with plasticine (remember to apply Vaseline to the plasticine). Then I build a Lego wall around it and pour the silicone.

Step 7: Separating both parts of the mold

Once the silicone is cured you will have to separate both parts. If you have remember to apply Vaseline it will be possible, if not you will have to cut your way to the model.

In the process of separating the two parts you will probably see that there are certain points where both parts are joined. Dont worry, pull both parts one from the other carefully but with a firm hand.

If you have made a "sock" mold you will have to cut your way to your model. Start by removing the plasticine, and if the piece that hold the model is free removed also. Then start cutting.