Workday 1 -- First of all, the sculpting job; below you're looking at my third attempt at it. The first was with some white clay on the right side of the photo -- actually pretty good, but I couldn't get it as small as the scale really required. Then I got the professional Blue/Yellow ("green stuff") clay and that does work much better. The one thing to watch out for is that it's very sticky -- it sticks to everything: fingers, desk, paper, plastic, sculpting tool, you name it. This job is pretty much at the very lower limit of what I could possibly accomplish; the fuselage of the fighter is just about as big around as the 1/16th inch wire I'm using for armature (stuck into a cork for support and handling). I have a magnifying-lens light, but I really don't like using it. Most of the work is with my fingers, sometimes using the pointy dental tool, and you can see I'm using an illustration from the SFKH rulebook (and the assault scout mini) as a guide. This took about 1 hour or less (the green stuff gets hard pretty soon after that), and came out much better than I had any right to expect for my first sculpting attempt. Certainly I was encouraged that my first project was based on a few simple, rigid geometric shapes, so I didn't have to deal with complicated organic stuff like skin, hair, fabric, etc.
Workday 2 -- Today I can start making the mold for the UPF Fighter miniature (my goal at the end is to have a mount of three fighters, but for now I start with one). One of my priorities is to save as much of the rubber compound as I possibly can -- it's really easy to waste a lot of it, especially on teeny-tiny projects like this one, where the leftover residue can easily be more than the mold itself. So while for the mold box I've seen a rigid container like a cat-food can suggested (and probably does have structural advantages), I'm making my own mold box out of simple poster board, sizing it exactly how I want (about 1/2" beyond the sculpture on each side), and that's working out just fine. Put the glossy side inwards to help a bit separating stuff out later. If you were doing a much larger project, you'd want a different technique, because the poster board structure obviously starts warping and bulging outward when it tries to hold a greater mass of rubber compound.
Take some Play-Doh type modeling dough and half-fill the mold box with that, or a little bit more. Press the sculpture down in the middle (being careful with more delicate sculpts) and try to get the play-doh evenly halfway up around the sides. You're also seeing what I've found to be single most important element to success with these two-parts molds: a good pour-hole. Once the mold is done, you need a space to pour metal in, acting a bit like a funnel, that's big enough to easily hit when you pour with no spilling hot metal, and has a very small connecting neck to the miniature at a location that you can snap and file off later. So plan this out carefully; I sculpt the pour hole out of a harder clay and embed a wire to guide the connection point where I want it. I cannot emphasize this enough, it's completely the key to whether my mold works for me or not. And you need to include the size of the pour-hole in the dimensions of the mold box, created in the prior step. Finally, take the end of a brush and poke two or three divots in the play-doh to serve as connecting slots for the final mold (this is the top thing I tend to forget about, as I did in the photo below).
Now I'm mixing the rubber compound for the first half of the mold. Once again, this Oomoo 30 brand rubber compound has been working very well for me. The official suggestion is to use 3 plastic cups; pour the required pink & blue parts into separate cups to visually equalize, then pour together in a third and mix. But that's going to waste so much material on the sides of the cups, it will be many times what you actually get into the mold. So to minimize this I spoon the separate parts into one cat-food can and mix them together there. I'm trying to get about the same number of spoonfuls of each, but the compound is very forgiving, so if I don't get it exactly right, I've still never had a mold fail on me.
Here I've poured the purple rubber compound onto the top half of the mold box. As you do this, the recommendation is to pour into a lower corner of the play-doh, and let the rubber seep in around the bottom of the sculpture and flood in up around all the details. However, it's sufficiently viscous that I usually have to finish it off (around the pour-hole, especially) by pouring over the top at the end. Then I very gently tap the box on the desktop a few times to release some air bubbles caught within (not sure if this makes any difference but I do it). Cleaning up the spoons & mixing rod takes a few paper towels (the mixing dish can be allowed to set and cleaned out once the rubber is hard). It takes about 6 hours for the rubber to set, so I usually set it aside overnight at this point. Be really carefully if you move the mold box, keep it on a large platform and use both hands -- I've dropped one at least once here and it makes for a catastrophic mess. (Literally, because it was caused by me tripping over the cat.)
Workday 3 -- Time to open up the first half of the mold. I use an exacto knife to split of the tape in each corner and peel down the sides from the rubber & play-doh. I'm going to re-use the poster board molding box, so I'm a bit careful with it.
Flip the block of rubber-dough-sculpture over; now we've got a cleaning job to do. The mass of the play-doh will peel off (it's still soft since it was trapped in there with no air), but then you've got to carefully pick out smaller bits and pieces by hand or with a dental tool or brush. You want every little bit of play-doh, especially anything on the surface of the sculpture, removed. Make sure to not move the sculpture or pour hole from the rubber as you do this. If you do, then the resulting cast will get "smeared" like a page that got moved on a photocopier while it was getting scanned.
There, I've basically got the first half of the mold cleaned up. I've also used an exacto knife to scrape clean the poster board mold box, so I can use that again.
So now I'm putting the first half of the mold back in the box and taping the sides back up around that. A few details: Since the poster board isn't perfectly rigid, that first pour of rubber will bulge out a bit at the side and be bigger than you originally designed the box; so now you've got to bend the sides a bit to take up the extra width (I used the dental tool as a straightedge for this). Consequently, the height of the box is now a bit less than it used to be -- so thinking about this in advance, that's why I suggested in the first step to fill the box a bit more than half with the play-doh (so the rubber was less than half, matching the less than half you'll get on this side, make sense?). Above all, you don't want to smash the wider mold into the smaller box, because the mold will bend and pop the sculpture up out from where its sitting (causing the "copy smear" problem). Also, you need to use a spray release agent on the sculpture and mold right before you pour in the rubber (I should have mentioned that in the first step above). This stuff is super toxic, you don't want to breathe any in -- I open my 3rd-floor apartment back window, set it on the ledge, stretch my arm out and spray it there. To date I haven't dropped any mold down to the ground doing that.
Once again, I'm mixing the two parts of the rubber compound together, estimating by sight how much I need in total. With a little practice I got really good at doing that for these very small projects.
And pouring the second half. The mold pour hole is sticking a little bit out (again, this side is lower the way things work out) but that's okay. Leave this to set through the day or overnight or however you're doing it (6 hours minimum).
Workday 4 -- Separating out the rubber mold from the mold box.
The mold looks like a single solid brick when it comes out at this point (rubber seeps down the sides of the box, sealing everything in), so I have to carefully feel where the two parts are separate -- the pour hole is a great place to start -- and peel it apart, kind of like a banana. This is what I get when I do that. I could throw the mold box away at this point, as I won't use that again.
Now there's a job of cleaning up the rough edges of the mold with the exacto knife. Mostly this is just around the sides where the rubber seeped down in vertical sheets that you don't want. In particular, the pour hole gets entirely shut off (see last picture), so scoop that out and make sure you have a nice accessible target for pouring metal. The point of connection to the sculpture might be shut off too, so I may use tweezers very carefully to free that up. Be very conservative there, we can play around with that more on a later step.
Putting the halves together, it looks like they line up nicely with a functioning pour hole and a nice visible connection down into the mold. I also cut out some cardboard braces to use around it when I rubber-band it together for an actual metal pour later on. We'll do that another day, but for now it looks like the mold is done!