Hi all,
Which is a typical value of vacuum pressure for a sandwich panel with foam core, wet layup ?
Thank you
good question… i have a couple more to add as i will be (or planning on) using divinycell for my project.
also, some people talk in terms of “atmosphere” (which I know is 14.7).
some pumps are measured in for example: 25" or Microns or CFM.
how do they all relate to each other?
thanks,
the noob - Rooski
Depends on the crush strength most foam cores in products we manufactue will take 29" no problem. If in doubt, go with about 10" of vacuum. This will provide adequate consolidation, and be safe even for honeycomb cores.
it depends on what density foam you are useing. however you will want as much pressure/Vacuum as you can pull.
lets say i am using 1/8" divinycell / divinymat:
http://www.fibreglast.com/product/DIVINYMAT_01024_A/Vacuum_Bagging_Sandwich_Core
I would recommend calling the manufacturer of divinycell with this one as it isn’t stated and depends on cure temperature.
I use 25+" with Divinimat all the time with no issues.
i didn’t think it would be - it takes a good amount of force to crush in your fingers… can’t imagine is crushing under a vacuum.
Thank you all
Atmospheric pressure is around 14 psi. so if you have an ultimate vacuum, there would be 14 psi pushing down on the bag/containers outside.
But when you need to measure vacuum, which is gauge and relative NEGATIVE pressure (so, 0, not 14psi), you switch units and conversions.
Micron is metric, " of Hg is standard (imperial?)
CFM is just the volume of air the pump can move. When bagging large parts, you would want a higher CFM, so it doesn’t take a year to pump down. For small parts, you can go with a smaller CFM…again, that is just SPEED of vacuuming the part, NOT the power of the vacuum!
Most pumps in composite industry will hit as low as 29.5" of Hg. You can go as high as 25" for small jobs, but if you want the best, get the best!
That said, composite pumps are ROUGH vacuum pumps!!! One does not need a cryo pump, or turbo molecular pump. These will reach near ultimate vacuum. No where needed for composite work 
what does Hg/" stand for?
my part i am bagging is 7’ long, maybe 2.5’ wide and at the most a 1’ tall (jetski hull).
this would be considered a “small” part?
as you state is low as 29.5" and as high as 25"… which is removing more… the 29.5" or am i backwards on this?
Hg is a measure of vacuum in mercury. " is the units that it is measured in so in this instance it is inches of mercury
my main pump in my shop is an old edwards vacuum pump that is mounted onto a pressure resiviouur, this pump easilly pulls down to 29" and will evacuate the tank to 29" within aprox 90 seconds, its a 100 littre tank, I dont know what the CFM on the pump is as I have not looked.
The main advantage of this big vacuum pump attached to the tank is that i can evacuate bags quickly and because I have the resiviour of vacuum it means that i have a little more time to find a small leak before loosing toomuch vacuum.
As for vacuum and sandwiches, none of the cores that I have used have suffered any problems with collapse due to the pressure that a vac pump alone will exert. I use a polypropolene honeycomb regually and have never had a problem with that as far as bonding it or colapse. I can see how water ingress maybe a problem with it but it would with any honeycomb if it is not sealed.
You’re right on. As Matvd said “in Hg” is a measure of pressure.
It means the amount of pressure exerted by a vertical column of mercury X inches high. 29.5in is approximately the pressure of the one atmosphere at sealevel.
Vacuum pressures also come in cm Hg, Psi, Kpa, Atm, Torr and Microns. Which one you use depends on which industry you’re in but damn it’s annoying. You need a different gas constant for every one of those, couple that with different volume and mass units and it’s enough to drive any fluids student crazy.
29.5" is removing more air, in effect, higher force/area.
thanks guys - cleared some stuff up for me.
sticking with one unit would be great for newbie’s like myself getting into this as a hobby - the internet is a wonderful thing!
Some additional info about vacuum
The West System manual pretty much covers the basics.
http://www.westsystem.com/ss/assets/HowTo-Publications/Vacuum-Bagging-Techniques.pdf