The subspace beacon is a very thin-walled part, intended to be cast hollow to allow room for electronics. Using traditional one-part or two-part sprue-and-vent techniques, it is very difficult to mold a geometry like this.
So I decided to try the squish molding technique. This is a two-part mold that involves no pouring sprue. Instead, the second part of the mold is effectively a "plug" that is squished into the first (bottom) part. The plug displaces the resin until it fills all the cavities and oozes out the sides. It's messy, but it works really well for creating thin-walled parts. In many ways, it's simpler than a traditional two-part mold with pouring gates and vents.
I began by cleaning up the master a bit with careful sanding. This proved tricky. Given the narrow joints and walls, the model was brittle and kept coming apart while I was sanding it, require more sanding, puttying, priming, etc. But I finally got it to a pretty good state.
The first thing to construct is what will be the top of your mold, or the "plug." This consists of a built-up clay positive underneath your molding master.
I built a mold box out of stock acrylic parts I keep around for this purpose, glued at the edges with 3M 3798 removable low-melt hot glue.
I then pour what will be the bottom half of the final mold. (I'm using Mold Star 30, which I like for squish molds because its high durometer gives it sturdy walls and because it only takes 6 hours to cure, meaning I can pour both halves of the mold in a single day if I'm diligent.)
Since I intend to pressure cast these, I vacuum de-gas the silicone before pouring
I also made squish molds for two fit-in parts, which I'm casting separately to make subsequent painting easier (avoiding having to carefully mask.) Because they had flat backs, no clay base was required. I just used some bumpons and styrene bits to create registration keys in the other (top) half.
I had to clay over some undercuts in the corrugated styrene.
I affixed these to the acrylic using a temporary adhesive
Six hours later, I have the first halves of my molds.
I removed the clay and registration keys and liberally applied Mann Ease Release 205 liquid brush-on mold release. and boxed them back up in acrylic. I also put some hot glue where the mold edges met the acrylic to prevent the second round of silicone from dripping down the side of the first half of the mold, which can be a pain (and sometimes impossible) to separate. (I've also gotten in the habit of putting release liberally along the sides; just in case the glue fails, it gives me a chance to separate the two halves anyway. Given that it took me hours and hours to prep these molds, a bit of extra insurance never hurts.) And then I poured the second halves (just a few minutes ago.)
I'll let you guys know how they come out!