Making a Mold, Part 1: The Silicone Jacket
Thank you to Lissa Carol for taking pictures for this guide.
Building props and costumes are awesome for making one-off items, but what do you do when you have a piece that you need to replicate multiple times?
How do you go about that, and where do you go?
Well, I am here to help you go about that: by showing you how I did my most recent mold for my Pip-Boy 3000 prop.
Getting Started: Think About Your Item (Wearable vs Non-Wearable)
The very first thing that needs to be thought about is: how do you want your piece to come out?
Do you want it a solid piece, such as a gun, or is it a mask or gauntlet that needs to be thinner to allow it to be worn?
With my Pip-boy, for example, it needs to be worn- so I needed to make sure to design my mold around specifically. There are several ways to make a mold for this prop, but we’ll keep it to the one method that I used which is a thin silicone jacket with a hard mother mold. A jacket is layer of silicone that is a 1/4″ to about a 1/2″ thick, that allows it to be rolled off the cast piece, and the hard mother mold, which gives it a backing so that the shape can be kept without warping.
Although there are many types of molding materials out there to use most folks usually stick to their favorites and will only do another type if needed depending on the item you are molding and what material you are casting the prop in.
Preparing to Mold: What You Need
My favorite is Smooth On’s Rebound 25, it mixes at a 1:1 ratio and can be done so by weight or by volume.
After I decided how my cast needs to come out and how I wanted to do my mold, I set out to get everything prepped and ready to go. Aside from purchasing your molding materials, you need to gather all your mixing supplies: mixing cups, stir sticks, wax paper, masking tape, nitrile gloves ( I use Nitrile as Latex gloves can inhibit some silicons from curing) a clean work area, a scale, oil or water based clay, and plenty of time.
Yep, plenty of time. Making a mold can become a weekend project and can take a good chunk of your time, so along with your supplies make sure you have plenty of time to give your mold the attention it needs.
Make sure your work area is clean and everything laid out within arms length to help keep everything simple and everything flowing along. Use the wax paper as a drop cloth for ease of clean up and tape it down with the masking tape to keep it from moving and slipping around.
When you have everything organized, it’s time to clay up your mold, which is the process of creating walls to either create two mold halves or keep the silicone from flowing where it doesn’t need to.
For the Pip-boy, I just needed to clay inside to build a wall to keep the silicone from flowing into the pipboy cavity (where I didn’t want it to), to elevate it above the surface, and to create the ‘negative wall’ for the silicone to be come the actual wall that you cast against. You’ll understand that last part as we go along so don’t worry if it sounded a little confusing.
Time to Mold: Getting Down to Business
Now that everything is clayed up and ready to go it’s time to start mixing silicone. I mixed up about three ounces of equal parts A and B of Rebound 25 together till I had a single cohesive color.
I like to use popsicle sticks for mixing up small amounts and for larger amounts I use paint stirs from Home Depot, as they are tough and free in the paint section.
After you have thoroughly mixed the silicone, it’s time to apply the first of many coats. The first coat is called the “print coat.” It’s a very thin layer that catches all the fine details and is very important. Here, you’ll see that I actually started with the bottom of the piece so that I could make sure that I got a good layer of silicone on before I started the top portion of the piece. Rebound 25 is a brushable silicone that can also be poured.
Time and Patience
Before, I had said to set plenty of time aside for mold making, and here is where it starts.
After I applied the first layer, I now have to wait for about an hour before I can put the next layer. Smooth-On recommends that you wait until the first layer is tacky, but not sticky, before the next coat is applied. It’s always best to have a small pile off to the side to test this.
Now that time has passed, it’s on to the second coat. See where this is going? Each layer is about a millimeter thick and with building enough material up to equal to about a 1/4 inch or more can take some time.
When you get to about your third or fourth layer, depending on your piece, you can use a thickening agent to thicken up the silicone so that you can trowel on your fourth or fifth coat. Thus, adding a thicker wall without having to do a thousand thin coats. But, I do warn you, as this is only to be done after about the third layer as with thickened silicone air bubbles can be trapped in between layers and cause problems.
After the thickened silicone has set enough, I poured two more coats without the thickener added. You do this so that you can create a cleaner and smoother mold so that you don’t have any problems with the mother mold application. Now that I have the top portion as thick as I want it, I flip it over and finish off the bottom. As you can see, I had to trim off excess silicone flashing to finish applying the final coats to the bottom.
Since I had done a few coats on the bottom, then switched to the top and then back again, you can see that a sort of moat was created. I just need to fill it in with silicone, and then we’ll be done with this part of the molding process.
Now that you have both halves done, you need to add “keys” to the silicone jacket so that your mother mold can lock onto the jacket and have a nice snug fit and help facilitate a non-warped cast. To create a key, you just need to find a rectangular or square shape to pour more silicone into to create a nice even piece. I used a tool box tray to give me a nice rectangle that I can cut into the smaller keys that I’ll need.
After you have mixed up a little silicone, poured it into the make shift mold, and let it sit you can pull it out and you have a nice piece to use. I cut mine down the center and into two little pieces for one half and a single piece on the other half. In order to attach these to the jacket, you need to mix up a little silicone and a little of the thickener to make a paste. From there, you apply it to the key and place it onto the jacket as seen in the following photo.
I used a popsicle stick to clean up the edges and smooth them out. Silicone sticks to nothing else but itself, so after the paste sets you’ll have a permanent key on you mold jacket as seen in the photo.
[box type=”info”] From there it is time for the final step, which I cover in Making a Mold, Part 2: Making a Mother Mold.[/box]
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