FRP Sandwich panel production

[ResiPeti]

I have been wanting to make an FRP sandwich panel (either HC core or foam core)in autoclave for a while. No pinholes are allowed on the tool surface. Co-curing is preferred.
So far I have made panels using CFRP and Nomex core, but only 2-steps (1.Front Skin + 2. Adhesive Film, Core and Inner skin)curing was succesful.
I guess pinholes are unavoidable when using Nomex HC in a co-curing process.
I also co-cured different PVC foams with CFRP. Results: sometimes core crushing another time pinholes on the surface.

 

[SWComposites]

Try using a surfacing film (adhesive, synskin) on the surfaces.

Are you using fabric or tape CFRP? what resin content? how many plies in the facesheet?

 

[ResiPeti]

The skin is composed of 2 layers of 40 % resin content, 200 gr/sqm woven carbon fabric.

 

[SWComposites]

Yes, pinholes are likely with that thickness and material. You probably need to add a surfacing film on the tool side. Or use a pin hole filler after cure prior to painting.

 

[ResiPeti]

I tried out two different materials as surfacing film while using NOMEX HC.
1. 48 gr/sqm glassfabric prepreg with 60% resin content
2. Redux film adhesive 150 gr/sqm

I could not believe it. The first one was much better. The second one rubbish.
What do you think about trying acrylic foam (such as Rohacell 50A)?
PVC foams did not work but I did not use surfacing film in that case.

 

[blakmax]

Nomex can absorb significant amounts of moisture from just atmospheric humidity, and this may result in micro-voids (pin holes). Try drying the core at 80C (180F) for about six hours before you lay it up.

Regards

blakmax

 

[ResiPeti]

Thanks for that.
Now I am more or less happy, with the middle section of the under core surface. (Surfacing film gave me acceptable result there.) The next problem is the area under the chamfered section of the HC (chamfer angle: 25-30 dgr). I suspect that there is a slight core movement causing much more pinholes and surface irregularities in this area. I intend to resolve this by using doubler plies under and above the chamfered core section.
The other idea might be to start the heating up just after the pressurisation has been finished with a vented bag at 0,7-0,8 Bar autoclave pressure.
Anything other that can help?

 

[cpinz]

The hc cells at the edge of the chamfer are not able to keep planar the layers; the cell crash and the layers tend to be wavy.

1-Remove the sharp corner of the chamfer (2-3 millimeters);
2-Fill the cells of chamfer, near the edge, with filler ( resin+microballons should work well);
3-In the area that you removed, put filler ( now resin+cotton) and drown some tows in it,all along the chamfer)

Now,doubler and outer layers have a solid base that can sustain them.

I hope this can help you

Cpinz