I am planning to cut some plugs with either XPS or EPS foam (whichever I can source cheaply) coated with a shell of fiberglass and body filler to make a polished finish to pull molds from. I plan to use a vinylester resin for the molds to helpfully reduce any shrinkage/warpage and am contemplating the possibility of vacuum bagging the mold for potentially a better surface finish/slightly stronger mold. Is there any benefit to vacuum bagging a mold vs just doing a wet layup, or is it more trouble and consumables than its worth? I don't really care about a lighter mold. I was just thinking that putting a vacuum on the mold would help reduce voids that need repaired before parts can be pulled and potentially help reduce warpage if excess resin is removed. 

Wet-lay vac bagging doesn't reduce void content compared to a properly wet-laid part. The idea is to give parts a better fibre:resin ratio, improving the strength to weight. For moulds as you said, this doesn't hugely matter. For moulds you actually generally want a bit of mass.

If you were infusing the moulds then that's a different story and well worth doing, but vac bagging no. 

I second that - with VE or polyester. Just make sure you nail your surface coat and get a nice void-free skincoat behind it before you start laminating and you should be fine.  Consider an all-chopped strand mat laminate done in two or three separate applications - thickness is the biggest driver of stiffness here - and it's stable and won't print much.

Wet-lay vac bagging doesn't reduce void content compared to a properly wet-laid part. The idea is to give parts a better fibre:resin ratio, improving the strength to weight. For moulds as you said, this doesn't hugely matter. For moulds you actually generally want a bit of mass.

If you were infusing the moulds then that's a different story and well worth doing, but vac bagging no.

Fair enough. So what is the point of wet lay-up vacuum bagging at all then? It seems like a less stressful process to just jump right to resin infusion. I thought that I had read that excess resin can move and warp in excess heat. My plan is to make car body panels with it and keep the molds in the garage attic that isn't conditioned if I need to pull another part if I need to replace a panel down the road. 

I was trying to stay away from VIP for the mold, just because I didn't really care about too much excess resin, it required more consumables for what seemed an equal quality mold, and most importantly, I feel like I would need to substantially beef up the plug material to be able to withstand a full vacuum for that long and not warp. I think the XPS foam readily available around here has a compressive strength of 15psi and that's at 10% compression, so i was thinking of just doing a partial vacuum of 10-12" that I thought would help prevent some bridging and force the layers of CSM together tighter, but it sounds like its overkill and less consumables anyway, so its a win-win. What are the benefits of VIP for a mold? Can you do resin infusion with CSM? 

I second that - with VE or polyester. Just make sure you nail your surface coat and get a nice void-free skincoat behind it before you start laminating and you should be fine. Consider an all-chopped strand mat laminate done in two or three separate applications - thickness is the biggest driver of stiffness here - and it's stable and won't print much.

I had planned to do a CSM laminate for the mold. For something like a rear clam shell, would 1 layer of .75oz skin coat, followed by 8 layers of 1.5oz suffice? Maybe with wood/rope/foam/whatever to build thickness ribs between layers 4 and 5?

I was thinking of glassing in some pieces of steel and bolting them together as well as sort of a cage to force the pieces to stay where they should be, especially on a multipiece mold, or is that just crazy overkill?