Air Breather is collapsing on an envelope bag for Autoclave Cure 1

[Cisnerjo1973]

Hello Everyone !

Once again here requesting for your valuable support as always, we are curing a Composite part on a Autoclave, the specification request a 60psi pressure and 350°F cure for 110 minutes, the part consist of 3 plies of carbon fiber prepreg, we are noticing the air breather is collapsed after the part its cured, so the valve is real compacted under it and the bagging film is clogging it, this situation is causing the Autoclave probes to show a false pressure under the bag readings (Bag Leak), at least that's what we assumed that is causing it since the parts are checked after the cure cycle for bag leak and we are not finding leaks on the bag.

Since the part is envelope bag we are using two layers of N10 Breather and also placing an extra 4 cushion pads of N10 under the valve, we placed four valves on each part (three are used as vacuum ports and one is the probe connected to the Autoclave) the part is a duct of around 3" diameter and a length of 50", but still seems like there's too much pressure that the Air Breather is collapsing.

My plan is to add two pads of marine cloth to provide support on the valves area, and maybe an extra layer of marine cloth on the bottom part of the envelope bag, but don't know if that could be a fix to my problems or not, so I would kindly ask for any advice on properly bag the part or maybe to use some other materials we are not considering, I really appreciate every comment and support on this topic.

Thanks a lot !

 

[Compositepro]

What you are observing always happens with N10 breather. The permeability is significantly reduced after cure. It has very good permeability in the early part of the cure cycle when permeability is most important. Are your parts turning out okay? If your guage is reading positive pressure then there is positive pressure at your sensor port.

 

[Cisnerjo1973]

Thanks a lot for your response Compositepro, really apreciate your valuable help, parts are coming out very good they look great with no indication of any issue at all, usually the customer specifications request a test coupon per part for each Autoclave cure but this one doesnt so we dont have a way to prove parts are ok.

Something really weird is that most of the specifications request to vent parts to atmosphere once certain pressure is reached (average after 15-20 psi are reached), on this case the specification request to maintain vaccumm during the entire run from 1 to 5 in/hg. The recipe is set to 2.5in/hg so we should be ok because besides the pressure, the vaccum is also applied to the part, unfortunately not even the vaccum kept the part from reading leak. But what causes more noyse is that the envelope bag is check on each part after being cured and they do not show leak after it. Thats why we believed the N10 callapsing its causing a certain type of clog on the valve that makes it to false read pressure under the bag without existing one.

 

[Compositepro]

The N10 collapsing is a type of clog. The permeability of the N10 gets significantly reduced but it does not drop to zero. The permeability of woven fiberglass is much lower than N10 to start with, but may be equivalent or higher after cure.

It is common to vent vacuum during cure to prevent voids caused by excess resin bleed during cure. The venting should only be done after the resin has warmed enough to flow a little. However, most cure procedures where vacuum is vented, only call for venting vacuum after pressure exceeds 20 psi, and do not mention temperature. The resin in the prepreg must be fluid to prevent air from re-entering by the same paths through which it was removed by the vacuum.

Nylon bagging material also has a small amount of permeability to gasses. This permeability increases with temperature and behaves like a small leak.

 

[Cisnerjo1973]

Great explanation Compositrepro. Thanks a lot for your help. Sorry I have a last question, so if the probe valve connected to the Autoclave suffers some type of clog by resin or reduced the amount of flow reading due a collapsed N10, this will be shown as leak (pressure under the bag) but not necessary would be a leak due a blown bag Am I correct? Im mentioning this because as mentioned before we had leak readings on the vaccumm probe but after they were cured we bag leak test them and none of them presented a leak.

Is there any book that I can read to learn a bit more about the autoclave principles, function of probes and how to interpret the readings obtained.

Really appreciate and value your help!

 

[Compositepro]

There is always some gas leaking into a vacuum bag from various sources. Pinhole leaks are the most common. Natural permeability of polymers films (

https://polymerdatabase.com/polymer physics/Permeability.html)

is something to be aware of but is not an issue if breathers are used correctly. In most infusion work films are placed in direct contact to the resin with no breather.

I've seen a number of composite processing handbooks over the years but not any that are particularly helpful. They are mostly very basic and usually have a few technical errors thrown in. Most of the technical literature will study one processing factor with intense detail while completely ignoring a multitude of other related factors. All I can suggest is that you study and understand the basic principles of physics and chemistry that are used in processing composites.