Newc

Since I am new to vacuum bagging, have seen a couple of great videos on the subject (Jim Young’s at http://www.rcgroups.com/links/index.php?id=4901 and one provided by Aerospace Composite Products that came with my purchase of their EZ Vac system) and have heard different things about the use of Gorilla Glue for sheeting, I conducted an experiment, the report on which follows. Note that the epoxy used for the experiment was not laminating resin, but was what I had on hand. This may affect any direct comparisons with Gorilla Glue, but I believe that the basic findings are valid. Comments are solicited.
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Vacuum Bagging Experiment – January 20-21, 2006

Four (4) pieces of 2” expanded polystyrene* were sheeted, both sides, with 1/16” balsa. Two (2) pieces each used Anchor Bond Epoxy (not a laminating epoxy, but what was readily available) and Gorilla Glue and each pair was split - vacuum bagging using ACP EZ Vac system and weighted down with 20#.

*Note that Styrofoam®™ (a term used very frequently in modeling circles) is a trademark of Dow Chemical Company for their extruded polystyrene, and is always blue in color. The expanded polystyrene is the typical white foam, and the density of this foam may vary from manufacturer to manufacturer.

The EZ Vac system is rated at 6” Hg vacuum, or approximately 3 psi. Based on the surface area of the pieces cured atmospherically but weighted down, the pressure on those pieces was approximately 2.2 psi.

On all pieces the bonding agent was spread across the balsa, wetting it, and then as much as could be was scrapped off, resulting in no visible bonding agent on the surface of the balsa. On the Gorilla Glue pieces the foam was dampened with a damp paper towel. On all pieces, following the application of the bonding agent, the pieces of balsa would not stick to the foam without weight being applied, indicating very little bonding agent was on the surface of the balsa.

Weights of each experimental piece were determined before applting the bonding agents, with the bonding agent on but prior to curing and after curing. Within the accuracy of the digital scale being used, and due to the samples each weighing approximately 5 - 10 grams, there were no notable differences.

Pictures of the four pieces, two in the bag and two behind, weighted down by sandbags and a 10# dumbbell, not visible, is attached, as are other photos, the captions of which are preceded by [Photo]

Following 20 hours curing, the vacuum was removed as were the weights on the atmospherically cured pieces.


[Photo] The vacuum bagged pieces were compressed 1/8” compared to the atmospherically cured pieces. This is a 6% compression of the 2” thick pieces.

Notice the compression of the vacuum-bagged pieces. This is likely to vary dependent on the density of the particular polystyrene used. It is also likely to vary dependent on the shape of the piece being sheeted and bagged. For example, it is likely that a wing shape, which is being sheeted on virtually all surfaces (the ends being a very small part of the area being subjected to the vacuum) will not compress as much as these blocks that had balsa on only two of the six available surfaces.

Each piece was sliced to be able to look at the bonded edge within the piece. Visual inspection of each piece with a magnifying glass revealed no visible difference in the interface between the foam and the balsa.

Following the above, an edge of each piece of balsa was loosened and then pulled off the foam, in each case resulting in breaking of the balsa to allow this release of the balsa. Following are the results of this:


[Photo] Notice amount of foam removed from each piece.


[Photo] Weighted pieces - Notice amount of foam removed from each piece.


[Photo] Vacuum-bagged pieces - Notice amount of foam removed from each piece.

Based on the difficulty in removing each piece of balsa from the foam (subjective) and the amount of foam pulled off with the balsa (which seemed to directly correlate with the subjective force required to remove the balsa), the relative bonding of foam to balsa – from lowest to highest - of each method was: Weighted Gorilla Glue, vacuum-bagged epoxy, weighted epoxy and vacuum-bagged Gorilla Glue.

While recognizing that the epoxy used may not have been the best for the application, following are my conclusions:

•The use of Gorilla Glue is an attractive agent for vacuum bagging when sheeting foam with balsa, and results in a better bond than the epoxy used.
•There is no discernable weight penalty – within the measurement accuracy - for the use of Gorilla Glue compared to epoxy.
•When not vacuum bagging, the epoxy used gave better results than Gorilla Glue.
•Vacuum bagging of the specific foam – ShelterSheath™ from Lowe’s building materials department – in the size blocks of foam used - resulted in compression of 6%.
•There was no visible ‘filling of foam voids’ with Gorilla Glue as has been speculated. This is not to say that this is not occurring at a microscopic level.

ChuckC

My Feedback: (24)

Now what I personally am REALLY interested in is a comparison of wing-type section strengths comparing Gorilla glue adhesive and epoxy adhesive while incorporating some strips of uni-carbon between the foam and the balsa. I'll experiment sooner or later with this, but if there's no significant strength degredation using Gorilla instead of epoxy, then there are significant production time gains to be had here as well as cost.

You seem to be familiar with typical scientific experimental technique, if not vernacular; sure would be interesting to see what I proposed above.... (This is a blatant bait). I'll go out on a limb and bet that if you wet out the unicarbon with gorilla glue, both sides, attach it to the balsa (also wetted out) and then vacuum bag - you would not see much, if any strength degredation over epoxy.

Anyway, thanks for sharing - it validates the rhetoric that I've heard. If you don't do it, I will - some day...

p.s. I'm surprised you had so much compression at such a low vacuum-may be the section shape?

daven

My Feedback: (1)

Chuck,

In my tests, I'd take any poly eurathane glue over epoxy.

I can get away with slightly less glue, and in my opinion, a stonger joint.

Not to mention the fact that its cheaper, and you don't have to mix it.

Newc

Chuck - Thanks for the opportunity, but I don't think I'll rise to your challenge/bait!

Some things you don't forget, even though it's been around 40 years since I developed and wrote up an experiment. Must admit that back in college many of the experiments were 'dry-labbed' - kind of like how a certain South Korean reported on his cloned animals and others have reported their discoveries of practical Cold Fusion. Except when I dry-labbed the answers I got were correct - since it was the same experiment that many years of my fraternity brothers had documented!

My best guess is that the compression is due to the section size and shape, especially the shape. When a wing is bagged it has the pressure applied all the way around the foam/balsa instead only two opposing sides having the foam. Another way to look at it could be that the 4 sides that have no balsa are allowing the vacuum to pull air out of the foam more than is allowed through the balsa.

ChuckC

My Feedback: (24)

On the compression; That's kind of what I suspected, using a cubic shape as I've pulled a heck of a lot more vacuum on some wings and got no discernable compression.

I agree that you'll get a better balsa to foam joint with urethane glues, just by their foaming action - my interest is when you need an ultra high strength wing, the typical method is to bond uni-carbon strips with epoxy between the cores and the balsa. Can you get the same strength with urethane? Specifically, when you use the uni-carbon on the tops and bottoms of wings in a spar arrangement, will it be as strong? Has anyone tried this comparison?

chelapa

id be interested in your setup. what is that little grey box to the right in the first pic? what kinda pump, plastic bag, and breather are u using??

chelapa

http://www.acp-composites.com/acp-vbs.htm

answered my own question. thanx! how good is the pump.. sorry to take off topic, but im very interested. will this vac be good for molds that have weird shapes? and can it handle bigger bags?

Newc

As you have probably already discovered using the ACP link you used, Chelapa, I am using the very inexpensive EZ Vac system, including the pump. The pump is setup for continuous running. I was a bit leary of this until I tried it, thinking that it would be noisier than I wanted and I was also concerned about heat generation of the pump. Both of these concerns have gone away, as the pump makes very little noise as it is pulling vacuum initially, and then it gets even more quiet! As for the heat build up, the pump housing gets only very slightly warm after 20 hours of continuous running. The bag that comes with the EZ Vac system is 18" x 9', and they also have 36" wide bags available. Based on the way that they have the vacuum being pulled through their reusable valve, there's no reason that even wider bags - or homemade bags from a suitable material - could be used.

As for the weird shapes, I would think that there’s no problem other than ensuring the proper alignments as the vacuum is being established. After that, the vacuum doesn't know the shape or orientation of the pieces. For detailed questions, I suggest that you contact ACP, as my limited contacts with them thus far have been very positive. The video that comes with the EZ Vac is excellent.

Chuck - Since the critical part of your question regarding the use of the carbon strips is actually asking if Gorilla Glue bonds well with those strips, I would think that it would, based on others and my experiences with the GG and other materials, includig plastics such as Depron. Additionally, GG claims on the bottle the ability to bond to such smooth and hard, non-porous surfaces as ceramic tile, plastic laminaqte, iron and aluminum. If you are truly concerned about this you could easily epoxy the carbon strip onto the balsa, allow it to dry and then proceed with the bagging with the GG. Perhaps ACP has a more definitive response for the question.

chelapa

thanx for the info! =) i will give them a call