phenolic sandwich panels

[Jim85]

Hi Guys,

Im after some help. Im have some problems with some glass phenolic sandwich panels. they seem to be buckling. Any solutions?

Im working off an aluminium mould, applying two plies of phenolic, curing then applying 19mm honeycomb core and a further two plies of phenolic. A pressure plate is applied before bagging. The weave of the glass phenolic is a satin weave 8 harness. There is also an airex border internally along with some aluminium inserts. The panels themselves are around 2 metres long by 1 metre wide.The amount of buckle seems to vary with every panel.

This is causing me some concern so any help would be much appreciated.

I look forward to hearing from you.

 

[SWComposites]

Is your issue core crush of the panel during cure?

 

[Jim85]

No the core is completely in tack. The panel itself comes out with a large bow in it down the length of the door.

 

[Compositepro]

Then your issue is warping. There are several possible causes.

The fabric weave is unbalanced. Are you using a balanced layup (e.g, warp face to warp face in each skin or warp face toward core in each skin)? Weave misalignment in the prepreg can also cause this,and is a very common problem. Check that the warp and fill yarns are straight and square with each other. Is the prepreg tacky or dry and boardy? Tacky prepreg can be difficult to lay down flat, without waves and bags. Pressing these flat will misalign the fabric.

Do you have a good release layer between the mold and and part? The CTE does not match so things will want tomove with temperature changes. Is the mold stiff enough to stay flat under stress?

Resin content of the two face sheets may be different due to variations in the prepreg or due to resin bleed into the core which flows to the lower surface. More resin means more shrinkage.

Phenolic releases water during cure which dries out later. Drying results in shrinkage.

 

[Jim85]

The mould itself is strong enough to stay stiff under stress. The lay up of the plies are matched. The CTE is a possibility as is the resin bleeding through into the core causing shrinkage.

 

[Jim85]

I'm using a release system called marbocote. Do you know it?

 

[Compositepro]

I'm not familiar with Marbocote. Since you are making flat panels, I would suggest that you use a sheet of Teflon impregnated fiberglass as a release film between your part and the mold and caul. It will almost always produce better surface finish with fewer pits than a mold released surface. It can be reused many times and is easily replaced. The only problem is it is difficult to apply to contoured surfaces. It may also help you warping problem because it will slide against the aluminum if there is movement. Mold releases will release in certain areas when the shear stress between the part and the mold exceeds a certain level (due to cure shrinkage) and not in others. This can cause uneven stress in the part that will show-up as warping.

 

[Jim85]

Thank you compositepro. I will look into that. You have been very helpful. I'll let you know how it goes

 

[SWComposites]

Why are you curing the facesheets in separate cures? The fact that the bottom facesheet is first cured, during the second cure the top facesheet may be shrinking during cool down while the bottom facesheet is not changing dimension - leading to a warped panel.

 

[Jim85]

That is a good point but I'm curing the first skins to get the mould definition. The panels have some tricky edges which will void. It might be worth trying a one shot process then?

 

[Hojo1122]

Your alumninum tool has a high CTE and is probably warping during post cure and returning to shape after it has cooled. What temp are you curing at?

 

[Jim85]

Curing at 140 degrees

 

[SWComposites]

140 F or C?

 

[Jim85]

Sorry. C

 

[Hojo1122]

I would try the same panel on a different surface such as tempered glass. See if you get the same amount of warpage. Or try a lower post cure temp for longer time if possible(check manufacturer). That should tell you if it is warping at higher temps.

 

[Jim85]

I will get in contact with the supplier to get alternative cure cycles for the material I'm using. Also I will try a test panel on a different surface to see if that helps the issue.

 

[Jim85]

Im back, still having issues with my flat phenolic panels. Ive gone back and worked through your suggestions. Im now curing top and bottom skins then bonding them together with resin film. This in theory should stop the top skins from shrinking, causing the warp. The weave alignment is definately matched on both surfaces. The only thing left that you guys have mentioned is that its an aluminium mould with a different CTE but I mainly work of aluminium moulds with a phenolic component. Ive never had any of these issues before.

 

[Compositepro]

Pictures and other data would help a lot. Up to now we can only guess.

 

[Jim85]

Ok I'll try and get some photos up over the weekend. What sort of data do you need?