Pre-preg Honeycomb Panel - Vacuum Concerns?

General Composites Discussions Composites Talk
#1

I’ve been doing some research about elevated temperature cures using honeycomb cores without autoclave and I ran across this quote in an article.

OOA=out of autoclave

“In its significant work with OOA-cured honeycomb-cored sandwich structures, ACG has identified some technical challenges. Chris Ridgard explains, “the bagging and layup techniques are basically the same as in autoclave curing, but venting of the core becomes important.” The high pressures used in autoclave curing, which do not permit air inside a honeycomb core to flow into skin laminates, are not present in OOA curing. Therefore, air must be removed from the core cells before the resin softens, because the lower OOA pressure might otherwise allow air to flow into the skin, resulting in high void content.”

I hadn’t really thought of this till now but I’m curious if there are special techniques required for properly evacuating the honeycomb before curing? When they say “before the resins softens” do they just mean before resin flows due to elevated temperature? Is this really much of an issue?

#2

Odd that they say, allow air to flow into the skin laminates…you want the air inside the cells to get vacuumed out, or stay IN while it’s being cured. I don’t see how, if you have vacuum in oven, or clave, why the evacuation of the cells is different. Pressure is hygrostaticlly pressing on the surface. The bag, to the laminate, to the cells the laminate is touching. Caul plates would have even pressure over the entire surface. I don’t think the cells would SEE the autoclave pressures. I have always wondered about what the pressure inside the cells are after curing. Unless you have porosity in the laminate, I would only imagine the cells would be under vacuum after cure, meaning the entire panel is vacuum cored! :slight_smile: However, if the prepreg seals off the cells before they can be evacuated, then it should always be ATM pressure.
Hmm

#3

Had ACG on the phone this morning, next time I’ll ask :wink:

#4

I would appreciate that.

Here’s the link for the full article: http://www.compositesworld.com/articles/out-of-autoclave-prepregs-hype-or-revolution

Any guidance to what they’re talking about would be appreciated. I’m glad that others are also confused by this.