Vacuum required?

[IFLOWATSEA]

I am new to vacuum bagging hand layup carbon fiber. I have watched many videos and successfully made some components. The question I have is how much vacuum to pull for a hand layup, 4 layers of 2x2 3K twill, epoxy resin. It is on a relatively flat surface 38 inches x 12 inches. Height above sea level is 800 ft and I can pull -28 inches with my pump. I am looking for max strength with max lightness, lightness taking preference. I have heard everything from max vacuum you can pull to -10 Hg. Is there an easy answer to this? Thanks Don

[Jeremy Senne]

Don,

Great question.  The answer depends a bit on some of the hairy details but I'll give it a quick shot.  It wasn't clear if your layup technique is hand wet layup (dry fabric that you are manually wetting out with resin) or hand layup of prepreg (material that is already impregnated by resin and generally requires freezer storage).  The answer is the maximum vacuum that you can pull for both but for slightly different reasons:

If you're hand layup of prepreg, the amount of resin is already set as a part of the prepreg process (unless it's very high resin content in which case you may still squeeze some out at high vacuum). In this case you really just want to squeeze as hard as possible with vacuum.

If you're hand wet layup, the amount of vacuum that you pull will actually help determine your final fiber volume (amount of fiber vs resin) as the more vacuum you pull the more resin you will squeeze out of the sides of the laminate.  This is a trick that infusion guys use to tune their fiber volume on big parts.  In your case, though, if you're looking for minimum weight you'd want to squeeze out as much resin as possible by pulling maximum vacuum you can achieve.  Regarding properties, your maximum LOAD in-plane won't appreciably change with fiber volume as the fiber will control that.  Your maximum STRENGTH (which is normalized to thickness) will increase as you squeeze out more resin and achieve a higher fiber volume content.  The same can be said about your STIFFNESS (load vs. deflection) which won't change, but your MODULUS (load vs deflection normalized to thickness) would increase as you squeeze out more resin.

Note for wet layup there's one additional complication to watch.  If you have bubbles trapped in your wet layup resin they can grow under vacuum if they can't escape the laminate during cure, which can create higher void content than if you used autoclave pressure (which would collapse the bubbles), hence the autoclave being the gold standard for curing.  Three tricks to help here:

1) After you mix your resin pull vacuum on it.  It'll look like boiling water because all the bubbles will expand and froth but then it will be air-free.  You can use a cheap vacuum chamber like this one: https://www.vevor.com/vacuum-pump-c_11109/vevor-5-gallon-vacuum-chamber-upgraded-tempered-glass-lid-vacuum-degassing-chamber-304-stainless-steel-chamber-for-stabilizing-wood-resin-degassing-silicone-degassing-and-plaster-degassing-p_010758564652?adp=gmc&utm_source=google&utm_medium=cpc&utm_id=15698890952&utm_term=&gad_source=1&gclid=Cj0KCQjwqpSwBhClARIsADlZ_TkMesI7iv86wsA6SLjUNuKEcRscf7o-uFqL3vsngUogx3WG_S07dT4aAvuGEALw_wcB

2) Ensure that the breather in your bag has a path all the way to the vacuum hose.  Some folks will erroneously only cover their part with release film then breather, but not put a breather path to the vacuum hose.  This results in the hose pinching off the vacuum bag to your tool and keeping your part from actually seeing full vacuum.

3) Pull vacuum during the entire cure as it will help provide a path for any bubbles still trapped to get pulled out of the laminate.

Hope this helps,

Jeremy