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Force of Vacuum?

06/30/2017 6:28 AM

My knowledge of vacuum is quite scarce.
I intend to produce an object made of prepreg carbon fiber but... since the object is completely closed I cannot use vacuum bagging method... instead of that I intend to press carbon layers mechanically.
Could you tell me what pressure/force is made by vacuum pump, i.e. what force is applied to carbon fiber layers by vacuum bag?
This is how the vacuum bagging process looks like:
https://youtu.be/cQ1Q4XmItN0
but it is not clear to me what force is applied to carbon fiber layers by vacuum bag?
Best regards

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#1

Re: Force of vacuum?

06/30/2017 6:51 AM

If there is water present, the maximum vacuum that can be pulled corresponds to the partial pressure of water at the process temperature.

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#11
In reply to #1

Re: Force of vacuum?

07/02/2017 12:53 PM

Water vapor pressure is less than 1 psi at ambient temperature. That would leave a maximum of 14.7 - 1 or 13.7 psi if you suck most of the air out.

http://www.engineeringtoolbox.com/water-vapor-saturation-pressure-d_599.html

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#15
In reply to #11

Re: Force of vacuum?

07/03/2017 6:25 AM

It is the Original Poster that needs the air sucked out, not the undersigned.

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#2

Re: Force of vacuum?

06/30/2017 7:20 AM

..."When the bag is sealed to the mold, pressure on the outside and inside of this envelope is equal to atmospheric pressure: approximately 29 inches of mercury (Hg), or 14.7 psi. As a vacuum pump evacuates air from the inside of the envelope, air pressure inside of the envelope is reduced while air pressure outside of the envelope remains at 14.7 psi. Atmospheric pressure forces the sides of the envelope and everything within the envelope together, putting equal and even pressure over the surface of the envelope. The pressure differential between the inside and outside of the envelope determines the amount of clamping force on the laminate. Theoretically, the maximum possible pressure that can be exerted on the laminate, if it were possible to achieve a perfect vacuum and remove all of the air from the envelope, is one atmosphere, or 14.7 psi. A realistic pressure differential (clamping pressure) will be 12–25 inches of mercury (6–12.5 psi)."...

..."Mechanical clamping or stapling applies pressure only to concentrated areas and can damage fragile core materials in one area while not providing enough pressure for a good bond in another. When placed in a closely spaced pattern, staples exert less than 5 psi of clamping force and then only in the immediate area of the staple. They cannot be used at all if you are laminating to a foam or honeycomb core because of the core’s lack of holding power. In addition, extra adhesive is often required to bridge gaps that result from the uneven pressure of clamps and staples."...

http://www.playwithcarbon.com/content/Vacuum-Bagging-Techniques.pdf

What is the shape of the part? It may be possible to make a mold of stiff foam rubber or styrofoam and then use band clamps to exert force evenly...

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#3
In reply to #2

Re: Force of vacuum?

06/30/2017 8:00 AM

...or you might consider wrapping it in electrical tape.....remember to put holes in the tape to allow excess epoxy or resin to escape....

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#4

Re: Force of vacuum?

06/30/2017 9:45 AM

The actually vacuum force may depend on the requirement needed to do the job, and this may be arrived by practical trials.

The info already given is good, but I suggest contacting a supplier, for more informed detailed information. I have used Quincy.

Because it will depend on the time such as; CFM rate for evacuation as well as vacuum requirements.

Good luck, and welcome to CR4

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#5

Re: Force of Vacuum?

06/30/2017 10:57 AM

Thank you SolarEagle and PWSlack,
My assumption turned out to be correct - clamping force is approximately 1 bar ~ 14.7 psi (my friends have been convincing me that the pressure is much higher).
The mould containing of two parts would be made of aluminium (cnc). I would put several layers of carbon on each part and before I put the moulds together, I would pour expanding adhesive inside (eg KLEIBERIT pur - 0,6 N/mm2)... There would be holes at the ends of the moulds to allow excess adhesive and air to escape...
On the top of that I would use bag and vacuum pump....
Thanx

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#6
In reply to #5

Re: Force of Vacuum?

06/30/2017 11:04 AM

(my friends have been convincing me that the pressure is much higher).

14.7 PSI is a lot when you think about it. On just (1) one square foot area, that amounts to 2117 pounds of force. Maybe that's what you tell your friends.

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#7
In reply to #5

Re: Force of Vacuum?

06/30/2017 2:49 PM

Actually probably averages ~8-10 psi...

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#14
In reply to #5

Re: Force of Vacuum?

07/03/2017 12:23 AM

I have some experience making carbon-fiber panels for amature built aircraft. Layup in mold then vacuum bagging. A most significant advantage I found in vacuum bagging is the elimination of bubbles and adhesive/epoxy voids. When I switched from hydro and mechanical presses to vacuum bagging, my rejection went from 10% to zero.

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#8

Re: Force of Vacuum?

06/30/2017 8:01 PM

If you need more pressure:

Autoclave (industrial ) - Wikipedia

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#9
In reply to #8

Re: Force of Vacuum?

07/01/2017 4:50 AM

Thanx, but I think I don't need more pressure... If vacuum bagging makes 0,1 N/mm2 - then pressure of expanding adhesive - 0,6 N/mm2 is all I need... In my case vacuum bagging will serve just to pull out air from mold (carbon and adhesive)...

Thanx to everyone... Now - practical trials!

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#10

Re: Force of Vacuum?

07/01/2017 3:19 PM

There is no force of a vacuum... there is only atmospheric pressure, or pressure created by say a head of water.
Del

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#12

Re: Force of Vacuum?

07/02/2017 11:32 PM

If you're looking to increase your knowledge of vacuum fundamentals, there's a 4-part series of no-cost, on-demand webinars/e-seminars available for viewing from Agilent:

http://www.agilent.com/en/training-events/eseminars/vacuum

(Viewing the Part 1 webinar may be a little flaky.)

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#13

Re: Force of Vacuum?

07/03/2017 12:21 AM

I have worked with pressure and vacuum for many years with my line of medical equipment.

And we have ALL come to the conclusion - vacuum sucks!!

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#16

Re: Force of Vacuum?

07/03/2017 5:18 PM

Force and pressure area related by area.

Vacuum (atmospheric pressure) can be quite damaging Vacuum sucks.

Hopefully, whatever closed item you are using can tolerate the compressive force and not collapse.

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Users who posted comments:

Anonymous Poster (1); Bruce Frank (1); Del the cat (1); GJM (1); Julian Grodzicky (1); phoenix911 (2); PWSlack (2); Rixter (1); Sid_Sidow (1); Skulptron (2); SolarEagle (3)

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