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.

Mass Casting a part that does not have a flat side requires a more complex process involving the creation of a parting line.

This is the model that we’re going to duplicate. As you can see, there are no flat surfaces, so no matter how we orient the model, if we just built a frame and poured on top of it, the material would flow underneath and the model would get locked in the mold.

Therefore we will create a two-part mold, which will have a parting line at the bottom of the car, about halfway through the wheels.

First, we machined four cavities in a piece of wood using a Bridgeport mill. This will allow our model to sit flush against the flat board. This board is now the location of our parting line and by sitting flush, it allows a tight seal and therefore a clean parting line.

We used Plast-econ modeling clay to secure the model to the plywood and also to fill in the slight gaps along the parting line.

On the bottom surface, we are applying the clay to seal off the bottom of the car. This will prevent material from flowing underneath the car.

Next we press the car onto the plywood. And you can the see the excess clay has squeezed out all around the model.

We trim the excess clay and then we press in some additional clay at the wheels and any other area that didn’t get completely sealed. To do this, we’re using one of the Freeman fillet shaper tools.

Next we apply some Freeman Wax release with a white bristle throwaway brush, then lightly buff the wax using a cotton cloth. We are also applying a layer to the model itself to aid in the releasing – technically, a release agent isn’t needed when working with silicone rubber, but the layer of Freeman Wax Release lubricates the surface and makes the demold process even easier.

Finally, using an exacto-kinfe, we trim the remaining excess clay around the detail of the wheels.

Here is our completed master model with the wheels set below the plywood and clayed in to create a clean parting line.

To contain the poured material, we have constructed a two part wooden frame. The first half is deep enough to cover the majority of the model and the second half is deep enough to cover the wheels.

Now we need a mechanism for the two halves of the mold to interlock. Additionally we need to create a path to pour the material into the mold, called a gate.

To accomplish this, we’ve chosen our Freeman half-round pattern wax, which is easy to handle and non-reactive with the silicone.

We choose a length and cut the ends at an angle so the surface will draw easily from the mold. And then we make an identical piece for the other side of the model.

We adhere the wax to the wood using clay and then trim the excess.

This will create an indentation in the rubber which will later be filled during the second half of the pour, producing a perfect alignment & locking mechanism.

Using a larger size of half-round pattern wax, we create a gate. We mark the end of the frame with a pencil and then cut a piece of the wax to fit exactly between the model and the frame. Again, we use clay to adhere it to the wood.

Our model is now ready for pouring.

We clamp the frame to secure it in place, making sure the gate is touching both the model and the frame.

We are using Rhodia’s V-340 silicone rubber, which has already been properly weighed, mixed and degassed. Please view our other mass casting video which details this process.

This material requires 16-18 hours to solidfy, so we need to wait until the next day to create the second half of our mold.

Day 2

It has been 16 hours since we’ve poured the first half of our mold and the Rhodia V-340 has now solidified.

Here we are taking off our clamps and then wedging apart the frame to reveal the underside of our pour.

The first thing we must do is remove the residual clay with our fillet shaper tool. Notice how the clay has prevented the silicone rubber from pouring underneath the model, resulting in a very clean parting line for our second half.

Next, we remove the locking mechanism. You can see, a little bit of material has flowed beneath the half-round pattern wax, which we can cut off with a knife.

To complete the cleanup, we use a small piece of folded sandpaper in the groove. The harder silicone rubbers like V-340 can be sanded fairly easily to remove small amounts of excess material. The softer rubbers are more difficult to work with sandpaper.


After cleaning the excess flash around the locking mechanism, we need to create the other half of our gate to form a wider, round hole through which the material will flow. On one end, we are angling the tip and on the other side, which will butt up against the frame, we cut at 90 degrees.

Before we pour the second half to the mold, it is very important to apply a release agent to the first half. Silicone rubber doesn’t stick very well to most materials, but it does stick very well to other silicone rubber, so we are applying Pattern Release 202, which is an aerosol form of Vaseline. A light, uniform coat will be enough to give us a clean release.

Next, we place the other half of our mold frame on top and clamp it down.

Now we’re ready to pour the second half of our mold. Once again, our V-340 has been weighed, mixed and degassed, and we flow the material into the mold as before.

Day 3

It has been 16 hours since we poured the second part of our mold. The silicone is now hard and ready to be demolded.

First we take the clamps off, and using the same wedges as before, we separate the frame at our parting line. Since we waxed our frame, it actually comes off first. And then we separate the mold.

Notice our locking device has transferred perfectly from one half to the other.

We push the first half out of its frame and then remove the lower part of the gate.

Notice that our gate needs to be cleaned up just a little bit, which we take care of with sandpaper, much as we did yesterday when cleaning the flash around the locking mechanism.

In order to pour a closed mold, we need more than just an entrance for the material. We also need a vent for the air to escape. We could cast these vents, but since V-340 is a harder rubber and cuts very easily, we’ve decided to cut the vents on one side of the cured rubber.

All we need are small vents, just enough for the air to escape but not too big to require a large repair on our finished part.

Since silicone rubber is self-releasing, it doesn’t need a release agent. However, in order to make the demolding process easier and to increase the life of our mold, we’re going to apply a light coat of silicone spray parting agent.

Notice how it gives the mold a higher shine as well.

Now we’re ready to put the two halves together, and since we’ve built in the locking mechanism, it aligns perfectly. We tape the mold together with masking tape. We could use clamps, but then we’d also have to use boards on each side to prevent any localized pressure that might distort the mold.

This is our Repro 10 Tan which has been agitated, weighed and mixed.

Because of its low viscosity, Repro 10 pours very well.

To make sure that we have no air bubbles on the surface of the mold, which might naturally occur around the headlights or other detailed parts of the model, we pour in a little material, then rotate the mold to make sure the entire surface is covered first. And then we finished filling the mold.

About a 30 minutes later, our Repro has partially cured, but it is still a little rubbery. We want to demold the part now so it is easier to remove the gating system.

After removing the tape, we separate the two halves of the mold to reveal our new model.

A little material did flow up into the vents, but notice that there is no air entrapment in the casting itself.

Now is the best time to cut our gate -- while the Repro is hard enough to handle and yet soft enough to cut with a knife. We also remove the part of the gate underneath our model. If we had let this fully cure, we would have had to machine or sand this off, which would take a lot more time.

Finally, here is our finished model car, along with our two part mold. Since we used a high quality silicone rubber from Rhodia, this mold will produce numerous high quality parts.

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.

Surface casting is a process where you use an inexpensive plug to serve as the core of your part, and then cast the surface of your part with a high quality epoxy or urethane elastomer.

Here, you can see a cross-section of the finished product.

Our goal is to create a part that performs as well as if we cast the entire part with the higher quality material, but at a much lower cost and with less weight. This process also creates a more dimensionally accurate part than when casting the part solid out of one material.

We start with our mold, which has already been mass cast from our original part, shown here. The mold is made of our Freeman 1040 flexible urethane.

In the cavity, we lay up our 1/8” thick high-temperature, adhesive backed Freeman sheet wax. By cutting & fitting the pieces, we form a uniform 1/8” layer over the entire mold surface.

Next, we apply Plast-Econ modeling clay to the seams. This fills in any slight gaps between the pieces of the sheet wax

Next, we must properly seal and release the sheet wax. One of the most common errors in working with sheet wax is assuming that sealing isn’t required. However, failing to seal sheet wax will result in your part sticking to the mold.

For more information on applying sealers and release agents, please see our other video.

Finally, we’re ready to pour our core. We are suspending a bar with two bolts over our mold. The nuts will be cast in our core. This will make demolding easier and enable us to suspend the core over the mold later.

Here we have chosen Repro Fast as our core material because of its low cost and quick demold time. Notice that we have not poured the core to the top, but rather stopped about 1/8 of an inch short.

A half hour later, our core is ready to be demolded. We begin removing the sheet wax, revealing our core, which is still covered by a layer of PVA mold release, as well as the modeling clay at the seams. We remove the clay with our fillet shaper tool and then wash off the PVA with water.

Finally, we rough up the surface of the core with sandpaper to help it adhere to the surface cast layer, which we are now ready to pour.

We have created a suspension bar to support the core over the mold. Once the core is in position, we have a uniform 1/8 inch gap between the plug and the mold so we secure the suspension bar to the mold box for stability.

After making sure the mold is level, we are ready to pour.

We have selected Freeman 1060 polyurethane elastomer for our surface casting material. This product, which is commonly used for foundry patterns, features superior abrasion-resistance and impact resistance. Available in either red or black, this is what the red Freeman 1060 looks like when completely mixed.

Although not a requirement for using the 1060, vacuum degassing is suggested for optimal results.

We start pouring in a corner and allow the material to flow between the core and mold surface. We have started pouring at the deepest part of the mold to minimize air entrapment.

As the mold fills, it may be necessary to pour along the length of the 1/8” gap to totally complete the pour. The pour is finished when the top the core is entirely covered with resin.

When possible, it is desirable to completely encapsulate your core as we have done here. This technique eliminates the possibility of the core delaminating from the surface cast material.

After 16 to 18 hours, we are ready to demold our part. First, we remove the nails that held the suspension bar in place. Next, we remove our shims and place wedges underneath the bar. By lightly tapping on the wedges with a hammer, our part lifts out of the mold easily.

Here’s what the finished part looks like.

The bottom of this part can now be machined flat for mounting purposes. A pattern constructed using these materials and this procedure will produce a highly abrasion-resistant as well as impact-resistant pattern.

Here again we show a cross-section of a part that was previously surface cast. Many different materials can be used for the core, including wood, synthetic tooling board, epoxies, and urethanes. To further minimize the cost of the core, you can add fillers such as Macrolite spheres or walnut shells, shown here.

If you are going to create three or more parts using this process, we recommend an additional step that will save considerable time. Notice here how we created an additional mold around our plug. This allows for fast and easy creation of additional cores.

Since the most time consuming part of this process is laying up the sheet wax, it makes sense to cast copies of the plug itself in separate mass casts instead of following the entire surface casting process each time.