Hi everybody,

I have all the time problems to get a perfect sealed vacuum bag. I think there was only one perfect bag in the last two months...

Now I am thinking about switching to an envelope bag in a pretty big size. I thought of backing the mould with breather to prevent little holes depending on the few sharp pieces on the backside. It also could be that the mould itself is not airtight because I had a crack to repair but I sprayed some layers of board sealer on the whole surface so it should be fine.

Anyone with experience on this or tips?

I do this every day and rarely have a leaking bag now. cleanliness is very important, and having molds big enough with enough dead space all around the perimeter helps too.

before you start bagging wipe down all the edges to clean off over-spray from airtack and get rid of all loose fibers. I try to avoid envelope bags (for vacuum infusion) if i can. I find it much easier to damage the vacuum bag  and it is hard to avoid epoxy resin going everywhere. especially when you cover the back of the mold with breather. and then the creases in the vacuum bag will leave sharp edges that really easily cut the vacuum bag of the next bag.

cheers, Lester

As others have said, make sure that the perimeter where you want to place your tape is clean and free of fibres and release agent/spray tack residue. Colder conditions can also make the bagging tape less sticky, so really work the taped bag down once you've pulled vacuum. If your thumbs don't ache, then you're probably not trying hard enough and it's worth making sure that the entire perimeter is pressed firmly down once vacuum is applied. Sometimes you'll know when you've found the leak source as you'll hear the pump tone change. 

Also, try to make your bags big enough to leave plenty of slack. A bag that is a bit tight once pulled down will be pulling harder on the edges and is more likely to leak, even after you've finished infusing. Pleated areas are usually the most likely to leak, so only create as many as you practically need and make them big enough to leave lots of bag to work with. Once initially pulled down, back off a little on the vacuum and try to release any unnecessary stress on the pleats by re-positioning your bag so that you can pull your pleats fairly upright and not have them pulled inwards. Pay attention also to the vacuum and resin inlet ports, as these can be leak sources if the bag is quite creased in these areas when inserting the tubing. I find that a small 'doughnut' of tape placed on top of each connector and stuck to the inside of the bag can help reduce any unwanted creasing here. 

Just a small number of tips, but hopefully you'll have your bagging routine nailed down with success with a little more perseverance. It is a bit of a 'last resort' to consider envelope bagging if you don't really need to, especially if you'd like to make more than a single part, so it's good not to need to go down that route

If your mould is porous, then sometimes it's only by actually infusing a part that you can find the source of the leak, but coating the back of the mould can help as Matthieu pointed out.