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Twi'lek Pam

Making a Two-Piece Silicone Shell Mold

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When you need to make a mold of a model which has two sides, deep undercuts, or openings that go completely through the model, the easiest method is to create a flexible two-piece silicone mold.



I am going to create tutorials for two common methods used to make two-piece molds. The simplest method is to make a "block" style mold. This mold is easier to make, but it uses considerably more silicone rubber and is therefore somewhat more expensive. The second style is called a "shell" or "case" mold. This type of mold is more complicated and more challenging, but it uses a minimum amount of silicone. Both mold types can produce high quality castings.


This tutorial is for a two-piece mold called a "Shell" or "Case" mold, which I believe makes the highest quality castings. It is also the most challenging type of mold to make... so be patent and be prepared for a bit of trial and error while you learn the process! I have demonstrated block molding in another thread.



For this tutorial, I am going to use my Mara Jade blaster for the demonstration pictures. The blaster was made of wood and styrene pieces, and has been painted with gray primer and then black paint.



Note: A shell mold can be made very compact in order to use as little material as possible. I prefer to make larger molds, however, and this tutorial will reflect that. I like to make a "base" for my molds to stand on so that I don't have to hold them upright when I'm working, and I like to make them larger around than they need to be so that I can easily use mold straps. These straps have a 'locking' design that snaps tightly closed, keeping the mold from leaking when I make castings later on. Mold straps are difficult to use on very small molds, so I keep things easy and make slightly oversized molds.






- First, check your model for problems. Fix any scratches or uneven lines, and make sure that you are happy with the way it looks. If you glued two pieces together, make sure that the silicone won't be able to seep into the space between them. The mold is going to copy and reproduce EVERY mistake... so it's better to make repairs once on the original model than to make the same repair over and over again on dozens of future castings!



- Seal 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.



- Draw a dividing line around your model with a permanent, felt tip pen. This line shows where the mold will be splitting into two pieces around your model. This line can be a true line, or it can be a row of dots. I like to draw dots and use a fine-tip Sharpie. Most of the time this line will stay in the center of your model, but occasionally the design will create a need to move the seam line to one side or the other. An example of this is the sights on my blaster. The center line was very small there, so I jogged the seam to one side. The key thing to remember for the center line is that it should follow the contours of the design so that the mold will be easy to remove. If there is a lump or undercut that might "lock" the mold onto the model, plan the dividing line to minimize that problem.



- Decide on the placement for a pour hole. Tip your model to different angles and find the best way to fill it with resin or whatever material you are going to use. Keep in mind that you should minimize the surface area of your model that you will be affecting by this hole in the side of your mold. It will leave a 'plug' of casting material that you will have to cut off later.



- While you are holding your model with the pour hole location on top, take a look at the rest of the model. If you can see any places where air might get trapped when you pour a casting, keep them in mind so that you can place an air vent in that location when you build your mold.



- 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 and hot glue will peel right off of it. Tile also has the advantage of being portable, easy to turn so that you can work on the different sides of the model, and easy to clean. A tabletop also works well if you don't have a piece of tile, or if the project is too large for one.



- Lay a large piece of Saran Wrap, aluminum foil, or plastic on your work surface. This will make it easier to pick up the clay base later on.



- Roll a 1/2 inch thick layer of clay across your covered work surface, making it several inches wider than your model all of the way around. I like to make this base wider than it needs to be, just to keep things easy. Ultracal likes to run off the model when you do the face coat, so having a wide base makes it easier to push the Ultracal back into place.



- Slightly scoop out the area where your model will be centered on the clay, and then press your model down into the clay. I prefer to use water based clay. It is soft, easy to blend, and is easy to clean up.



- Add small pieces of clay if needed until the dividing line you drew on your model is lined up with the top edge of the clay, and then level out the top surface of the clay with a damp sponge or scraping tool. This does not have to be perfect, but I'm a neat freak so I always smooth everything out anyway.








- Cover the model evenly with about 1/4 inch of clay. Smooth it out and make the line where this clay touches the base clay a sharp right angle.



- Add two or three "registration bands" of clay. These stripes of clay will help the silicone mold stay in perfect alignment with the Ultracal support shell later on. Again, make the connecting edges sharp angles.



- Add a cylinder of clay near the center of the model. This 'plug' will create the hole where you will later pour silicone into the mold. Make the plug at least 1 inch wide. I prefer to make them about 1 1/2 inches wide so that it will be easier to pour the silicone.



- Use small 'worms' of clay or a drinking straw to create vents that will allow air to escape from any places inside the mold where air might be trapped when you fill the mold. These vents should run to the other edge of the clay. They do not have to be very big around. Air doesn't need a lot of space! Angle the vents so that they will go toward the top of the mold when you hold it with the pour hole on top. You don't want them to angle downwards, because that would simply trap more air inside your mold.



- Use clay to build up 1/2 of the resin pour hole. This lump of clay, also called a plug, should be wider near the outer edge of the clay, and should be smaller where it touches the model, like a funnel. Make the funnel slightly larger than you think it needs to be, since the silicone will be taking up a bit of space inside.




- Using rectangular pieces of clay, add 'keys' to various places around the outer edge of the clay. The keys should have sharp right angles, and can vary in length. Make them about 1/4 inch tall and wide. These keys will ensure that the two halves of the mold will fit together correctly when you begin making castings later on.



- If you want to, you can build up a small retaining wall around the outside edge to hold in the Ultracal. I prefer to have a comfortably rounded mold for ease of holding, so I don't do this.



(Note: In this picture you can see that I placed a piece of cardboard at the "bottom" of the blaster. I will make the Ultracal support shell wider in this area, giving my mold a flat bottom that will allow it to stand when finished. You can also see that I chose to place my 'pour hole' plug at the end of the blaster grip. Air would have become trapped in the two raised 'hammer' areas, so I also added a vent to those places with a drinking straw.








- Mix enough Ultracal to cover your model about two inches deep all of the way around. Start with a small amount of cold water (For this project I started with about 1/2 inch of water in a one gallon bucket), and then sift in the Ultracal. Do NOT mix the Ultracal into the water! Instead, let it sink to the bottom on its own. Gradually the water will become saturated with powder, and the Ultracal will begin forming a crackled "dry riverbed" look on the surface of the water. Allow it to continue soaking for a few minutes longer, and add more Ultracal if it starts looking soupy anywhere on the top.



- Stir the Ultracal with your hands (wear gloves to keep them from getting dried out!), breaking up any clumps until the material is smooth.



- Using your fingers or a chip brush, spread a 'face coat' of Ultracal over the clay. This first coat should be about 1/4 inch thick. As the Ultracal stiffens, it will be easier to keep it on top of the model area and within the boundaries of the keys you made.








- After the face coat has stiffened enough to stay in place and has begun losing it glossy look, begin dipping pieces of burlap into the remaining Ultracal in your bucket. Saturate each piece, and then spread them over the top of the face coat. This burlap will add strength to the support shell. Cover the top area with about 3 layers of burlap. I add an extra layer or two to the outer edges, folding them over to make the edges stronger. If you run out of Ultracal, mix a bit more.



- Cover the burlap with a final "beauty coat" of Ultracal. This coat will cover all of the fibers, which tend to get stiff and scratchy if you leave them exposed. Some people use a wet sponge to make this final coat very smooth, but I prefer to leave a bit of texture on it so that it will be easy to grip later on.



Note: You can see in this picture that I added a considerable amount of burlap to build up the base area of my mold so that it will stand upright and not topple over. Folding each piece helps build up the bulk faster there.








- Allow the Ultracal to set for at a couple hours.



- Pick up the whole thing... clay, model, Ultrcal and all... and turn it upside down. This is where you will be glad that you spread a layer of plastic underneath the clay! Peel off the plastic, and then remove the clay base. If it's still wet and clean, you can toss it back into your bag for reuse.



- If there are any rough edges in the Ultracal, now is a good time to clean them up. Use a plaster rasp to file the edges smooth, but be careful since the Ultracal is still soft and can be damaged.



Note: In this picture you can see the keys that were created with the clay earlier, and the air vent that will release air from the hammer area. I left the clay that surrounds the blaster intact, as well as the clay that is filling the pour hole for the resin.








- Now you will need to repeat the process of covering the blaster with clay, exactly as you did on the first side. First, cover it with 1/4 inch of clay.



- Add the registration bands.



- Add a cylinder of clay for the silicone pour hole.



- Add more clay for the top half of the resin pour hole. Remember to make it slightly oversized.



- Fill in the vent holes with clay to keep them from getting filled when you make the second half of the shell.



- Check the keys. If there are any places where an undercut or poor angle might later 'lock' the second half of the shell onto this half, now is the time to fix the problem.



- Using a stiff brush, APPLY A COAT OF VASELINE to the top surface of the Ultracal. This is very important! It will keep the second half of the shell from sealing itself to the first half. If you don't do this, you will never open the mold without breaking it.



- Add small pieces of clay around the outer edge of the Ultracal. These will created 'pry points' that will be used later on when you crack open the mold. Without them, it can be very difficult to open the mold!



Note: In this picture you can see that I added a line of hot glue along the edge of the cardboard base. This will keep the base from pulling away or allowing Ultracal to leak down. So tell me, does that thing look like a chicken thigh and leg to you, too?








- Mix and apply the Ultracal exactly as you did for the first half. Apply the face coat, let it set a bit, and then add layers of burlap. Once again I built up extra layers of burlap at the base before smoothing down the outer beauty coat.








- Allow the Ultracal to set for a couple hours.


- Use the plaster rasp to clean up any rough edges and to make sure that the second half of the mold hasn't run down over the first half, sealing them together. I like to rasp all of the way around the edge, making sure that I can clearly see the dividing line all of the way around. The Ultracal will be VERY hard once it cures fully, so now is the time to do the cleanup!



- I prefer to allow the mold to cure overnight before opening it. This will keep it from chipping or cracking when it is opened.



- Use a flathead screwdriver to open the mold. Insert the screwdriver into the pry points. You can ignore any clay that is still in them, or you can clean it out first. It helps to have some small scraps of wood or styrene on hand to shove in the crack as you open the mold. Many mold makers use pieces of cut up paint stir sticks. Open an area and place a piece of the stir stick inside to keep it from closing again. Then move to the next pry point and open the crack a bit wider. Add another stick, and so on until the two halves of the shell come apart.



- Try to keep your model in one side of the shell. This will make things easier later on. If it comes out, you'll just need to put it back in one side again, making sure to line up the dividing line with the edge of the Ultracal.








Now you have two halves of the support shell. One side will house the model, while the other side will need to be cleaned out. Pull out the clay on that side and SAVE IT in a sandwich bag! The clay will help you determine how much silicone you will need to mix up later, and the bag will keep it from drying out.



On the side of the shell that now holds your model:



- Make sure that the model is still sitting with the dividing line even with the edge of the Ultracal. It might have been lifted too high when you opened the mold.



- Adding pieces of clay wherever they are needed, build up the clay that the model is resting in until it reaches the dividing line all of the way around the model. Make the clay surface as smooth as possible, and make the edge where the clay touches the model a clean and sharp right angle. This will give a sharp edge on your mold that will give you the highest quality seam lines possible.



- Use clay to fill in any air vents in the Ultracal so that the silicone won't run into them.



- Add clay to make one half of the pour hole where you will pour in the resin. This clay should not go all of the way out to the Ultracal. (This is why you made the pour hole oversized earlier.)



- Smooth the clay to a glassy finish, and make sure that it does not overlap the Ultracal. The edges should be perfect.



- Use a small rounded tool to cut a 'gutter' into the clay around the edge of your model. The gutter should be midway between your model and the Ultracal. Most mold makers use a rounded loop tool for this, but I like to use a hair pin that is bent into the shape I like.



- Smooth the clay again, being sure to keep every angle sharp.



- Clean the top and sides of the model thoroughly, making sure to wipe away all clay residue. If you leave any clay on the surface of your model, it WILL show up in the mold, and in every casting you make afterward. Even a fingerprint can be reproduced by the silicone! Damp Q-tips and paintbrushes make good cleaning tools. Don't use tissue; it will leave lint on your model. Use as little water as possible. Using too much water will soften and weaken the clay surrounding your model.



- Spray a coating of silicone release agent on your model. This will help you remove it from the silicone later on. It is not necessary to spray the clay, but overspray on the clay surface won't hurt anything. Give the model a light coating, and then use a soft brush to spread the release into every nook and cranny. Make sure the brush doesn't damage the clay! Give the model another light coating of release, and then let it sit for ten minutes or so. Don't spray too much, as this can cause problems.









On the side of the shell that is now empty:



- First, make sure that all of the clay has been removed. The last traces can be cleaned away with a damp paintbrush.



- Find two or three points that are the lowest in the mold when you are looking at the inside. (These will be the highest points when the mold is turned so that you are looking at the outside.) The lowest points are usually inside the registration bands. Drill small holes through the lowest points, drilling from the inside of the mold to the outside so you won't risk chipping the inner surface. This will allow air to escape when you pour in the silicone later on. Two holes are usually enough, but you can add more if you want.



- Put a thin line of Vaseline around the inner edge of the mold. The theory is that it will help create a seal or gasket that will keep the silicone from leaking between the two halves of the shell.








- Fit the two halves of the shell together and strap them tightly.



- Some people put a line of hot glue around the seam at this point to keep any silicone from leaking out, but I don't. Using an oversized mold and mold strap locks the shell halves so tightly together that I've never had a silicone leak.








- Cover your mold straps with a plastic bag or some Saran Wrap to keep the straps from getting silicone on them. It tends to cause problems later on if they get coated with chunks of goop that won't come off.



- Have a flat piece of clay on hand, ready to cover the opening at the top that has been left for the resin pour spout and the air vent holes. Don't cover them now... they will serve as an escape route for trapped air until the silicone reaches that area.



- Find out how much silicone you will need for the first half of your mold. Using the clay that you saved in the sandwich bag earlier, you can see exactly how much volume you will need to fill inside there. 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. If you are creating a large mold, you might want to consider mixing two or three smaller batches of silicone instead of trying to pour the whole thing in one shot. The silicone gets thicker and stiffer once it has been mixed, which might cause problems as you pour. By mixing smaller batches, you avoid the risk of having the silicone start to thicken in your mixing cup when you're still trying to pour it.



- Pour the silicone into the hole in the top of your shell. Do this slowly, in a small stream or it will fill in the pour hole and create a trapped air pocket inside. It helps to have an assistant bump the mold gently against the table, or thump the table itself with a rubber mallet as you pour, which will spread the silicone across your model faster and can release any air bubbles trapped inside. Keep pouring until the silicone fills the mold. When it begins to leak out of the air vents or the resin pour spout, use small pieces of clay to seal them closed.



Note: I like to fill the mold about 3/4 full from the silicone pour hole, and then I turn the mold to stand upright and I continue filling it from the top, using the opening where the resin will be poured in later. It seems to give me better results, and I can wait until I see silicone leaking out of the air vents or pour hole before I seal them, assuring me that the mold has been truly filled. I don't know of anyone else who does this, but it works well for me.



- Leave your mold undisturbed overnight. If the room is cold, place the mold in a warmer area to help the silicone cure.



- Remove the mold strap and carefully open the two halves of the shell. One side will now have silicone around the model, and the other side will still be covered with clay. Try very hard to keep the silicone sealed against the Ultracal shell, and keep the model sealed against the silicone. This will keep the mold edges sealed against leaks when you pour the second half of the silicone mold. If either of them come apart, you should be able to press them back into place.



- Clean every trace of clay off the model and silicone. Save it again so you will know how much silicone it will take to fill the second half of the mold.



- Because the trigger would trap air when I pour in resin, I added a small line of clay that connects the trigger to the trigger guard. This will create a small piece of resin that I will have to cut out later, but it will act as an escape vent for any air caught in the trigger.



- Using small rolls of clay, fill the air vent channels again. Make sure the clay fills in the entire vent, and touches the model. This will keep the silicone from filling the vents when you pour the second half of the mold. This clay only needs to fit down inside the channel. It does not have to go higher than the channel.



- Use clay to fill the pour hole plug in the bottom half of the mold, and then add more to it so that you create the second half of the plug. The clay should now be a round plug, with the bottom half of the plug down in the silicone, and the top half rising above the silicone. It should look very much like a small funnel.



- Clean all of the clay out of the empty side of the shell.



- Drill air vents in the low points of the shell again, being sure to drill from the inside to the outside.








- Spray a coating of RELEASE AGENT. Give the model, and most importantly, the silicone mold half a coating of release agent. This step is very important! If you do not use the proper release, the two halves of your mold will glue themselves together, and you'll be stuck with a silicone brick! Use the soft brush again to spread the release agent over every surface, and then give everything a second coat. Allow it to dry for ten minutes before continuing.



- Strap the two halves of the mold together again, and have pieces of clay ready to seal the air vents and pour holes as the silicone begins leaking out of them.



- Mix another batch of silicone, exactly as you did for the first half, and then repeat the process of slowly pouring the silicone. Pour until the mold has been completely filled, and then let the mold sit overnight again.



- Open the mold and clean away the last traces of clay. Carefully remove your model from the mold. If possible, try not to pull the silicone out of the shell. Leaving it there will always keep things perfectly lined up, and the silicone that ran into the pour hole and air vents will help to hold it in place.



- If all went well, you now have a perfect silicone mold with a Ultracal shell! Shell or case molds tend to have very, very good seam lines. There should be little or no flashing when you cast copies of your model in this mold, and everything should stay perfectly lined up.








- Strap the mold closed again and fill it with water. (If you added a base like I did, this is when you will really appreciate having a mold that stands on its own!) If your seals are good, the water will only run out of the air vents and the rest will be perfectly watertight. Cover the air vents to hold the water inside, and measure how much water it takes to fill the mold. That will tell you how much resin you will need to mix later.



- Label the mold with a permanent marker. I like to add the date so I will know how old the mold is later on, and I write down how much water it took to fill the mold so I don't have to check that again each time I cast a new blaster.









- Now you can cast copies of your model! Silicone molds will not stick to most casting materials, but you should always use a release agent. It will greatly increase the lifespan of your mold, and will make it easier to remove your castings from the mold.



- Add a coat of Vaseline or paste wax around the air vents and around the pour hole to keep resin from sticking to the Ultracal. Reapply it every now and then since it will wear off with each casting.



- When you pour casting material into your mold, have some lumps of clay or pieces of tape on hand. As you pour, casting material will begin to leak out of the air vents. Allow it to push out all of the air, and then press the clay or tape against the holes to seal them so that no more material will be wasted. It is not necessary to seal the pour hole, since that should be at the very top of your mold.



- Have someone gently bump your mold against the work surface and tip it a little from side to side while you pour in the resin. This will help to remove any air bubbles that may have been trapped inside. It also helps to have someone thump continuously on the tabletop with a rubber mallet as you pour. It jars the air bubbles out of the casting and makes them rise to the top.



- Allow the casting material plenty of time to cure. You don't want to get impatient and open the mold while the material inside is still soft.





And, that's it. Wow... that tutorial took forever to write. Hope it helps someone out there!


Pam :-)

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