Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

What is Vacuum Venting

What is Vacuum Venting

What is Vacuum Venting


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,

RE: What is Vacuum Venting

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.

RE: What is Vacuum Venting

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.


Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Research Report - How Engineers are Using Remote Access
Remote access enables engineers to work from anywhere provided they have an internet connection. We surveyed our audience of engineers, designers and product managers to learn how they use remote access within their organizations. We wanted to know which industries have adopted remote access, which software they are using, and what features matter most. Download Now
eBook - Managing the Context of Product Complexity Using the Digital Twin
Keeping track of changes to complex products is difficult—think Aerospace & Defense equipment, new generations of commercial aircraft, and software-based automobiles. A new way to managing the digital context of the physical product is required and the answer is the Digital Twin. This ebook explores the opportunity available for Operations and Maintenance for the Digital Twin. Download Now
White Paper - Trends in Industrial Filtration
Substantial progress has been made in filtration technologies in recent years. New filter media materials, designs and processes have led to filters that are more efficient, reliable, compact and longer lasting. This white paper will discuss the various trends that are impacting operational responsibilities of MROs today and the resources that are available for staying up-to-date on the latest filtration solutions. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close