why does the bag loosen up when resin starts flowing???


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mscomposites
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Trying to figure out how to keep the bag to stay tight all the way through the infusion process. there’s absolutely no leaks before we start infusing the resin. But as soon as we open up the resin inlet line the inlet side/area of the bag loosens up and it really never get a tight until the resin starts to gel. Anyone have this issue or does anyone know how to fix this issue. I’m finding small pitting on the surface and only on the inlet side so i’m guessing g it’s happening from the bag being loose on that side.
Hanaldo
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It's caused by uneven vacuum inside the bag. Can you post a photo of your layup?
mscomposites
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I don’t have a photo as of now, But basically if you had a 2x2 square and on one side had a resin inlet manifold running along the entire edge and then on the opposite side of that had a vaccum manifold running all good the entire edge. Once i start flowing resin the inlet side always loosens up and stay that way the whole time and slowly starts to tighten back up once I lock off the resin inlet and after a couple of minutes I lock off the vaccum side too. But to me the bag never goes back to full tightness. No side
if it’s caused by flowing the resin to fast. or the amount of air that’s not the line between the resin and the lock off clamp? not really sure what’s going on....0
mscomposites
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had a lot of auto corrects in that last message please let me know if it dosnt make since... 🤦🏻‍♂️
Lester Populaire
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mscomposites - 11/20/2019 4:44:56 AM
had a lot of auto corrects in that last message please let me know if it dosnt make since... 🤦🏻‍♂️

Is the resin pot on the same height or higher than the part? In this case resin will flow into the part by itself which results in pooling.
Keep the resin pot lower, and maybe don't fully open the line if needed.

mscomposites
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yes the resin pot is always siting on the same table as the part, which is the same height. I will set the pot lower if not in the ground and slowly open the resin inlet hose and slowly flow the resin in. I have heard that it’s ok to fully open the resin inlet hose and pump away, but also heard flowing to fast can cause surface pitting. Any feedback on that?
MarkMK
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The unevenness in bag tightness is quite normal

Basically, once the resin begins to flow, the inlet side will only be at the ambient pressure of the incoming resin.  

Once the whole set-up is clamped-off, the negative vacuum pressure closest to the exit port will begin to equalise across the surface of the part. I don't think that this alone would cause issues with surface quality once cured and everything should tighten-up after infusing

If you suspect that your part might be infusing too quickly, you can (if not already using) create a 'resin-break' just ahead of your exit line. This involves cutting your flow media short of the exit edge to leave a gap of around 20mm or so before another strip of flow media along that edge of the set-up that the exit port sits upon. This should help ensure that all areas of the part have enough time to wet-out prior to the resin starting to travel up the exit line and help slow things down a little. This is especially helpful on small-ish moulds

Leaving the inlet port open for 30 seconds after clamping the exit to let-in a little more resin should also help to make sure that there won't be any dry spots




Edited 5 Years Ago by MarkMK
mscomposites
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Thanks for the info MarkMK, Yes we do already give it a resin break, but one thing we are doing is clamping off the inlet line first and a minute later clamping off the exit. No resin is traveling down the exit line when we clamp. But I do notice that the small pits we are getting are in the inlet side. So i’m guessing it has to do with the bag being looser on that side? Or maybe since there’s more resin on that side at the time it gassed off differently and shows signs on the surface?
Lester Populaire
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mscomposites - 11/20/2019 2:30:32 PM
Thanks for the info MarkMK, Yes we do already give it a resin break, but one thing we are doing is clamping off the inlet line first and a minute later clamping off the exit. No resin is traveling down the exit line when we clamp. But I do notice that the small pits we are getting are in the inlet side. So i’m guessing it has to do with the bag being looser on that side? Or maybe since there’s more resin on that side at the time it gassed off differently and shows signs on the surface?

No resin gasses off only at the flow front where the pressure is low. The resin break, while being a very good thing, does not do anything for your problem...


MarkMK
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In that case, it's possible that the inlet side is being a little starved of resin and it's more likely if your part is quite small. 

Try doing things the other way around and leave the inlet port open a little while after clamping the exit. Assuming that everything's air tight, I wouldn't worry too much regarding the looseness around the inlet, as things will consolidate well once the vacuum level equalised after close-off and any excess resin will likely be removed along with your stack materials



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