Thick UD carbon infusion

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#1

Hi,

Does anybody on this this forum do very thick Uni directional Carbon infusions - say finished laminate thickness of +10mm? Common applications for this type of work would be the manufacture of carbon aircraft wing spars, yacht spars, turbine blade spar caps etc… large structural beams type of stuff.

Im looking for information pertaining to the wetout of these tightly packed fibres which result in a very low permeability preform and ways of approaching the infusion to minimize the possibility of fiber lockout.

Any reference material would be much appreciated.

#2

I do thin unidirectional infusions up to three plies with two bidirectional under and over them and even that is problematic

#3

It is important to use a UD that is recomendet for infusion. Most stitched fixed UD’s work in infusion. Than 10mm should not be a problem. Some customers use unifilo between the UD layers.

#4

Yes, the only idea i could come up with was to use CFM interleaved in the laminate to promote flow - but this increases resin content and reduces Vf of the total laminate. Id like to avoid its use for this reason if possible buts probably not the end of the world provided the resin has very good shear properties.

#5

Maybe some construction grade spar but certainly not an aircraft spar,

#6

Prepreg might be the only way. Just talked to one of our experts in UD and carbon, he said that unless your resin is like water, the only other wya is to prepreg your way into the part.
Either use existing prepreg, or self-preg the fabric, and then stack the layers as needed, mold and cure.

He did mention that Toray has a tow that allows it to be infused better, but no information besides that was given.

Also, maybe try looking at Justin Alms’s work into localized flow in VARTM/SCRIMP. It involves using less vacuum pressure in the part, and a higher vacuum thing (think of it like a flat vacuum cleaner head) that actually sucks UP the bagging and relaxes the fibers, allows resin to flow into the relaxed part, and then the outside vacuum ssytem is taken off, and the resin/normal vacuum pressure is equalized in the part.
It was to avoid racetracking and void areas (like a corner…suck the bagging film up, relax the fibers, floods with resin, then bag vacuum is returned and boom…no void)

#7

Where can I find more info on this? Sounds interesting.

Even when the resin is like water, it’s still not fun.

#8

Sorry, but that is bullxxxx! The fabrics always relax when the resin gets in, because the resin gets in with ambient pressure. For infusion always full vacuum and notging else. I often infuse UD layers up to 40mm made from 600g/sqm UD from Seartex. No problems with BASF Baxodur resin, Momentive RIM 145.
It only depends on the UD, thats everything. No magic, just physik.

#9

The next problem I was going to ask you about, assuming you can wet out 40mm thick UD carbon, is how to deal with the fibre waviness? Unless the fibers are kept very straight, there is a significant loss in mechanical properties in the laminate. There are several texts on this problem for the wind industry spar caps.

The rim145 resin does seem like a brilliant resin… Maybe I should look into making a heated mold… Do they sell this resin in smaller quantities so I can do tests with it?

#10

You can prestretch you UD. I think you have a good chance to get the RIM145. Much better then getting the BASF resin.
And if you only lay UD it is most times not such a big problem layimg the fabrics straigt and flat. I don’t know if you can get the Saertex material. There is the option Searfix, it is a 1K epoxy adhesive so that the UD is tacky like a prepreg.

#11

So I have a question about this. A) if the resin, especially at the infusion front, is at ambient pressure, why does it reducing vacuum once the infusion is finished help reduce pinholes?

B) I’m trying to decide in what order and when to clamp my lines (I’ve never seen a definitive explanation on when to do so) so I’ve decided it’s best to clamp the vacuum draw line first and then shut off the resin feed. I’m doing this in the hopes that pressure inside the bag will relieve just a bit in order to reduce the magnitude of whatever pinholes might be apparent.

Do you guys think that makes sense, and can you comment on the order? All of the videos I’ve seen on youtube show the operators cutting off resin feed first and then clamping the vacuum line so I’m not sure if I’m totally messing it up or if I might be on to something.

#12

Thats why we developed the MTI hose. In a infusion with a normal evacuation media like spiral hose you always have a vacuum working on the resin. As long as the resin feed line is open resin can get in the part and the vacuum difference is not so big. When you close the resin line the vacuum is stil sucking and working on the resin(and air in the resin). Than air wants to expand and pushes resin to the evacuation line. Also capilary forces let the resin travel from the crossing points through the fabrics to the evacuation line. Now pinholes and voids are created.
You make a brake zone and reduce vacuum to avoid this, but you can only minimize this effect.
The MTI hose is not permeable for resin so either void than pinholes can be created by this effects.
The point when you have to close the resin feed is mostly experience. A good thing is to place the resin pot about 1-1.5m below the part so the gravity does not allow resin pooling and sucking too much resin in the part. I close the feedline when the part is completely saturated.
To make this easier we just made a self regulating valve, here a linkMTI Valve
I hope that helps.

#13

With all due respect, I dont think that is valid reasoning DD. Im not saying that MTI hose isnt a useful product, but i can infuse perfect pinhole free laminates, with 100% vacuum applied via spiral hose at all times, using no MTI hose, clamping only the resin inlet, and achieves Vf ~68%. Here is the proof;

This laminate is 2 layers of 750gsm triaxial each side of 20mm foam core, with 3mm recessed edges, 20ft long x 6ft wide… no veil cloth, no gelcoat, nothing but raw epoxy/e-glass laminate - no pinholes and i did not even degas the resin… Pinholes are caused by other problems - of which there are too many to discuss in this “unrelated topic”

#14

No degassed resin, a VF of 68% and no pinholes? No way! It is possible to make pinhole free laminates without MTI, I always say that. But what you write is even not possible on a flat surface. And on the picture it is not possible to see if there are pinholes or not.
And I am interested how pinholes are created if not by air or capilary forces.

Just realized that you used a NCF Triax. Than it can be on bigger surfaces. But never with a woven fabric.

#15

Hi Groper,

Thats one giant infusion. We too do such things the largest being 6m x 1.5m in one shot. I’m interested in the kind of core you have and the resin absorption on the core faces. Whats the resin viscosity that you are using? Wanted to add that we have done 18mm of 90 ply UD solid layup in one shot from 179gm UD. Beyond that thickness our resin’s exotherm goes haywire.

Thanks,
N

#16

So, assuming that one isn’t after the lowest Vf as possible, is it possible to reduce the number/size of pinholes by clamping the vacuum draw line off first, and letting a bit of extra resin pool into the part? Does the ‘pooling’ only occur in the immediate area around the resin feed spiral wrap, or does its increased pressure permeate and average through the part (I’d imagine that it does)?

I’ve noticed that even after a successful infusion I can still have a couple of minor pinholes (so I guess not a very successful infusion…:() at the very far end of the part near the vacuum draw line. I have no idea where they are coming from.

I’ve used the MTI product and I know it works great, but it’s just too expensive to use in a production environment. For one off, single shot parts that you want to pull out perfectly the first time around it’s an amazing product, but for production purposes I think it’s better to sort out whatever infusion errors are cropping up to be able to perform a good, repeatable infusion without it.

#17

The core is just 80kg/m3 divinycell from diab which is perforated only, holes at 20mm offset spacing. Resin to fill core surface is measured as 300grams per m2. Flow media on the bag side is 30% shade cloth and p16 film. Viscosity mixed is 300cps.

What considerations are there for infusing the heavy UD laminate?

#18

Groper,

We use a patent pending process developed by us in these infusions, and I cannot reveal that method. However, the general considerations that we’ve discovered after extensive testing are as follows:

  1. A compatible sizing on the fiber that enables the resin travel in a tight layup.
  2. A resin and viscosity that is right for the infusion. In your case 300cps is quite ok.
  3. Slow enough resin front, you need to consider that the resin has to go deep inside the laminate. Lets say that you draw a line between the top surface and bottom surface of the resin front in the laminate, that line should ideally be in the plane of the resin flow and not be any more than 45deg from that that plane. If the drag of the flow in between top and bottom surface is very high, slow the front down.
  4. Very slow hardener, that keeps the exotherm at bay till the entire laminate is done.
  5. Extremely tight control on parameters, such as temp, humidity, vacuum (back pressure as well as in front of the resin front).
  6. Well degassed and mixed resin, dissolved (rather than visible air), will blow up into a void even after infusion if you have sufficiently high vacuum.
  7. Cleanliness of the laminate, work area and environment. This may not affect the look of the part, but if you imagine that your fingers have body oils that can contaminate the dry fabric’s sizing. High quality sizing will grab onto whatever oils it contacts. This will embed those oils in the laminate and reduce the quality.
  8. An infusion strategy that allows you to have an undisturbed resin front for the entire infusion.
  9. Debulk the laminate a few times before the start of infusion to have good compaction of the laminate.
  10. Lots of testing and patience.

Hope this helps.

N

#19

All of the above makes perfect sense, but it is nothing i havnt already discovered via the infusion work ive already done before. Could you be more specific in terms of;

  1. Infusion flow front speed - assume we are talking about 10mm UD laminate thickness, what kind of speed would you expect the front to move; over 0-100mm, 100-200mm, 200-300mm. If i know a range, i can select an appropriate flow media (or none at all), to get in the ball park and follow your idea of the < 45deg z-axis flow front from there…

  2. How do i select a fiber sizing thats compatible in this type of layup??? Most of the composite suppliers and their staff over here would not have a clue about this if i asked them this detail… what would you suggest?

3.Viscosity - the 300cps resin i have, is just general pupose epoxy that i use for standard e-glass thin laminates - this is not the resin i would expect to use in this specific application of spar caps. The exotherm alone rules it out. Im looking at resins which would be more suitable for this purpose. Some say viscosity is not important, others disagree - my personal belief it should be quite low visc in the 100-150 range???

  1. I was tempted to not debulk the laminate too much for fear of making it even harder to wetout. Obviously it will be a better structural quality if its very well de-bulked, but does this make it more difficult to infuse?
#20

To your points

1: You can not change the size of the fibers, that depends on the type you need. What you should pay attention on is how they are fixed. Thats what I have already written The stitched UD enable the resin to travel in Z where it is stitched. For example a thermoplastic fixed UD will not make it posible for the resin to travel in Z.

2: 300 cps is for UD infusion on thicker laminates very thick, I would prefer a resin between 30 and 100 cps. Also important is the polarity of the resin.

3: Slow resin front is always important, but is always self regulating on bigger parts. Than bigger the part than slower the resin front at the end. Im some cases this is important, but most times other things like vacuum, fibre type, fixing etc are more important.

4: correct, best are latent systems that need an initial temerature to cure

5: Correct

6: thats why many wind energy blade manufactorer use our MTI hose. You do not have a vacuum after infusion.

7:The fat on the finger is a b***h. On UD it is extreme how less performance can become just becuse you touched the carbon with your fingers. Always use gloves.

8: correct

9: I can not see why that should make sence, just let the vacuum work for a while to be sure that all air and humidity is out of the laminate.

10: Yes, Yes, Yes