Simple Silicone Rubber Molds (no parting line)

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Rhodia silicone rubbers are used extensively for applications where a flexible mold material is required. A truly high-quality line of silicone systems, these products offer high accuracy, high heat resistance, and excellent tear strength and elongation properties.

And since silicone rubber compounds, by nature, are self-releasing, and therefore make an excellent choice for a variety mold applications.

Preparing a mold

In preparing a silicone rubber mold, we are first putting clay around the edges of our model to make it adhere to the mold board.

Next, we aligned the model in the center of our mold, removed the sides of the mold, and then pressed the model onto the wood.

Next, we cut the excess clay from the edges of the model.

Technically, you do not need a release agent with silicone rubber, but in order for the mold to slide off easier, we decided to apply a light coat of Freeman Wax Release. Here we are using a white-bristle throwaway brush to apply it, and then a cloth to gently buff it.

Note that the wood has already been sealed with the wood and plaster sealer so the surface itself is not porous – otherwise, the silicone rubber will reproduce the surface of the wood, which is not what we want.

For more information, refer to our other video on sealing wood.

Finally, we screw on the sides of the mold

And now we’re ready to pour the silicone rubber.

For this demonstration, we’re showing Rhodia’s V-340 and the CA-55 catalyst – the other option here is the CA-45 catalyst, which would change the hardness.

Here is what the base looks like. The catalyst, which can be shaken inside its bottle, will be mixed using 10 parts base to 1 part catalyst by weight.

Here is what the mixture looks like.

For more information about weighing and mixing, please see our other video on this topic.

Once the base and catalyst are fully mixed, we poured the mixture into a larger bucket for the degassing process. Since the mixture will rise while degassing, you need a larger container to hold the silicone rubber.

Once it is properly weighed and mixed, our V-340 is ready to be degassed. While degassing not necessary for many simpler or less demanding casting applications, if you require an optimal, completely void-free mold, we recommend degassing the material.


When pouring a silicone rubber, you want to pour it at a low point first. Notice that this is thicker than a urethane like Repro.

When you pour the material, you do enter a little more air into the mixture, but these bubbles will break on their own.

This mold will take 16 to 18 hours to cure. An addition Cure rubber can be accelerated only by heating it. A condensation cure rubber can be accelerated chemically.

Notice the larger bubbles that were introduced while pouring are already breaking on their own.


The next day, our mold is ready. We remove the screws and then wedge the whole thing off with very little effort.

Next, we pushed out our model from behind. Notice how accurately the silicone reproduces the surface of our model.

Next, we pushed out the mold itself and pulled off the edges with our fingers.

Our mold is now good for 30-150 uses, depending on the material. Anything that gets really hot will shorten the life of the mold. Here we have a mold that is starting to show its age. You can see the absorption of the urethane into the rubber – thus creating a discoloration. While still perfectly usable, this is an indication that the mold is reaching the end of its life.

As we pour our thoroughly mixed Repro into our mold, we typically like to create a smaller stream in order to break the bubbles better. Regardless, given its thin viscosity, bubbles aren’t much of a problem with Repro and it is never necessary to vacuum degas Repro.

After allowing our part to cure for 60 to 90 minutes, depending on the mass of the pour, we can now demold it and immediately begin using the mold to pour another casting. Even though the Repro is hard enough to demold in 60 to 90 minutes, full hardness and strength are not achieved until 24 hours. The part should not be placed into service until then.

To demonstrate pouring and demolding epoxy mass cast plastics, we’re showing Freeman 801 epoxy casting resin.

When pouring an epoxy mass cast material, it is important to pour in a narrow stream to eliminate bubble.

The aluminum filler used in Freeman 801 will create the separation of color that you will only see at the bottom end of the pour

Pouring and Demolding a Polyurethane Elastomer

When pouring a polyurethane elastomer mass cast material, it is important to begin in the lowest part of the mold and pour in a narrow stream to avoid creating new bubbles in the mixture. The thinner stream also helps when directing the pour into intricate patterns.

Since we are using silicone rubber mold, it is easy to flex the mold to remove the cured part.

Notice that the smaller mold isn’t as shiny because the master model isn’t as shiny – thus showing how well the Rhodia Silicone Rubber transfer the surface features of your original part.

This little model also displays the flex in our Freeman 1060 semi-rigid urethane elastomer, and its superior impact resistance