Vacuum Bagging

Wet layups work great for quick reinforcements, however they aren’t exactly ideal from a strength-to-weight perspective. While this may not be a big deal on a small part, on larger parts towards the aft end of the airframe (read: CG/CP relationship matters here…), it makes sense to try and minimize the weight of the layup. The weight of the cloth is fixed; so the only thing that you can control is the amount of resin in the finished product. Since the strength comes from the fiber and not from the resin, it’s safe to remove resin from the layup, as long as the fiber still sticks to the part you’re working on.

Enter the vacuum bag. It’s an easy way to not only remove resin from the finished part AND, at the same time, make sure the fiber stays stuck to the working piece. It’s a nice twofer that makes all the work pay off. The general premise of vacuum bagging is to use atmospheric pressure to squeeze an absorbent material onto the resin-soaked laminated part you’re working on to remove excess resin, and at the same time hold the lamination in place. The challenge comes in to play when you realize that (1) vacuums suck (literally, ha) and (2) epoxy is sticky. Vacuum bagging has a few tricks to make sure everything goes well and you only stick stuff to your rocket that you plan to.

The basic components of a vacuum bag setup, in addition to the items mentioned in the Wet Layup section, are:

• A vacuum source (usually a vacuum pump, this one from Aircraft Spruce is my weapon of choice, since it’s quiet)
• A way to hold the vacuum in (vacuum… surprise, bag). The vacuum bag is usually made from sheet material — Airtech comes through with a nice cheap bag material that’s good for low-temperature cures, and they also have a stretchy pink high-temperature version that’s good for up to something crazy like 350 or 400 degrees F. The bag is sealed using copious amounts of bag seal tape (this stuff rocks). An alternative is using a FoodSaver to seal the bag and pull a vacuum at the same time — John Coker has a great article on that topic, along with a great description of the full process.
• A thick, absorbent layer to soak up epoxy and provide an air path out of the laminate (“Breather” or “Bleeder”… ususally “Breather”). Breather is usually made of thick material similar to cotton batting, sold in large rolls. I like the cheap stuff (“Econoweave“) from Airtech, mostly because… well… it’s cheap.
• A non-stick release layer to keep the breather from becoming one with the laminated part. There are a few possibilities for your releasing pleasure:
  • Release film is perforated and coated in release, to simply leave a slick layer underneath the bag. It’s used mostly inside of plug-molded parts to guarantee an easy release, as it’s the simplest product to apply. However, it leaves a surface finish that mirrors its application — or, in the heat of a lamination, often lumpy — and leaves behind a thin layer of release on its non-porous surface. It works great, however, for doing flat parts in a bag that need a finished surface (eg panel layups) and for the aforementioned plug lamination jobs, thanks to its almost brainless application and use. (Note that parts made using release film need to be sanded and solvent washed to prepare for bonding again — see page 1.)
  • Peel ply is a smooth, woven nylon or polyester fabric that magically doesn’t stick to the part. Better living through chemistry, as my high school teacher Mr. Ingalls always said. Peel ply is my favorite release fabric, if it’s possible to have one, as it leaves a smooth surface textured by the weave of the ply, with no release layer on top. This basically means that it produces parts that can be bonded again without any bond prep — yay! Peel ply also generates a nice high resin transfer, which leaves an incredibly light part. It also can be “laminated” into the laminate with resin, to make sure it lays in flat and even, leaving an even surface that’s really pretty. When doing tip-to-tip lamination on external fin cans, I use peel ply extended slightly over the front of the laminate — it leaves a nearly imperceptible “edge” where the laminate material ends and the rest of the airframe begins, and it prepares the surface for a few coats of sandable primer and a perfect finish.
  • Mylar (I have no link — somebody help me here) leaves a glass-smooth surface finish, while promoting very little resin transfer. It is similar to the above release film, however it is a bit thicker than the film (think plastic sheet vs. saran wrap), and as such is less inclined to leave wrinkles. It can be used as a barrier layer over carbon fiber for a dramatic three-dimensional finish. Some users have reported troubles with Mylar sticking to the laminated part — a quick coat of release agent should take care of that.

In hindsight, that sounds like one big Airtech ad. I don’t work for them, I just have used their products (a lot) with great results. Recommend what you know, I guess. Anyways, underneath all that warm, fuzzy, vacuum-bagged affection lies a part completed using the previously-mentioned wet layup techniques. A typical vacuum bagging routine might look something like this:

1. Laminate your part, using your l33t laminating skillz. Don’t worry as much about the resin-to-fiber weight ratio; in fact, apply a little extra to guarantee you’ll get a nicely wetted-out part.
2. Apply the release layer directly to the lamination. I usually cringe at this point; I’ve just spent an hour (or more) applying a beautiful, bubble-free, even fabric layer to an expensive rocket part, and I feel like I’m going to mess it up by putting on a thick, stiff, resin-repellent layer of release. Don’t worry. Vacuum bagging solves all your problems. If it’s peel ply, it’s even safe to use your brush to apply a little resin on top of the release layer and attach it sort of securely to the layer underneath.

   Cut the release significantly larger than the part being laminated; the worst thing in the world is to come back, de-bag a part, and find it impossible to pull the breather layer off because it got glued to the laminate. Been there, done that, no t-shirt. (So not worth it.)

3. Next, lay breather over the entire part. Be generous here, and be especially careful to cover up any sharp corners (Competitor fin tips, I’m looking at you) with an extra layer or two to prevent them from puncturing the bag. I usually tape the breather in place (not using masking tape! the adhesive can migrate, especially in curing ovens, and ruin your day!) to keep things from getting too out-of-hand. Airtech (surprise, surprise) makes a really good silicone-free tape for this purpose. It’s cheap, too.
4. Finally, wrap the entire thing in a layer of bag film and seal it with bag seal tape. Don’t forget to include a vacuum takeoff valve to be able to pull air out of the bag. I usually put a folded up square of breather under the takeoff valve, too, to make sure that air can flow freely out of the laminate.
5. Double check all your vacuum taped joints to make sure they won’t leak. Ha. Attach the vacuum hose and plug in the vacuum pump. By this point, if I’m working alone, I’m usually scrambling to finish up before the resin kicks. (West System 209 is a GODSEND!)
6. Look at the vacuum level on the gauge in the pump. If the needle is down in the 0-10 PSI inHg (thanks Rick!) range, you’ve got a leak. See above step and repeat. If the needle is in the 10-15 inHg range, you’re almost there. Start focusing more on getting the bag to lay down evenly over all the parts — make sure it doesn’t get stretched over any corners (“bridge”). Wrinkles here are OK — they will usually get taken out in the breather layer and not be transferred to the part.
7. When you’ve sealed the bag well enough to get ~20-25 PSI inHg , you’re all set. Double check that nothing is bridged and the bag isn’t pulled tight over any corners, and you’re done. Throw it in the curing oven, or walk away and wait for the epoxy to cure.

After writing this (and re-reading it), vacuum bagging is a lot more work than I remember. But it goes quickly, and the finished product is TOTALLY worth it. It makes for a fabulously light part that’s incredibly strong, as the fabric is guaranteed in contact with the part being reinforced, with no air bubbles. The work put in to vacuum bagging usually pays dividends when finishing, anyways, as there are no bubbles to sand off, dips to fill, and very little weave to finish. Oh, finishing. There’s another topic to write about.

Coming Soon: Vacuum Bagging, Curing Ovens, Finishing Hand Layups, Prepreg Techniques, Filament winding. And pretty pictures!

Pages: 1 2 3

Posted December 26, 2007 in Rockets. Add Comment / Trackback