Hi guys

I have read on some of these posts that some people use vacuum chambers to remove air bubbles from their epoxy before the lay up. I have just made one and on testing I have found that the bubbles just get dispersed into millions of tiny ones, What have I done wrong? does my vacuum pump need to be more powerful, at the moment the vacuum gauge reads about 400 mm of mercury

Thanks

Mike

Mike,

I am not quite with you here. Are you casting epoxy? or is this your normal glass skin over substrate (balsa or foam)?

If you are laying glass, and have problems with bubbles then the answer might lie in how you set up the complete assembly in the vacc bag. There are plenty of web pages, and threads here as well, on vaccing and these should help. What is important is that you are using atmospheric pressure as a press to squeeze excess resin and anything else like air out of the surface as it cures.

There is nothing better than a bit of TLC as you work the resin and glass. Preparation and practice is everything.

If you are casting solid resin, then I just can't help - way out of my league.

Hi

Sorry for the confusion. What I`m doing is mixing epoxy in a plastic container ready for a normal lay up in an epoxy mold, But when I mix the epoxy I get bubbles in it, So I put the container in a vacuum chamber to try and draw the bubbles out of the epoxy, but they just get dispersed into lots of tiny ones. The whole problem lies in pinholes forming in my finished moldings so this is one way I thought I could avoid this.

Mike

Hey Mike
I suspect that you do not have the vacuum that you need...In order to debulk your gonna need to pull absolute vacuum...or pretty close anyway...it appears your only getting between 15-16" of mercury I doubt that will do it...Depending on the viscosity of the materials you may get away with slightly less than absolute vacuum...

I believe that most epoxy laminating resins are 600 - 1200 cps...I have debulked much thicker materials...casting resins, sealers, urethanes, paints. All ya need is pressure...lol

I don't know what sort of equipment you have...make sure your system has no leaks...good luck...

Chuck

Hand mixing, treat it with care. use a shallow bowl to mix in so that you do not have to "lift" the mix so much from the bottom toward the top.

Mix slowly

Use a broad paddle and rub the mix on the bottom of the bowl.

The answer is to not get the bubbles in it in the first place. Easier said than done I know.

Hi all I do a lot of epoxy fuselages and stuff and from my experiance it is not the air in the resin that is your problem ,I am probably going to get shot down with this but who thinks that they can get a 1/16 diameter air bubble through 2 or 5 oz cloth on to the surface of a mold dont think so . It is getting the air out of the glass cloth which is your problem and putting the layup and mold in a vacuum bag wont really help as it draws the resin out to get to the air bubbles produceing more air bubbles . You would need to put the whole mold and finish layup in a vacuum chamber to achieve the desired result and realy I doubt it would get them all.
To stop the resin voids on the surface of a layup and they are actual voids where you could not get the resin not real air bubbles , put something down first and let it cure a layer of resin or a coat of primer .I use epoxy primer as the surface coat for all my molding the finsh always comes out a lot better than just laying up straight onto the mold .
Have fun Terry.

Hi

I think your right!!!!

However, I do use a surface coat and this does give a good finish, but the things I make are usually clear so you can see the carbon weave, unfortunately you can also see the air bubbles between the weave. and these are what I`m trying to get remove, I`m going to try again today with a lighter cloth.

Mike

Terryoc has a point..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...

Hi all
I do some parts that are left clear they are wheel spatts that incorporate the u/c leg and wheel pants in one they are made of carbon only and have no surface coat but I have to have a lot of resin in them to ensure it bleeds through the carbon to stop the air holes .The resin I use is quite thin and has a 90 min pot life and I have to brush over the carbon every 10 minutes or so for about an hour to ensure I have worked the bubbles out .It is just luck that this suits the situation as I make 4 at a time 8 halves so I keep going from one end of the bench to the other.
Terry

Terry,

Have you tried using a heat gun (aka heat lamp or hair dryer) to try and pop the air bubbles?

Yes I have tried a hair dryer to warm it up and it worked to a degree but the resin gets thiner and runs to the lowest point of the mold and leaves voids at the outer edges .The pot life of my resin also stays thinner longer when it is warm the colder it is the quicker it thickens so it just was not practical.
How I am doing the lay up works well for the application as I lay carbon cloth in and wet it to soften it up to make it go into the bends of the mold then go onto the next one and down the line of 8 molds then a layer of unidirectional for the internal U/C leg wet it and down the line then another layer of uni and down the line and then a piece of roving across the wheel opening to prevent cracking and down the line .
So just the laying up works most of the air out but it does stay rather wet in resin content which I think is about the only way to really stop the air voids I have tryed bagging them and it suck out more resin than air I even tryed puting it in a bag and then that bag in another bag so the vacuum was not on the actual part as this gave the best result but I had an implosion of the interal bag a couple of times which made the molds hard to trim and filled the thread holes in the mold with resin.
As I do these to sell I have to keep the time to a minimum with a very low regect rate and the old style wet layup just produces the easiest and most reliable result .I do get some very small hles in the outer surface but thats life .
Maybe the best way for the original poster to fix there problem is to warm the resin first as it will penetrate the carbon a lot better when it is thiner .
all the best
Terry

> ... hair dryer to warm it up and it worked to a degree but the resin gets thiner and runs to the lowest point of the mold and leaves voids at the outer edges

Yup that can happen. From your description so far, I suspect you are struggling with an epoxy that has a higher than desirable viscosity and also what sounds like limited hardener selection to control pot life. Both of these parameters are working against you in a hand layup (non bagged) application. So what sort of epoxy are you using? What is the rated viscosity at room temp? What is the ambient temperature when you are doing the layup?

Ideally you want a thin viscosity resin/hardener mix so that it will wet out the matrix properly. The thinner the cloth & tighter the weave, the more important this is if you want to acheive a low weight layup. From my experience, a R/H mix in the 300-500 cp range at room temp is perfectly fine & you will get excellent results. But if for example you have a mix with 2500 cp viscosity (like very thick honey), even by heating it moderatley, you may still be at 1500 cp - still too thick. If you are using a resin rated at 1000 cp at 70 deg F & your shop is at 60 deg F, you can expect proportionately thicker viscosity - say 1300 cp. A good resin manufacturer will provide this data. Reduced viscosity will also help your entrapped air problem. Consider what happens with an air bubble 'trapped' in a glass of water (viscosity = 1 cp); the bubble immdeiately races to the surface & pops. It is very difficult for it to hold the air from doing otherwise. (ok technically water has a higher density which helps a bit, but not to the same degree).

It is also important to have a good range of hardeners in order to control set up time (pot life). If you can get a fast hardener like 15-20 min & a slow like 90 min, then you will have the ability to custom blend hardeners to suit the layup job. The resin:hardener ratio stays the same. Even if yu have a thin resin, it will be difficult to keep it in place with a long pot life for the reason youve stated - drain off. There are lots of epoxies on the market, but you have to select the right one for the job.


> The pot life of my resin also stays thinner longer when it is warm the colder it is the quicker it thickens so it just was not practical.

Hmmm... does not sound right. Heat promotes the curing, pot life is reduced, exotherm is accelerated, molecular chains are building meaning the mix viscosity is increasing & heading towards solidification - the opposite to what you are saying.

Hi PTXman I am happy with the results that I get I was just answering a question .The resin I use is Ceiba or Araldite LC3600 and LC3600 hardener there are other hardeners available allthough the reps over here dont really like to sell just the hardener by its self and really I find haveing lots of different hardeners a pain .I use this resin for every thing I do molding wings,tails and fuslage layups both glass only and balsa composite.
The resin at about 25deg C is about the same thinkness as milk maybe a touch thick so a thiner resin would be pretty much useless the temperature range I use it in is from 10deg C winter to 34deg C summer they are the coldest and hotest temps that I have used it in my workshop.
When I said the resin thickens quicker when it is cold it is not going off it just seems to thicken so the working pot life dosnt really change just the thinker resin in cold temps is harder to work.I suspect the mixing action to start with and holding the mixing cup in a warm hand makes it it normal viscosity to start with ,I still get about 45 mins before it looks like being a pain.
I dont have any trouble with air in the resin with how thin it is once it is mixed I can leave it for 5 mins and 98% of the bubbles are on the surface and bruching the top gets rid of them all.
One other thing with this resin is it must be post cured at a minimum of 30deg C for 24 hours so I normally go to around 50deg for 10 . I am more than happy with the results I get but as I make kits I have to watch the amount of time it takes to ensure I still make money as I make them as a hobby to pay for my own model aircraft addiction.
While I know there are many different methods to make parts lighter and stronger many of which I have tried if I can make a part simply that I and customers are very happy with I will go the simple route which I often find is also the most reliable for a home workshop.But thank you very much for your replys and ideas as they are all put into the memory bank for when a problem does arrise they may be usefull.
Terry

I should have mentioned the reason I do the lay up that way is that to get into the compound curves the cloth needs to be soft and the resin softens the cloth ,The following layers are 300gram unidirectional which is quite thick and it takes a few minutes for the resin to soak through if you were to put all the layers into the mold and try to wet it out you would get sick of trying and lose the plot before resin even got through to the mold.Also I find it easier to control the resin content by doing one layer at a time as you can tell when each layer is wetted out .
Terry