Carbon fiber vacuume bagging

[Pontiackid73]

I've been working the kinks out of our new resin infusion process and I've come the " this is the best its going to get" stage of trial and error. I can not figure out why It isn't coming out correctly. I am laying these test pieces on a pane of glass so the surface should be perfect(ish) and I can't get it right. I am pulling 28 in of mercury, it has a good seal, and it is taking the resin about 8 minutes to get across a 16" long part of 6 layers of CF. see the attached photos http://i.imgur.com/cG7jDJB.jpg
http://i.imgur.com/OTUql1f.jpg http://i.imgur.com/YhaPcYq.jpg

[Pontiackid73]

My bad for the giant pictures. The divots are just small dimples on the surface, they do not go all the way through. Part size is 12 inches by 18 inches (300x460mm)

[Francis Gagné]

First, some questions :

What resin do you use?
What is your release agent? How do you apply it?
What is your process (When do you start vacuum, for how much time)?
How do you leak test? Do you vacuum degass your resin?
What is your stack (Peel-ply? Flow Media?)?

Knowing how you do it might help to pin-point where it is going wrong.

[Pontiackid73]

Resin is US composites' 635 thin epoxy resin system with the slowest hardener they have, about 50-60 minute pot live at shop temperature.
Release agents are part-all wax paste buffed on and off once since it is glass I don't think it needs much to be sealed. Following that is Partall #10 liquid mold release agent ( brushed on ).
Vacuum starts about 10 minutes before I introduce the resin and once the resin is pulled all the way across the part (8-15) minutes I leave the pump on for about 1.5-2 hours. It seems to take a while for it to compress the part and pull out the excess resin.
Leak test is done with a gauge before resin is introduced. I leave it for about 10 minutes and if it doesn't loose any pressure I say it is good to go. Gauge is mounted to the overflow pot.
I do not vacuum degass the resin, but post mixing i leave it sit for about 5 minutes and most of the bubbles reach the surface. I make sure to shove the pickup tube as far down as possible. ( not sure if that is good enough)
The stack is carbon, peel ply, this nylon mesh I picked up from a hardware store that is meant to be screen door material, then vacuum bag material. I attached pictures of the stack and resins. Could it be that the pump is not strong enough? I like to think it is the flow media but I have tried several types all with the same result. One was a much larger plastic mesh and the other was a finer mesh screen. http://i.imgur.com/malxLCH.jpg http://i.imgur.com/LFiayi9.jpg http://i.imgur.com/fQjbzlM.jpg

[Ben K]

I found at NU that vacuum Degas-ing makes a huge difference in quality. As you are pulling vacuum all the volatiles are coming out--its not just about the bubbles you see. Be careful while degassing--you'd be amazed how much that stuff bubbles up.

We also degas-ed on a mixing plate so that the bubbles would break. You might also want to consider a viscosity modifier to make your resin less thick.

[Francis Gagné]

Ok, I have never worked with this type of mold release (We use Frekote 770-NC, it's a charm!).

The important thing with most mold release is to let it out-gas properly. The solvents will continue to evaporate during the curing, even more when heated by the epoxy curing, which could be the source of your pinholes.

One of Rexco documents recommends to let the wax coat out-gas one hour before applying the first PVA coat. Make sure to let sufficient time between PVA coats, (10-15min). A tech lab at Gurit told us to always wipe-in wipe-off the mold release (Frekote 770-NC). This leaves only a thin coat, needing less time to out-gas.

Degassing the resins might also help, with the resin I have here (PTM&W PT2712) we don't need any, it is a 320cps infusion resin. But it is probably very dependant on the resin... For degassing you can put your pot in the vacuum resin catch can if you have one for some minute just before the infusion.

Another thing to look out, that I learned the hard way, vacuum should be applied to infuse the part. Then vacuum tube clamped off, and vacuum slightly released (maybe 15hg). Until the resin start to gel. The you put the vacuum back again. The the right timing is resin dependant, some resin datasheet will indicate the timing to turn on and off the vacuum. The helps not drawing more air into the mold, and keeping the resin flat. If you have full vacuum on one side, you may have a small pressure differential pulling the resin more to one side.

If you want a faster infusion you'll need a thinner epoxy, or use multiple feeding lines. But the duration you have looks ok. Also, look at how your mesh deforms under vacuum, if the patterns becomes completely flat, it slows down the flow.

You'll have to try the different methods to see what is causing you trouble. Maybe its something else also.

[David_uconn]

1) Yes always degass the pot of mixed resin/hardener - give it a good 10min under vac and you'll be shocked how much the pot fizzes up, there is no excuse not to. We rigged a degass chamber out of a large pipe and some plexiglass laying around, get creative it's not expensive and will help.

2) You're pulling resin way too fast - throttle the inlet down so it takes at LEAST 20min to finish wetting out - that should help eliminate the voids (voids are getting trapped as the resin front moves across them.) - If you're pot takes an hour to gel you have more than enough time.

3) To prevent that fabric print through (not getting the glass finish):

--first let the part cure in mold for at least 24hrs+ (be sure where it is curing is a good 80 degrees or better) Any less and it might pull and look nice then as it finishes hardening the texture appears.

--second when running the infusion: as the resin starts hitting the vac line clamp off the vac line first, wait a min or two and let more resin into the part then clamp off the the feed line (resin rich will give a far better cosmetic finish than resin starved which it looks like your part might be - weight difference is minimal) - leaving the vac running for an hour is pulling out all the resin you want still in the system into your catch can.


Hope this helped.


David
UConn FSAE

[Ben K]

Originally posted by David_uconn:
1) Yes always degass the pot of mixed resin/hardener - give it a good 10min under vac and you'll be shocked how much the pot fizzes up, there is no excuse not to. We rigged a degass chamber out of a large pipe and some plexiglass laying around, get creative it's not expensive and will help.

2) You're pulling resin way too fast - throttle the inlet down so it takes at LEAST 20min to finish wetting out - that should help eliminate the voids (voids are getting trapped as the resin front moves across them.) - If you're pot takes an hour to gel you have more than enough time.

3) To prevent that fabric print through (not getting the glass finish):

--first let the part cure in mold for at least 24hrs+ (be sure where it is curing is a good 80 degrees or better) Any less and it might pull and look nice then as it finishes hardening the texture appears.

--second when running the infusion: as the resin starts hitting the vac line clamp off the vac line first, wait a min or two and let more resin into the part then clamp off the the feed line (resin rich will give a far better cosmetic finish than resin starved which it looks like your part might be - weight difference is minimal) - leaving the vac running for an hour is pulling out all the resin you want still in the system into your catch can.


Hope this helped.


David
UConn FSAE

Might I suggest a resin catch chamber? All it takes is to have a small cylinder with an inlet and outlet where the inlet drops pulled-through resin into a cup while the vacuum is still being pulled on the outlet. As long as you know your gel time, you should be able to just watch rather than worry about resin getting into your vacuum pump.

[Pontiackid73]

Thanks for all the info guys. I will put together a de-gassing chamber for the resin pot and Slow down the infusion with the resin rich trick at the end. I will update with new part pictures in a couple of days.
I already have a catch chamber. Very homemade (well the whole setup is) but it works well and for a total cost of 17 dollars it does the trick. http://i.imgur.com/iyQb7Xu.jpg

[Pontiackid73]

If you see any other problems with this let me know. I'm not flying blind but I am the only one on the team with the ability and interest in making the composite parts so you may see something I am overlooking.

[Pontiackid73]

Quick Update: Did the next run. Changes I Made for this run were de-gassing the resin, slower feed rate it took 28 minutes to wet out fully, and the resin rich trick. I am keeping the vac running for 2 hours and will wait until tomorrow evening to pull it. I keep a big shop light on it to take the temp up to about 120 degrees Fahrenheit. What are your opinions on raised temperature cures? COuld that also be a source of the problem?

[Pontiackid73]

Another point of interest:
While the part is wetting out I have these little air bubbles behind the inlet tubes that spread across the part and bubbles in the inlet tubes ( see pics ) without me manually coaxing them out of there is there a way for me to make this not happen? also a pic of the sweet resin de-gassing chamber... Just like you said, 5 inch schedule 40 pipe , some scrap 3/16 plate, and a slab of plexiglass. Everyone in the shop always gets worried when I break out the oxy torch for some serious business.
http://i.imgur.com/9Fkjxgf.jpg http://i.imgur.com/4qgVDg3.jpg http://i.imgur.com/oSNXv40.jpg

[Francis Gagné]

The bubbles might either be a small leak at your inlet tube joint or it can be trapped air in the bag before infusion that released at the end of the infusion into the resin. It shouldn't be much of an issue though as they seem on the top surface only.

[David_uconn]

A small amount of bubbles is normally acceptable/expected. As Francis said it's likely air leftover that was trapped or gas from the resin (degassing will never get it all just reduce it drastically.) Your degass setup looks a lot like ours with the pipe. http://fsae.com/groupee_common/emoticons/icon_smile.gif

For applying heat - it's probably overkill for a sample part. I'd say let it cure 24hrs at room temp 70-80 degrees then pop the part.

If you do want to add heat do not pop the part, instead do a post cure heat ramp (most resins have instructions on what they recommend) it's usually ~2 degrees per hour til you reach the recommended temps then hold for another 8 hours then ramp down to room temp again. Don't take it out of the mold to do this because you'll deform the part and get print through again.

Again likely overkill but you do squeeze a bit more strength out of the part.

David
UConn FSAE

[Pontiackid73]

Ok next test run is done, The de-gassing helped a bit but it is still a failure. I called the manufacturer of the resin and they basically told me their resin is to viscous to do what I am doing so it looks like the problem the whole time is just my oversight in product selection. You know one of those moments when you just feel plain stupid? This is one of those moments. I will just go back to normal wet layup and then vacuum.