Re: New vacuum bag setup
So, the next day I had to undo all the bag stuff and pull the panel out.
I had a little too much epoxy in the layup, and it hadn't stuck to the flow media or peel ply, but rather it had pooled enough to go "around" it and grab on when it hardened like a whole bunch of little hands. I had to really work to peel it off (pic is sideways):
The glass was pretty nice as far as I could see, although the force needed to pull up the flow media at the corners delaminated a bit there. I suspect it didn't have enough resin there for some reason.
Once I got the flow media off most of the way, the peel ply came off in strips... I think the polyethylene threads weren't strong enough to hold the nylon, and they ripped.
So, I couldn't get it all cleaned up, and the finished panel, while strong, was too flexible anyway. I wanted something less "giving". So I gave up on cleaning it. The pain helped.... plastic mesh plus epoxy = cheese grater:
So... after thinking things over I decided what I'd done wrong was A) Not checking the peel ply to see if it worked before use B) too much epoxy (although this wouldn't be a problem if the vacuum worked) and C) I'd let the vacuum bag crinkle too much, leaving me with a lot of folds and creases that leaked.
I started fresh with a piece of 1/2" plywood. Stiffer, needing less glass. It also saved time cutting.
I cut two pieces of 8 oz cloth to laminate on either side.
This time I cut the bag bigger, and was careful to avoid folds and crinkles... there were still a few places where the bag had too much material, but I did my best to avoid leaks. I didn't know how to pleat the bag, I only learned that later.
I sealed the board with epoxy, laminated glass, flipped it and did the other side, then put it in the bag. Same sticky tape, fitting....
and again, no luck. I just could not get the bag to seal on three edges well enough to avoid running the pump. Not even the expensive sticky tape helped. Argh.
So, rather than let this all harden into a useless mess, I tried one more thing. I took a scrap of bag from my earlier cutting (a big scrap) and took the panel out of the bag it was in (fitting off, sticky tape off, etc).
Then I put the panel on my big table, directly on the waxed melamine, and put the bag piece over it. I wiped the table down, and sealed it with red tack tape around the edges:
Success, sort of. The bag held a tight vacuum to a point... I set the pump to pull about 15 inches of vacuum, and it ran maybe twice a minute.
But it was pumping hard/fast enough to "jerk" and it would turn on/off at least 2-3 times per cycle. Kinda hard on the pump.
I fixed what leaks I saw, and a couple I didn't see clearly, and still had the same problem. In fact, it seemed to get worse.
I scratched my head for a while, and walked around to look at every part of the setup.
Oddly, oil was "boiling" out of the top of the pump. It wasn't hot, it just looked like the pressure was low enough for it to give off gases. I'd topped off the pump with compressor oil since I ran out of vacuum oil (as the pump runs it throws out a vapor of oil, which means I had to fill it often). I'm not sure that was a good idea. Maybe the compressor oil had a component that would boil off at low pressure.
Weird.... ok, so I decided to check what happens if I used a valve to cut off the bag from the pump. The vacuum gauge decreased, but the bag seemed to lose only a little vacuum. So it seemed like the pump was putting gas into the system... odd.
But there was a solution for that... I used a vise-grips to pinch the hose shut between the pump and the tank, after running it up to a 20 inch vacuum. That meant only the tank and bag were connected.
Then I did evening things like watch some TV, and went back to check later.
It still was holding pretty well... very little leakage on the bag it seemed. I decided to leave it overnight.
This morning it's still doing ok, no sign of leaks in the bag.
So.... my conclusions so far are as follows:
1) Don't try to build an "envelope" bag without either a tubular bag roll, an impulse sealer, or something similar unless I have to.. it's hard to seal three double edges.
2) If you get epoxy resin on the sealing surfaces, it's even harder to seal.
3) Avoid folds/creases at all costs. If you have extra bag material, form a pleat.
4) Get a check valve for the pump, and move the vacuum controller to measure bag vacuum instead of the pump circuit. That way the pump runs when pressure in the bag drops, which should work better, and the vacuum in the tank and bag won't cause cavitation or moisture in the pump to gasify. Someone suggested the check valve above somewhere, and I'm sorry I didn't pay better attention.
5) Cut cloth, bag, and flow/peel ply carefully...extra gets in the way.
6) Get some real peel ply.
Comments welcome.
Erik
