During my work at a composites workshop I got some experience with composites and vacuum-bagging. I decided to make a foam core wing for a model aeroplane. I checked for some internet info on it, and what I found is that instead of the classical arrangement we would use at the workshop:

  • core
  • impregnated fiber layers
  • perforated peel ply
  • breather layer
  • vacuum bag

In wing vacuuming guides, they put an additional layer of Mylar between the fiber layers and the peel ply. The Mylar is removed after curing.

Why do this? Is this to have a smoother surface for the wing? Mylar can be used to efficiently contain gases (baloons, food industry etc). Wouldn't this mean that it also blocks the excess resin that should go trough it? Or if I assume that resin can go trough the Mylar layer freely, then what's the point of having a perforated peel ply at all? Is the Mylar layer essentially doing the work of the peel ply?

  • $\begingroup$ Odds are that this prevents vaccum bag material from sticking to model. $\endgroup$ Oct 1 '20 at 18:03
  • $\begingroup$ @StainlessSteelRat : the bag material should be multiple layers away from the model. (the "peel ply" is a release layer, while the "breather layer" is a fluffy mat that lets the air move freely and prevent bubbles getting trapped. Unfortunately, I haven't done vacuum bagging in ~25 years, and mylar wasn't something we used. $\endgroup$
    – Joe
    Oct 2 '20 at 16:15

From Vacuum Bagging Wings Instruction Manual Purdue University

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Inner layer is foam with Kevlar at wing tip. Next fiberglass resin. Then Mylar with tape at tip.

Then Perf Ply (perforated release film) and Breather. Both of these allow air to be removed from Mylar covered wing. Finally Vacuum Bag.

In assembly procedure:

  1. Lay the fiberglass on the Mylar. Pour resin on near the middle and squeegee out the resin
  1. Wrap the Mylar/fiberglass around the foam core. Tape trailing edge and tips to keep the foam core in place

They state Mylar has a release film to allow separation of wing from Mylar.

  1. Turn on the vacuum pump. The bag will pull down quickly over the part. Check the reading on the vacuum gauge. It should read between 18 and 24 inches of Hg depending on your altitude. Use a squeegee over the outside of the bag to smooth out the bag surface and help the bag pull tight around the edges of the part. The squeegee will also smooth out any edges in the layup under the Mylar and move any excess epoxy off to the edges.

In preparations:

Try not to scratch or put any gauges on the Mylar because any of the defects will be transferred to the wing later on.

So reading between the lines, the Mylar, which is reusable, has three functions:

  • To separate the resin from the vaccuum.
  • To provide a smooth surface (no scratches) for the fiberglass to adhere to the foam.
  • To allow excess resin to be squeegeed to edges of wing for easier removal.

How to Vacuum Bag a Wing

  • $\begingroup$ This is the primary source I found for this Mylar thing. From your conclusions, point 1 makes no sense to me: why would one want to separate the resin from the vacuum? Isn't the point of vacuuming at all is to suck up excess resin? (and to get rid of air bubbles) $\endgroup$
    – GZoltan
    Oct 4 '20 at 15:40
  • $\begingroup$ But if you suck up all the resin, what gives you the smooth surface. Mylar holds resin in place until it hardens. Vaccuum compresses mylar to wing. Excess resin, bubbles are squeegeed out. I could be wrong. $\endgroup$ Oct 4 '20 at 21:22

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