Making Simple Rigid Molds

Watch The Video

Rigid molds are ideal for models with a flat side and no undercuts where a negative draw can lock the model and mold together. To demonstrate the construction of a simple, rigid mold, we’re going to use our Repro NS fast-cast urethane. Our model is made out of our Repro One fast-cast urethane.

The three greatest advantages of using fast-cast urethanes to construct simple molds are:
- ease of use—because of the1:1 mix ratios and no vacuum degassing required
- the low cost—lower than
urethane elastomers or silicone rubber
- and the quick demold times, usually just over an hour, enabling the construction of multiple molds per day.

Our mold board is made of
MDO plywood and our mold frame is made of sugar pine, glued and nailed together. We purposely did not seal or release the mold frame so that the Repro will adhere to the mold frame.

Here we screw our mold frame onto the mold board.

The model is then bolted down onto the mold board. Since the model fits flat onto the mold board, we did not need to apply clay around the edges.

When constructing rigid molds, proper sealing and releasing is very important. We have already applied two coats of wood and plaster sealer and here we applying our second coat of
wax release. We were careful to apply this to the detailed areas first, and then immediately buff those areas to avoid buildup before moving onto the less detailed areas.

Next we apply our two coats of
PVA, and then a final layer of wax release. For complete instructions on proper sealing and releasing, please see our other video on this topic.

We are going to use Repro NS Non-Settling fast-cast urethane as our mold material. This material requires no shaking or mixing prior to use. Simply stir each side with a paint paddle and it is ready to go.

We’ve chosen Repro NS because of its blue color, which will make it easier to distinguish the model from the mold.

We weigh and mix our material – in this case a total of 3200 grams is required to fill our mold so we are weighing 1600 grams of the A side and 1600 grams of the B side.

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

Finally, we are ready to pour the Repro in the mold cavity. Notice how the thin viscosity allows for easy pouring, even into the highly detailed areas, without entrapping air.

It has been a little over an hour, so the Repro NS hasn’t fully cured. In fact, it is still a little warm and very slightly flexible. This is the ideal time to demold, since the material will give a little during the process.

We remove the screws holding the mold frame to the mold board. Then we use wedges to slowly and evenly separate the mold from the mold board.

Next we clean off the PVA mold release with a damp paper towel.

Now that we have our mold we must apply our release agents so that our parts do not stick to the mold. However, since we are pouring a urethane into a urethane, the release procedure is different than the standard two coats of wax, two coats of PVA, and one coat of wax. Here we will only apply three coats of wax, buffing after each application. For more detailed information on sealing and releasing, please see our other video on this topic.

Here we are setting our support bars above the mold. The nuts attached to the bolts will serve as aids in the demolding process. We attach the bar to the mold frame with clay to raise the bar slightly over the surface of the part, thus making sure the Repro won’t touch the support bar when pouring.

Here is our Repro One fast-cast urethane. Repro One, like Repro NS, is non-settling, meaning that it does not require a mechanical mixer.

We measure 260 grams of part A and 260 grams of Part B and mix them together. Then we pour the mixture in a new cup.

Finally, we pour the Repro into our mold. Again, we pour slowly in a narrow stream in order to make sure the material flows easily into all of the highly detailed areas.

Once the Repro cures, we use wedges to lift the cured part from our mold. Notice how it demolds cleanly and easily.

Finally, we remove the bolts from our part. Our mold is now ready to be put back into service.