Surface Casting

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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.