What is Vacuum Venting
What is Vacuum Venting
(OP)
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





RE: What is Vacuum Venting
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.
RE: What is Vacuum Venting
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