MOLDING & CASTINGSAVERIO SILLI

RUBBER MOLD & CASTING RESINTIPS ON HOW TO MAKE A

PLUS “COLD CASTING”TECHNIQUE

MAKING THE ORIGINAL PIECE (MASTER)The piece we want to cast can be anything, in this case it is a model made with plastic sheets and tubing, but it can be a 3D printed part or a CNC milled part. It could be something that was already existing like a toy or a part of a device, but in this case be sure not to infringe any copyright (in the prop-making industry it is called “recasting” and if you are caught doing it your reputation is dead).

PREPARING THE TWO VALVES MOLDThe master is surrounded by clay around its parting line. We use clay to make the channel used to pour the resin. We build a simple box with wood and hot glue and we make “registration keys” into the clay, so the two valves will match automatically later.

POURING RUBBER IN THE MOLDThe first layer of rubber is brushed on the mold to avoid bubbles being trapped on fine details. Then the rest is poured in the box which stays on a leveled table. When the first valve is cured, you turn the box, remove the clay and pour the second valve. Remember to add demolding compound on the first valve or…

COMPLETED TWO VALVES MOLD - ISSUES…you’ll end up with a solid block of rubber which you’ll have to cut with a knife like in the picture. By the way, if you manage to align the two valves well when you assemble the mold for casting, you can get nice casts with no flashing on the parting line.

“COLD CASTING” AN OBJECT IN A ONE VALVE MOLD"Cold-Casting" is a term used to describe the process of mixing metal powder with a translucent casting resin and applying the mixture into a mold. The resulting cast give the appearance and weight of solid metal. I started downloading a Star Wars' Jedi model and I modified it to be used as a cufflink, then I used the DWS stereolitography 3D printer to achieve a great level of detail.

MAKING THE MOLD BOXThis time I used poor material I had laying in the lab: a block of MDF as the base, four rectangles of cardboard and hot glue gun. Once the box is made, I taped the master to the box using double sided tape.

MIXING THE SILICONE RUBBERThe first thing to do is to mix the silicone rubber alone, then you weight it following the mixing ratio described by the manufacturer, and mix the rubber with the hardener. Use a sturdy stick of wood to mix rubber, cause it has high viscosity and you’ll spend five minutes stirring, so better use something comfortable. Remember to scrap many times the sides of your cup, you want ALL the rubber in your cup to be stirred and mixed.

POURING THE SILICONE RUBBERAs usual, brush the first layer of rubber on the master, then pour the rest of the resin. Remember to pour it holding the cup high over the mold box, so the rubber will go down in a thin string. Start from the bottom of the box and let it grow up until it submerge the highest point of the box.

Wait a day, then break the box and demold: cross your finger for bubbles!

COLD CASTINGFor the process you need urethane resin, aluminum powder and a black tint that works with urethane (similar to Smooth-on So Strong tint). Put a small amount of aluminum powder in the silicone mold and "flour" the mold to lay a thin layer of pure aluminum.Mix the resin with the black tint a little amount of aluminum powder and pour it in the mold.Optionally you might sink something into the resin when it is still liquid. I put the cufflink support in the mold and used a piece of wood as a support.

BRINGING OUT THE SHINE AND FINISHINGAfter an hour, you can take out the piece and you see the finish is dull because of the first layer of pure aluminum.To polish the piece you can use steel wool or work it over a fabric disk mounted on a drill press. You can use polishing compound for a very shiny effect.

STEP BY STEP COLD CASTING PROCESS‣Put a small amount of aluminum powder in the silicone mold and "flour" the mold like you would do with flour when preparing a cake pan. This will lay a thin layer of pure aluminum that will dramatically increase the finish. Aluminum powder sticks to the silicone very well. ‣Pour the amount you need of part A in a cup and add one drop of black tint, and stir well. ‣Put in the same cup the same amount in volume of aluminum powder and stirr well. ‣In another cup pour part B, it has to be the same amount of part A in volume (mixing ratio is 1:1, the aluminum powder is an extra). ‣Pour part B in the cup with part A that you already mixed with black tint and aluminum, and stirr very well: the reaction and pot life starts now ‣Pour the mix into the mold. ‣Optionally you might sink something into the resin when it is still liquid. I put the cufflink support (bought at a cheap jewelery supply store) in the mold and used a piece of wood as a support to keep it in place. ‣After an hour, you can take out the piece and you see the finish is dull because of the first layer of pure aluminum ‣To polish the piece you can use steel wool or work it over a fabric disk mounted on a drill press. You can use polishing compound for a very shiny effect.

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