Making RTV Molds for Spincasting

When I first started casting, I was using gravity casting with RTV molds. Essentially, what this means is that I would have a small rectangular mold with the cavity of what I wanted to duplicate inside. It had a hole in the top, in to which I would pour the molten metal. The detail wasn't the greatest, because the only thing forcing the metal into the mold was gravity. However, the method had the advantage of being cheap, and I could make my own molds.

How did I make my own molds? I used RTV rubber, which is short for Room Temperature Vulcanizing rubber. I could build a mold box just big enough to cast what I needed, so it was very economical, and the rubber I used, a putty substance called Castaldo Quick-Sil wasn't that expensive to start with.

Now, however, I use spin-casting to get much greater detail from my castings. This method of casting uses a large rubber disc shaped mold, which is spun in the machine at high rpms. When the metal is poured in, centrifugal force causes the metal to fly outwards into the mold cavities. This provides much more pressure than gravity casting, and consequently provides much more detail. As far as I know, all professional metal minis you can buy in the stores are cast using this method. It is far superior than gravity casting in every way.

Except...the big round rubber molds are expensive, and they require very expensive equipment to make --which I don't have. I have to send out to get my molds made. This is fine if I need 15,000 units, but what if I only need a few? Or what if I want to cast up my own masters to make my mold-maker's (Hi Jeff!) job easier.

Well, I have come up with a way to use the RTV compound to make spincast molds, using old rubber molds as a frame. I've heard that other people out there have had some success doing this, but I thought I would share my method in the hopes of opening doors for someone else out there that want to try it, or perhaps learn something from someone out there who already does it better.

Anyway, they say a picture is worth a thousand words, so I have prepared a visual tutorial to hopefully save on some words.

1.

This is an old rubber mold I have been using for this experiment. The mold used to be used to make my gaming tokens, but then I resculpted them, and so the mold was retired and I didn't feel bad about cutting it up. The little blue rectangles are my first efforts, and they work! 

2.

Here's what it looks like on the inside. You may be ale to see on the left-hand disc where I have marked out a small rectangle where I will be placing my next mold on the disc. 

3.

I'm using an x-acto knife to cut out the circle. It ain't easy. First I make several passes along each line of my box, cutting deeper each time. When I've gotten deep enough, I force the knife all the way through and then work it back and forth down the line until I've made my cut. 

4.

I have successfully cut one side out. Note that on the right hand side, I cut in a little triangle at the top to help act as kind of a 'Mold Key'. Owing to the fact that all these holes are hand cut, they are all extremely individual anyways, but I figure this will help me recognize which molds go in which holes later on after I've been swapping them out for a while (That's right--you can pop these molds out, put 'em back, and they still work great!) 

5.

Finished with the hard part! After cutting the first hole, I put the mold halves back together, and then trace the shape of the hole over onto the other side. Then I cut it out using the same tedious knife work.  

6.

The sulpher in the rubber mold apparently acts as an inhibitor to the RTV...it just won't cure if it is touching the original mold rubber. My solution: paint the inside of my new mold frame with gesso. Gesso is an acrylic sealant used by artists to seal canvases or board prior to painting on them. It creates an impermeable layer, so the paint sits on top of the board instead of soaking in. It also works pretty good here. I put on two coats to be sure. Normally, I would have used regular white gesso, and you would have been able to see it better, but my white was so old and crappy that it was pretty much unusable. The brown stuff was still good, so, as I'm cheap, that's what I'm using.

7.

Here I have placed the bottom half of the mold onto a big  plywood  circle I got at the Home Depot. I then fill the mold cavity with plasticine modeling clay.

8.

I've just finished sculpting a 15mm skeleton footsoldier, so that is what I am going to be making a mold of. I set him into the clay, and carefully build it up all around him, using the clay to control where my parting line is going to be. Note that I've also formed the spue from clay as well. 

9.

On the other side of the mold, I fill up the sprue entrance with clay, so the rtv doesn't go down in there when I fill up the mold and put it under pressure. 

10.

Now it is time to make the first half of the mold. I fit the two rubber halves together, and then break out the mold goo. I scoop out roughly equal portions of it, of an amount that looks like it will fill the space I have. A little bit more, actually, because it is better to have too much mixed up than too little. Note that it still is not a lot at all. Imagine if I had to make that whole ring from rtv! 

11.

I mix the two colors together, and now I'm on a time clock. This stuff cures completely in about fifteen minutes, so I really only have about 2 minutes from the time I mix it until I have to have it all packed into my mold and clamp it down. I mix the two colors until they make a more or less uniform light green color. It doesn't have to be perfectly mixed for it to still work properly. 

12.

I pack the mold goo over the skeleton, and fill it up till it is just a little bit taller than my vulcanized rubber 'mold form'. I try and make sure I really press it in all around the skeleton, and try to avoid making any air bubbles in it. 

13.

Since the cured rtv would have a tendancy to stick to the porous pressboard, I throw a piece of acetate on top of it. 

14.

I then put my other pressboard circle on top of the mold, and use C-clamps to apply a good amount of pressure. The This is why I overfilled the cavity a little bit...the pressure makes sure the rtv gets into every little detail of the figure. 

15.

15 minutes later, I carefully pry the two halves apart. If I've made my mold right, my original should be stuck in the rtv half now, not the clay. Success! If it had stayed stuck in the clay, I would have just had to dig it out and transfer it to the rtv half, but I hate to do that, because you never know if you don't get it back in there quite perfect, it may cause some distortion in the cast. 

I should point out at this time that the back side of the mold is now pretty much perfectly flat. The rtv under pressure ends up flush with the vulcanized rubber mold frame. The reason is that the outer rim of the frame around the hole I cut will bow out a little bit under the pressure, making enough room for all the rtv. I have learned that this can cause problems later on, as the the vulcanized rubber pushes back on the rtv, and can cause some distortion. We'll fix that problem in our final step.

16.

First, however, we need to make the other half of the mold. I carefully apply a thin coating of mold release over the rtv. Not the miniature, though. This stuff pretty much just sticks to itself, so that's the only place I need to apply release. 

I have also removed the clay from the other side of the mold frame, and cleaned out any stray clay bits from the frame and from off of my skeleton mini. Also, note that I have filled in the indent I made for the sprue with clay as well, so that after I cast this next side, there will be a hole for the metal to go down, and I won't have to cut one. 

17.

I fit the two halves of the frame together, and simply repeat steps 10-15 to create the second side of the mold. 

18.

Here we have the two finished halves of the mold! 

19.

As I mentioned earlier, we have to correct the problem of the outer section of vulcanized rubber mold frame putting too much pressure on the rtv. To solve the problem, I simply pop the rtv mold out of the  frame, and trim off about 1/8" - 1/4" off the back side. I pop it back into the mold, and it still fits great, but now I won't get any distortion. I also cut some air vents at key places, and then it is off to go try it out!

20.

From the one, comes many! It takes me a little work to get his sword to start coming out good, but I finally get it, and now my itty-bitty skeleton army is well on it's way!!!