I need to replace some nomex honeycomb on a flat chassis where it was burned. I’ve never worked with honeycomb so my questions are 1. How do I bond the honeycomb to the old solid surface and 2. How do I apply layers of carbon on the top side without resin going into the honeycomb? Do I make a flat plate and then bond it to the top side of the honeycomb? What do I use to bond with? Thanks for your help.
Ro
Their are resin films and special adhesive films for this. Airtech carries the adhesive and so does Cytec. The adhesive film is NOT cheap, I think the roll I ordered a month ago was $5,000.
The resin film is basically prepreg without the fabric and not as expensive, I’ve see it available from online shops but can’t remember which one. On a side note about resin film is you can get some that’s for resin film infusion which is a neat way to make parts and I havn’t seen it mentioned on here before.
Without going the prepreg route, you will be best to make a plate up first otherwise if you infuse the resin will fill the cells. You do want a small amount of glue to form a fillet in the corner of the cells to get a good bond.
Last time I had to do this I made a thickened epoxy bed (about 1-2mm deep) in the hole before putting the Nomex in, then I vacuumed it down and waited for the resin to cure. I then did the same with the premade plate - bed of glue on the plate and vacuumed on. I had to sand the Nomex flush before putting on the plate but that was because I didn’t have the right thickness on hand.
Worked pretty well but it’s much easier with prepreg or can the Nomex and use foam if you can.
Typically that is a standard core repair. New core would be inserted and backside cured under slight vacuum using a film or foam adhesive mentioned earlier. After cure new cells would be filled with microballons and adhesive… Cured. Potted repair would be sanded flush and a scarf repair bonded topside.
Plates are an option but in all honestly once the cells are broken or damaged the nomex loses integrity. Potted core repairs are pretty typical in aerospace.
Side note: nomex collapses very easily as you probably already know… If you plan to try an plate over the new core do a printing technique and use a reinforced film adhesive.
For example. Bond new core in repair area using your film adhesive or foam. Then cure… Then. Take the film adhesive and lay flat on the plate and lightly pass over with a heat gun so it sticks. Leave the plastic sheeting on it and don’t press it down with a spreader… Kinda leave it tacked on. Then place the plate on the core repair and apply light vacuum. The core will print on the adhesive… Telling you where you need more film adhesive. Cut and apply and then print again until you have as close to 100% contact with core and plate… Foam adhesives expand when cured and alleviate this crap… Much better IMO.
On the flip side… A thick layer of resin and microballons… Set in new core and a generous portion of resin on a piece of fabric over the repair area for a first ply to close cells would not keep me up at night either so long as you tap test each cell afterwards looking for disbonds… Then layup repair over the closed core… Disbonded cells would then get injected with balloons and resin…
Hope this helps…
A layer of glass to close the cells should not be a problem in the stack up… Most film adhesive already have glass in them. You will get a crappy surface closing the core but fill with more resin and ballons and sand flush as you like. You can scarf the rest of the plys in and honestly never notice the repair if you take your time.
Moke was on point with his potting advice… A pointer would be using a plunge router to level the nomex until the plate fit snug… But… Vacuum can collapse good core surrounding the repair area if the part is too big to bag… So be careful… Example would be pulling vacuum on a local area… Say a aileron or speed brake… And there is a hole on the far end… Your pulling vacuum and suddenly your not… Ndi verifies that your vacuum has tunneled itself to your repair ripping through cell after cell… Just putting it out there…
Good luck man
Sorry off topic but wow you are getting ripped off there. We pay NZ$2500 for a roll of 50 linear meters of glue film. And that includes the fact it’s made the other side of the world to us. But then I also hear materials cost more in Australia than New Zealand too.
Back on topic:
The method of repair will be determined by what you want. A prepreg glue film and unfilled Nomex will be the best for weight and strength but highest cost and you need an autoclave or heat box. Potted Nomex or foam will be cheaper but heavy.
Thanks a lot for the advice. I found the film adhesive last night on the internet. I like the idea of micro balloons too.
Here is one approach,
[ame=“http://www.youtube.com/watch?v=_-pMlHAJ1a8”]Composite / Carbon Fiber Repair Training Video Introduction - YouTube[/ame]
Why was he wetting out the peel ply??
Is that just to ensure a uniform surface finish when the peel ply is removed?
If using that method on a large scale, wouldn’t it just add lots of extra resin to the job?
He wet the peel ply so it would look uniform… Kinda cheated IMO
I didn’t watch the video, but if you want to have a bondable surface and are trying to tightly control weight wetting out your peelply or using prepreg peelply will keep the peelply from sucking out to much resin. If you were to use a perforated release ply you would have to prep the surface before secondary bonding
He didn’t use vacuum.