Ski Builders Forum

Hello,

Does anyone have experience with prepreg film adhesives?

Nomex Honeycomb is generally not compatible with wet layup (due to the cell structure filing with resin). But prepregs work, because there is no excess resin, and they have a very short liquid state.
The objective is to continue to use dry cloth (with liquid resin) in layup. But to place a prepreg film adhesive directly above the core. Between the core and the first fiber layer, and then to continue with traditional wet layup above.

Also curious about curing 250F prepregs at a lower temp say 190F

Thanks,
Pete

No experience here but happy to kick the idea around.

Do you have examples of what you're thinking for the film adhesives?
Something like this from Henkel maybe? Cure temps seem high on most of these.
https://www.pccomposites.com/product/ea7000-0325m/
https://www.pccomposites.com/wp-content ... 5H_TDS.pdf

Oh this 3M product looks a little better and has a really good applications section (worth reading for that alone). Starts to cure at 82C-- prob could just run a longer routine at that temp to cure. Wants a lot of pressure-- but if you've got 40-60psi on in your press and the knife edge from the honeycomb i dont see an issue there.

https://www.pccomposites.com/wp-content ... 6M_TDS.pdf
https://www.pccomposites.com/product/af6/

You may have already gone the cheap route on this adventure... but you could always try tack gluing some craft paper over the top of the honeycomb? I'd try a small sample piece obviously, but it might do the job and would be a much easier "x" to throw into the mix.

Something like this: https://compositeenvisions.com/prepreg- ... 50-f-cure/
Lots of versions though, many of them lightly supported with a nylon mesh (this one says you could cure at 150˚F)

What I'm really curious about is if the prepreg resin will bind into a wet resin system(like of they are compatible).


Interesting with the craft paper, I'd actually thought about capping in with 1/16" balsa. But that brings up shear forces, something that I'm not familiar with. Would that bond, between the balsa and nomex honeycomb, fail before the upper fiber layer?

I think the idea with the craft paper is that it wets out but prevents tons of dripping/filling of the honeycomb. I would think that might be worth a try.

I had a similar thought of using something like a medipore tape, or maybe doing a super thin wet glass veil as the layers outside the honeycomb so there's less epoxy to squeeze out on that layer. You could also glass the honeycomb with a thin veil prior to and let it cure but that seems like a PITA.

Wood on honeycomb direct doesn't feel right to me intuitively.

The henkel product i referenced above actually has wording in the spec sheet about co-curing although it does mention with prepregs. Seems like it's likely to be chemically compatible maybe.

Someone on here must have used a wet layup on a prepreg at some point-- i wonder if we can get your question answered. My hunch is that if its an epoxy type pre-preg you'll be OK but there are other prepregs i think too.

One other thing has been bothering me on this approach--
If the wet layup is up against a honey comb or even something that has some compliance, you're going to have less squeeze out since you won't have even force over all of the glass. Might be part of why prepreg works well with honeycomb.

I agree, I honestly think the honeycomb needs a prepreg of some sort on both sides. That's why I was looking into the film adhesives, don't really want to entirely go down the prepreg route.

Could put a prepreg adhesive on both sides of the core, but this may be more trouble than it's worth

hows about precured sheets? Phenolic. Just givem a good sanding, bonds really nice.

You could also use pre-cured fiberglass or carbon sheets. These could easily be made by just vacuum bagging or pressing single or mulitple layers of cloth on their own. If you want to save weight, you could make a pre-cured sheet thin fiberglass veil and use that as a separating layer.

For better bonding I would use peel ply when pressing the pre-cured sheets.

I once came across a small German ski manufacturer that used pre-cured fiberglass sheets in all of his skis.

That is an interesting idea, might have to give that a try.

What about full width vds sheets top and bottom of core? this would relieve the issue of perhaps a low quality bond.
Does anyone know how much mass (grams) this might add to a ski overall?

Full width vds = heavy. So you loose the advantage of honeycomb again. Friends do titanal skis and they told me their skis are heavy because of the vds

ive used precured on the base side, cut to fit inside the edge tangs. this way there is no need to rabbit the core.
to answer your question about weight, ive ripped apart some skis to test bond strength. out of curiosity i weighed the triax that came out of the ski.
The .5 mm precured weighed almost excactly as the 22oz triax. both measured about the same thickness as well. 0.5mm/ 0.020"