This will be a 2 part mold. 2 part molds show detail completely around the object being molded. Each half of the mold will cover half of the object you chose to copy.



PREPARE



1. To start, begin by making a wire ring that fits the container you're using. Nothing fancy, it's only there to help pull the clay and rubber out of the cup after it's cured. A wire coat hanger will suffice.



The ring isn't totally necessary, but makes things easier when removing the first half of the mold.



2. Next, take a small lump of clay and drop it into the container. Press it down, getting it deep into the corners and all around the ring. Make it as flat as you can on top. I modified an old coffee tamper to fit my candle holders and that seems to work quite well.



3. Place your object in the center, on top of the clay. Press it in about half way. If your part has protruding appendages, you'll need to build the clay up a bit under the parts that stick up. Make sure you don't cover anything over. Ideally, you want to be able to look straight down at the clay from above and see the entire perimeter of the part.



If the clay is covering a portion of your master, push it back so the part is exposed, even if it's below the surface. Be sure the part is snugly surrounded by clay, that any openings or raised parts are exposed and supported and there aren't any large undercuts.



4. At this point, using a pencil eraser or similar object, press a couple of locating holes into the clay. These will keep your mold halves lined up when it comes time to inject plastic.



5. if you're unfamiliar with the container you're using, now is a good time,to determine what the volume of molding compound you'll need is. Don't worry, there's no math involved... Just coffee. Because coffee is dark, it will be easy to see in light colored molds.



6. pour coffee into the mold container until it's level with the top. Then transfer it all into the measuring cup you're going to use to mix your RTV. That's it. Note the level and mix the same amount of rubber when it's time to fill your mold.



6. Now carefully wash the mold with a soft brush, soap and water. Because it's dark, it'll be easy to insure you've gotten all of the coffee out before pouring RTV. Clean the dirt and grease from your hands as well as long as you're at it.



7. The half of the mold you're pouring first, will determine when to mount the sprue you made. The gun carriage I'm making will have the top half of the mold poured first. That means I'll have to mount the sprue to the master before the first pour. You can see where I mounted it, using super glue, pointing straight up and somewhat centered in the container.



Note: If you're using your great aunt's priceless gold and diamond earring as your part, don't ruin it with superglue. Use blue tack, or whatever they call it in your neck of the woods that parents use to hang all the stuff their kids bring home from school without ruining the walls. This stuff is incredible. I'd be lost without it.





MIX & POUR



Mixing silicone RTV is a sorcerer's mystical combination of weight, volume, time and luck. I never bothered too much with it all, discovering early on that "more is better". We already know how much rubber we'll be needing. The problem comes when attempting to guess how much catalyst needs to be added to cure it. My method seems to work pretty, pretty, pretty well.



The instructions say "add 10% by weight"........................ Ya, Right.



I totally guess the 10% part. Then I add half as much again. Seems to work every time. Of course, the RTV sets up pretty fast the way I do it, which means the bubbles have to be removed ASAP. No problem. I have that covered as well.



You need to keep bubbles to a minimum. You can help by pouring the mixture from the side, allowing the thick liquid to flow into and around your part. Don't pour it directly on top of your part, whatever you do. You'll carry a lot of air along with it.



Once you've filled the container with silicone, you'll want to get as many bubbles, trapped in the mixture out, or at least up from the bottom where they can't do any harm. You can repeatedly slam the mold down on a hard surface if you're OK with possibly breaking things free. I do that a couple of times just to break some of the more stubborn bubbles free.



Then, I use a hand sander to vibrate my mixtures. Placing the sander upside down on the bench (with no paper in it), turn it on and balance the ceramic cup on top, close to an edge, letting it bounce around like a happy goat. You'll see bubbles come to the surface and burst open. A few minutes of this and the bubbles will drastically reduce both in size and number.



Now, if you measured the catalyst according to the instructions, you'll have to wait here a little longer while your rubber cures... It should be ready in about 24 hours.







The rest of you, read on...



