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help with sandwich construction

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Old 03-22-2004, 12:04 AM
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aeroanalysis
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Default help with sandwich construction

I need help guys. I am making a composite fuselage with nomex honeycomb core. I have the first part layed up, fiberglass/carbon against the tool surface and the core on top of that. That part goes fine and the test parts have come out perfect. The question is, how do I get the inner layer of fiberglass onto the core? The wetted out fiberglass doesn't want to stick to the core at all - the core almost repels it. But the fiberglass loves to stick to the release ply and the vacuum bag and everything else but the core, so everything gets all wrinkled up and shifted around before I can apply vacuum. I would love some suggestions. I was thinking of spray cementing the cores first to make them tacky, but I am not confident that this will work.
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Old 03-22-2004, 11:24 AM
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buttuh
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Default RE: help with sandwich construction

You're missing something. You need to apply a layer of adhesive film in between the nomex and the graphite cloth on both sides of the laminate. This film is supposed to come with or can be purchased from the same people who you bought the nomex from. Without this adhesive layer, activated by heat using a heat gun, you will not get a proper bond plus the excess resin that will soak through and collect inside the closed cell structures. That probably explains why you're laying the whole thing up in 2 steps instead of all at once.
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Old 03-22-2004, 11:51 AM
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aeroanalysis
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Default RE: help with sandwich construction

Can you tell me more about the adhesive layer? I know the folks in the shop used something like this at Scaled when I worked there, but I didn't think it was essential. We have made several test panels using the two step process, without the film adhesive and they came out stiff and light with good core adhesion.
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Old 03-22-2004, 12:10 PM
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Default RE: help with sandwich construction

The only reason you got good adhesion is because the excess resin ran through the closed cells and bonded to each other on both sides. Even then it's inconsistent depending on how much resin you applied to certain areas. Plus your overall surface area is still very small. It might look good and strong now but one or both sides of the laminate will probably come off under stress/impact. Any vibration will cause the resin to slowly break off from the graphite and the honeycomb since there isn't a whole lot of surface area for it to hold on to in the first place. Anyway, I'm just being technical. In actuality you might still have a good strong structure that will hold up.
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Old 03-25-2004, 03:36 PM
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Default RE: help with sandwich construction

Okay, anyone have any more ideas? Getting wet fiberglass to stick a bit to the core while I get the vacuum bag on it. Anyone?
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Old 03-25-2004, 05:04 PM
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Default RE: help with sandwich construction

First of all sheet adhesive is generally used for elevated temp cures and I see your performing wet layups versus pre-preg lay-ups. I can only presume you are allowing your cure cycles to happen at room temp. Before you start your actual lay-up, trim your core slightly oversize of final core dimension, tailor .5 oz. cloth oversize for both sides of core, prepare bagging, place first ply down in tool with resin, place second ply with resin down on core, place core down into the tool, place appropriate release media, bagging, ect. Place down lead shot bags or equivalent, don`t be to concerned at this point about critical placement or small wrinkles. All you are trying to do is create a vail on each side of the core bagged to the desired shape you want and allowed to cure. after cure you can trim to exact tolerances perform perfect 45 degree bevels and abrade. You will have a light custom tailored and detail tool fitted honeycomb core ready for your wet lay-up part.

Bob
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Old 03-25-2004, 06:54 PM
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Default RE: help with sandwich construction

ORIGINAL: sensei

First of all sheet adhesive is generally used for elevated temp cures and I see your performing wet layups versus pre-preg lay-ups. I can only presume you are allowing your cure cycles to happen at room temp. Before you start your actual lay-up, trim your core slightly oversize of final core dimension, tailor .5 oz. cloth oversize for both sides of core, prepare bagging, place first ply down in tool with resin, place second ply with resin down on core, place core down into the tool, place appropriate release media, bagging, ect. Place down lead shot bags or equivalent, don`t be to concerned at this point about critical placement or small wrinkles. All you are trying to do is create a vail on each side of the core bagged to the desired shape you want and allowed to cure. after cure you can trim to exact tolerances perform perfect 45 degree bevels and abrade. You will have a light custom tailored and detail tool fitted honeycomb core ready for your wet lay-up part.

Bob
Thanks for the help. I wonder if you have actually done this? The problem is the wet fiberglass doesn't want to stick to the core at all. It grabs at the release ply and the bagging plastic and gets all fuddled. I'm going to experiment with a smaller lay-up tomorrow.
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Old 03-25-2004, 07:15 PM
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Default RE: help with sandwich construction

I have been in the composites business for over 30 years. I have used this method along with many of my employees for ages. Just get that weight bag down on top of the bag and place Hg on it as soon as possible. Try a couple of samples ,you will see


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Old 03-26-2004, 07:12 AM
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Default RE: help with sandwich construction

It would be greatly appreciated if someone could take a series of pictures explaining how to work with Honeycomb material. I always thought you needed to use Pre-Preg composite for HC. Anyone know have a link showing the process?

Bill


[link=http://www.paonline.com/hayman/video.htm]Made Easy DVD's[/link]
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Old 03-26-2004, 08:04 AM
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Default RE: help with sandwich construction

This may help.

http://download.r-g.de/handbuch/kapitel_09.pdf
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Old 03-26-2004, 11:00 AM
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aeroanalysis
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Default RE: help with sandwich construction

OMG! That is a kicking article!

I will post pictures of our work later today.


ORIGINAL: DMP!

This may help.

http://download.r-g.de/handbuch/kapitel_09.pdf
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Old 03-26-2004, 10:09 PM
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Default RE: help with sandwich construction

Nice site! That appears to be just one chapter.

http://www.r-g.de/

Ted
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Old 03-26-2004, 11:23 PM
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Default RE: help with sandwich construction

Sensei, I'm confused, are you making a core preform with your technique. If so how are you prepping the surfaces for secondary bonding in the final wet lay-up? This sounds interesting but seems like twice the work. Why not just do it all on the primary lay-up/cure?
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Old 03-27-2004, 03:17 AM
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Default RE: help with sandwich construction

Yep That is one nice company. That is where I get all my fiberglas stof.
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