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Clinic - Advanced Resin Casting

Working with Two Part Molds

     In this clinic we will work with two part RTV molds.  Our first step is to take some modeling clay and create a block of clay that is at least 3/8 of an inch bigger than the part on all sides.  The surface that will face up needs to be as flat as possible.  Next, place your part into the clay until 1/2 of the part is buried in the clay.  We then use the principles I taught in the introduction clinic to build a box around the block of clay.  Build the box so that it surrounds the part and is taller than the part by at least 3/8 of an inch.  Once the box is sealed mix the RTV and pour it into the box.  Allow this to cure overnight regardless of what the direction say.  There are RTV's out there that say that they are ready to use the mold in 4 to 6 hours.  Don't believe it.  It's better to wait.

     After allowing 24 hours for the RTV to cure we pull the mold out of the box.  Remove the clay but be very careful not to remove the part from the RTV.  Clean off any clay that might be left on the part.  I then cut a couple of holes into the mating surface of the rubber.  This will create 'keys' so the two pieces of rubber will go back together the same direction every time.

     You will want to cover all of the rubber that will touch the rubber being poured in to create the other part of the mold with a mold release.  It is crucial that this step be done.  If you don't cover the rubber around the part with a release agent the mold will become one big piece of rubber that you will have to cut up to get your original out of and thus ruin your mold.  You can buy mold release but I use plain old Vaseline.  Be very careful not to get any on you part.  Just cover any rubber surface that will come in contact with the other part of the mold.

     After the keys are cut and the mold release is in place, build another box around the lower part of the mold.  Remember to build the box tall enough to clear the top of the original part by 3/8 of an inch.  If you were careful taking the first half out of the box you might be able to reuse the pieces.  Mix up the RTV according to it's directions and pour into the box.

     After letting the second half cure for 24 hours you can remove the mold from the box.  You then remove the part from the mold and clean off the Vaseline.  Next, you will need to pick a location for a 'gate' and cut it out with hobby knife.  A gate is the location from which you pour the resin into the mold.  Try to pick a side of the part that will be facing down to away from the viewer.  Start at the edge of the part and cut to the outside of the mold.

     Once this gate is cut you are ready to pour your first casting in the mold.  Place the two halves of the mold together and rubber band the two halves together (or clamp...lightly).  The idea is to keep the sides together and keep the resin from running out before it hardens.  

    That's all there is to it.  From here several parts that you might have thought would be impossible to mold and cast yourself become possible.

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