Coleagues
Just another issue I´m unsuccesfull.
Trying to laminate a sandwich panel  with nonconstant thickness in 2 steps process. 1st step there is a cca 0.7mm thick panel based on UD carbon prepregs - 1x54+2x100+2x200gsm UD. On 2nd step apply the Rohacell core and inner skin based on twill 200gsm carbon prepreg, see the attchd. sketch. The curing process starts on vac-1bar@100°C for 4.5hours, cooling slow down to 30°C for 6hours. After demoulding the panel is going to distorsion about 10mm.
Any tips for process changes?
Thanks a lot, Rosta

So the bottom skin was pre-cured... was it released from the mold/tool before the core and top skin were applied?  What type of adhesive did you use to bind the Rohacell to the carbon skins?

What you are describing is (probably - without knowing fiber orientations) an unbalanced laminate - made more interesting by varying core thickness.  Was the top skin placed in one piece over the top of the shaped foam?  Are the ramps in the foam short as drawn or longer and more gradual?  Are the prepregs all cured with the same cycle?   Any of these will cause the problem!  

The Rohacell is pretty stable so that is probably not the issue.

You have either the top getting longer or the bottom getting shorter!  From the information it isn't clear which is happening.  Is there a way to try curing it all in one shot?  Over 400mm is doesn't take much to get a 10mm curve. 

Chris
1. Yes, the bottom skin was released from mould before.
2.The core chamfer is 45° all around the shape.
3. Yes, all the prepregs are cured with the same cycle
4. I have made an experiment where all the plies were cured in one shot, no surprise, the effect is totaly same.
This is my conclusion, you right, the laminate is TOTALY UNBALACED.
The picture below shows the basic experiment and the deformation results. If the sheet flatness is the crucial parametr, the laminate has to be strictly balanced and symetrical across the plies. Because the thermal expansion goes in fibers direction, on plain/twill ply goes 0°+90°, on UDs goes 0°. This fact generates the internal tension penpendicular to UD direction. On sample 3 the laminate is balanced and sheet is finaly flat.
On panel above there are  5 plies UD0° + 1 ply twill which generates the tension because contains fibers goes 90°to UDs direction and UDs plies has no 90°fibers...
That's the result I hope so...

Looks like you found the problem!  I wouldn't lay all the blame on the unbalanced laminate - tapered core can be an issue too even with balanced skins - but most of it. 

I'm interested in your 54g uni though - what is that and where did you get it?

Chris, that's a standart UD 54gsm carbon prepreg, bought in CN, contact here: cangzhouzhongli123@163.com