Method for closing honeycomb

General Composites Discussions Composites Talk
#1

Hi all,
I saw a post from a year or two ago that touched on the subject of closing honeycomb so the inner face can be bonded without dimpling.
I have a similar use case that involves bonding a very thin inner face to honeycomb. The catch is that the part is specific-stiffness critical so the extra weight for materials to close the honeycomb is problematic.

My sandwich application is a small part (2 " X 6" X 50") with curved faces. It’s essentially a bent rectangular tube facricated in two halves. The faces of the tube walls are slightly curved and very thin (0.010") from 100% uni and the wall comb is 0.250" thk. Unfortunately I don’t have the necessary autoclave to use pre-preg and film adhesive so my proposed approach is to use a combination of lower tech methods as follows:

Step 1: Infuse and post cure the outer face in a female mold.
Step 2: Bond the core to the inside of the tooled face.
Step 3: Close the honeycomb’s remaining open/inner side. This is the challenge!
Step 4: Infuse the cut materials for the inner face on a flat surface to optimize resin uptake. Remove materials from bag and lay the wet inner face over the closed honeycomb, re-bag, cure and then and post cure.

So again the challenge is to close the inner face without adding a layer of glass and micro-balloons that will weigh more than the structural face itself.
So far all I have come up with is the idea of filling the h-comb cells with a removable material. Sanding flush with the edges of the h-comb and then protecting it with plastic film so the inner face can laminated, removed and then bonded after the filler has been removed from the comb.

I’m looking for any ideas on how other users may have solved this problem….so thanks in advance for any advice….cheers

#2

How are you going to bond a preformed skin to a honeycomb and under laying skin and not have delams. Only thing I can think of is using a foaming adhesive. Are you using aramid or aluminum? Is this a one off project?

#3

Look at this product. http://www.henkelna.com/product-search-1554.htm?nodeid=8797798334465

#4

Also why could you not use a foam or film adhesive for your application. If you are going to preform tool side and co cure the outside skin over a mock core then sandwich it all… I’m confused why you couldn’t use a standard foaming film in a vacuum bag and cure in a box. A part of that size could easily be cured with a few standard heat guns a some thermocouples.

My idea is this…

Infuse tool side.
Use a mock core and infuse outside skin.
Remove outer skin and apply adhesive films to both skins and insert core
Place in an envelope bag and cook the whole thing together.

Would that not work?

I’ve reached temps well up to 200F using an electric heat gun and a cardboard box

#5

My original plan was to make matched tooling with the correct gap for the honeycomb. But I was warned by another composites guru (not on this site) that achieving a high quality bond to honeycomb between two tooled faces that were not “flat” was a dubious proposition at best. This is what led me to propose using filled honeycomb as a “mock” tool surface. Given that my skins are so thin and flexible - a point I did not emphasize initially - I’m inclined to give the “mock core” approach a try…thanks Dallus

#6

Well it’s deffinately not the preferred way to do it… I will say.

#7

The Hysol EA 9815 looks perfect…thx for the tip…

Can you briefly describe the way this is done with the proper tooling and autoclave?

BTW: I do have a 3’ X 5’ X 1’ oven driven by two good quality adjustable heat guns and 4 probe thermometers. I needed this for post curing the Proset 117LV I’ve been using for infusion. This oven will go up to about 300F but it does not have a proper controller - yet. I can fit the entire part in the mold in a v-bag in the oven for post curing…typically about 16 hours at 120~140 F.

#8

This. Even if you can not achieve high temps needed for the film adhesive, you can try using a THIN layer of paste on the skins to bond the core.

Remember to use peel ply on bonding surfaces :slight_smile: