Let’s talk about mold making. The process of reproducing one object into multiples. There is some basic terminology associated with mold making which will help with the explanation. The first one would be the Master. The Master is the name of the original object that you want to make multiples of. If it’s a sculpture, it’d be called a Master Sculpt. This object needs to be prepped so that we can make the Mold. The Mold is the Negative (or reverse image) of the Master. When you pour something into the Mold, you create a Positive, or the reproduction of the Master.
So the process looks like this: Master Sculpt > Mold (Negative) > Reproduction (Positive)
There are many types of molds that can be created. The most basic is called a Box Mold, or a one piece mold. There are also multiple piece molds typically called by how many pieces make up the mold (2 part mold, 3 part mold, etc.). Today, I’ll walk you through the process of creating a box mold. A couple notes about what a Box Mold would be ideal for. Simple shapes, something with a large flat base, or something that does not have a lot of undercuts. An undercut is an area that is parallel to the base or less than 90 degrees from the base. Picture yourself standing upright, arms to your side, legs together. That would be ideal for a Box Mold. Now, if your arms are out in front of you, parallel to the ground, then that would not be ideal, because the undercut (your arms out) would have a hard time being removed from the mold without cutting it up a lot.
You see how the jaw is less than 90 degrees from the base? That’s too severe of an undercut for a Box Mold. When you mold something you are typically going to make the base the bottom of the mold. But when you cast the objects the mold will be upside down. So areas like the chin will actually create an air pocket. Think of water finding it’s level. That’s what will happen when you pour the material into the mold. Let’s look at the dragon upside down-
The red line indicates a parallel line from the bottom, where the material will be poured. The chin is above that line, but not connected in any way to an area to allow the air to escape, so when you pour the Positive, there will be a void there due to air being trapped. The green lines represent how the Positive will be pulled out of the Mold when it is cured. You see how the mouth and the horns fall outside the green lines? Those are areas that would be hard to remove when pulling the Positive from the mold. When something won’t release from the mold, it’s called Mechanical Lock. Mechanical Lock is bad. We want to avoid that. Pieces that would create that would be better suited for a multiple piece mold.
So the piece that I chose is a crystal shape. It has some undercuts, but they are 45 degrees from the base, so they will come out of the mold easily.
Knowing the object we want to mold will work with a Box Mold, the next step will be to make the box. The materials that you use to make the box aren’t terribly important, they just need to stay rigid during the molding phase long enough for the mold material to set up into that shape. I’ve used folded card stock, playing cards, wood, cardboard, and foam core. For this example I’ll be using foam core again. It just needs to be rigid. Scraps work great for this.
The first thing we’ll need to do is set the piece onto a base. When creating your base, you’ll need to account for the thickness of the walls of the Mold. Typically, you’ll want the mold walls 1/2”-3/4” thick all the way around. This will allow the mold to be self supporting. If you don’t have thick enough walls, then the walls might deform when pouring the material for the Positive.
So set your piece on a base and measure around it 3/4” from the furthest points. In the example provided, the point and edges of the crystal stick out further than the base where it will be attached to the box, so I’ll want to measure from the furthest point out. To attach the piece to the base I’ll use super glue. You can use hot glue, but I personally like super glue because it is thin and will hold tight. Use a glue, but not a tape. You want an adhesive that will cure completely and tape won’t. Once you’ve glued the piece down centered on the base, you’ll notice it that there is a gap between where the piece connects to the base.
You’ll want to fill that gap to ensure that the mold material can’t get underneath the object. To do this you can use some regular Sculpey that you have lying around. It doesn’t need to be anything special. Roll out a thin rope of clay and fill in that gap, using just enough material so that the gap is filled, but not so much that the material covers part of the sculpture. Remember, whatever you place here will appear in all of the castings.
Once you’ve filled in that gap you can use a little rubbing alcohol on a soft brush to smooth it out. Next we’ll create the sides. The top of the box needs to be 1/2”-3/4” or higher from the top of the object. Create two sides of the box first. You want the sides to sit around the base, not on top of it.
For the other two edges, the easiest way to measure the proper length is the lay your material so that the edge lines up with the edge of the base you want to create a wall for. Then, take the two walls you cut and set them on edge next to the base and mark where the fall on the wall you are cutting. This will give you the exact width you need.
You’ll want to tape up your edges using masking tape. The masking tape will help seal the edges so that the mold material won’t leak out. Make sure your edges are completely covered.
(The above image the box is upside down so that I can show you the taped up bottom)
Once your box mold is assembled, and all the edges are sealed you are ready to start prepping the piece for molding.
You’ll want to add a little mold release to the interior of the mold and the piece to be molded to ensure that the mold material comes out easily. If you are molding with silicone, it is not absolutely necessary to add a release, but it will ensure an easier removal process. You can use a product like Mann’s Ease Release, which is an aerosol spray, or you can thin some petroleum jelly with a little rubbing alcohol and apply it with a soft brush.
Set the mold box aside for 5 minutes to allow the mold release to set. While you wait you can set up your mold material.
We will be using Rebound 25 from Smooth-On. It is a silicone mold material. A note about silicone. You MUST wear gloves when working with silicone. BUT, the gloves can NOT be latex. Latex and latex powder will cause cure inhibition, meaning the mold will not set. Use Vinyl or nitrile gloves. The other items you’ll need for mixing the mold is a large clean bucket, stir stick (I personally like wooden chopsticks), the mold material and a gram scale if required. Some silicones can be mixed by volume as well as weight. Please read the instructions that come with your mold material and follow their directions.
In our example, I’ll be using a gram scale to measure out equal parts A & B by weight. You can pour parts A & B into separate containers and mix them together, or into the same container, just make sure that your volumes or weights are accurate to your instructions. Always pour and mix silicones over a smooth surface, and cover an fabric or carpet with plastic sheeting. Silicone will not stick to smooth surfaces, but will bond permanently to any fabric or porous surface.
Once the two parts are added together it’s time to get mixing. Using your stir stick, be sure to stir the mix thoroughly. You shouldn’t see any strips of color in the mix, it should be a uniform color. Be sure to scrape the sides of the bucket and the bottom. If your container is clear, you can carefully lift the bucket above eye level to check for parts that aren’t mixed correctly.
Now comes time for the pour. You’ll want to pour the material to the lowest point of your box and let it find it’s level. The other thing you’ll want to do is pour from a height of a foot or two above the Mold Box. This will create a thin amount of material to pour into the container, called a Ribbon Pour, that will help eliminate any air bubbles trapped in the silicone.
Once the mold box is filled to the line that you have marked above your object, set the box to the side and let cure. Follow the directions for your mold material for the cure times, but typically overnight works. Some silicones can take a day or more. Always follow the enclosed instructions. If some excess silicone has fallen onto the workspace, DON’T try and wipe it up. Let it cure and it will peel right up.
Once the mold material has set up, it’s time to remove your Negative from the Mold Box and the Master. The mold box isn’t precious and can be torn away. The Mold should now be firm enough to hold it’s shape. Then remove the Master. Now you have a working mold that you can pour material into to create as many duplicates you need. Be sure to show us what you plan on molding by tagging it on Instagram with #HowDoYouSculpey and tag me @PaulPapeDesigns.