Vacuum Bagging honeycomb core

May I butt in?

Honeycomb is considered as an advanced composite material. So does foam core as the properties of the materials is best used to advantage.

Honeycomb is best used with prepreg but you need a glue sheet in between the honeycomb and the prepreg. Honeycomb is best when vacuum bagged.

It is not a good practice to bond immidiately the honeycombed to a wet laid up curved surface as the wet laminate will be disturbed during the vacuum process. Best bet is to let the outer laminate cure until tacky then apply a light coat of resin to the honeycomb then bag it. The resin acts as a glue.

For flat laminates or very slight curve, you can lay the honeycomb directly to the wet laminate then bag it after peel ply, perforated release film, bleeder cloth, ect.

Unless you are working with prepreg, you cannot make a double skin laminate (inner and outer) in one go. You need to make a partial laminate first of outer skin and honeycomb. The inner skin will be a little bit tricky and is a different process for flat and curved laminate.

 

I use -20 to -25 inches of mercury. 27" is hard to achieve with a 2 Hp. vane pump. Aerospace and racing parts use large vacuum machine that reads up to 75" of mercury. I have never seen it but they say you cannot talk to each other when it runs. at any rate, I have never crushed a honeycomb with -26" of Hg.

You have to cut the edges of the honeycomb to 45 degree. Use a very sharp utility knife. If you want a perfect edge, you may fill up the eges with microbaloon putty then sand it before the inner skin is bonded.

Use ridgid honeycomb for flat panels, "Hex" hexagonal honeycomb for curved surfaces, "OX" over expanded honeycomb for compound surfaces.

 

Thanks. I am learning a lot from this thread.

 

At last someone has given me the answers and information i have been asking about .
Not everyone has used Honeycomb , i have laid lots a foam , lots a balsa but never used Honeycomb .
Getting an informative answer is like getting blood from a stone !
Thank you so much !!

 

Thanks for the nice words tunnels and hoytedow. I do not consider myself an expert. Just 17 years in the composite industry.

I am glad I am no longer bounded by "confidentiality" so I can share my knowledge. Some are trade secrets which prevented me from sharing in the early days. You will not find that in the books.

 

We should add: less is more in common prepreg or resin film layups!

8psi or (when I remember right 20Hg) are a lot of vac. pressure on a average core layup (too much for most applications in boat building).
On a wet film layup you would print through from 4 or 6 psi upwards. (depending on material of course)

A honeycomb "sucks" resin into the root when in contact with a wet layup, but the cells are not filled with resin. First, there isn´t enough resin to fill the cells, second there is no way (almost) to suck the resin out of the wet fibre.
At say, 25 psi and above you would deal with that problem amongst others, sure.
But there still is not enough resin to fill the cells to some extend. The layup though would be for the landfill.

And to come back to the original question:

No, it is a MUST.


Regards
Richard

 

Thank you !
At last there is some good and useful info starting to surface .The process for vac bagging is basisly easy to do ,just i know it all came to work with us and stuffed a couple of big panels we were making .
I ill explain a little later the whats and whys when i find the pictures i am looking for on my dvd disc.
Keep the info flow going this is all good stay !! There are little traps for people to watch out for and i totally agree there are secrets that you wont find in books . Some of the panels we were making were 16 mtrs long x 4 mtrs wide x 50mm cores with 56 layers of glass each side some even had Carbon as well . Sometimes took 30 guys to get the panels off the table for the crane to be able to pick them up .

 

I cant remember the conversion right now but the tech rep says we are applying about 50 lbs/inch2 to the laminate, that is prepreg laminate.

The root of the honeycomb gets about 0.002 to .003 of resin in our coupon test section.

Most likely you will get print through on the laminate. Even WR will show a print through.

I won't reccommend Honeycomb on the hull of the boat. Not even on the deck. It is great for interior panels.

 

Tunnels,

If you are making 16 meter long panels, watch out for curving of the laminate when it is pulled out of the mold. I have described the heat absorption of the tool somewhere in this forum before. That is the tool sucking the heat out of the resin while it is exotherming.

 

Going to sleep

 

I dare to disagree, (in some details)

It is rubbish!

Replace it with a light wood and be fine. Several times the shear strength and no rot. (opposed to common belief)

In high tech applications and aircraft, were every single gram counts, these composites have a high value.

In boats for average (including circumnavigations) use, it is wasted money, complicates the whole stuff and never pays back. (the opposite)

For a homebuilder it is nothing but mad, even to think about it.

Regards
Richard

 

For Racing boats that is out of the water most of the time, honeycomb will do but for commercial applications it is not advisable.

Honeycomb has air cells. Fiberglass is known to have osmosis after 5-10 years. The air void tends to fill with water after some time.

Some core manufacturers offer foam filled honeycomb. This might be a solution, but personally, I have not tried this so I cannot offer even an opinion.

 

Tunnels,

With 56 layers on a side of a cored laminate, that would be 0.65 x 56 layers=36 mm. of laminate of pure WR600. What are you building? Bridges?

But you say 30 men needs to lift it so I beleive it is not a typo error. Drop me a line in the private message forum. You might be overdoing it a lot and wasting your material.

Rx

 

On to the original question of the topic starter:

Get yourself the quarter sheets of Core-Cell. They are great for your application. You can easily butt-join them in the works, but make sure there is some resin or glue between them. If you wish, you can pre-coat (hotcoat) the sheets, this will make bonding more bullet proof. Although a slurry-type of bonding paste should do the job without hot coating. (you can always run a test)

About honeycomb: do not bother, unless weight is a more then critical factor.

 

Re. bagging pressure (vacuum):
29.9 inches of mercury, 760 torr (mm Hg), 1 atmosphere. That's the most you'll get: pull a perfect vacuum in the bag, and your bagging pressure will be that of the surrounding atmosphere.
Reports of bagging at 75 inches Hg are for parts that are cured in pressure autoclaves. Take your normal vacuum bagged mould and part, and seal it inside a giant pressure vessel that goes up to five atmospheres and 120 degrees Celsius during the cure. The only place I've ever seen/used such a machine is at a helicopter factory where we were curing a solar car's body/chassis assembly.

Re. resin in the cells of the honeycomb: If the mould is airtight, the resin is not subject to any pressure differential, and is not forced in any particular direction. The pressure of the bag is transferred only through the solid components: the honeycomb and the fibres. The vacuum does not suck resin into the cells. Do note that this applies only to standard vacuum bagging: infusion systems are carefully designed to ensure that the resin IS subject to a pressure differential, by keeping one end of the resin system at higher (ambient) pressure, and attempting to infuse with honeycomb in the layup will probably result in resin-filled cells.
The cells will, however, tend to fill with resin purely due to gravity, as it drips down from the layers above. Unless you know about this and plan around it, you might end up wetting out the top layers over and over again until the piece is heavy and solid.
Of course, I'd have a pretty hard time recommending most honeycomb materials for wet layups; they're really designed for the heat-cured prepregs used by the aerospace industry.

IMHO, honeycombs are better left out of most boat applications, although they do have some merit in win-at-any-cost racers where the engineering costs are easily borne and the long-term downsides are less noticeable.