How to remove air bubbles from prepreg panels

[MegComposites]

Does anyone have any advice on the best way to remove air bubbles from panels of prepreg? We have been using a vacuum table, but that just pushes the bubbles together and seals them in between plies. (We cut pieces from the panels and use them in a bladder molding process.) We have also tried using a roller that perforates the panels to allow air to escape during vacuum. However, that pushes the backing paper into the perforation holes, which contaminates the material.

 

[Compositepro]

Trapping air between plies of prepreg is a very common problem and is probably the most common cause of voids in parts. It sounds like what you are trying to do is debulk a lay-up. There are a number of ways to do this and the best really depends on the specific prepreg you are using. You may want to warm and cool the panel. You are probably doing a very common mistake which is to put you panel under a vacuum bag without any edge dams around the edges of your panel. If the vacuum bag simply drapes over the edge of your panel down to the tool plate you will pinch the edges and prevent air flow from the center of the panel to the vacuum. Use edge dams that are thicker than you panel so that the vacuum bag is lifted up off the edges.

 

[wes616]

First and foremost, it sounds like you are pre-stacking your plies, then laminating your mold with the pre-stacked plies. THIS IS BAD PRACTICE!!! and if this is what you are doing, you should see a considerable decrease in the phenomenon you are describing.

If you are not pre-stacking and I mis-understood your original post, could you describe the type of consumable materials you are using. For your delbulk, consider using a porous material between your laminate and your breather material to aid in air flow during the debulk.

Wes C.
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[blakmax]

You could also try looking at the humidity at the time you are exposing the material. Epoxies and many other resin systems absorb moisture from the environment, which turns to steam as the laminate is heat-cured. That causes micro-bubbles which may coalesce into larger voids. As a guideline, materials should not be exposed above the humidity and temperature limits of 65% at 18C (65F) to 45% at 23C (75F).

The same goes for elevated temperature cured adhesives, where lap-shear strength losses of 50% and flatwise tension strength losses of 28% have been reported for film adhesive FM300 exposed at 29C (85F) for 1 hour at 80% RH. See 39. Arnott, D.R., Wilson, A.R., Pearce, P.J., Mathys, G., Kindermann, M.R., Camilleri, A., DAVIS, M.J., Swan, G., Void Development in Aerospace Film Adhesives During Vacuum Bag Cure, Int. Aerospace Congress, Sydney, 25-28 Feb. 1997.

Try reading this link:

http://www.adhesionassociates.com/papers/Managing Micro-Voiding of Adhesive Bonds.pdf

Regards

blakmax