Nomex and wet lay ?!?!

[ostraker]

Hi Guys
Im planning on building a 5m dinghy over the christmas break and am about to start work on the hull and deck moulds. Ive spent a while collecting nomex offcuts from work and have enough to do the job. Minimum wieght and stiffness is the priority with this build.

Due to the limited facilities I really dont want to make an oven and the mould prob wouldn't hold up anyway. I've been experimenting with wetlaying the nomex but have no real way of testing it or knowing if what im doing is right.

The best way seems to be to comb the bog on the first layers and vac the nomex to it. I have various plans but the final layers with the bog is a bit hit and miss.

Ideally I would prefer not to use the bog at all and lay it up like you would with phenolic prepreg (ie minus the resin film).

Can anyone help me out here. All pointers appreciated

Cheers Olly

[marshmat]

Hi Olly,

Wet-layup with Nomex honeycomb is a royal pain in the backside. There's no way around that.

It's not impossible, though. The most successful (of many) attempts that I've seen to wet-lay over Nomex involved making flat plates with the stuff. The outer skin (against the mould) was done normally and the Nomex pressed onto that. Then the first layer of the inner skin was wetted out on a plastic sheet with a fast catalyst, and transferred to the Nomex just as the resin began to kick off. (Thus, the resin was at just the right viscosity to adhere to the Nomex properly without flowing into it and filling the cells.) The rest of the layup was then done normally and the whole thing bagged- very quickly- with peelply, breather, bag, etc. that had been pre-cut to fit. The key is to transfer that partially-gelled skin at exactly the right moment, when that layer is sufficiently solid to stop any resin from flowing through, but still wet enough to acheive a full bond.

A similar procedure could be adapted to suit a more complex mould, although it would take a fair amount of planning. For a boat hull shape in wetlay-over-Nomex I would probably want to lay up the outer skin and bag the Nomex to it while it's still wet, then wait until that is fully cured before doing the inner skin in a separate layup.

If you have access to prepreg, an 80-celsius oven is really not that hard to make. Scrap aluminum, some 2x4s, a few bags of Roxul mineral wool insulation, and a few shower-stall heatlamps on separate circuits do the job just fine. Just be sure the roof is well supported- speaking from experience here, you do not want any part of the temporary oven coming down on top of your bag.

[ostraker]

Hi Matt

Thanks for the advice - a few different angles to think about there.

With the off the job process - Im thinking ampreg 22 with a std and slow hardener mix should give a couple of hours working time and be less viscous enough to do the job.. The only thing is it would need postcuring, maybe I will flip over the mould and then heat it.

Prepreg
Its good to know resin film isnt neccesaryly neccesary.
Im not too opposed to this option but Im worried the polyester moulds I will make wont hold up to the heat. Shrinkback etc...
Ive left emails with my reps so will see what happens.

Layup
My origional plan was to do a 1/core/1 layup with carbon twill, but as you have mentioned filling up the honeycombs with resin, Im now worrying about the finished product having holes in the cloth letting the water into the honeycomb. Maybe non perferated breadwrap when i flip it over to post cure?
Does anyone have any thoughts and or recomendations on that.

cheers Olly

[marshmat]

I am not aware of any prepreg that will bond properly to Nomex without a core-splice adhesive film. They just don't have enough resin content, and what's there isn't flowable enough.

Polyester moulds may have trouble at 80-plus Celsius, hopefully your supplier will have more specific data on the product you're using.

Have you done the calculations to find the proper skin thickness? I'm not sure what weight of carbon twill you're thinking of, but a single layer of typical 300 gsm carbon on either side of any amount of Nomex will certainly not be strong enough.

Water getting into the cells of the Nomex is a major problem. Those skins have to be perfect, completely non-porous everywhere.