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Making a One-Piece Silicone Mold 
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Unread post Making a One-Piece Silicone Mold
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When you need to make a mold of a flat or gently curved model, the easiest method is to create a flexible one-piece silicone mold.


For this tutorial, I am going to use my Mara Jade knee guard for the demonstration pictures. The original model was carved from rigid foam and then was coated with a facing of fiberglass resin. A few low spots were filled with green modeling putty and the back of each circle was covered with a piece of styrene, and then I glued an oil clay ridge around the outer edge.

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- Begin by sealing your model with an acrylic spray. I recommend Krylon Crystal Clear. It is widely available and works well. Let the sealer dry for a full day before you move on to the next step.


- Select a firm surface for working on your mold. When making a small mold, I like to use a 12x12 ceramic or masonite tile. The textured surface is rough enough to 'hold' clay and hot glue, but it's also sealed so that cured silicone rubber will peel right off of it. Tile also has the advantage of being portable and easy to clean. A tabletop covered with plastic works well if you don't have a piece of tile, or if the project is too large for one.

(Note that the model extends down past the actual design edge. The knee guards actually stop at the bottom edge of the brown clay. The fiberglass section that you can see below the clay area is just a base which will not be included when I cast copies of the model. The extra height allows me to give the mold a clean and sturdy edge.)


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- Place your model on the work surface and seal it down to that surface with modeling clay, water based clay, or hot glue. I like to roll out a 1/4 inch layer of water based clay, and then press my model into the clay. Because I added extra height onto the bottom of my model, pressing the model into the clay will not cover up any of my design. I then use more clay to seal any openings under the edges. The knee guard model is curved upwards at the ends. I had to put clay under the ends to keep the silicone from running underneath the model and being wasted. The layer of clay has been trimmed so that it is about 1/2 inch wide all the way around the model. This gives a good edge for applying the retaining wall.



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- Build a retaining wall around your model. The wall should be at least 1/2 inch away from the model, and should be a few inches taller than the highest point on your model. I like to use thin cardboard or tagboard. It is cheap, easy to cut, easy to fold or bend, and easy to glue. Cut the pieces to fit all of the way around your model, always staying about 1/2 inch away from the model edge. You can use chunks of clay or pieces of tape to hold the retaining wall in place until you have everything fitting correctly. Then, use hot glue to seal the wall down. Glue the wall to the work surface, and seal every seam closed so that no silicone will be able to leak out when you pour the mold.


- Spray the model, the clay, and the retaining wall with the proper release agent for the type of silicone you are using. Check the information that goes with your silicone to find out what is the correct release to use. This step is important. If you do not put release on your model, or if you use the wrong kind, your mold may 'glue' itself to the model. Or, using the wrong release agent such as oils or wax may keep the silicone from curing, so you will be left with a gooey mess. Since I prefer to use Smooth-On silicones, I usually use Ease Release 200 or 800 on my model. I spray it on, brush it into every nook and cranny with a soft brush, and then I spray on one more light coat. Don't use too much release, or your mold will take on a bubbled, spongy surface.

(Note: It's wise to cover your work surface with some scraps of paper or plastic to protect it from overspray before you begin to spray the release. This will keep a buildup of release from developing on your work surface and will help you avoid future problems. Hot glue won't stick to a surface that has been coated with release!)


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- Measure out your silicone according to the instructions, by weight or volume. Then pour both components into a mixing cup or bowl, and stir it well for at least three minutes. Be sure to scrape the bottom and the walls of the cup. When you're sure you've stirred it enough, stir it some more! The most common problem with silicone is not getting the two components mixed completely together. If you have a degassing chamber, use it after the silicone has been thoroughly mixed.


- Pour the silicone into the mold. Do this very slowly, in a small stream. Do not pour the silicone directly onto your model. Instead, pick a low point beside the model and continue pouring all of the silicone at that one point. The silicone will spread across the base of your model and then up over the edge, filling every detail and pushing air out of the way as it goes. If you were to pour the silicone directly onto the model, it would tend to trap air, leaving you with bubbles on the surface of your mold.


- Set aside your mixing bowl. The silicone will cure inside, and then you can peel it out and your bowl will be clean and ready for the next batch!


- If your model is flat, you can simply pour in the silicone until it has been covered completely. The mold silicone should be at least 1/4 to 1/2 inch higher than the highest point of the model. This will give you a strong and sturdy thickness of silicone, and it will level off naturally to a flat surface. Let the silicone sit overnight until it cures. Then peel off the retaining wall, clean away any clay residue, and you are finished with your mold. There is no need to create a support shell, since the mold will lay flat on a table when you cast something in it.


- If, however, you are making a mold of a rounded surface, then it is wise to shape the mold so that you use less material. Pour in the silicone, and then use a brush or plastic spoon to push it over the highest points of your model. The liquid silicone will run back down to the base, but as time passes it will begin to thicken and will remain against the sides and top of your model. For this mold, I worked on other projects in the same room so that I could return every 3-5 minutes to spoon the silicone back over the top of my model until it set, which took about 20 minutes. Each time I pushed the silicone to the top, a bit more stayed until I ended up with an even 1/2 inch coating over the entire model and the area around the outside of the model. Because the model was rounded, if I had simply filled the retaining wall area until the whole thing was covered, I would have had to use twice as much silicone. By spending a few extra minutes shaping the silicone over the rounded surface, I was able to save some material. (Other options would have been to use a brush on type of silicone, to thicken the silicone with a product such as Thi-Vex, or to use an accelerating agent. I didn't have these products on hand, so I just used a spoon and some patience.)


(Note: Because I made the silicone have a rounded surface, I would not be able to set it upside down on the table. A rounded mold needs a support shell that will have a flat base. So, when the silicone was thick enough to support the weight, I pressed some lightweight plastic shapes into the surface around the edges beside the retaining wall. I left them there until the silicone cured, and then I took them out. In this picture you can see the indentations that the shapes left. These indentations, called "keys" will help me line up my support shell later on.)


- After it has begun to set, leave your mold undisturbed for at least 15-20 hours. Letting it sit overnight is wise. If the room is cold, place the mold in a warmer area. Silicone actually strengthens if you place it in a 150 degree Fahrenheit environment for a few hours. Do NOT use a household oven. The chemicals in the mold can contaminate any food you cook later.


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- If your silicone mold needs a support shell (or if you just like to have something that is easier to hold on to than a flexible mold), you can pour plaster into the retaining wall after the silicone has fully cured. This is why I recommended earlier that the wall should be several inches taller than the model! Mix the plaster well, and then pour it over the silicone and tap the work surface to release any air pockets. This will also level the plaster so that you will have a flat surface on top.


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- After the plaster has set, you can peel off the retaining wall and pick up your mold. The model usually comes free from the clay base and sticks inside the mold. Set the plaster support base aside, and then peel the silicone away from your model. If you applied the release agent correctly, the silicone will not stick to the model at all.


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- Now you have a durable and flexible mold... and it's time to start casting copies of your model! Silicone molds will not stick to most casting materials, so a mold release isn't absolutely necessary. It is wise to use a release agent when you begin casting, though. It will increase the life of your mold, and will make it easier to remove your castings from the mold.


Have fun!


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Pam :-)

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Wed Jul 15, 2015 5:54 am
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