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Twi'lek Pam

Making a Two-Piece Silicone Block 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, but it uses a minimum amount of silicone. Both mold types can produce high quality castings.


This tutorial is for a simple block style two-piece mold. I will demonstrate case 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.






- 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.



- 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. I like to 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 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.



- Roll a 1/2 inch thick layer of clay across your work surface, making it at least 1 inch wider than your model all of the way around. 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.



- Adding pieces of clay wherever they are needed, build up the clay 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.



- Trim away the excess clay so that it is 1/2 inch wider than the model all of the way around. Follow the contours of the model. Remember, sharp angles are better than rounded ones. They help the pieces of the mold and shell lock together tightly.



- Use clay to build up 1/2 of the pour hole. This lump of clay, 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.



- Use small 'worms' of clay 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 touch the model as unobtrusively as possible, and 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.



- Clean the top and sides of the model thoroughly, making sure to wipe away all clay residue. Trust me, 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. Use as little water as possible. Using too much water will soften and weaken the clay surrounding your model.



- 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.



(Note: In this picture you can 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 vents to those places. Because the trigger would trap air, I also 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.)








- Build a retaining wall around your model. Make the angles sharp, following the contours of the clay that you trimmed earlier. The wall should be at least 1/2 inch away from the model, and should be a couple 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 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. Check the information that came 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 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!)






- 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. Fill the mold until there is at least 1/4 to 1/2 inch of silicone covering the highest point of your model.



- Leave your mold undisturbed for at least 15-20 hours. 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.



- After the silicone is cured, carefully pull off the retaining wall.



- DO NOT take your model out of the 1/2 mold you have now made. Keep that seal tight, so it won't be damaged before you pour the second half of the mold! Very carefully, pry the model off your work surface. I have found that it works best to use an old cooking spatula. Slide the spatula between the work surface and the clay that is underneath your model, lifting the clay and everything that is sitting on top of it. Then turn the whole thing over and set it down with the silicone on the bottom. Carefully remove all of the clay. When you have removed all of the big pieces, use a brush or small tools to clean away the last residue. I sometimes put my model in the sink and use the spray hose attachment to wash away the last tiny bits of clay residue. If you do this, dry your model and the silicone well with paper towels, being careful not to pull on the edges of the mold. As long as you're cautious, you won't break the seal between the silicone and your model.



(Note: In this picture you can see the empty channels that were created by the pour hole plug and the air vent 'worms' that were created with clay. You can also see the notches that were left by the clay keys.)






- Using small rolls of clay, fill the air vent channels. 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. I also added clay to the vent that connects the trigger to the trigger guard.



- 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.






- Build another retaining wall. In my experience, if you're not using a professional quality mold box, it's better to make a new retaining wall rather than trying to reuse the old cardboard one. The wall should fit as tightly as possible against the silicone. If there are any places where the retaining wall is not tight against the silicone, then use a bit of clay or a squirt of hot glue to seal the crack. You don't want silicone to leak into the holes. The wall should be a couple inches taller than the highest point of the model.



- Spray a coating of RELEASE AGENT. Give the model, the retaining wall, 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 a soft brush to spread the release agent over every surface, and then give everything a second coat. (If you forget this step, it is possible to use an exacto knife to slice the rubber back open again... but it would create a lower-quality mold and you would probably need to start over again. You also run the risk of damaging your model with the knife.)








- Mix another batch of silicone, exactly as you did for the first half, and then repeat the process of slowly pouring the silicone over a low place beside your model. Pour until the model has been covered, and then let the mold sit for 15-20 hours.



- After the mold is cured, remove the retaining wall. DO NOT open the mold yet! Keep those two halves sealed tightly together! In theory, you now have a complete mold. But, silicone is flexible. While that's great for removing it from your model or your castings... it's not great when it comes to keeping a specific shape. If you were to cast something in the mold now, you run the risk of the casting becoming warped. It's also difficult to tightly strap together the two halves of a mold when they can flex. You could get the outer edges clamped closed, but the inner area could warp outwards, making your casting 'fat' in the middle. It's safer to create a support shell that will hold your mold in the exact position that you want, and in order to make that shell as form-fitting as possible, you should make it BEFORE you open the mold!



(Note: In this picture I hope you can see the keys that will help line up the two halves of the mold. I'm finding it difficult to take good pictures of all-white semitransparent molds!)






- Now you need to make the two-piece support shell. Begin by placing your mold back on the work surface. Build up a layer of clay around the outside, making it as high as the middle seam of your silicone mold. The clay should be about 1 inch wide.







- I'm sorry I don't have pictures of the next steps, but I will continue to describe them for you. Next time I do a two-piece mold project, I'll make sure to take pictures of the shell building. Sorry!



- You do not want the shell to get in the way of your pour hole or the air vents, so you will need to add extra clay to these areas. I angle a lump of clay outward just like I did with the first pour hole, so the shell will become a part of the 'funnel' I created in the silicone. For the air vents, I add clay 'worms' so that the vents keep going from the silicone and right out of the shell.



- Here's a tip that might make things easier later on. Consider the location of your pour hole. When you hold your mold so that the pour hole is on the top... will the bottom of the mold have a flat surface so that you can set it down on a table? If it does not, you have two options. You can set the odd-shaped mold on the table and wedge things around it so that it will be supported and won't fall over. (But you risk spilling casting resins on your support materials.) Or, you can widen an area of the support shell to create a flat 'bottom' for your mold so that it can stand on it's own!



- Add keys that will help you align the two halves of the shell later on. Rigid products do not like right angles, so don't use rectangular keys this time. Instead, create half-circle domes in several places. The domes should be about 1/4 inch tall, and about the size of a nickel or quarter. Smooth them so that they blend into the surrounding clay.



- Build another retaining wall. This wall needs to be at least 1 inch wider than your silicone mold, and it should be a couple inches taller than the top of the silicone.



- It is not necessary to use a release agent, but you can if you want to. The shell material will not stick to the silicone or the damp clay. It might stick to the retaining wall a bit if you are using cardboard that doesn't have a smooth facing, though, so if you don't want fragments of cardboard stuck to your mold shell, you can give the retaining wall a light coating of Vaseline or mold release wax to prevent that from happening.



- Fill the retaining wall. You can use plaster, but I don't recommend it. Plaster is soft and will wear down each time you clamp or bump your mold. It is also prone to breaking easily. Instead, I recommend using Ultracal or Hydrostone. Or, to save a bit of money, you can make a 50-50 blend of plaster with one of these harder products. It is also wise to press some burlap strips into the shell material before it begins to harden. The burlap will help protect the shell from breaking apart.



- Allow the shell half to cure overnight, and then remove the retaining wall. Once again using the spatula to lift the clay off your work surface, pick the entire collection of materials up and turn it over so that the whole thing is resting on the plaster shell. Clean off all of the clay and use a damp brush to wipe away any bits that stay in the edges.



- Once again you will need to fill in the pour hole and air vents with clay to keep the second half of the shell from filling in those spaces. Continue making the pour hole into a funnel.



- Build another retaining wall or reuse the wall from the last step if it is still in good shape. You will simply need to turn it upside down. Seal the edges, making sure the wall is tightly against the shell in the bottom. Fill any gaps with clay.



- Coat the shell with Vaseline or mold release wax. Once again, if you forget this step, the two halves of your shell will seal themselves together and you'll never open it without a hammer! You can also coat the retaining wall if you wish.



- Mix and pour in the material for the second half of your support shell. Allow it to cure, and then remove the retaining wall.



- NOW you finally get to open up all of the pieces and find out how the mold looks inside! If everything went well, you will be amazed by how well the silicone reproduced every detail of your model. You will 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.



- To find out how much casting material you will need, remove your model, seal the mold closed, and fill it with water. Then pour out the water into a measuring cup, and find out how much water it took. Write that amount with a Sharpie on the side of your mold, so you'll never forget. I recommend always mixing a little bit more casting material than the amount you wrote down, just to be on the safe side.



- When you pour casting material into your mold, have some lumps of clay or pieces of strong 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.



- Gently bump your mold against the work surface and tip it a little from side to side. This will help to remove any air bubbles that may have been trapped inside. Now, aren't you glad you didn't use fragile plaster?



- 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.



(Note: This is a casting made of black resin. You can see the plug where the pour hole was located. It will be a fairly easy task to cut the plug off and smooth the area. The mold was tightly sealed around the outer edges, so very little resin leaked out between the two pieces of silicone. The leaks, called 'flashing' were less than paper thin, and broke off as soon as I started handling the blaster. I have already snapped off the two air vents in this picture. Sorry, I should have left them for a reference, but I didn't think of that in time! The area inside the trigger guard has the flashing still present. The layer of resin you can see around the trigger is actually very thin. You can see sunlight through it. It will be an easy task to dremel the flashing out of that opening.







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