vacuum bagging

[velocity dreamer]

has anyone used vacuum bagging in there construction. I hear it helps make the bonds stronger and reduces weight by helping remove excess resin. what are your ideas. If bagging the wings do you have to do each layer seperatly or can you put all layers down and then bag it.

 

Lynn

[Jack Morrison]

Lynn

Learn the process, it will give you very consistant , strong and light parts. It is not hard to do and I feel the benefits by far exceed the small cost of material. I have a high vacuum system I use but the low vacuum sysem the C Girrls use is also very effective especially for small parts. It is simple, low cost and very effective. I use it on all glass and carbon fiber I do now.

 

Jack

E Racer 113

[Steve Innova]

I just tried my first vacuum bagged part last night, and can report great results! I vacuum bagged the entire upper nose (don't try to correlate the part, I'm building a one-off design). Although I was only able to get 5psi due a leaky bag, the part turned out great. Lighter than expected, perfectly conforming to the foam core. Very pleased, well worth the effort.

 

I've assembled a set up using a 12 gal air tank connected to the in port on my air compressor using Home Depot fixtures & hoses. I have a vacuum guage to monitor the vacuume, and a bleader valve to control it. I also have a vacuum switch to turn the air compressor on/off, but it's not hooked up yet.

 

All told, cost me around $150-180 for all the stuff. It could be done cheaper, but I wanted an air compressor as the vac pump, since it's useful for other things.

 

Where it really pays off is in the large parts, where you can never squeegie enough of the excess out, due to many layers, large area, time constraints. You just lay the plies up really wet, put down peel ply and the bleader, and vacuum bag it. All the excess gets sucked out. Learn it. Try it.

[Jon Matcho]

I have experimented and used the low pressure technique the Cozy Girrrls have championed. Read more about my use/interpretation of that here.

 

My biggest issue right now is that I can't quite figure out how NOT to make my parts too dry/light. I've had to trash some work because I had too many faint white spots within the layup (at least I thought they were too many). I've heard from others about similar experiences.

 

The girls call for 2 layers of absorbant (paper towels), which provide a conduit for air removal as well as soak up the epoxy. Maybe I should use only one layer of paper towels? Adding excess epoxy tends to only migrate up the vacuum tube. I've since read that the girls turn the system off after the epoxy gells (just past pot life). Maybe that's what I need to do -- I've generally left the vacuum on for 6-8 hours.

 

Maybe I need to read a proper reference on vacuum bagging!

 

Any advice is appreciated...

[Steve Innova]

you need to use a real breather/bleeder layer for any larger part. This allows air and epoxy to evacuate out of the layup without getting saturated.

 

It's more expensive, but will produce better results. The paper towels will get saturated and won't distribute the vacuum evenly.

[Jon Matcho]

This allows air and epoxy to evacuate out of the layup without getting saturated.

That's actually my problem... too much epoxy is leaving the fiberglass.

 

How many inches of pressure are called for in "real" vacuum bagging?

[Steve Innova]

That's actually my problem... too much epoxy is leaving the fiberglass.

If you have dry patches, that means that the vacuum is not uniform. Use a real breather layer (maybe $5 a yard from AC spruce -- save money by getting fake "fleece" from Walmart), to get a uniform vacuum. Or, perhaps the paper towels are wicking up the epoxy before you apply the vacuum.

 

An important distinction might also be that I used the dry micro method to prep my core. I applied micro and glass in two steps. First I applied the micro to the foam, let it cure, and sanded. I then applied my layups to the cured micro.

 

Why is this important? Well, if you coat the surface with micro, then lay up your glass, then vacuum bag it, most of the "pure" expoxy will get sucked into the breather layer, and the micro will get sucked up into the glass plies. This is bad, because your replacing glass w/micro bubbles, weakening the part. If you don't want to try the dry micro method, then lay up your parts with just pure epoxy on the surface--no wet micro. (Wouldn't recommend this for Divinicell / Blue Foam, too heavy).

 

I don't know if it makes a difference for you, but it worked for me.

 

How many inches of pressure are called for in "real" vacuum bagging?

My pump setup wasn't working properly, so I only had mayb 7" HG. Next time I'll try it w/15" HG. From what I've read, you don't want to go over 15" HG on most foam core substances. Between 10-15" is better.

 

For the blue styrofoam on the wings, even lower. Don't remember, but probably not more than 10" HG, max, or you'll start crushing the foam. You need a vacuum guage and bleeder valve if you want to do this right. They're cheap. $12 for the vac guage, you can use an air compressor bleader valve from Home Depot. All my fittings and hoses are from Home Depot, cheaper that way.

 

For single layer components (no core material) you can go up to 28" HG.

 

Lay up your parts very wet, put down peel ply, coat the peel ply w/epoxy so that there's no dry spots. Then lay down your breather layer (unlike paper towels, a real breather layer won't wick up the epoxy UNTIL you apply vacuum.) Then apply vacuum. I used a heat tent to speed the cure.

[Len Evansic]

I've been contemplating vacuum bagging, but the more I read, the more I hesitate. It's not the investment, but the fact that we use foam for our reinforcements. Think about it, the foam is a ready supply of air to bleed into the layup. It might be closed cell, but I'm sure even a slight vacuum could coax quite a few bubbles out of the foam to make layups too dry.

 

Thinking back to the Cozy Girrrls demo at Oshkosh, the NACA scoop that they used was a solid, non-reinforced layup. Basically, an open mold. This is what vacuum bagging is traditionally used for.

 

Also, vacuum bagging is used to squeeze pre-preg onto core material like Nomex honeycomb in autoclaves. As mentioned, our core material will crush at those vacuum levels, and also they won't withstand the heat.

 

If I were to build a mold for my fuselage, wings, or strakes, then yes, I would definitely use vacuum bagging on a large scale. Right now, I'm even afraid to use lo-vac for all but the simplest parts.

 

-- Len

[Steve Innova]

I've been contemplating vacuum bagging, but the more I read, the more I hesitate. It's not the investment, but the fact that we use foam for our reinforcements. Think about it, the foam is a ready supply of air to bleed into the layup. It might be closed cell, but I'm sure even a slight vacuum could coax quite a few bubbles out of the foam to make layups too dry.

-- Len

Len,

 

The entire part, foam and all, goes into the vacuum bag. Obviously the peel ply / bleader layer only goes on the layup side, but the entire part is subjected to vacuum. If it's properly sealed, there's no way for air to get into the layup.

 

I'm not trying to sound arrogant (or cast myself as an expert) or anything here, but it's really an undisputed fact among composites experts that vacuum bagging produces higher quality, lighter layups and parts. There is a learning curve and it requires more skill than wet, hand layups, but vacuum bagging is superior.

[Marc Zeitlin]

...Think about it, the foam is a ready supply of air to bleed into the layup. It might be closed cell, but I'm sure even a slight vacuum could coax quite a few bubbles out of the foam to make layups too dry.

 

Thinking back to the Cozy Girrrls demo at Oshkosh, the NACA scoop that they used was a solid, non-reinforced layup. Basically, an open mold. This is what vacuum bagging is traditionally used for.

 

Also, vacuum bagging is used to squeeze pre-preg onto core material like Nomex honeycomb in autoclaves. As mentioned, our core material will crush at those vacuum levels, and also they won't withstand the heat.

First, I'll say that I'm not a big proponent of vac-bagging for our aircraft - I just don't believe that the slight weight savings justify the extra work. Others disagree - that's fine.

 

However, foam cores of the types that we use in our homebuilt aircraft are used regularly with wet layups and vacuum bagging, at the vacuum levels that Steve has indicated. Since the cores can withstand up to 140F, we can heat tent our parts to 100 - 120F while curing under the bag (or NOT in a bag) if desired with no problems.

 

With respect to pulling air out of the foam and into the layups, there is no evidence that this happens. Measurements have shown good epoxy/glass ratios in foam core vac-bagged layups using standard techniques. In fact, it's FAR easier to get a quality sandwich layup using foam, micro, and a wet layup than it is with honeycomb core, which is a real PITA.

 

This is all for wet layups - pre-pregs are an entirely different story.

 

So, if you're willing to do the extra work for whatever weight savings there MIGHT be, don't be worried about these issues - there are enough issues to worry about :-).

[Len Evansic]

I'll take from these replies that my fears about vacuum bagging foam are unwarranted. Trying to rationalize postings like Jon's, noting dry layups after even lo-vac are what brought me to my conclusion.

 

I'm still going to hold off on doing anything vacuum bagged until I have more confidence. Speaking of which, I may be able to do my practice work this weekend, as it's supposed to almost hit 60° on Sunday (my garage is still under-heated, and I'm not allowed to do it in the house).

 

-- Len

[Steve Innova]

If you want great instruction on vacuum bagging, go to this site:

 

http://www.fibreglast.com/contentpages-Vacuum%20Bagging%20Equipment%20and%20Techniques%20for%20Room-Temp%20Applications-230.html

 

I've found the www.fiberglast.com site to be a great resource on all-things composites. They have in-depth instructions on mold making and many other topics. One of my favorite sites.

 

If you want to explore a really neat variation of vacuum bagging, check out resin-infusion: http://www.fibreglast.com/contentpages-Vacuum%20Infusion-316.html This technique allows you to do layups without EVER touching the epoxy. Although it requires considerable skill to do properly, for those suffering from epoxy allergies, it's a way to finish your plane.

[Marc Zeitlin]

... Since the cores can withstand up to 140F, we can heat tent our parts to 100 - 120F while curing under the bag (or NOT in a bag) if desired with no problems....

After some prodding from Wayne Hicks, I did a bit more research to figure out where I got that 140F # from.

 

Wayne wrote to me:

>----> Reading chapter 25, room temperature cure epoxies soften and lose

>rigidity at moderate temps. (160F.) Foams soften and swell at moderately

>elevated temps. (250F.)

 

That sure is what it says, doesn't it. For some reason, I thought that I had read that the wing foam had lower temperature capabilities. In fact, the Dow web page says that Styrofoam Brand Insulation's maximum continuous operating temperature is 167F (75C). Most of the "Last-a-foam"'s have a max. cont. service temp of around 200F.

 

I'm not sure where I got the 140F from..... Aha - close. From,

 

http://www.netcomposites.com/education.asp?sequence=47

 

the max cont. op. temp. of some PVC foams can be as low as 120 F, with processing at up to 150 F.

 

So, Nat's probably not being conservative enough if folks would use his #'s for fabrication temps.

 

At any rate, as long as you're not cooking your layups, either while bagging or not, you're extremely unlikely to harm the foam.

[Wayne Hicks]

So, Nat's probably not being conservative enough if folks would use his #'s for fabrication temps.

 

----> I'm thinking it might be RAF who is not being conservative enough. We all know that Nat's plans closely follow the Long-EZ plans, word for word in some sections. And I think Chapter 25 is one of them.

[argoldman]

So, Nat's probably not being conservative enough if folks would use his #'s for fabrication temps.

 

----> I'm thinking it might be RAF who is not being conservative enough. We all know that Nat's plans closely follow the Long-EZ plans, word for word in some sections. And I think Chapter 25 is one of them.

 

I seem to remember that some tests I did years ago on wing foam that it liquified close to 200F.

 

This was many years ago and I could be wrong, but I was testing the material to see if post curing at 180F was damaging... Sorry I can't remember the details.

[Maarten Labordus]

Vacuum bagging will not automatically improve your laminate quality. If you've done a poor job hand laminating your part, vacuum bagging will only make it worse. You're decreasing the pressure causing air bubbles to increase in size (the well-known Gass law: P*V=constant). So the first thing is to remove all air during lay-up with proper de-air rollers or similar.

If your pressure in your vacuum bag is too low (=too deep vacuum), dissolved air from your epoxy can come out of solution. Similar to a beer bottle where the CO2 comes out of solution when you open it. Degassing resin before you use it will solve the problem. Bagging at higher pressures (=less vacuum) will reduce the problem.

The next thing that can go wrong is if you use a very low pressure (=too deep vacuum), is that you have squeeze out so much resin that the laminate is resin starved. During curing, the epoxy will shrink (up to 7% by volume) and that will cause shrink voids.

If done properly, vacuum bagging will give a far more consisted laminate quality than laminates cured at astmospheric conditions. Even beter would be to vacuum infuse your parts!

[Jon Matcho]

If your pressure in your vacuum bag is too low (=too deep vacuum), dissolved air from your epoxy can come out of solution. Similar to a beer bottle where the CO2 comes out of solution when you open it.

I wonder if this is what I have experienced on occassion.

 

Degassing resin before you use it will solve the problem.

Not really an option in this crowd. If there are any little bubbles that become bigger bubbles, I think it's more likely a result from mixing the resin and hardener.

 

Bagging at higher pressures (=less vacuum) will reduce the problem.

What do you think of this technique involving 'less vacuum': LoVac? The vacuum pump is supposed to be "weak" (less vacuum), but I'm starting to wonder if it draws more than I expect. You can see a layup in there that looks a bit white/dry. I need to get a gauge to measure the vacuum, but what is a range for a "higher pressure" (less vacuum) vacuum?

 

Even beter would be to vacuum infuse your parts!

I saw your other post, and this sounds interesting, but I don't see how it's going to happen with me or most anyone here. You do need some sort of tooling/mould as well as a reservoir of mixed (ready-to-cure) epoxy (more than you'll use). Stocking up on another type of epoxy (low-to-super low viscosity, w/very long pot life) for infusion isn't going to add up for many people here. I'm sure it's a great technique, but for our purposes I don't see the bang for the buck. Maybe I'm wrong.

[Jack Morrison]

Jon

I think you are correct on bagging V infusin. I have vacuum bagged complex shapes for 10 years and have had very few problems. Vacuum bagging for our application in my opinion seems to be the ultimate bang for the buck. I have had only 1 dry part in many many parts and some are quite complex. Do not misinterpet my message here, you can get extremely quality parts without vacuum bagging, I just find it improves the end result a bit.

 

Jack

E Racer 113

[Jon Matcho]

...you can get extremely quality parts without vacuum bagging, I just find it improves the end result a bit.

That somes it up quite well.

 

Where I really appreciate it the most is where you have to glob on the epoxy a little thicker, or wait until it gels to keep the glass wrapped close to compound curves. Anything that is a complex shape -- which are few and far between in these planes by design -- is a candidate for the vacuum.

[Jon Matcho]

That somes it up quite well.

I hate when I do that! That sums it up quite well!

 

Maarten, I'd appreciate any feedback you have to offer us. You obviously do this for a living, but this is also our passion. Of foremost importance to me are approaches/changes that can do two things: (a) allow parts to be built as fast, if not faster, as hand layups, and (b) to consistently ensure quality parts. We have many flat parts, boxes, foam core airfoils, and some parts w/compound curves.

 

The only way any significant tooling/moulds could be created would be for 1 building to invest, and be able to send (easily) to others when finished. Otherwise complex tooling is a waste for our one-time needs (well, we do make 2 of many parts...).

 

Feedback is welcome.

[Maarten Labordus]

For all flat parts, you basically need nothing else then a flat plate like a sheet of steel. I find it very convenient to use a glass plate. First: it's perfectly flat and absolutely airtight, but second: you'll have the possibility of a visual control of the resin flow at the bottom side of your laminate. But the point is: investment in tooling is very low for flat parts. For double curved parts, it get's slightly more complicated. But since most curved parts on a aircraft will probably have something to do with aerodynamics, I would rather spent some more time on getting the right geometry. And it does not really matter so much whether you spend your time on sanding and filling a tooling surface or on working on actual part. O, but adding filler on the actual part also increases the weight.... But as soon as you have the tooling finished, making the actual part is peanuts!

 

It seems to me all of you are more or less building the same aircraft. Why not use one finished aircraft and use that as a master mould for getting a complete set of tooling? And have that tooling circulate among all the builders. There might even be some business in doing that! That will probably safe all of you a lot of time! You will be able to infuse a complete wing structure in a couple of hours, rather than spending weeks shaping your wing. Or to take it to the next level, would the regulations for kit-aircraft allow complex parts being build by one company which supplies them to all the home builders who'll perform the assembly? In that case, I'll build (and vacuum infuse!) the parts for you!

[Jon Matcho]

For double curved parts, it get's slightly more complicated. But since most curved parts on a aircraft will probably have something to do with aerodynamics, I would rather [spend] some more time on getting the right geometry.

Curved parts are not always airfoils, but the airfoils are a critical shape.

 

And it does not really matter so much whether you spend your time on sanding and filling a tooling surface or on working on actual part.

I think this is the best argument for creating a tooling, but then more questions:

• We hotwire our wing cores from blue Styrofoam®. Won't that create a problem given that the foam needs to be bonded to the outside skin? I'm sure the foam could be treated with micro -- as you do anyway with a hand layup -- and then fit into the mould. Voids are a concern.
• The Berkut used our same airfoils, and evolved into moulded wings using foam+fiberglass ribs. Otherwise it was hollow.

My question is whether you can still use foam-core AND get the benefits of an almost-ready-to-paint outside skin? Also, there are spars that run through the length of these wings. I also wonder whether it would be best to only mould the skins???

 

It seems to me all of you are more or less building the same aircraft.

Yes, many are building the same model, but there are a few varieties. What is interesting is that airfoils are shared between most models (whereas fuselages are not).

 

Why not use one finished aircraft and use that as a master mould for getting a complete set of tooling?

It's been done with a couple of our designs. Check out www.aerocad.com who have created a kit based on a plans design.

 

And have that tooling circulate among all the builders. There might even be some business in doing that!

You'd have to crate and ship -- a project in itself, which I don't see happening -- forget I mentioned it.

 

Or to take it to the next level, would the regulations for kit-aircraft allow complex parts being build by one company which supplies them to all the home builders who'll perform the assembly? In that case, I'll build (and vacuum infuse!) the parts for you!

The majority of us are building from plans and raw materials (not a kit). People buy wings, parts, and entire projects (abandoned) all the time from "vendors".

 

Here's an idea... you scratch build wings using our plans, finish nicely, take the mould, and then build me molded wings (or give me the plug). I'll test fly to prove your quality and establish reputation, and then you sell to more builders.

 

I can't imagine shipping from the Netherlands is cheap, and so as an alternative you can tell us how to make a mould (and use it). For a moment I thought this would make sense for making my wings, but I'd still have to make TWO different moulds -- one for left and one for right. No savings for 1 plane.

 

There is a builder/flyer who makes a few wings each year and sells them. He has an outstanding reputation. www.aerocad.com has recently changed owners, and is working on establishing their "new" reputation. Buying someone else's work is a leap of faith, which many do not make for financial reasons (they *can* build it themselves) and other factors (mainly 'trust'). Some just want to build every piece possible, whereas others want to get into the air in MONTHS instead of YEARS.