Moldmaking With Clear Silicone Rubber

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Moldmaking with clear silicone rubber is the ideal way to construct a mold with a complex parting line. In this demonstration, we will be duplicating this part, which was taken from a bumper assembly. Given the irregularity of the shape, we’ve decided to avoid the difficult and time consuming task of developing a parting line and pouring two individual mold halves. Instead, we will pour the entire mold in one process and then cut our complex parting line after the clear silicone has cured. This method will not only be easier, but it will also save us a full day in making the mold.


Here we’ve constructed a mold box out of plywood. The two-part L-shaped mold frame allows us to clamp the mold together securely, which will make the demolding process easier than if we had screwed the frame together.

We made sure the mold is high and wide enough to guarantee that at least 3/4 of an inch of silicone will completely surround the part.

Next, we place clear tape along our chosen parting line. This tape serves the purpose of providing a thin, uniform vent for reducing air entrapment when pouring the final part material as well as establishing the parting line away from the actual model for ease of cutting the parting line. After the tape is applied, we cut the edges for a neat, uniform application.

Next, we cut small pieces of plastic to cover the threads, and then fill the ends with clay. This will prevent the silicone from locking around the threads when it is poured.

Here we are gluing our downsprue onto the part itself. This will create a cavity in the mold that we will later use to pour our urethane.

Next, we use a marker to darken the edges of the clear tape. This will provide easier visual identification of this edge which will become necessary when we begin to cut our clear mold in half.

After we affix the piece of wood that will enable our model to hang in the mold box, we are ready to drop the model into place. Note that we’re using a thin wire to secure the other end of the model. This wire, which has been coated with vasoline, will also serve as one of our vents.

Here we are weighing and mixing Rhodia V-3040 clear silicone rubber which features a 10 to 1 mix ratio by weight. We’re weighing 2500 grams of base to 250 grams of catalyst.

This material is more viscous than our other popular addition cure rubber, V-340. Also, since this is a clear material, it is difficult to tell the base and catalyst apart, making it especially important to pay attention to how thoroughly it is mixed.

Next, we vacuum degas the material to minimize air entrapment. For more information on this process, please see our other video on that topic.

Finally, we begin to pour our V-3040 into our mold box. We pour directly onto a low point and pour slowly enough so that we do not distort our clear tape parting line or move our model.

Upon completion of the pour, note the bubbles on the top of the mold, which were introduced during the pour. Most of these bubbles will pop on their own and since they are far away from our model, they won’t affect the quality of the mold itself.

Lastly, we lower a couple more pieces of wire into the rubber until they reach the clear tape – this wire will form a couple more vents in our mold, which will be necessary when pouring our urethane parts.


The next day, we unscrew and then lift the piece of wood that held our downsprue.

Next, we are pulling two of our vents out of the mold.

Then we remove the first two clamps and remove the mold box from the mold bottom.

Next, we remove the last two clamps and pry the two halves apart, revealing our clear mold.

Here we are removing the flashing around the mold, which was created when the silicone rubber seeped between the plywood sections.

Notice how we can easily we can see the clear tape parting line because we had marked the ends of the tape with a black marker.

Now it is time to cut our parting line. We mark the end of our gouge with a marker to help us avoid cutting too deeply.

And then, following our parting line, we begin to cut the silicone rubber.

Note that we aren’t trying to cut all the way to the tape with our first pass, but rather we are planning on taking two passes with the gouge. This conservative approach not only makes it easier to cut, but it also establishes a more complex registration that will ensure proper alignment of the two mold halves later.

Our first pass simply establishes the outside of our parting line.

With the second pass, we place a wedge in this established parting line, which makes it easier to access and then cut the remaining rubber all the way to our clear tape.

After the second pass, there are a few remaining small sections of rubber not completely cut through and therefore keeping the two halves together. With a brief, final pass with our gouge, our mold comes apart.

We remove the clear tape from the model and pull then pull the model out of the mold.


Now we are ready to cast our part, which requires us to pour our Freeman 1060 semi-rigid urethane around a metal core. The metal core and the threads make this particular part more involved than most. Therefore, most other clear moldmaking projects will be less complex and easier to complete.

Since we don’t want the threads to be covered in urethane, we secure plastic sleeves around the threads.

Notice the space between the core and the mold where the urethane will flow. In order to make sure the material will fill the entire mold, we drilled a hold through the core. This will allow the material to flow to the bottom side of our core, and not just the top.

Next, we assemble the two mold haves and securing them with electrical tape. We are using electrical tape because it stretches and therefore it won’t deform the mold as much as stronger tape might.

Next, we pour our Freeman 1060 semi-rigid urethane into the downsprue. Note that this material has been vacuum degassed to ensure a virtually void-free casting.

We fill the entire cavity, including the pouring sprue. This excess material is necessary to accommodate for the added pressure while curing and slight material shrinkage in thicker sections.

After filling the mold, we put the mold into the pressure pot to cure under 15 PSI of pressure.


The next day, we are removing the tape, separating the mold halves and then removing the part. Notice how the urethane formed a slight flashing around our parting line, where our clear tape used to be when we created the mold.

We remove the plastic covering the threads and then use a utility knife to remove the remainder of the vent material.

Here is our finished part with the sprue cut off.

And here is our part mounted on the chrome bumper piece, where our original model came from. Again, had we not used a clear silicone rubber on this project, it would have required a lot more time to prepare the mold and at least an extra day to create it.



As you can see, most of the surface bubbles that appeared at the end of our pour have broken as the mold cured.