sealing a split mould for vacuum infusion .. advice from experts needed !

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sealing a split mould for vacuum infusion .. advice from experts needed !

Author	Message
Paul Bristo	Paul Bristo posted 4 Years Ago HOT Topic Details
Group: Forum Members Posts: 3, Visits: 22	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,
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Fasta	Fasta posted 4 Years Ago HOT Post Details
Group: Forum Members Posts: 468, Visits: 3.5K	If you are making gel coat or painted parts then you can consider a wet layup skin across your whole mould including the flange areas, then set up your whole infusion and seal the vac bag to the skin layer on the flanges. We do this with larger moulds, using gelcoats and 225csm for the skin layer. Fastacraft Facebook page Edited 4 Years Ago by Fasta
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Paul Bristo	Paul Bristo posted 4 Years Ago HOT Post Details
Group: Forum Members Posts: 3, Visits: 22	Thanks Fasta but I wasn't planning on using gelcoat, I've been mulling this over and i'm tempted to just envelope bag the whole thing this time.
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Hanaldo	Hanaldo posted 4 Years Ago HOT Post Details
Group: Forum Members Posts: 2.5K, Visits: 28K	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).
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ttpb7paul	t ttpb7paul posted 4 Years Ago HOT Post Details
t Group: Forum Members Posts: 4, Visits: 11	Thanks Hanaldo, any brand of RTV silicone you can recommend ? its a big mould so i'll need a fair quantity
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Hanaldo	Hanaldo posted 4 Years Ago HOT Post Details
Group: Forum Members Posts: 2.5K, Visits: 28K	+x ttpb7paul - 4/13/2021 12:36:52 PM Thanks Hanaldo, any brand of RTV silicone you can recommend ? its a big mould so i'll need a fair quantity I use Würth because it comes in a pressure pack rather than a tube, but its all the same stuff. You don't actually need much, I used about 100-150ml to do a full Lotus front clam which was a big 4 piece mould. I guess if in doubt, more is more, but the more you apply the bigger the clean up job so really you just want enough to seal the flanges. If your flanges are flat and straight then its easy, a thin bead (I do two beads, one along the top of the flange and another along the bottom, as well as a circle around each bolt hole) is enough. If your flange goes around corners or is a bit rough then you might need a bit extra to seal it up. Just watch for it squeezing out the top onto your mould surface, have a clean rag on hand to wipe any of that away. Be aware it doesn't work every time, I would say its about 90% successful. I feel the times it isn't successful is when I have to drag the flanges across each other to get the bolt holes to line up and then it disturbs the silicone and creates an air channel. A few times it can still be sealed by smearing some extra silicone over the joint on the outside of the flanges, as well as over and around your nuts and bolts. Sometimes it just doesn't seal, and in these cases it is very difficult to know if it is actually leaking through the mould or whether you have a leak in the bag/bagging tape. This is when I would double bag and place the entire thing in an envelope bag.
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