Your objective is to produce a simple open-faced mold, possessing many identical cavities, from which to cast resin parts.
Select a mold medium that is quick to work, will capture the masters shape faithfully, and will not require any special attention in order to assure a quick, clean separation of the cast resin parts once they’ve cured hard.
Yes, your air-dry clay will work as a medium, but it will require application of a mold release and, because of that mediums rigidity (once hard), can make cast resin part removal problematic, particularly so if the cavities have high draft angles.
But, why suffer all that hassle of impressing the shape of your master onto a medium that requires you to wait for it to change state from a malleable semi-solid to a solid; and then having to introduce a part-release system?
Too much work.
Instead, you can achieve a simple, open-face type mold quickly, and use it immediately, by using oil-based clay as your mold medium.
The following shows how I employed this clay to produce a tool from which many tear-drop shaped resin pieces were cast, in mass, in one casting cycle. I suggest this technique to you as a means of producing all those little raft/float parts required for you current project.
A few years ago a job called for tiny, tear-drop shaped parts, each representing a streamlined zinc anode, attached to the sides of a submarine. And I needed a lot of them. I employed oil-based clay as the tool from which those many parts would be cast from Alumilite brand polyurethane resin.
Below you see the end-game: The cast resin tear-drop shaped zincs have been painted, and transferred to the sides of the model submarine, adhered with CA adhesive.
I produced nearly one-hundred of these parts in one, quick, casting operation. This exemplifies one of the many virtues of this tool making technique.
I rolled out a slab of oil-based clay. Onto the face of this slab I impressed the desired cavity shape(s) with a simple master of the zinc anode, attached to a handle.
As the ‘hardness’ of this non-hardening medium is a function of room temperature, the environment is adjusted to a temperature that permits easy impression onto the clay, but does not produce a sloppy mess of the cavities(s) formed once the master is withdrawn.
Make the flange face of the clay tool even with the base of the eventual resin part formed. That means the master has to be impressed into the clay to a specific depth. I achieved this goal by gluing a ‘depth stop’ to the handle of the master. I press the master into the clay till the depth-stop makes contact with the face of the clay; I stop, and withdraw the master. And move on to the next cavity forming operation.
Casting resin is mixed up and poured over the work. Gop it on, don’t be shy!
Place wax-paper over the resin and screed over the top of the wax paper (a popsicle-stick used here). Use a light touch. The objective is to push the resin into the cavities with very little projecting atop the flange face of the clay tool. Leave things alone till the resin cures hard.
If you have the facilities, pressurize the work to two atmospheres till the state change is completed – this will crush any small bubbles that got entrapped within the tool cavities. If not, make sure to use fresh casting resin – this will alleviate any off-gasing induced bubbling in the parts.
Yank the fret of resin parts off the clay tool. Then scrub the still attached parts with a lacquer thinner saturated piece of abrasive pad. This to remove oil the parts picked up off the tool, and to etch their surfaces for priming.
Place the fret atop a sanding block (outfitted with #240-grit sandpaper) and abrade the thin layer of backing resin away, releasing the parts from the fret/backing resin.
And there you have it.
David