Thick UD carbon infusion

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#21

Groper to answer your questions:

If you are infusing a UD spar of a significant length, I would use a very slow hardener, and say infuse laterally, from the sides and keep say around 30-45 minutes of time for a 100mm travel (and this will be across the fiber). The slower the better. I wouldn’t use any flow media on the top. However, the best will be you try it out on a small A4/Letter sized sample. Always infuse from the outside to the center in standard infusion techniques. BTW we infuse spars with a resin of that viscosity and it works very well. If you want a thinner resin, then first check whether it is approved by FAA or EASA or wherever you are their regulatory body. Thinner resins that I’ve seen do not have the toughness needed for main structural parts. We’re limited in our choice of resin by (a)FAA/EASA certification (b)price.

Oh btw, Boeing uses a lower pressure during infusion and they flush the part with epoxy and then they increase the vacuum to take out any excess epoxy. There’s an old thread in which I’d mentioned the patent number of Boeing. In this case if you have any miniscule amount of air dissolved into the resin, it will play havoc once you start drawing out the excess resin by increasing the vacuum.

About sizing, most fibers come with silane sizing on the fibers. However, the quality of sizing varies. Try to see with a bunch of UD fibers stuffed into a plastic tube, do the fibers have a capillary action with the sizing, if so then the sizing’s surface tension with the resin is what you need. If on the other hand the resin beads on the sizing then you’ve got trouble at hand.

You could also use a carbon veil in the middle to enhance flow if that becomes an issue. You could also heat the resin to reduce viscosity during infusion, if your hardener is slow enough and you get enough gel time.

Of course, this is based on our experience. Someone else may have other ideas.

#22

30-45 min for a 100mm length? That is way too slow, will cause voids, even if you don’t use a flow media. Maybe it looks good, but make a cut and microgrindings.
Flow media helps to degas resin in infusion process.
The certification also does not prove the resin quality! There are many new thin resins that are much better than most certified resins, but they are not certified so in some cases not allowed.
Also infusing from the outside to the center is never good, even if you have a UD that is not easy to saturate. Possibility for voids, dry patches where the resin fronts hit is very big.
Also most CF do not come with a silane, mostly glass comes with a silane finish. More important is what kind of fiber you use. A ht fibre is much easier to saturate than a hm fiber. hm fibers have a smother surface so that they are more compacted under vacuum.

And Boeing does not use lower pressure for infusion, most people just do not understand the patent! What they do is to flood the part and than put it in a autoclave to push the rein out with pressure. There are some processes like SLI, ARI etc

#23

Very interesting comments! Thanks
In case I place poltruded carbon rods at the spar caps but these are wrapped around and inbetween with ht carbon for shear purposes up to the shear web do you think the resin would flow between the porltruded rods? The rods could be either round or squared. (6 or more).
I thought placing compoflex against the mold wall (U shaped mold-C spar) and on top to facilitate the shearweb sandwich impregnation.
Which infusion resin would you recommend for a spar?

#24

There is a fundamental issue which you keep promoting, the idea that with vacuum on the resin under the bag, the voids will grow. I don’t agree with this at all - and neither does the boeing patent make any sense if you beleive otherwise. The autoclave simply increases the pressure beyond what is available with our atmosphere so the compaction is greater, but the principal is the same in either case. The positive pressure acting from OUTSIDE reduces voids, NOT negative pressure from INSIDE the bag growing voids…

The way I see it, atmospheric pressure presses down on the bag once the part has infused, the pressure pushes down and evens out the resin rich areas near the inlet and pushes it out towards the low Vf areas nearer the outlet.

I don’t see how it can behave as it does in a degassing pot - because the bag is flexible!!! With a rigid chamber, the negative pressure can build within the fixed cavity and the air in solution will thus expand and bubbles form to offgas - as it becomes supersaturated at the lower pressure. But in a bag, the resin is never subjected to negative vacuum pressure after the resin front passes, once its wetout the vacuum is gone as vacuum cannot exist in an incompressible fluid… all that is left is positive pressure outside the bag from the atmosphere above. I do not have any issues with voids despite clamping the inlet early and letting the resin even out, holding full vacuum on the part until cure. Never pin holes, even with woven fabrics… As the infused laminate compresses with the atmospheric pressure and the excess resin is squeezed out toward the edges, any voids should shrink - not grow.

Do you have any theory or proof which backs up the claims you keep posting in this forum, of the vacuum pressure should be reduced after infusion to prevent entrapped air from growing into larger voids? And that keeping vacuum on the resin after infusion does the same??? Where is your reference material and how credible is the source? This practice effectively reduces the pressure acting on the bag, and allows any entrapped air to EXPAND in a flexible environment… Its no wonder so many people keep asking questions about issues such as pinholes… the only solution seems to be - Buy some MTI hose???

I’m my experience, I think Nash has it correct, the speed must be very slow. I’ve heard the same from experienced engineers who’ve infused very thick laminates. I think zero flow media is the correct move…

#25

Fiber nesting. Each vacuum cycle relaxes where the fibers are, and then pulls them together, shifting slowly. This creates a very dense laminate. This is what he is talking about, not getting air and humidity out.

And at another point somewhere in this thread. You say that resin is at ATM pressure. not fully. Even after a infusion, you have a full part pressure gradient and thickness gradient in the part. Leaving the resin lower than the part always helps, but It doesn’t mean the part is at ATM. The bagging is tight on the preform, else you would have flooding. Using a lower preform vacuum, and a higher one outside, you are helping the normal VARTM process relax an area in the preform, letting resin fill this. Again, this was for racetracking and part no-fill areas. But the concept can be used for thick laminates.

So no, it is not BS. 7 years have been devoted to this process for research and testing. Automated systems have been built and used.

pic: standard layup. mold, fibers, DM and bag. not all shown.

#26

I understood why he is doing that, but in infusion it does not make sense. Uncompacted your layers have 100% thickness. If you debulk the once they have lets say 30% thickness, if you debulk a few times after that procedure lets say 20%. But when you infuse the resin the thickness becomes if you have a perfect VF lets say 50%.
If you have perfect vacuum why do you want to compact them more than you need? This will just make it harder for the resin to travel through the fibers.

And with atmospheric pressure (ATM) you are right. The resin is pushed in by ATM, but if you do not flood your part the fibers are still compacted. And they want to expand completely. They act like a spring. The force you need to keep them compacted to the level where the perfect VF is can be changed to pressure. At a VF of about 55% this is about 150 mbar. This 150mbar will be the difference between the ATM and the pressure in the part.
The VF can also be controlled by the pressure difference.

There is also no issue with the vacuum working on the resin. The air that gets in the part has its volume at ATM. If it is infused and you put your part in an autoclave, the overpressure between the ATM and the autoclave pressure compacts the entrapped air. Thats why autoclaves work.
Even if the bag is flexible, if you work with spiral you have a space in your spiral/catchpot where vacuum is. I understand what you want to say, but you missed a detail. The resin does not care if there is vacuum or not, but the air in the resin.
You have air in your resin in the bucket. Now you start your infusion. The resin is pushed into the tube. That happens with the same pressure than the airbubble has, so it keeps it size. When it travels to the flow front you can see it groving. After your theory this should also not happen because the bag is flexible.
When you clamp your resin feed after the infusion there are different forces.
The ATM working from the outside on the bag, the resin that was pushed in with ATM works against the bag from the inside, the pring forces of the fabric, the inner pressure of the airbubble, the vacuum in the spiral, capilary forces and the friction of the fluid in the fabrics.

the ATM and the resin have the same pressure, the spring forces push against the inside of the bag and want to create a bigger volume. The air inside the bubble also wants to expand and can do that by the forces created by the fibre spring forces, but only a little bit becaus it is only about 150mbar pressure difference.
The air inside the bubble wants to expand further, the resin is pushing from all sides on the bubble, and from the bagging film outside is the ATM pushing.
But there is a vacumm inside the spiral. So the resin is incompressible, but the air not. It pushes against the resin with ATM minus the 150mbar, but in the spiral is vacuum. To keep the forces equal there has to be ATM. So the ATM in the void pushes resin into the spiral and it grows. If you scale it down also a fabric, and more than ever in the crossing points, is a solid chamber.
If you now disable the resin to get in the vacuum line you have your mechanic force that disables the resin to travel away so that your void can not grow. Thats what the MTI hose is doing.

#27

This i agree with - and is why we get a resin rich area near the inlet, and a gradient toward the outlet…

The air inside the bubble wants to expand further, the resin is pushing from all sides on the bubble, and from the bagging film outside is the ATM pushing.

This part here is where the problem is… why does the bubble want to expand further, it is already at 1atm+150mbar! The bubble doesnt have a mind of its own or have a choice in the matter, it will respond by the pressure around it accordingly.

So when the resin line is clamped off, the pressure must then equalize the +150mbar differential. So to equalize the volume and thus the +150mbar spring pressure, something has to move to change the pressure -the air or the resin? In my experience, the air finds its way out first because its more mobile. If the air remains like you say it does - i guess your saying it will expand by “only a little bit” 15%? =(150mbar/atm)

15% is a pretty small amount of volume when your talking about tiny bubbles so small that we can not even see them… Subtract whatever volume of air becomes mobile and escapes, the result will be less than 15% void size growth… This was the point, it doesnt make bugger all difference if you reduce vacuum or not. There are far greater influences on the formation of voids and or pinholes. Reducing vacuum or using MTI can only change the size of the entrapped air by 15% or less. Preventing the entrapment of air in the first place is where the real gains are.

But there is a vacumm inside the spiral. So the resin is incompressible, but the air not. It pushes against the resin with ATM minus the 150mbar, but in the spiral is vacuum. To keep the forces equal there has to be ATM. So the ATM in the void pushes resin into the spiral and it grows. If you scale it down also a fabric, and more than ever in the crossing points, is a solid chamber.
If you now disable the resin to get in the vacuum line you have your mechanic force that disables the resin to travel away so that your void can not grow. Thats what the MTI hose is doing.
No disagreement there, this is why i think MTI hose is great for infusions which have leaks or other poor procedural errors which allow large inclusions of air… it allows the air to pull through the laminate (usually in the high permeability areas) and then escape out the MTI hose rather than accumulating in the laminate and growing into large voids.

#28

No this happens because of friction. Then longer the way the resin has to travel than more friction. Friction is a force that keeps the resin in the part. Thats the trick of a peelply brake zone. Air has such a low viscosity that it can pass the brake zone quite easy, but resin has a lot of friction so that in an ideal case it stops traveling. That is this ideal case you have the same effect than with MTI hose. But the world is not always ideal.

Air is even at ATM compressed with ATM. So you need the ATM to keep it at ATM. So it has an overpressure of 1bar wich wants to expand the bubble. The spring forces of the fabric work negative to the ATM outside, so the bubble pressure will be AT-150mbar, and that means it will become bigger. And pressure to volume is not a linear function, so 150mbar are not 15%, it is much more.

The spring forced can only be equalized by ATM from the outside. But even than the spring forces will work on the airbubble, so the bubble will expand by the created pressure difference, independent from MTI hose or spiral. If you want to eliminate this pressure difference crated ba spring forced you have to put more resin in the part. That will lower your VF but also lower the expansion of air.
And the air will not find its way out, why should it. The air is surrounded by resin wich is pushing/pulling with the same force from every side. (don’t mix up with inner pressure of the airbubble.)
There is only the lifting force of the airbubble. But this will not move the airbubble to the vacuum or out of the laminate.

As I wrote, it is more than 15%. And the “15%” are only the groths caused by the spring forces of the fabric. Depending on vacuum, friction, viscosity, brake zone and so on it can be much more. As long as vacuum is working on the resin front this will able the airbubble to expand.
Reducing vacuum helps because you minimize the pressure difference so that the force working against the pressure in the airbubble is maximized. Thats why I say that if you know all parameters, have ideal conditions and a ideal world you can make perfect laminates without MTI. But MTI eleminates a lot of parameters and makes it much easier to get perfect results.
And preventing entrapped air in the laminate is the best way, because if there is no air it can not expand.

NO! Even for good results with MTI a leak is fatal! And air can not be pulles through the laminate! It can only be sucked out at the resin front but not some where out in the middle of the laminate.
If you make a infusion your system has to be 100% tight!!

#29

Well i have visibly watched air escaping from a laminate everytime i infuse something… this is well behind the flow front, how can you say that it will not escape??? If there is a leak in the tool, it will move right to the edge on the tool side, same if there is a leak in the bag, it moves right across the top in the flow media. The larger Air bubbles can and will be mobile, some thesis on infusion describe the effect of “washing” out of air bubbles… it is a fact.

Smaller bubbles, can be physically trapped between tows where there is physical barriers sure…

So i guess im just lucky that i dont get problems with voids, as i keep full vacuum on the resin until cure…

#30

You see air escaping when resin is moving, you need a volume flow to get the air moving. Or the air grows that much until the air reaches a open vacuum so that it can escape.
Or have you ever seen air traveling to the spiral after infusion? This only happens if you have a leak and a volume flow of air from the leak to the vacuum. But that means the vacuum is stil working:rolleyes:

And the smaller bubbles between the tos are the problem. They cause microgrooves that can cause major problems.

In a lot of thesis about infusion are written many wrong things. Washing out airbubbles is not fact, its bullxxxx. You can do that but it is not really a solution.