Atlanta 60

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Hey...I posted this in another thread...I wondered if it fit there???...so I decided I would make a new thread for anyone who may find it useful...so here it is...

I would agree that a surface coat is a necessity...I envisioned a resin rich surface coat...and the tiny bubbles were in there...if that is the case then the vacuum chamber (debulking) the resin would be a benefit...I also agree with Probligo and his comments to avoid stirring the air into the resin...that is a valid point...however...off gassing of the resin is something that you cannot overcome...vacuum bagging is a real plus for us doing this at home...at my day job we vacuum bag everything!!..there the purpose is to debulk the part (remove the air) and control the volatiles as off gassing occurs...then the parts go into an autoclave (a pressure/heat chamber) which actually does all the work...it applies 100psi to the part as well as putting the part through whatever the heating cycles the materials require for cure...for you and I doing this at home an autoclave is pretty unrealistic...but vacuum bagging your stuff is a good alternative...

I hope that I do not talk down to anyone here...but I would like to explain what this vacuum stuff is all about...by putting something in a vacuum chamber or a vacuum bag you are debulking (removing the air) from that environment...this in turn allows the weight of the atmosphere to act on the part... assuming that you get a good bag (no leaks) and your equipment is capable of pulling absolute...you would be able to remove all the air surrounding your part...On a standard day (at sea level and 72deg F the barometric pressure is 29.92 inches of mercury)...given those conditions your absolute vacuum would be 29.92 inches of mercury...that would put 14.7psi of force on your part...The barometric pressure on any given day, where ever you are is the force you are capable of applying on your part by vacuum bagging...That is pretty kewl if you ask me!!!!...

There are other important factors involved...

1) your vacuum generator/system capabilities...there are systems that are not capable of pulling absolute vacuum....some may only be capable of achieving 20" of mercury or less...some are designed to pull 30"....you need to consider this when setting your system up....If all your doing is skinning wings you could save some cash by getting a less capable system...you need less pressure for something like that...but if you would like to do wet lay-ups and bag them or pre-preg stuff you would probably want to get a more capable system...you can always regulate it down to do your wing skinning...etc....

2) the qualities of your materials and workmanship...Use materials designed for bagging...(your common trash bag and masking tape is not the way to go in my opinion)...the real materials are a tad bit more expensive but with a little planning and patience will give you a better end result...they will work if used properly so you save the money in rework...

3) Attached is an example of how you should set your bag up...these components are important in making a successful part...it is a very generic sketch but it will give you an idea of what your bag set up should look like...I recommend you do not omit any of these materials as they all have their function in the grand scheme of things...(notice: I do not think this diagram is complete please read on)...

4) The peel ply layer is important...it will basically become a layer of your laminate...then after the part has cured you will peel this ply (hence the name) from the back of your laminate...it will leave you with a "fractured" surface so you could do additional bonding or painting without having to sand...or if you are finished you can sand the slight texture off to achieve a smooth surface if you desire...

5) The perforated release sheet (plastic with holes in it)...this is what controls the resin bleed...it is a very important component of your bag!!!..this is an area you may have to experiment with to "fine tune" the amount of resin that is sucked through this layer to the bleeder... without this component it is possible that too much resin can come out of your laminate and leave you with dry or resin starved areas in your work....your goal is a good resin to cloth ratio to achieve the max strength to weight ratio...too much resin left in the part=too much added weight... too little resin=a weak part...I can't tell you how to figure this out other than experiment with test articles...the viscosity of resin plays a part in resin bleed so your manufacturer may have some suggestions if you read their data... this material is available in different configurations (holes per inch) so you can fine tune it...it is also made to release from your laminate...it will not stick like peel ply...it is a barrier between your wet part and the rest of your bagging materials...it will not allow the other materials to stick to your part and make a big ole mess...lol...

6) The bleeder ply...this is the layer of your bag that absorbs the excess resin...you can also monkey around with the thickness of this layer to control resin bleed...it is available in varying thicknesses...another important role of this material is to provide ventilation for vacuum to flow consistently across the part...(This is where I disagree with this diagram!!! please read on)...

7) Here is my suggestion...it is possible for the bleeder to become saturated with excess resin...in turn sealing the bag off...I suggest adding two more items in addition to what the diagram shows...the first being another layer of peel ply or perforated release film on top of the bleeder...either one you choose would be fine...then on top of that...an additional layer of the bleeder material...but it will actually be used as a breather ply...this method is the aerospace standard and will help ensure consistent vacuum across the part...

8) then as shown the bag itself along with your vacuum port and chromate tape as necessary...


I realize this is long...if you have made it this far thanks for reading!!!..I hope I am not talking down to anyone and maybe my suggestions could help someone out...I have wanted to write this type of thing in this forum for a while...I realize that there is a very broad group of readers of this forum...from guys like me (your average Joe) to real life rocket scientists...this is not meant to offend either end of the spectrum...it's just I do this on a daily basis so I thought I might pass on the info...I am not forcing my opinion on you...just providing info that you may read and find useful...and sorry...I am not done yet...lol

vacuum is vacuum...be it in a bag or a chamber...all your doing is removing the air and letting the weight of the atmosphere do it's work...A vacuum chamber is a very useful tool for debulking liquids that are in a cup, can, bucket or bowl...something you just cannot bag....a bag is useful for most other applications as it is "custom fit" for the part your working on...another benefit is that you get direct pressure to your part that will hold things in place as it cures...keep in mind...vacuum bagging will not fix a sloppy laminating job...if there are voids in the laminate the bag will not fix them!!!...if you do a nice job during the lay-up...your properly prepared bag will hold everything in its place until it cures...as well as provide an escape for the volatiles...I believe it is the best way to apply 14.7psi evenly to your part for 12 hours with suction for the gases to boot!!!...

You want to pay attention to your bag as it is initially pulling or sucking down to the part...it is helpful to regulate the bag during this process...go slow and watch for "bridging" of the bag...it is useful to put "pleats" in your bag in areas that you think will be a problem...sharp corners for example will require extra bagging material to avoid bridging...if you add a little pressure at a time then you have the opportunity to move the bag and work it around as you need to....

OK...now I am done...whew...thanks for reading this...and hope it has helped...Good luck...

Flyingmike-inactive

Hi

Thanks for the info, it sounds like my vacuum pump is well under powered. something I will have to sort soon, In my gallery is a picture of my carbon spinner and mold, is it possible to bag such a shape.

Mike

filtr-RCU

Great Post. I have used the vacuum bagging system for appling balsa wings. This will help with my next project.

Thanks

ron

Atlanta 60

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Thank you Ron for your comment!!...and yes Mike you can bag your spinner...I would use an envelope bag...and make sure to have enough excess to conform to your mold...Good luck!!