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Dry prepreg after VBO cure

Dry prepreg after VBO cure

(OP)
Hello.
We recently manufactured a larger CFRP sandwich article using OOA VBO technique and we were a bit surprised that the resulting prepreg appeared to contain much less resin than usual, and by this I mean that the fibers are visible in many places. This was especially appearant in the prepreg just above the honeycomb sandwich. In another thread on this forum from years ago(thread327-171582: pinholes and surface defects of any kind) it was suggested that a leaky tool might be to blame for this. I believe that this is the cause to our problems because; a) we had almost no bleeding and b) there were indications that tool wasn't completely sealed. Could someone please explain the mechanisms which by which a leaky tool can cause a dry laminate please?

I will provide some more background information. We did the lay-up and cure on a epoxy board tool (several boards glued together and milled to correct shape). The cure was a two-step cure at 130°C and a post-cure at 180°C. The vacuum pressure during cure was <30mbar but with a flow rate of >6 l/hr (hence our concern for a leaky tool). There was an adhesive film between the core and the honeycomb and the honeycomb was sealed on all sides by solid regions of CFRP. Initial resin content was 42%. I can provide some more information upon request if necessary.

Any input on the matter would be highly appreciated.

RE: Dry prepreg after VBO cure

You need to develop a deeper understanding of how a vacuum bag works and what prepreg actually is. A tool leak will cause a loss of compaction pressure around the leak. In effect there is no vacuum bag. Try curing a piece of bare prepreg in an oven it will appear just as you describe. Without compaction the fibers fluff-up and absorb the resin like a sponge. Squeeze a sponge and you can squeeze the air out and bring fluid back to the surface.

VBO of sandwich structure is challenging. Normally, full vacuum must not be used. If the core is fully evacuated then the resin in the prepreg will boil during cure and run into the core. If not enough air is removed from the core before cure, it will expand from heat and volatiles, and lift the skins off of the core. 10-15 in of Hg is typically used. If you see core print-through on the outside surface but the surface is dry then the cause was likely too much vacuum.

RE: Dry prepreg after VBO cure

(OP)
Thank you Compositepro for your input. If I may, I'd like to ask some further questions on this topic.
We cut our product into pieces the other day the to see what it was like on the inside, and the results were both positive and negative. On the positive side, our final CPT came very close to the predicted value on the flat surfaces and the porosity levels were at a tolerable level. If we had a loss of compaction pressure due to the tool leaking I would expect a slightly higher CPT across the laminate but that does not seem to the case...

Continuing, the article has a female corner and this has been an area of concern since day one. Early tests showed that this problem was manageable. However, on the product, there was a great increase in both CPT and void volume fraction in the corners compared to the flat surfaces and especially to previously manufactured test corner samples (mind you, these were manufactured on a non-leaking steel tool). I'm suspecting that the flat surfaces did turn out good despite the reduced compaction pressure because they are less sensitive. However, because of the leak, the area in corners (which already are sensitive to reduced pressure) the low compaction pressure had a much greater effect. Does this seem reasonable or am I on the wrong track?

I don't know if it's of any importance but the core turned out good, with no dimpling, nice fillets and no resin.

RE: Dry prepreg after VBO cure

I agree with your reasoning. If CPT is correct and void content is acceptable in the flat areas, then your problems in the female radius is caused by bridging of materials over the corner because they cannot slip or stretch into place. The solution is usually to place a 1/2" overlap splice in all prepreg plies in the radius area. Also pressure intensifiers are very helpful. This is a solid rubber insert that fits over the prepreg in the corner, acting sort-of like a two sided mold.

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