What is Vacuum Venting

[vivekpdd]

Before loading the part I have verified the bag for leak checks. Pressure (vacuum) inside the bag is maintained at 20 inch Hg.
After initiating the autoclave cure I have been asked to Vent Vacuum once pressure reaches 20 PSI.

I would like to know how much pressure will be there inside the bag during cure once I vent vacuum after the autoclave pressure reached 20 PSI.

Autoclave pressure during cure : 80 PSI

How does the volatile and other gases escape if I am not pulling any air from the bag once the vacuum is vented.

Please guide me.

Thank you,
Vivek

 

[Compositepro]

What actually happens to your prepreg can be very complicated because prepregs have variable amounts of permeability/porosity for gas flow. The breather materials under the vacuum bag are designed to be permeable so it is easy to says what is happening there. When you apply 20 inHg vacuum, that is what you will have in your breather. That is 10 inHg of absolute pressure (the pressure above a perfect vacuum. This is the gas pressure in the pores of the breather

Venting vacuum when the autoclave reaches 20 psig (above atmospheric pressure, or 35 psia) is a common, but very poor practice. Air will flow back into your breather and into your part if there is still porosity/permeability in the part. Prepregs do not fully consolidate until the resin is fluid enough to flow into pores, which is usually around 140F. That is when vacuum should be vented and autoclave pressure increased.

The purpose of venting is to maintain a minimum hydrostatic pressure on the resin during cure. If the resin bleed during cure is not carefully controlled by bagging technique used, the resin pressure will fall to the vacuum pressure in the breather. At high temperature and low pressure the volatiles in the resin will cause it to boil, and result in major porosity in the part.

I have written longer answers to this question in the past. You can do a search for these posts.

 

[blakmax]

The reason for venting the vacuum is to reduce the porosity content. There is a perception that volatiles are always drawn out by vacuum, but that is not always the case. If the uncured material has been incorrectly exposed to a humid environment prior to cure, then as the temperature increases that moisture will be liberated as steam. When vacuum is applied, initially any air will indeed be removed by the vacuum. But once the laminate ceases to consolidate any volatile trapped will in fact experience low pressure and will expand in size. By backing off the vacuum the static pressure applied to the void will cause a reduction in size. We had a similar procedure for on-aircraft adhesive bonds where full vacuum was applied until the flow temperature for the adhesive was achieved and then the vacuum was backed off to 10 inches Hg. That procedure considerably reduced porosity.

With regard to your cycle, I'd suggest that you maintian full vacuum until the resin has flowed and before the resin starts to gel back the vacuum off then.

Regards
Blakmax
Blakmax