Thanks - I'll check out the links. I realized I didn't show a picture of the casting resin, only the molding silicon. See below.
I'm not an expert at all in molding, but I started with a kit from
www.Aluminite.com. It came with Aluminite low-viscosity liquid casting plastic. The kit included a useful, albeit brief manual on techniques. I don’t recall what mold-making compound came with it. But when I used it up, I bought silicon RTV, type 1328 from
www.Silpak.com. Various distributors also carry it, including Amazon and Kit Kraft Inc.
Things I’ve learned:
Make the plugs for the mold as nice as you can. Every detail, even wood grain, will show up.
Build a box for the mold pretty small around the part (but not closer than ¼ inch) to save material. RTV is expensive (I think mine is a pint for around $25)..
I like using shiny-paper-backed foam board to make molding boxes. I get lots of it from the garbage in our marketing department after every trade show. Graphic artists use it for rigid posters. Tape the box together and seal the seams with a glue gun. I CA the plug to the bottom of the box.
I use powder, sand, or even water (if the plug is waterproof) to check the volume of the mold material needed to cover the plug (again, at least ¼ inch over the top surface).
I measure the Silicone RTV on a postal scale. The mix is 10:1 (1 part catalyst) by weight. Tare the scale!! The shelf life of the silicon is about 45 minutes. You only need a few minutes to pour, so use the time to mix SLOWLY so you don’t get air bubbles. I sometimes tip the cup so the mixture is shallow, so bubbles reach the surface quickly. Tapping and can help bring bubbles up. Then pour slowly into the mold. Thin stream to prevent trapping bubbles. It takes overnight to cure.
I use water again to judge how much casting plastic I need to mix. For my flanges, I only needed about ½ oz each. I put ¼ oz of each part in separate measuring cups, and pour them at the same time into a 3rd “pouring” cup. Using 3 cups helps ensure the final mix ratio is right (versus pouring one into the other, which will never produce the right ratio because of the film left behind). I can then keep reusing the “measuring cups” and also the pouring cup, by pulling out the residue out when it hardens.
This stuff cures FAST so mix carefully for 15 seconds and pour into the mold. Maybe sooner for a bigger part. Try not to poor too quick, or directly into small features. Did I mention bubbles? It’s hard in about 1 minute. But I wait 30 minutes for it to cool so it won’t warp when I pull the part out of the mold.
I use Silpak mold release every couple of castings. But I’ve never cast enough parts to wear out the Silicone, so I’m not sure it’s needed. Parts seem to pop out easily either way. In a pinch I’ve used epoxy in the mold. That tended to stick and mold release helped.
It's pretty easy and kind of fun!