Molds with Complex Parting Lines

Sometimes, due to irregular part geometry, creating a straight or curved parting line is either impractical or very difficult to complete. In these instances, one of the best ways to create a two-part mold is to use a soft material such as modeling clay to temporarily simulate one half of your mold, as we’ll demonstrate here with a model of a vertebra, or segment of a human spine. This video will also show how trial and error can play a role in creating an adequate venting system for a complex mold.

We prepare this irregular shape for the mold making process by shaping pieces of our sulfur-free Plast-Econ modeling clay underneath the model in such a way that we eliminate all, or at least most, of the undercuts that would occur when the silicone rubber is poured from above.

This process may require a bit of imagination and experimentation. And as you see here, we spend some extra time using our fillet shaper tool in order to form the clay exactly how we want it.

Here we have the finished application of our modeling clay. As we will see later, the volume of the mold cavity that is occupied by the modeling clay will be the same exact shape of the second half of our mold.

We are now ready to complete the construction of our mold frame, which we do by clamping our mold frame onto our mold board. Then we pour our properly weighed and mixed Bluestar V-340 silicone rubber into our mold cavity and allow the silicone to cure overnight.

The next day we separate the mold board from our mold frame, thus revealing the inside of our mold.

Next we remove our model from the mold board and then peel off all of the clay that we had applied to the model the day before. Since our model has a very irregular shape, this takes a bit of time for all of the clay to be completely removed.

Next, we place our model back into our mold, which shows us all of the volume that the modeling clay had occupied. Then we spray the first half of our mold with Pattern Release 202. Silicone rubber doesn’t stick very well to most materials, but it does stick very well to other silicone rubber, so we are applying a light, uniform coat of Pattern Release 202, which is an aerosol form of petroleum jelly.

Before we pour the second half of our mold, we need to create and align our gate and our venting system.

Here we have two small metal pins attached to a wood bar that we will affix to the top of our mold frame. These will form two of our vents, which will permit the air to escape while our urethane fills the mold cavity.

We also have a small dowel rod that is connected to another wood bar. This will form our gate, which is where we will pour our urethane into our mold cavity when casting our parts.

With the mold frame clamped down and the gating and venting system nailed into place, we are now ready to pour the second half of our mold.

The next day, we remove our vents and our gate. Then, using a couple of wedges, we remove each half of our mold frame.

And then we pull apart the two halves of our mold, revealing our model.

Casting our parts in done using the same method as shown in our other casting videos. We use electrical tape to hold the two sides together and form a small reservoir over our pouring sprue with modeling clay. Then we properly weigh and mix our Freeman 1070 polyurethane elastomer, and then pour it into our mold cavity.

Thirty minutes later, we demold our casting. In this case, our first casting did not come out as well as we had hoped. There were a couple of areas of our mold that weren’t completely filled, thus leaving our part incomplete. Fortunately, this is easy to fix. In fact, since we used a firm silicone rubber, we can cut and/or drill additional vents in the areas where the air entrapment occurred.

First, we cut two small vents in one of the ends of our mold using a sharp knife. This vent may produce a little flashing is that area which can easily be removed, but it will enable our urethane to completely fill that end.

Next, we use a drill to create a new vent that will allow air on that end to escape through the top of the mold.

As you can see, we now have three vents on the top of our mold, plus extra relief on the one end of the cavity where we experienced voids in our castings.

Now, as before, we weigh, mix, and pour our urethane into our new, improved mold. Thirty minutes later, we demold our casting. This time, we get a complete fill with no air entrapment. And since we produced our mold with a high quality silicone rubber from Bluestar, our mold will have a very long shelf life and perform well through numerous castings.