Autoclave Processing - Vacuum Drop During Ramp

[Skyhawk5PT]

I work for a company who is relatively new to autoclave processing. We are working through an issue during our cures in which the vacuum pressure is decreasing during the ramp phase. The adhesive is condensation polyimide supported film adhesive.

The cure parameters are as below:
Apply full vacuum (minimum of 24 in Hg)
Apply 25 +/- 5 psi external pressure
Heat from 75F to 350F at 2-5F per minute
Hold at 350F for 90 minutes
cool under pressure below 140 at 2-5F per minute

For our process, we vacuum bag the part and perform a leak check. At this point, the bag holds 29-30 in Hg. We then apply external pressure and begin the temperature ramp. The vacuum pressure holds steady until the temperature reaches about 180F. At this point, the vacuum pressure begins to decrease and usually stabilizes around 24 in Hg. As the cure progresses, the vacuum pressure increases and by the time the full soak is complete, the vacuum pressure is back up to 29-30 in Hg. We are scratching our heads as to why this is happening. We have looked at how we are bagging the part and tried many configurations but the condition still prevails. Has anyone faced this issue before?

 

[3DDave]

How large is the part? Rough guess is that at temp the adhesive viscosity drops and the vacuum pump isn't keeping up with the additional flow until it hardens and stops flowing. If this isn't possible, then more details may be required.

Edit: Perhaps the problem isn't adhesive flow, but that lowered viscosity allows trapped air to escape with the same problem - the pump isn't keeping up.

 

[SWComposites]

Try slowing the ramp rate.

But is the part free of voids and porosity, and thickness ok? If so, maybe there is no problem?

 

[jotunn]

I've seen what sounds like similar things...multiple jobs/places. Example below is out of autoclave (oven cure, prepreg under bag)...

Initial vacuum drop (green oval in photo) during first temperature ramp which recovers, I believe to be moisture related... Moisture in the nylon bag, nylon breather, tool (material dependent), etc. being baked off. Running a bagging set-up (no part...bag an empty tool) recreates this.

Second series of drops (red box in image) are a bit harder for me to understand. Definitely functions of temperature...or, at least, correlate to temperature. Best guess is oven vacuum plumbing expanding at temperature and creating small "leaks". Big powerful vacuum pumps...but high differential, low flow... Recovers as everything cools down. <edit: we've also discussed bag permeability increase as a function of temperature as a potential contributor...>

Don't know if this helps or hurts...it's all marginally educated guess work.