Hi guys,
I’m writing up a procedures checklist for vacuum infusion.
I believe I have accounted for every aspect of the job from start to finish except for post infusion pressures.
Assuming a successful infusion…resin supply is shut-off, vacuum pump remains on.
Should the vacuum pressure be adjusted to some higher value (maybe 150mbar absolute) to shrink any remaining air bubbles? Or should the assembly be kept at max vacuum for the duration?
Thanks in advance!
db
I wouldn’t lower the vacuum pressure by adjusting the pump, because that way you let air back into your system, which will cause under-infused parts= air voids, way worse than te initial bubbles you might have had.
I don’t let the pump run during the curing of my parts, because i think this will create a degassing effect at the vacuum end of your infused part when all the excess resin is drawn out. This will lead to small air bubbles growing way bigger than they were, but this may depend on your resin.
The way i had most succesfull infusions is by clamping off the vacuum end, and then clamp of the resin side, with cosmetic parts you can even let the infusion side suck up some more resin for even smaller air bubbles and a less “pressed down” surface cloth. How long i let the resin side suck depends on layup, how good the flow is inside your (soon to be) part etc.
my theory is that at the vacuum side you have -1 pressure, at the resin side 0 (atmospheric) anywhere in between, the pressure is somewhere between those values. when you clamp both sides off, the resin doesn’t stop flowing, but continues flowing to the side with lower pressure, until there is a ballance over the entire part of about -0.5.
But i’m no expert, this just works for me, after a lot of trial and error.
hope this helps you a little
I did a test doing just that a few days ago. I infused a small test panel, then decreased vac from 29 inches to 20 inches. The result was a ton of air introduced into the system. Bad idea. Glad I did it on my test piece, not a real part.
Always maintain the highest vacuum, until the part gels and cures. If you want, infuse at a lower vacuum, then raise it higher, but why bother in the first place? Just vacuum, and infuse. Leave pump on.
as Marcel mentioned, yes, there will be a pressure gradient, because one side is vacuum pressure, the other is ATM pressure. However, once you clamp your resin line, it evens out more. You will have a difference in thickness from vacuum to vent side, but unless you are doing precise parts, then it doesn’t matter too much. You shouldn’t be doing VARTM in the first place if you need to be that precise.
Clamp early, and let your resin flow into a delay line, and things should even out. Of infuse aroiund the outside, and vent in the middle, or vice versa.
But don’t stop the vacuum, because if you get an air leak around your tape, you will lose the part. Vacuum staying on, gives you a chance to have the air leak go around the part, not in it!
I prefer to infuse at 5 mbar or less (ATM = 1000 mbar) then bring back vacuum to 50 mbar, which does not have a big effect on fiber springback, but does tackle eventual pinholes. 50 mbar is well above the boiling point of water (20 mbar) at ambient, so no real problems expected from that, or other stuff boiling off.
Or use MTI hose in the vacuum end…
Thanks Herman - I was pretty sure I recalled hearing something about raising the pressure post-infusion, but couldn’t remember the details.