Pinholes in surface after resin infusion 2

[crud]

Please help me before i go insane... After every infusion i do (practcing on glass using carbon fibre and plain weave e glass) There are small gaps between every tow in the weave. I have tried everything i can think of to stop this. I am using carbon that is stored in bubble wrap, epoxy infusion resin, i acheive full vac and no loss of vacuum. As i do it on glass i can see where the problem lies.... During infusion it works perfectly, but as soon as i clamp the feed line small voids form in the weave. I also de gas the resin for 10 minutes first. Any help would be very much appreciated.

 

[Compositepro]

How do you define full vacuum? The only meaningful definition, in this case, is absolute pressure - not gauge pressure. Absolute pressure is measured relative to a perfect vacuum (zero is a perfect vacuum), and requires a special gauge like a U-tube manometer. Gauge pressure is measured relative to ambient atmospheric pressure (zero means no vacuum), and is what most gauges measure.

There are two reasons for your pin holes.
1. Residual air in fibers tows that were encapsulated by resin flowing through the weave openings. After resin plugs the vacuum port, no more air can be removed. Capillary action causes resin in the weave to wick into the tows, displacing the air into the weave openings. The solutions are use a better vacuum (so there is no residual air to trap), or to slow down the infusion so that the wicking of resin into the tows closely follows the flow front through the weave (so the resin can push the residual air out the vacuum port).
2. You may be boiling the resin by applying too much vacuum after you close the resin inlet. Resins will always have some volatiles (including the resin itself). After closing the resin inlet and the vacuum line is full of resin the vacuum should be reduced to 10 inHg. This provides 10 inHg of compaction pressure on the fibers while maintaining 20 inHg on the resin during cure.

 

[crud]

Thank you for helping compositepro. Your the first person to give me something to go on. I have tried infusing at 'full' vac then dropping to 15inHG, in this case the void content was dramatically reduced however the resin was so thin over the finished part that some fibre was visible outside the resin (i am not using any gel coat). So i did it again but reduced the vacuum after infusion to 20inHG, in this case i got almost perfect results. But again there was a very slight show of fabric but the resin skin seemed much thicker. On the third attempt i reduced to 25inHG... this is where it gets interesting, There was no 'print through' of fabric but the voids had returned in the tows. Also bubbles had formed on the top of the laminate and in the infusion mesh. It seems as if the resin is still emitting volatiles even though i had spent a long time de gassing. (i am only using around 70 grams of resin for my test pieces).

To go back to your suggestion. If i reduce to 10inHG would i see more 'fabric print through'?

Oddly the guys i bought my pump and gear from say i should always infuse at 'full' vac from beggining to end. I am also using their own resin which they say they use themselves.

I am seeing 'full' vac only using the supplied vacuum guage fitted to the catch pot. Also the pump goes quiet on full vac.

In all cases i reduced the vac before clamping the feed line.

I wonder what your thoughts might be on this?

Regards, Chris.

 

[berkshire]

Crud,
stub in another vacuum gauge near your inlet port, so you can see what the vacuum on your layup/infusion really is away from the pump. However be aware that residual air from the gauge hose can enter your layup.
As Composite Pro says slowing down the rate of resin transfer to get better wet out may help.
B.E.

 

[crud]

I dont know whether this would be the same thing but i tend to only have a small amount of infusion mesh (flow media) then just the laminate/ peel ply. As soon as the resin hits the laminate without the mesh it slows down(i am only making pieces around 6 inch's by 8 inch's).

I have also tried infusion mesh under both inlet and vacuum ends. But i think my parts are too small and thin for that.

Or would you still recommend controling the speed of the infusion using the clamp?

Thanks, Chris.

 

[berkshire]

Yes if you are using a vice grip type clamp where you can throttle the resin flow.
The other thing to look at is the viscosity of your resin.
You can alter this a little by warming or cooling the mixed resin. Cooler resin, higher viscosity, slower wet out rate.
Warmer resin, lower viscosity, faster wet out rate.This comes with the penalty of faster gel time if you over do it. However with a small piece like yours, a lower viscosity may get you better wet out and push the air to the outlet of your mold faster.
B.E.

 

[Compositepro]

When you reduced the vacuum you apparently allowed air to flow back through the vacuum line into your laminate. The vacuum line needs to be full of resin and remain full when you reduce vacuum. You should be able to see this through a transparent plastic line. With VARTM the big problem is that during infusion the area around the resin inlet will become very resin rich and thick because of high resin pressure relative to the area around the vacuum line. When the inlet line is closed the expectation is that the thickness will equilibrate throughout the panel. It is the "sponginess" or compressibility of the fabric that controls the panel thickness in concert with resin content and viscosity. Viscosity resist flow and therefore resin pressure can vary from one area to another. Resin pressure plus fiber compaction pressure is always equal to bag pressure (which is ambient air pressure). And thickness is directly related to fiber compaction pressure.

 

[crud]

Thank you very much to you both for your support. I will put some of your points together tonight when i carry out another test piece ( then i will be out of resin untill saturday). I will let you know how i get on. I will try infusing at full vac (or as full as i can achieve with no proper guage). Then when the vacuum line becomes full of resin with no air i will drop the vacuum to 21-22 hg's and clamp. I have tried at less but it didnt seem to have quite as good results. I think the big thing is that i didnt wait for the vacuum line to be properly full on my other test pieces.

Chris.

 

[crud]

ok, test done and out of resin untill the weekend. The results were average. Roughly the same as last time.

If i am acheiving full vacuum (lets say i am for the purpose of this question), even on a part as small as 8x6 inch's, should i leave the stack for any longer than a couple of minutes to ensure all the air is removed?

As i mentioned in my other post the part appears to infuse perfectly (looking from the under side on a piece of glass) it is only when the feed line is clamped that the problem occurs. I will post a picture of a typical layup of mine for some scrutiny, infused with peel ply etc still in place.

Could any one here tell me if it is possible to infuse a single layer of carbon fibre? (not that anyone might want to) just a question.

Chris

 

[berkshire]

It takes a while to remove the last vestiges of air, a couple of minutes may not be enough.
Also with the second gauge I mentioned, after you have pulled "full Vacuum", turn off the line from the pump to your layup and observe the gauge on the layup stack for at least 10 minutes, it should not move. If you do see movement
this will indicate a minuscule leak that can create some of the problems you are having. Your pot clamps and spigot valves should be capable of supporting full vacuum or at least 29"Hg.
B.E.

 

[berkshire]

I just re read your original post where you said you had no loss of vacuum.
Did you determine that by the method I just mentioned, or were you using another method?
B.E.

 

[crud]

The method was probably not sufficient really. I pulled as much vac as possible and closed the 1/4 turn valve. Then turned the pump off. I left it for about 3 hours and when i returned the bag was as tight as it was when i left it. But i guess i should probably stick a guage on it so i can see if it drops at all.

 

[crud]

Got a question.... i read a post on the net saying that some bagging plastic can be too stretchy for infusion, therefore filling the flow media/ laminate with 'bag' where the air would normally flow through thus restricting air flow. Does this sound like it could happen??

Regards, chris.

 

[Compositepro]

It may be possible but it would pinch-off resin flow long before pinching-off airflow. It isn't really a good practice to have the vacuum bag directly in contact with the resin. Any permeability or pinhole in the bagging film will leak air into your part. There should be release film and breather between the part and the bag. That way all leaks (resin or air will be into the breather. The breather should be connected to full vacuum all the time. All films should be assumed to have pinholes.

 

[MotoMike]

Crud,

I've spent the past 8 months doing a lot of trial and error to learn VARTM. At one point I think I was having the same problem as you. Are you using a vacuum bag around the sheet of glass? When you clamp off the inlet do you have epoxy in the vacuum tube going to your pump? I am wondering if your epoxy is going up the creases of your bag and right into the pump line. At that point any air leaks causes the epoxy to percolate in your part and causes pin holes and voids. If you have any creases in your vacuum bag the same thing will happen, the epoxy will follow the path of least resistance and go right to the outlet. This may cause voids because the epoxy does not flow through the fabric. Rather than using resin flow, make your part a little oversize and surround it with a perimeter of breather material on peel ply. Pull the vacuum on the breather material and infuse from the lowest point or the center. You can also do some fabric flow tests to see which point is the best to pull from and to infuse from using fiberglass and some other liquid - I've used coffee before so I can see the flow pattern. Another thing to consider is that if you see the epoxy percolating in your exit tube - there is a leak. Also, look at the geometry of your part. I like to set things up so that I use the vacuum to pull a void in the fabric and let gravity fill things up. If you keep your exit end higher and keep enough epoxy in your feed tube you'll usually be okay.

Good Luck!

Mike

 

[crud]

Thank you moto mike, i think i am having the same problems as you were having. I have been trying for about four months now too. I have never tried a breather layer around the perimiter so i will definety try that. (what would the purpose for this be though)?

I do indeed butyl tape the bag to the glass.

I have done tests where the resin flowed into the vacuum line and tests without it flowing into the vacuum line. Neither seemed to make much difference.

On full vacuum bubbles appear as if the resin is out gassing but i degas the resin first so i can only assume they are trapped air? but they dont seem to want to make their way to the vacuum line they just sit in the mesh or peel ply.

 

[blakmax]

I suggest that your problem may be moisture related. If your resin was exposed during high humidity, then the moisture absorbed by the resin is outgassed as steam during the cure cycle. Was it raining the day you did the process viz. high humidity?
High vacuum levels exacerbate the problem by expanding the voids. Despite the common perception that vacuum removes voids, it can make them worse by the low pressure expanding the voids. The resin flows away, not the voids.

 

[MotoMike]

Crud,

Can you upload a picture of your layup? Also what type of epoxy are you using?

You shouln't have to degas the epoxy because you do that with the vacuum pulling while it cures. Get yourself a resin trap with a guage. Hook it up tp your pump and then clamp off all the lines to see how long it holds vacuum. If that holds then put it in line to your layup. After pulling it down clamp off from the pump and see how tight the layup is. If that is all okay then I think the problem is some thing that I also ran into.

You are getting air in the system when you infuse. Because you are not using breather, the air in the inlet tube goes into the fabric. The epoxy takes the path of least resistence and goes around the air and hits the outlet tube which encapsulates the air. The air just percolates in the part. Have you seen in the outlet tube when you get a little resin in and it moves up and down but doesn't travel too far? The same thing happens in the part.

Make sure you wrap the vacuum tube in the bag with breather, otherwise it stops pulling once the epoxy gets there. You use the breather to pull air and excess resin through the part. This keeps an open path to the vacuum pump. Also, you can put breather strips to pull the epoxy in specific directions if the geometry of the part moves it away and you get a void. Remember that even with vacuum, gravity will affect the epoxy flow.

Hope this helps. I've read a lot of textbooks, whitepapers, etc. On this and set up several hundred parts in the last few months using many different epoxies and resins. I'm still learning everyday. This stuff is not all that easy. Good luck.

Mike

 

[crud]

Motomike,

Yes thats exactly what happened yesterday the resin creeped into the vacuum line (i have a catch pot with a guage), i thought it was odd when i saw the resin fall back down the vacuum tube then rise again then as soon as a drip went into the pot it fell again. I have set another one up today using some breather around the vacuum line and i timed it just right so the part infused without hardly any going into the breather and so far none in the tube. Ill let you know how i get on.

Thanks again mike. Regards, chris.

 

[berkshire]

Moto mike said "You shouldn't have to degas the epoxy because you do that with the vacuum pulling while it cures."

The problem with doing that is that your resin is now degassing inside your mold producing bubbles in your part.

I will reiterate what Composite Pro said " 2. You may be boiling the resin by applying too much vacuum after you close the resin inlet. Resins will always have some volatiles (including the resin itself). After closing the resin inlet and the vacuum line is full of resin the vacuum should be reduced to 10 in Hg. This provides 10 in Hg of compaction pressure on the fibers while maintaining 20 in Hg on the resin during cure."



Reducing the vacuum pressure allows the the gas bubbles to collapse on themselves.
B.E.