Dear all
Just working on the project based on carbon prepreg honeycomb structure. The procedure below is OK, the final rigidity is excelent, but there is a problem on visible surface goes to be non flat after vacuum bagging. See the procedure:
1. Hit the first  layer, carbon prepreg 210gsm, 1 layer, vacuum bagging, autoclave 100°C@-0.98bar/+8bar. Perfect results, excelent flat visible surface, no pinholes, final thickness cca 0,25-0.28mm.
2. Apply the epoxy glue RG (5min.) for better adhesive properties, put on the nomex honeycomb OX4.8, thickness 3mm.
3. Apply the outer layers on honeycomb side, 3x carbon prepreg 210gsm.
4. Vacuum bagging, autoclave 100°C@-0.98bar, no overpressure.
Final structure is fine, but on the mold side - the visible side of structure, there are after vacuuming around all the surface where the honeycomb was applied..how to say that..hollows? The surface is not absolutely flat like in step 1, but deffected by inner honeycomb core. See the picture. Any ideas what's wrong?
I'm convinced, the problem is vacuuming. If I hit the outer and inner side of prepregs separately in autoclave and glue the sandwich together in vacuum, the defect is quite same, if the thickness of outer visible side is 0.7mm instead 0.25mm, the defect is quite same.
Many thanks for support.
Rosta

It is print through of the core due to the heat and vacuum. Try using a lower vacuum, you don't really need full vacuum for bonding core you just need enough to hold everything in place - normally around 50% vacuum.

I would also suggest not sending it in straight for a 100° post cure, as this is essentially a free-standing post cure and should be ramped quite slowly even though the skins are already cured. The skins are quite thin and easily affected by the shrinkage of the adhesive. I find you will get this even with 2mm thick skins if you aren't careful. Are you sure the adhesive you are using can actually handle 100°? Most 5 minutes epoxies can't handle that temperature and would be shrinking pretty violently. If yours can, I would still suggest it needs to be ramped up slower.

Hanaldo, for better understanding, in step 3 the prepregs are not already cured (In this case if the structure isn't flat panel, needs two mould - inner skin / outer skin and bond together). If the shape is quite comlicated I put the outer skin prepreg not cured and full vacuum bagging for best copy the surfaces.
You right, the  epoxi I have used has temperature resistance cca 60-70°C (in accordance with TDS), but I think that's no reason why the core print through...

I doubt its print through
My guess is your not running a vented cure ?
If you maintain vac during the cure each cell essentially becomes a vac chamber and pulls your uncured laminate up
Called golf balling in the trade
At 15psi vent your cure

+x

 

f1rob - 1/21/2020 4:49:20 PM

I doubt its print through
My guess is your not running a vented cure ?
If you maintain vac during the cure each cell essentially becomes a vac chamber and pulls your uncured laminate up
Called golf balling in the trade
At 15psi vent your cure

What is 'vented cure'? Do you mean vacuum =0 and overpressure +1bar? It makes sense.

Yeah i think as well that it is a mix of the core not able to vent to ambient and the thermal expansion of the glue meniscus that pulls the skin down while cooling.

Its hard to tell from the picture but could it be some pre-release of the mold-side skin from the cooling off after the first cook?  I've had that happen esp. on aluminum tooling.  If that happens the golf ball thing is more likely - even with 3mm cell nomex - not sure what your cell size is...

F1rob mentions vented core and he may be onto something - that .28mm skin isn't much - guessing its woven if just one ply.  I am a big fan of slicing the top of the cells (nomex) or vented core (aluminum) and in either case perforating the top skin every 50mm or so to release pressure as the cook ramps up.  Can you get vented nomex?

Can you cook the whole thing in one shot?  You might need to warm debulk the first skin to get a nice surface but it is totally possible to get a really nice surface finish from a one-cook cored panel.  It's corners and core fitting that make multiple cooks necessary most of the time... unless you're doing something crazy complex.

Lester's theory about the glue I agree with, especially if you are using an ambient cured epoxy to bed the Nomex and it gets up near/above it's Tg during your top skin cook... Can you get some glue-film that's compatible with your resin system?

Good luck - I hope you figure it out!

Most calves enable you to vent the cure
At 15psi the vac is turned off and the job side of the pipe is vented to atmosphere
If you cant do it with your current system it's very easy and put a gate valve in an a t piece with a valve and run a very basic manual version.

@Chris: Vented Nomex probably not possible to use, the shape is 3D machining, I have no idea how to machine the vented slots for each chamber of honeycomb. I have to use better adhesive bond, with Tg over 100C. that's right.
@f1rob:  I see, my clave enable to vent manualy. I'm just a little bit scared, if the vacuum is -1bar, clave pressure increasing up to +1bar, before the vac is turned off there is a 2bar relative pressure, I'm not sure if the honeycomb will not be pressed. Better to decreasing the vacuum and increasing the overpressure continously to keep the relative pressure 1bar?
Many thanks for support

Sorry, I misread f1rob's vented autoclave cure for vented cell core - with little holes between the cells to allow cell pressure to equalize and leave out the edges.  This isn't an option with Nomex but aluminum cores can be got this way.

You don't have to vent the autoclave right at 15psi - you could wait until the pressure gets higher so the differential is greater... I'm not convinced you have to vent it at all, but I am not experienced with it and have only ever worked in places and had my own autoclaves where we left the pump on - mainly just for volatiles and moisture control.  Worth a try though!