I have a large, but shallow one-part mold to make.
It's 60in x 24 x .5 - .75 at most (the original is 60x24x<.25)
I will be casting platinum silicon in the mold, so no tin options.
Years ago I tried a Smooth on product, a kind of liquid plastic but it was so temperamental (not curing despite plenty of mixing) and fragile (would crack if you look at it funny) that I don't want to go that route again. I often use Ultra cal for smaller projects, but this would be way too heavy.
I would prefer a flexible, long term final mold. Looking at the Plat 40 for this. Would this be a good choice?
With that information, yes it sounds like the Plat 40 would be good choice. Be aware of a couple things though:
1) the Plat series are platinum cures and as such are a bit "picky" on curing. Be sure your master is clean and dry. Platinums inhibit easily (which sounds like what you encountered before) so be sure of what your master is made of and what is on the surface of the master when you make the mold or it will not cure right. There should be a list of things that inhibit the Platinums on the Alumilite website.
2) "Silicone sticks to silicone" - this is a phrase you will hear almost everywhere on the forum. If you are casting silicone in a silicone mold make sure to use a special release agent between them or you'll end up with one big chunk of silicone that you can't separate. I've been known to use Remelt as a mold box when I cast some silicone molds but I always put a layer of release agent on the Remelt before pouring the other silicone since I wouldn't be able to remove the Remelt from the other silicone.
I hope this helps, if not feel free to ask more questions.
You're welcome. Plat is only picky if you try to use it on just anything, I usually use the HS series when I don't know the exact makeup of an item or if I know the item won't work with the Plat (like the Flex won't work with the Plat - ask me how I know this...).