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Vacuum Bagging Thick Parts

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wsimpso1

Super Moderator
Staff member
Joined
Oct 18, 2003
Messages
11,355
Location
Saline Michigan
I have been vacuum bagging my doors and roof structure in a female mold.

The normal technique for building male and female parts using a mold or caul plate works great. The bag is put on using short extra sections of mastic to put in extra folds of bag film, which allows the bag to conform and follow concave and convex surfaces, cover places where the laminated part has thickness greater than just a few thousandths, etc.

But when I went to build my doors, well the parts are thick. The door frames are built like beams with UNI tape in top and bottom caps and BID at +/-45 for a hat section shear web. Foam cores an inch thick separate the caps. Conventional layups worked fine for getting the skin and skin side UNI tape plies on the mold. But I only wanted 4 BID where the windows fit. How to make a clean break with the vacuum bag moving the plies around a bit as it drew down?

Trick Number One! – A tool was built in the mold with 2 BID against the mold, and then foam to get 1 3/8" thick. It was trimmed to just slightly larger than I what I wanted for windows, buttered with micro, sanded to perfection and covered in release tape. Then several wooden blocks were hot glued to the mold to establish position of the tool. When I did the first door layup, I put down peel ply, 2 BID, and then set the new tool in place over the 2 BID, and duct taped the whole thing to the mold. Then we applied the several plies of UNI tape and vacuum bagged it all. The tool kept the UNI tape from reachign the places where I needed everything to stay thin. After it cured, the tool was removed, the glass was trimmed and faired, and the tool was reinserted.

The cores were applied on top of the UNI tape in layers using hot glue and rested against the tool too. Then came applying the inside caps and shear web. Making the vacuum bag conform to the 1 ¼" deep section began to be a problem. We deliberately left lots of extra bag film in the areas where the par changed thickness. And it worked OK on the doors.

To do the roof, I built it right over the doors in the mold - that way we know they will fit each other. I applied ¼" foam as a surrogate for weather strip, and then followed that with duct tape, both as an adhesive and as a release layer. We then proceeded to laminate the roof in the same manner as the doors. But now instead of the surface that we were bagging being single steps from mold to the top of the part, we had two raised areas with ¾" space in between that was much lower.

The bag could not follow the profiles and ended up bridging the gap between door and raised parts of the roof! And excess epoxy would run down to the local low spot and pool there! And the glass had excess epoxy in it!.

Trick Number Two! – Well, after going in with a mallet and chisel and very delicately chipping the pooled epoxy out of the part, and doing lots of itchy sanding, my creative juices got flowing. When each layup was finished, we started applying peel ply and perforated ply in short strips overlapping each other and that ran across the new and old raised areas. In this way, the peel ply and perforated ply could be driven into good wet contact with the entire new shape. Then we twisted "ropes" out of batting to force down into the gaps between door and roof just like caulking a wooden boat hull, except that we did not use a mallet to put it in place. Then the rest of the batting and bag film would go on, and the parts came out nice, with minimum epoxy and no pooled epoxy in local low spots. We figured all of this out and demonstrated how well it worked just in time for the last layup of the roof...

Ah well, I now have two big gullwing doors at about eight pounds and a fourteen pound rollover cage that is ready to fit to my tub and decks, so I should not complain.

Billski
 
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