Making a Mold, Part 2 : The Mother Mold

Making a Mold, Part 2 : The Mother Mold

Last week, we covered “Making a Mold, Part One: Making a Silicone Jacket”. Check it out before you read Part 2 down below!


IMG_0100Now, I’ll show you how I did the “mother mold” using Smooth-On‘s Free Form® AIR Lightweight Epoxy Putty. Free Form Air is a two-part epoxy putty that sets overnight into a sandable, carveable, and paintale rigid epoxy. It very tough and very light, which facilitates an easier time slush casting items by hand.

We will be creating two sides on the mold, which will allow us to be able to remove the silicone jacket and the cast piece without damaging it.

Determine How to Split Your Mold

Before you start mixing up the epoxy, you need to decide how you will split your mold.

For the Pip-Boy, I settled on two halves with a separation straight down the middle, where I had originally put the keys on the silicone jacket.

Measuring and Mixing

Free Form Air comes in two parts that measure out by volume. The preferred method, as taught by Smooth-On, is to have two equal balls of part A and B and smoosh them together, then mix by folding it into itself (as seen in these photos).

It’s very important to mix it with this method, as it’s easier and gives a more even distribution of material.

While you were mixing up Part A and B, you will have noticed that once mixed you have a very sticky, thick dough in your hands. It takes a little to get used to how this material works, but you learn pretty quick as you get used to it.

Forming the First Half

Once you have thoroughly mixed both parts, and have an even color of grey with no white or grey streaks, you are ready to apply the first half of your mother mold!IMG_0107

To start applying the putty, I break it up into sections (instead of just slapping the whole amount on). This allows me to control the thickness of the mother mold so as to not waste material.

I wanted to keep the thickness down to a 1/4 inch all around, which is suitable enough for this type of mold.

Taking little bits of material, I start to press and pat the epoxy onto the silicone along my center line (where I will create mother mold keys – yes, more keys!) to help lock the two halves together.

I also make a taller wall down the center, where the keys will be (and, later, bolts can be added to help lock the halves together).

Once you have the material on the mold, you’ll want to use water to help smooth out the material and make your center line and edges sharper.

In the photo you’ll notice a wet block of MDF board. I used this to help create the sharp lines on the center and also the bottom. Make sure you keep it wet so it doesn’t pull material off while you use it to smooth the flat areas out.

Wait for 12 Hours! (I’m Not Kidding)

Now that you have one half all smooth and exactly the way you want it , it’s time to wait.

Free Form AIR takes about 12 hours to set: the other half will have to wait for the next day.

Carve in Keys

While it’s setting up, you can carve in your keys into the epoxy. I usually wait about an hour or so when the epoxy has become more rubbery and stiff to carve in my keys.

As seen in this picture, I am using a cheap razor knife to cut into the epoxy and carve out little squares. After you cut one out, you can clean it up by adding water to smooth it back out and use the flat part of the razor blade to help flatten the indented areas.

Adding the Second Half

With that done, it’s time to wait for it to completely cure before you add the other side.

To do this, just repeat the above process with a few extra steps as seen in the photos below.

In order make sure that each side releases from itself, it’s important to add a release agent to the first half so they will not adhere to each other. I use petroleum jelly for a release agent and brush it in with an art brush to make sure I get it into all the little nooks and crannies on the mold.

Now, mix up some more putty and apply as before, while making sure to get it into the keys you had carved out earlier.

Let It Sit Overnight

Now that I basically had a turtle shell and everything the way I like it, I let it sit overnight to cure.

Remember when I said before that it can take a few days to do a mold? This is why! 🙂 Set times of your materials play a huge part in how well your mold comes out and how much time you must set aside for this process.

Finishing Up

Now 12 hours later from the last step, it’s time to pry the two halves apart, clean up the edges, and gently pull the silicone jacket off of your master piece. I usually will use something like a large flat tip screw driver or a chisel to help separate the two halves. I gently tap  the chisel to help split the halves and I pry one half off slowly and methodically to keep anything from snapping.

After one half is done, it’s time to remove the second half. For this, I just apply pressure and wiggle the other half off. It can take a little time to do this, as the halves get suctioned onto the silicone during the curing process.

I hope this has helped everybody understand a little bit more on the molding process and hopefully made it a little less of an intimidating process. If you have any questions on the products I use or on anything I have done please feel free to contact me here or on Facebook!