mold material ?

I'm not familiar with Delrin, what material is it made of? From what I can see online, it looks like a hard surface item so I'm going to go from that assumption. Normally I advise using a soft part/hard part way of molding - using a silicone mold (soft part) to cast an RC-3 resin casting (hard part). Or casting silicone (soft part) in a RC-3 mold (hard part).

I believe it can be done but when you use two hard surfaces, you do need a release agent and even then there is a chance you can scratch or possibly break either the RC-3 or the Delrin. Personally I would either use a more flexible resin (or use silicone) to cast a part in the Delrin mold or make a silicone copy of the mold to cast RC-3 in instead of the Delrin mold. Since it's a 3 piece mold, it CAN be done but you do need to know that there is a chance you can damage the casting and/or the mold.

Other than being aware of the drawbacks of using two hard surfaces, the RC-3 should have no problem with most molds. I haven't actually seen it have trouble with most things, it's not normally a picky resin so it should work. I *have* seen it bubble up in a new mold but that's normally just the mold needing a little extra time to fully cure (I've done it when using a mold I just made a couple hours before) or if the RC-3 has a bit of a moisture problem but keeping the tops tightly capped usually prevents most moisture problems.

Feel free to ask more questions and post pictures of how things turn out, we're here to help.

Ah you should definitely be able to get the resin out without much trouble from that - just be sure you have a release agent like petroleum jelly or a mold release of some sort (I've even seen cooking spray being used in a pinch). If you don't have some sort of release between the resin and the mold, it might bond to the mold and that will be a big mess, I use an countertop cutout (where they cut the piece out to put in a sink) as my "pouring" surface so I get drips of resin on it just about every time I pour and I use a plastic scraper to get them off after they harden and I can say it does leave scratches on the surface of both so a release is absolutely needed. I hope this helps.