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Lessons Learned in a Vacuum Bag Job

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wsimpso1

Super Moderator
Staff member
Joined
Oct 18, 2003
Messages
11,358
Location
Saline Michigan
Attached is the letter I wrote to my friends and assistants as a followup to a layup we did on a mold we have been building up. We were applying a layer of BIAX cloth as additional stiffening, and vacuum bagging it. I had taken some liberties from our normal practice, and they nearly ruined our efforts for the evening. In all, it worked out OK, and we spent the next two hours talking, drinking chewy beers, and eating excellent Spicy Mediteranian Pizza.

I hope that you folks can benefit as much as we did.

Post Brief on the Epoxy Part 17 March 2004

All in all, the result was OK. I looked over the tool this morning, and there were a few places in between the foam cores that the cloth did not lay down, but most places it was well bonded. If it had been an airplane part, it would be scrap. As it is for tooling, it will be just fine. There were pools of excess epoxy along the mastic, in the wrinkles of the bag film, on the edges of the mold, etc. I had turned up the heat to 80 F overnight, and the entire part had reached initial cure, so I turned off and disconnected the vacuum pump, and reset the heat to a more nominal 65 F.

Speaking of bonding, the suction cup fitting that I used to re-connect vacuum and the first seven inches of hose were heavily lined with epoxy pulled out of the part, and thoroughly glued together. I may be able to remove the epoxy, maybe not. In either case, it was about a $0.30 fitting, so it would not be a huge loss if it is toast…

Now for the critique of the planning, leadership, and experimentation – that would be a critique of me, not you my friends …

First off, while I have used a vacuum bag to glue together things, this is only the second time that I have vacuum bagged tooling with glass cloth involved, and the first time that I deviated from what we do when we make parts. So this was as much experiment as anything else, sort of like the whole rest of the adventure... We were also trying to vacuum bag cloth on top of another pre-existing structure, without peel ply, perforated ply, or batting, another first for us.

Usually, we are wetting out glass and foam with a layer of peel ply thrown in that sucks off some of the epoxy, then a layer of perforated ply that will allow excess epoxy to bleed off, and then batting to pick up the extra epoxy. We have also found that if we squeegee a thin layer of epoxy onto the foam layer, the glass only needs to be wetted to the point of not looking white.

Even under these circumstances, we all have had the tendency to want to wet out the cloth and get it to lie down. Well, we have found by experience that the knitted cloths (BIAX and TRIAX) are sponges, and will require 2x their weight in epoxy to get them to wet out and look good. This is 3x the amount of epoxy that will remain in a well bagged laminate. So I let you folks pursue the look of the well wetted out look, and then we fell victim to TOO MUCH EPOXY. I should have reminded everyone to stop once we are just past having white (dry) spots. Once heard that the epoxy resin was running out, I knew that we were way fat on epoxy. I had calculated that we needed less than a whole reservoir to do the job with excess…

Being as we were working with the bag film on one plane and the mold being made on a somewhat higher plane, I had figured that the step plus the many wrinkles in the bag would serve to adequately channel vacuum around the mold. This works OK if the step and the wrinkles stay clear of epoxy. As it was, vacuum did communicate everywhere, but only at a low level.

We did not use the peel ply/perforated ply/batting layers, so we had nothing to help trap and contain the excess epoxy. So, we had excess resin running down the part and onto the mastic around the bottom.

The last benefit of batting is that it protects the bag film from the epoxy. So by omitting the batting, we exposed the bag film to being wetted by epoxy and made for an impossible job of sealing the bag. Bad choices lead to much fuss.

So, lessons learned about vacuum bagging up cloth :

White (dry) cloth is bad, but so is over-wetted cloth. Stop adding epoxy as soon as the white disappears. There STILL is no need to add epoxy beyond the point of getting rid of white cloth….

Perforated ply and batting or some other bleeder layer is necessary, if for no other reason than to protect the bag film from epoxy and to give excess epoxy someplace to go besides down to the mastic;

So, your leadership learned a bunch from last night. I hope that my manner during this process did not offend, and I apologize if I did. We will certainly do better on the subsequent parts, and hopefully, the really big layups on tools are now over. Thank you again for your assistance. I do believe that we have to play with some more Half-and Half again soon.

Billski
 
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