I’m trying to inject some kind of rubber around an aluminum strut to form “feet” for a robot. I’ve already milled the mold, but I’m having trouble finding an inexpensive and readily available rubber compound that will cure without exposure to air. Ideally it should cure to about the consistency of a silicone O-ring. I’ve tried silicone gasket-maker (the automotive stuff), however a week later it hasn’t cured in the mold, as there is no exposure to the air. Is there anything out there with a similar consistency to silicone, but doesn’t require air to cure? Or is there a way to get what I’m currently using to set up without waiting a millennium? There aren’t any real mechanical requirements, I’m just trying to clean up the look of the robot and prevent its legs from scratching my table.
You need a two-part resin of some sort. Search on "room-temperature vulcanized" rubber, or "RTV rubber". There may also be epoxies that are designed to dry rubbery.
You want stuff that comes with an activator -- if it's single-part RTV it depends on drying or on oxygen in the air to kick off, and you're back to never curing.
Your challenge will be to find something that is available in small quantities to the hobbyist. I can't help you with that, alas -- I just have the name floating around in my head from some prior research, and new what search terms to type in.
Here's an example, but I couldn't even say if they'd sell to you: http://www.contenti.com/products/casting/179-050.html.
Instead of trying to mold a liquid, you could try to form a viscoelastic solid. That way, the uncured material would keep its shape without an airtight mold.
There's a product called Sugru that you might try; it's hand-moldable silicone rubber that dries in air. (I am not affiliated with their company.)
Sugru is great for this kind of thing, but it can be a little pricey. An alternative is Oogoo, a DIY mixture of silicone caulk, cornstarch, and (optional) pigment.