Vacuum bagging it sounds like the next. In Part 7, we tackle vacuum bagging. BY BOB FRITZ

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1 Dave Saylor holds the finished window. Two pieces of carbon fiber were made using the bagging technique described, then bonded together in a bag. It s super strong and light. In Part 7, we tackle vacuum bagging. BY BOB FRITZ Vacuum bagging it sounds like the next stage of self-expression after you ve gotten a couple of tattoos and been to the bodypiercing shop. But, no, it s a process for creating very low weight, complex shapes using composite materials. In previous articles we looked at wet layups where one soaks the fabric in epoxy and then drapes it over the mold. You ll recall that much effort was expended in getting air out from between the layers and reducing the percentage of epoxy in the product. You ll also remember in our first article in this series that the strength of this technology comes primarily from the fibers; the epoxy s job is to prevent shifting of the fibers relative to one another. It follows, then, that when it comes to the epoxy/fiber ratio, the adage, If enough epoxy is OK, more is better, and too much is just right is wrong. Once the job of nailing the fibers together is done, more epoxy just adds weight. Is it easy to have too much epoxy? A beginner at wet layup might have an epoxy/fiber ratio of 90/10. Someone skilled in wet layup will reduce it to less than 70/30. In a sophisticated aerospace component this will drop to 40/60. However, achieving this requires excellent process control, without which you might find uneven distribution of the epoxy to the point of having dry spots. A reasonable target for us is in the neighborhood of 60/40, and vacuum bagging is a great way to get there. 48 KITPLANES November

2 Jesus Gomez and Harold Bunyi demonstrate the value of teamwork in laying out the bottom side of the bag. Note that this is not exactly the same as the schematic/ cross-section drawing. Here, they re going to squeeze two rigid parts together with epoxy in between rather than force a glass/epoxy matrix into conformance with a mold. The principles, though, are identical. The vacuum tape consists of white paper separating a strip of putty-like adhesive. The guys are creating the bag. You can see that Gomez is about to set the upper side of the bag into position over the tape that Bunyi has laid down. But how important is it to keep the pounds off? If you re building a Light Sport Aircraft you re limited to 1320 pounds maximum gross weight. If you re building an ultralight, well, the name says it all. Keep it under 254 pounds. Rule compliance aside, there is still the issue of performance. Kelly Johnson of the Lockheed Skunk Works quantified it best: Each pound adding to the airplane s overall weight cost us one foot of altitude, so in building the U-2 we were ruthless weight-watchers. Even if you re not building the U-3, if you ve ever taken off with full tanks, all the seats full and the baggage area maxed out, you have a feel for the advantage of permanently removing 50 pounds from the airplane. (That s why I m on a diet.) Getting there consistently, though, requires a bit more than a pin to get the air out and a squeegee to remove the excess resin. (That refers to the epoxy, not the diet.) That s where vacuum bagging comes in. It uses air pressure to squeeze out the excess epoxy and trapped air. This improves the bonding between the layers, decreases the cure time, and will frequently get you down to that target of 60/40. You re thinking, Oh, sure, but I can t do that in my little shop. Ah, but you can, Grasshopper, and it s neither difficult nor expensive. If you ve already done a wet layup you ll need just a few more items, the most expensive being a vacuum pump. However, even that can be had without too much pain. In fact, in a future article, we ll show you how to build a nice vacuum pump that is powered by compressed air. You ll notice that I did not say, Use a vacuum pump to suck the air out of the bag. This is so often stated that the engineer in me goes nearly apoplectic every time I hear it. Hold this thought: Air does not suck, it blows. A moment s cogitation to consider the tensile strength of air should convince you that the vacuum cleaner you use on the carpet is really a pressure-differential cleaner. The vacuum in vacuum pump, as opposed to a compressor, is simply a sloppy way to describe whether Photos: Bob Fritz KITPLANES November

3 Composites, Part 7 continued the machine alters air pressure to above or below ambient. It s important to understand this concept, because without it you ll not understand the why if something goes wrong in this process. The Setup The basic idea is simple: In doing a wet layup, we used peel ply and a brush to soak out the excess epoxy. This time, we ll mechanically squeeze it out. You could try to sit on the bag, but as you do so you ll remember the last time you tried to squeeze the air out of an air mattress. Tough to flatten out those random pockets of air wasn t it? Sort of like playing Whack-a-Mole at the amusement park. Instead, let s get Mother Nature to sit on the bag. All we have to do is push a bit of the air out of the bag with a vacuum pump, and the rest will take care of itself. The two components received a generous layer of epoxy-flox and were taped together. They didn t need the perforated plastic sheet, as there simply isn t a lot of epoxy to squeeze out and it all comes to the edges. The bleeder cloth, though, is required to give the air a path for leaving the bag. After placing plenty of bleeder cloth in positions that will channel all the air out of the bag, it s time to close up. Again, a buddy to help in doing a part this big is essential to keeping the wrinkles out of the tape zone. A wrinkle there is a leak. 50 KITPLANES November

4 One more digression into science. Air pressure is simply the load exerted by the weight of the air above a specified area. Go up, and the air pressure decreases because you re now above some of it. At sea level it s 14.7 psi; at 18,000 feet above sea level it s half that. If you don t think air has a weight, put a flattened basketball on a gram scale, then pump it up and weigh it again. What that means is that unless you live in a very deep hole in the ground such that the air pressure is greater than at sea level, the absolute limit you re going to place on the bag is 14.7 psi. Now, 14.7 psi is not a trivial load; on a part measuring 12 x 12 inches, you d have to stack pound bags of sand in perfect distribution to get the same effect. The reason you don t feel that weight is because you re 99% water, which simply transmits the load through you without crushing you. Taking a look at the diagram, we see that there are a lot of components to the bag. The vacuum bag, fabric, peel ply, wax and mold are all familiar by now, and a vacuum pump is obvious. But how do the sealant tape, perforated plastic and breather material fit in? Clearly, to transform the bag material into a bag you have to seal it. That s the job of the sealant tape. It s a putty-like material that adheres to the bag material. It comes as a roll and is roughly one-eighth by onehalf inch when laid out. You strip off the paper that protects its sticky surface and simply push the bag material into it. Wrinkles should be avoided as much as possible because they are leak paths. Vacuum tubing is vital. Obviously, Tygon tubing will simply collapse, so a more rigid material is required. Size is not critical; just keep it short, 4 to 5 feet maximum, and unkinked. Last is the pressure differential generator, aka vacuum pump. This can be anything from a vacuum generator (no moving parts) that hooks up to your compressor, to an aquarium air pump, to a big-bucks industrial vane or screw-type vacuum pump. The size of the pump is entirely dependent upon the size of the project and the integrity of the bag. You should be able to achieve a solid squeeze Automated Engine Monitoring.....of up to 29 parameters with 62 alarms. From RPM to peak-detection leaning, the EIS does it all. Includes graphical and digital displays, customizable screens, and alarms with external warning light. Models for all engines up to 9-cylinders. Find out why the EIS is the choice of thousands of pilots. $473 2-stroke $553 2-cyl 4-stroke Prices include probes. $995 4-cylinder Actual Size 6"W x 2.75"H x 2.5"D All-cylinder EGT/CHT analyzer functions for 4, 6 or 9 cylinder engines. Grand Rapids Technologies, Inc Madison Ave SE, Grand Rapids, MI Fax Kitplanes.com/aircraftdirectory KITPLANES November

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6 Composites, Part 7 continued in 5 to 8 minutes. Too small a pump and you won t keep up with the inevitable leaks. To explain the function of the other parts, let s go into the bag to see what happens when the pressure is on. First, the bag itself starts to collapse, pushing against the bleeder cloth. Remember that air mattress? Tough to get out those random pockets of air. That s where the bleeder cloth comes in. It s a thick, porous mat that gives the air a path by which to escape the outside push. The bleeder cloth has another use. The gases and excess epoxy pass into it with the liquids being trapped, and the gasses continue through the breather material and out through the vacuum pump. It s somewhat akin to wearing socks on a wet carpet. The water gets squeezed out of the carpet and into your socks through a combination of capillary action and a pressure differential. Speaking of pressure, the bleeder cloth also serves to distribute it evenly. Without it, any wrinkles in the bag would define the surface contour of the finished part just like a wrinkle in your pillow leaves you looking like you ve fought a saber dual. As the bleeder cloth collapses, it pushes against the perforated release film and the peel ply. Epoxy, being a liquid, is non-compressible, so it squeezes through the weave in the peel ply, through the small holes in the release film, and soaks into the bleeder cloth. The air bubbles in the epoxy also are forced through the release film and, as the layers descend, they push the fabric against the mold. The epoxy/air between the fabric and the mold gets pushed through the fabric, through the holes in the release film, and it also deposits into the bleeder cloth. Just like that, you re at the 60/40 ratio. I should note here that some of my sources tell me that peel ply is optional, but that it s good for fine-tuning the ratio of the epoxy to fiber. It s also a good filter to keep strands of the breather material away from the epoxy-fabric, but it means the part will assume the texture of the peel ply. The fabric has been soaked in resin and then assembled with all the rest of the components. All that remains is to turn on the pump, let the epoxy set, and then pull out the finished part. They take the time to carefully push the upper bag into the tape. The hose goes in with special attention to keeping the bag material away from the inlet. Gomez has even wrapped the end of it with bleeder cloth. KITPLANES November

7 Composites, Part 7 continued Here s what it looks like if the bleeder cloth is excluded. The inevitable wrinkles in the bag and release film define the surface finish. Choices and Tradeoffs Is this the only way to vacuum bag a part? Of course not! That s the fun of this process. It s open to invention and variation once you understand the basics. For instance, to make a flat layup such as an extra thick spacer, you could use painter s plastic for the bag, kitchen sponges to soak up the excess epoxy, Dacron material from the fabric store (that s what peel ply is made from), duct tape to seal the bag, windshield washer hose for the vacuum line, an aquarium air pump for a vacuum source and skip the perforated release film if it s a part smaller than about 10 square inches. I did it, and it worked great. Another variation was demonstrated by AirCrafters Guru of Glass, Harold Bunyi. He s a bit more elegant in making up the windows for a Lancair when he bonds two existing shells into a single component. The bag goes completely around the assembly and, when the pump is turned on, it gets about 4000 pounds of perfectly distributed pressure while the adhesives do their thing. And that vacuum pump that uses compressed air? Here s a hint: Bernoulli. We ll show you how to build one next month. For more information about AirCrafters builder assistance, call 831/ , or visit A direct link can be found at Now they add wrinkles. The bag material will stretch just so much, and then it either displaces the parts or won t fully collapse around the assembly. Placing wrinkles is a skill, though. You have to see where additional material will be needed. It started out as a square bag and ended up looking like something from the Bat Cave, but these two parts are getting squeezed with about the same load as if a Hummer parked on them. 54 KITPLANES November