Making Simple Urethane Rubber Molds

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Urethane rubber molds are widely used when a flexible mold material is required and silicone rubber is not desired. Urethane Rubber, while generally less expensive than silicone rubber, is not self-releasing and therefore requires the mold-maker to follow the full sealing and releasing procedures in order to obtain optimal results.

To demonstrate the construction of a urethane rubber mold, we’re going to use our
Freeman 1035 to make a mold around this model of a leaf.

The mold board is made out of
Chili-Form, a heavily laminated plywood known for its for high strength and stability. Chili-Form features a phenolic-infused face, making the surface non-porous and moisture-resistant. Therefore, it doesn’t require a sealer.

Our model is made out of our Repro One fast-cast urethane. It is mounted onto the mold board with clay.

Our mold frame is made of pine, nailed and glued together. And it has already been sealed with Wood And Plaster Sealer and released with
Freeman Wax Release and Partall PVA mold release.

Here we are applying two coats of wax release to our model, followed by two coats of PVA, and finally one more coat of wax release. For complete instructions on proper sealing and releasing, please see our other video on this topic.

Next we attach the mold frame onto the mold board and then make sure the mold itself is level.

We’re now ready to weigh and mix our Freeman 1035 urethane rubber. Here we are measuring 2200 grams of Part A and 2200 grams of Part B and mixing them together.

We’re actually weighing these materials in two different containers so we don’t overfill them. This also allows room in each container for the material to rise during the vacuum degassing process.

For complete instructions on weighing & mixing of materials, please see our other video on this topic.

Here we are vacuum degassing the material in order to remove nearly all of the air entrapment that occurs while mixing the two sides together. While vacuum degassing isn’t critical to make a Freeman 1035 urethane rubber mold, it is always a good idea to do so in order to virtually guarantee the absence of bubbles in your mold.

For more information on vacuum degassing, please see our other video on this topic.

Here we are pouring the material into the mold. We pour in a narrow stream, paying special attention to areas of higher detail before filling the complete mold.

The second container of material fills much faster since all of the highly detailed areas are now already covered. When the mold is filled, we allow the material to cure overnight.

The next day, we unscrew the mold board from the mold frame. Then we begin hammering our wedges between the mold board and mold frame. Since we sealed and released the mold frame, it easily separates from the mold, allowing us easy access to the mold itself so that we can begin slowly peeling it from the mold board, revealing our original model.

Notice the excellent detail reproduced with Freeman 1035 Urethane Rubber.

With a damp paper towel, we remove the PVA mold release, which had formed a thin film between our model and our mold. We are also able to run the mold under water, allowing for faster removal of the water-soluble PVA.

After drying the mold, we apply three layers of wax release.

Next we weigh and mix our
Repro One fast-cast urethane, and then we pour it into our mold. Repro One if Freeman’s top of the line filled urethane. Like all Repro products, it features an easy one to one mix ratio, adequate working time, and does not require vacuum degassing.

As soon as the Repro One cures, which takes a little over an hour, we turn the mold upside down and peel the mold off of our part.

Here you see the final part with our Freeman 1035 urethane rubber mold.

Again, notice how accurately the rubber reproduces the detail of the model.

Our mold is now ready to be put back into service.