Casting Molds & Models with Intricate Detail

One of the greatest challenges in making molds and casting parts is air entrapment. If air becomes entrapped while pouring a liquid urethane or silicone rubber on a model or mold, it forms small voids that become defects on the surface of your mold or part.

In previous videos, we have demonstrated several methods of minimizing air entrapment These include applying pressure to a mold while the liquid cures, vacuum degassing the material prior to pouring, and venting a mold. All of these methods allow air that might otherwise become entrapped in the mold, to be eliminated.

However, some highly detailed or intricate patterns pose some particular challenges that cannot be solved with simple venting, degassing, or applying pressure. In these cases, adding an extra step or two to the casting process, as demonstrated here, can make a significant difference to the quality of the final part.


Here we have a miniature replica of a Michelangelo statue, which has been adhered to our mold board with Plast-Econ modeling clay.

The highly irregular and detailed surface of our model features many places where air can easily become entrapped during the mold-making or casting process. Therefore, before we pour our silicone rubber into our mold box, we take a small amount of our mixed silicone and gently brush and dab the material all over our model, paying particular attention to those areas where air can become entrapped easily.

After we have covered the entire model with a thin layer of silicone rubber, we then finish constructing our mold frame. In this case, we are simply using a paper cup with the bottom cut out of it, which is adhered to the plywood with Plast-Econ modeling clay.

Now we are ready to pour the remainder of our silicone rubber into our mold cavity. The layer of silicone rubber that we had applied with a brush has actually smoothed the surface of our model and thus reduced the likelihood of air entrapment.


The next day, we remove our mold from the mold board and peel away our cup, revealing our silicone rubber mold. Next, we cut the flashing around the bottom edge of our mold.

Then, using a sharp knife, we carefully begin to cut one side of our silicone mold until we can remove our model.

Here you see our new silicone mold next to the model that was used to create it. Notice how well the silicone rubber re-creates the detail of the model and how easily it returns to its original shape after we pulled it apart.

Now we will use our Freemen 1085 polyurethane elastomer to create a duplicate.

We close our mold and then use electrical tape to hold the parting line together.

After weighing and mixing our urethane, we apply some urethane to our brush and cover as much of the surface of our mold as possible, much like we did with the silicone rubber, except that this is a bit more difficult because we are applying it to the inside of a mold rather than the outside of a model.

Next, we pour a little bit of the material into the mold cavity and spend about 30 seconds rolling that material all around the mold cavity so that it coats the entire surface of the mold with a thin layer of our resin.

Finally, we pour the remainder of our urethane into our mold cavity and allow the urethane to cure, which in this case takes about an hour.

When the urethane has fully cured, we remove the tape and demold our new casting. By following these simple techniques, we were able to re-create the intricate surface of our model both with silicone rubber and our urethane casting material and minimize any air entrapment that would have formed imperfections on the surface of our casting.

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