What type of material were you casting with? And what type of mold? Others have
mentioned Alumilite mixing, but I recently found out that some tin cured silicones
can react with some types of PR, giving you a sticky outer edge.
Specifically, Silmar41 used with a tin cure silicone can come out tacky. US Composites
suggested washing out the mold with denatured alcohol before pouring to help
alleviate this.. and also that using a platinum cure silicone mold will eliminate it.
I found this out after ordering both Silmar41 and TinSil 70-25. They told me that there
might be a compatibility problem.. something about alcohols in the silicone. (I didn't
understand it all) But a post cure in the toaster oven can firm it up.
EDIT .. sorry, I just saw that you said urethane. Helps if I read the whole thing, huh?
But I'll leave the mistake here, since it's still good info.