Introduction: Making a Silicone Mold 'Zombie Head'

  • Hi, my name is Ken and this is my first Instructable. Today I will be showing you how to make a silicone jacket mold. This is a silicone mold that will have a hard casing around it to help it keep its shape. Our mold today will have a plaster jacket. A jacket mold is a little more time consuming and more of an advanced mold but what it is good for is storage and price. If you have ever priced RTV silicone it gets pricey. This will minimize the silicone needed for a project. Our subject: a 1/3 scale zombie head I sculpted in 'Roma Plastalina' clay.
  • You will need:
  • Plaster
  • Modeling clay
  • Foam blocks or sheets
  • Burlap
  • Vaseline
  • Howard Feed-n-Wax
  • RTV silicone
  • Crystal clear spray paint
  • Mixing cups,
  • Paint brushes,
  • Tongue depressors or popsicle sticks
  • Weight scale

Step 1: Prep the Sculpt

    1. First thing give the sculpt a coat of crystal clear spray paint. You want to seal your work. This sculpt is in Roma clay and it is sulfur based so if you don't seal it most silicones will not dry properly. Give it a good 2 or 3 coats. Be careful not to get any runs or put so much on you lose detail.

    Step 2: Parting Your Mold

    1. This being a head I will part this front to back. This is so you don't have a seam in a spot with a lot of detail. You will want to part your sculpt at the apex. This is so you don't get any under cuts and the part won't get stuck in the mold. It is always a good idea to place your sculpt on a board other then your table this keeps the work portable and you can rotate it. It also let you do things like screw it down if it is needed, in this case I did. I used foam blocks to start the build up for my parting line. You want to make the foam come up to about 1/4" to 1/2" below your intended parting line. If you placed it on a board you can hot glue your foam blocks down. Now the foam is in place you can finish the build up with the clay. I like to use a softer clay of a different color for this Kleen Clay is a good clay for this it's cheap and available at any craft store. Build up the clay to your desired parting line carefully working the clay to the sculpt. This is when you start to realize the usefulness of the sealing clear coat. It makes your sculpt a lot less fragile and can protect it from minor hits from your sculpting tool. Once you have this done you now have a dam. Now you need to add keys to locate the molds two sides. You need two or three for a mold this size and shape. Take some clay and roll it into a long tootsie roll a few inches long and about 3/4" around. Now cut it 1-1/2" long press them to about 1/4" flat. Place your now flat tootsie rolls on your dam. I did one by each ear and at the top of the head this placement will keep it from doing any X or Y motion. Take a knife and cut your tootsie rolls on three sides at a 45 degree angle. Last thing is to add a key for the silicone, make a small groove around the sculpt about 1/4" out from the sculpt on the dam. This can be seen in the pics.

    Step 3: Making the Inner Core

    Okay, this is the part that I might lose a lot of you, hang in there. Now that we have put a dam around your sculpt that will separate the front from the back we need to make a cavity for the RTV silicone will be. First get out your crystal clear spray paint and give your dam two coats and let it dry between coats. Once it is dry take wet paper towels and cover just the face of the sculpt but not the dam. Now you need a flat surface two sticks of wood 1/4" thick 1" or more wide about a foot long and a rolling pin or something like a rolling pin. Take some clay place it between the two sticks and roll it flat until it is a consistent 1/4" thick. Cut off the ragged edges and start to place them onto the face of your sculpt over the wet paper towel. Cover the whole face one layer only in sort of a patch work quilt look. After the whole face is covered you need to add a small flange of clay on the dam. Take more of your 1/4" clay and cut it into 3/8" to 1/2" strips and lay them flat on the dam contouring around the 1/4" build up over the sculpt. Now fill in the 'quilt lines' with more clay and make a nice concave transition from sculpt to flange. The last thing you need to add is a fill spout. Roll some clay into tootsie roll about 1" around and 2" long. Place it sticking up at the highest point and blend the edges.

    Step 4: Plaster Jacket

    Now it is time to start making mold parts. On this example I'm using plaster but any hard drying material will work. I like fiber glass personally or Ultracal 30 which is a lab stone 'aka super hard plaster'. Mix your first batch of plaster and with a brush start coating over the surface slowly building up the thickness. Try not to cover the top of your pour spout. You can work plaster until it reaches the point it no longer compacts and just starts to crumble. Stop with the first batch at this point. Try to make it smooth for the next layer. In this coat you will be adding burlap to it. Mix your plaster on the thin side not watery and then put a strip of burlap in the mixture, pull it out and lay over the first coat of plaster, cover the whole surface, except the top of the pour spout with overlap then add a second layer change direction if you can. Make sure your flange is thick, add a third layer to it to make sure it is thick enough. As the plaster is drying try to clean up any rough edges.

    Step 5: Getting Ready for Silicone

    After letting the plaster dry for a few hours it is time to remove it from the sculpt and dam. You want to be gentle doing this. My sculpt had a wood stick holding the head up at the neck so I was able to start prying from here. If you don't have this place a block of wood that is the same height as the top of the dam and go around the edge with a screw driver or other pry bar and work it loose. You want to break the seal at the dam and sculpt surface and lift the plaster jacket straight up off the both. If all goes well the clay that you put over your sculpt will stay on the plaster side. The wet paper towel should have protected the sculpt and if the prying wasn't too bad the dam should be intact. Peel the clay out of the plaster side of the mold and place it in a mixing cup that will fit all of it, I will return to this later. Clean up any imperfections. If you need to you can use sand paper to clean up any ridges inside the plaster jacket. Once clean blow or brush out the plaster jacket and coat it with a wax. I've been using Howard Feed-n-Wax, it is a wood polish but works very well and you can buy it at Home Depot. Brush a generous coat on all of the inside of the plaster jacket and let it sit to soak in. Now you want to repair any damage that may have happened. Make sure there are not any gaps between the sculpt and the dam, if there is carefully fix them. After the wax has had a chance to soak in fit it back over the sculpt and onto the dam. Make sure there are no gaps between the jacket and the dam it should but right up to it. If you can smear some clay between the dam and the jacket on the out side. This will help seal and keep the jacket from moving.

    Step 6: Pouring Silicone

    Now we are returning to the mixing cup with the clay that was removed from the jacket of the mold. Press the clay in the cup down flat trying to get all the air pockets out of it. Once this is done look at the measurement on the cup, this is roughly how much silicone you need to mix. Go slightly over 20% or so to make up for fine detail that was not filled. If your molding something with a lot of cavities increase to 30%. You don't want to mix a ton more then you need and this is alway a great starting point. Pour your silicone to the amount you need and catalyze it to the manufactures specs usually 10% and mix well by hand. A mixer would put too much air in the mixture. Now start your pour, you want to take your time. Pour it in like you are trying to waste as much time as possible. A very thin stream stretches a lot of the trapped bubbles out of the mixture and allows the silicone to fill the mold as to not trap bubbles making air pockets. As the silicone nears the pour spout stop and let any bubbles vent out for a few minutes and start pouring again just short of the top. Stop again, check and see if any silicone is leaking out. If it is plug it with clay. now watch and wait. Silicone is quite viscous and all the little bubbles should be coming together at the spout. As they vent out the silicone level will drop and you need to top it off. After a half hour and no more bubbles you're probably good, keep an eye on it until the silicone starts to gel and let it sit over night. Side one is done.

    Step 7: Second Side of the Mold

    Day two. If everything was done right the silicone should be full to the top of the spout and set. If you screwed your sculpt to a board it is time to remove it and flip the whole thing over. Carefully remove the foam blocks you used to build up the dam and now the clay. Do not try to remove the sculpt from the silicone, keep this all in place until the mold is complete. Look over everything, repair any damage if any to the sculpt. If any silicone got between the sculpt and where the dam was very carefully remove it with an Exacto knife or small scissors. Now you need to repeat everything from "Making the inner core" through "Pouring silicone". The only new thing you need to add is you MUST Vaseline the flange on the first part of the mold, or good luck getting it apart.

    Step 8: Opening Your New Mold

    Opening a new mold can be tricky sometimes. If I'm doing a fiberglass or plaster mold without the silicone I like to do it when it is still a little warm, the clay is soft and makes it easier. That is not possible with this kind of mold, unless you want to put it in a oven at 150 degrees for an hour. You want to see your work done now. I will use a screw driver to start the opening process but you don't want to pry too hard on any mold. You can usually see the separation point of the two parts. Put the screw driver in one of those points and give it a little tap with the handle end of another screw diver. I say this because I don't want you to beat up all your work with a hammer. All you want it to do is pop the seam between the two halves even if you could only stick a piece of paper between them this is good. Popsicle stick are great now. Sand one end so it looks like a chisel insert it into the largest opening in the crack and tap it in then move along the seam till you have done the whole flange start this over again on top of all the sticks.Iif the mold has not popped open add a third stick or a paint paddle sanded in the same manner. Once you can get your fingers between the two halves you should be able to get them apart. Most of the time the sculpt is going to stick to the silicone and the jacket will slide off the silicone, baring any bad under cuts. After the jacket is off slowly peel the silicone rubber off starting at the parting seam. Once it starts to separate the two halves should peel apart easily. After the parts are separated you can trim any excess off with a small pair of curved scissors. Put all your parts in place and you are ready to start making parts. Congratulations. If the silicone wants to keep sliding out of the jacket use Vaseline on the inside of the plaster jacket, the sticky nature of it will keep it in place. A silicone mold can be used to cast most any material, it can cast silicone parts but make sure to use a good release agent.

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