Hi -
If I’m laying up a pre-preg on top of a honeycomb structure, and then vacuum bagging the whole thing, how to do I get sufficient ply compaction in the open spaces of the honeycomb cells? It would seem that the vacuum bag will just push the pre-preg down into the cells, leaving a dimpled surface, with no ply compaction, because there is nothing pushing up on the bottom of the plys to squeeze them.
What am I missing?
Thanks!
Jon
Your missing the film adhesive that has thin stands of graphite or glass that gives it support. There’s many brands and types. There’s expanding foam adhesives. Impregnated thin films ect.
There’s also nothing wrong with bonding thin glass to the core during core forming to close the cells
Interesting. Can you explain how that works? The film adhesive bridges the cell gaps, and supports the laminate to keep it from being pushed into the cell cavaties? What keeps the film adhesive from simply being pushed down into the cavaties?
Thanks again,
Jon
If I were to go this route, how would I ensure a good bond between the glass and the honeycomb? I assume I’d have to use a vacuum bag - which leads back to my confusion(!) - since the bag would push the glass down into the cell cavaties.
Thanks
Jon
Don’t know if this helps but when I make flat honeycomb panels I cure both prepreg skins in a separate operation ahead of time so that they are flat without any dimples that could occur had I tried bonding the honeycomb directly to them. Then I lay the fist cured prepreg sheet down and coat it with hysol 9430, an epoxy with excellent shear specs, Then I lay the honeycomb over it and I vibrate the honeycomb with an orbital sander but without any sandpaper on it, that is to coat the cell walls with the hysol to ensure a good bond. then I do the same for the top layer and vibrate it as well then I vacuum bag it. Because the prepreg is cured, it doesn’t dimple from the cells in the honeycomb. and I also don’t use very much vacuum when bonding the skins, If I use too much it actually forces resin back through the prepreg because of all the air left trapped in the honeycomb, which is going to try to find a way out through your prepreg skins and it may draw resin with it depending on how porous your skins are.
Just like mando said …
Here’s and example senario:
Spray tack peel ply to prepared mold. Apply adhesive film or a pre impregnated glass. Lay core. Apply pre impregnated glass then peel ply. Perforated film. Breather. And bag. Cure.
Carefully remove peel ply from the top side. Give the peel ply a tug under the core and pop the whole core out. Remove peel ply and trim the closed core. Low vacuum in the bag is ok.
You shouldn’t have dimples and if so subsequent plys in the later layup will fill.
If you do have dimples…
If engineering will allow mix microballons and resin and wipe the core flat like bondo. Should be good to go.
Mando - yes, thanks; this is helpful. One side of my part is flat. So I can pre-cure the laminate on that side before applying the honeycome core.
The other side, however, is not flat (i.e., the honeycomb itself is shaped). And I have to lay the pre-preg on top of the shaped honeycomb. So I can’t pre-cure that side of the part. Any recommendations for that side?
Thanks again,
Jon
Dallas - so, basically, what you are doing here is bonding a thin layer of glass to the core under a light vacuum. And then after the cells have been closed with that thin layer, applying a full vacuum on subsequent layers of reinforcement so that you get good compaction. Right?
Thanks,
Jon
That works so long as the core is supported and doesn’t collapse. If it collapses… Goodbye part. But to answer your question. Yes the core has to be closed.
The other thing to be wary of is air removal from the honeycomb core. While bridging the gap with a layer of film adhesive or secondary bonding of pre-cured panels, i would always try and put a little hole above each cell (with a spiked roller) to ensure the air can be removed and will not cause delamination of the part as it expands under heat. what temperature are you curing at?
what about a solid male and female mold that acts as caul plates. should help with the dimples and give the prepreg a good smooth surface to cure against.
im more on the side of manufacturing raw product so my actual curing/paneling skills lack but it seems like this should work.
best thing seems like the cure than secondary bonding like previously stated
Hi there,
It’s been a while since this discussion thread was active but I thought I should re-awaken it because I am attempting the same “wet carbon layup with honeycomb core over a curved surface” use case…but with slightly different constraints…The honeycomb is 1/4 " the in my app and minimizing part weight is critical.
I’m wondering if there may be alternative methods for “closing” the honeycomb that are lighter than using a layer of light cloth - which still seems like it would dimple under even the most modest vacuum when wet and require post filling. I can see how a material such as a temperature activated film adhesive that contains stiff filaments for bridging across the open cells would work but I would like to avoid oven curing since I do not have the necessary equipment.
I have a couple of ideas for you collective comment as follows:
Could some sort of very low density foam - perhaps 2 lb urethane - be used to fill the cells after the h-comb is bonded to the first skin? This would provide a fairly smooth solid surface over which the inner skin could be v-baged. It would ad some weight but probably not as much as a layer of glass.
A variant of this approach would be to fill the h-comb cells with a removable filling agent that can be sanded flush with the honeycomb. The idea would then be to cover the filled h-comb with a release film - perhaps shrink wrap - and then v-bag the second pre-wetted carbon skin down. After the skin has cured, it is pulled off so the h-comb filler can be removed. Once the h-comb filler is gone, the skin should be a perfect match fit to the honeycomb for directly bonding with proper contact.
Thanks in advance for sharing your thoughts and experiences……cheers