Hi,

So I am making some rather large car parts with vacuum infusion. Due to the shapes, the moulds are split into several pieces, bolted together.

What eludes me is what is the best way to achieve an airtight seal when assembling the mould ?

On my first try I used a fairly generous amount of silicone sealant on the flanges. This worked OK, airtight seal, but was a heck of a job to take apart again after, breaking the silicone was very hard. I had used 6 coats of easylease release agent all over, but the silicone stuck to it very strong.
On my second try, I used a rather less generous amount of silicone .. and I just could not get an airtight seal.

My third attempt is coming up soon and i'm looking for any advice from those that have done this before. Would PVA on the flanges where the silicone will go work ok ?  I've only really used easylease so not much experience with PVA.  Is there a better solutiuon than silicone ? The moulds are already made so I can't sink a trench for a proper rubber seal.

thanks in advance,

You need to use the right silicone. Blue RTV gasket silicone is ideal. Don't get me wrong, it still makes a mess of the moulds and is a little bit of work to clean up if you are using the moulds several times, but it does work easily and is the most reliable method I have found for sealing split moulds. 

You can also use regular gelcoat in the same fashion, which is significantly easier to clean up afterwards. But it's less reliable, if you get the timing wrong applying it and bolting the moulds together then it can be too thin and not seal properly or it can gel too quickly and prevent the moulds bolting together properly, etc. This is a good alternative though, and I would recommend trying it if you are really put off the silicone.

Envelope bagging im not a fan of. For one, it is no where near as reliable as you would hope, in my experience you'll have leaks 75-80% of the time. Secondly, resin goes absolutely everywhere, and really makes a wonderful mess of the moulds. I only do it when I am double bagging, so I've already bagged to the flanges in the regular fashion and I want to improve my vacuum integrity. That way the resin is contained in the first bag (split flanges still need to be sealed to prevent air/resin coming through the flange).