Hello everybody I'm experimenting with resin infusion for only a year, so maybe it's just a beginner question but I'm really stucked with it.
I'm making composites with jute+epoxy resin and I can't find the logic under the changing of color of the finished part: sometimes I get light colored part with clearly visible and contrasted jute texture (which I like), sometimes jute seems deeply soaked,and looks darker and uglier. In these bad cases it's also more likely to have problems like bubbles or voids.
I did some tests trying to understand why it happens, but without results.
Jute and resin (supersap biobased epoxy) are always the same, temperature and humidity don't change too much, mold is the same.

The only things changing is the vacuum bag: sometimes a standard bag, sometimes a custom self-made silicone bag (made with Smooth-on EX-Brush silicone), but I have color changes even using the same bag type.
Just to give an example: with a silicone vacuum bag I have a good result, with a identical silicone bag I have the "dark effect".
Again: with a standard vacuum bag I have a good result, the next time I change a bit the layout of flow mesh and I have the "dark effect".

I noted that if I do a hand-layup test it always comes light-colored (good), and this gave me the idea that the dark effect could be caused by the to high vacuum level: I imagined that in a perfectly sealed mold under a full vacuum the resin struggled to move, and was forced to pass INSIDE the jute fiber, wetting it more deeply and thus darkening it more (this would go along with my theory that the first silicone bag I made worked better, maybe because having less experience I did not make it perfectly sealing).
So I bought a SMC Vacuum Regulator, nothing seemed to change. Also, EasyComposite support informed me that it is not intended for resin infusion.

Has anyone ever come across such a problem?
I really can't find the cause, I'm sure I'm really missing something important!

Thank you very much,
Marco

I post a pic of the different results I get

The mold+silicone bags I'm using. Even if the seem identical, someone gives good results, someone gives the dark effect

That's super strange.  The thing that puzzles me is why bagged wet layup always look light.  Is it always either light or dark or do you get a range of shades of dark?  You may be on to something with the vacuum infusion wetting out more thoroughly, because the wet layup will always have some trapped air, especially with natural fibers that are so hard to fully wet out and trap so much air. 

Are there any surface defects or porosity in the light ones?  

Is the thickness repeatable?  Are the light ones measurably thicker?

There's no way this is a material issue - like roll to roll or edge to edge this still happens randomly?

A vacuum regulator is fine for infusion, but you need to (very generally) reduce the flow speed if you are using lower vacuum level.  With natural fiber especially, water vapor is an issue and it boils off into gas at near full vacuum.  Some other people do not agree with me, but I like to reduce the vacuum on epoxy infusions from really high during the shoot to about half right before clamping the resin off.  I also ALWAYS leave the pump / vacuum on the part until it cures - another thing that is 100% normal in every factory I've ever been in but not with many people on this forum - who I must admit seem to get good results turning the vacuum off!  Clearly there are lots of gray areas in this composites business.

Good luck!  Very interested to hear what you learn!...

Hi Chris!
First of all thank you for your earlier big help in beginning 2020, I emailed you and you gave me a lot of useful information. Then the world begun to go wrong and I realized only months later that I didn't respond you, sorry I didn't want to be rude. I hope I can make up for it sharing some info gained in these months' experiments.

Once again you had a good idea: I focused on weight (that doesn't seem to vary with a logic depending on dark/light) and only today I try to measure the thickness of laminates. I cut 10 bad experiments with my bandsaw in order to be able to measure thicknesses also in the center part.
Every dark shell is 2.2mm thick (sometimes 2.0 in some areas).
Every light shell is about 2.5mm thick. Sometimes 2.4, sometimes 2.6 and it seems to goes with a proportionally slightly change of shade.

So now we know that the color change (at least also) with the thickness of the stack. Since the layers are always the same I think we should talk about the compression of these layers.

About surface defects: even if I guess theory should say the opposite the light/less compressed shells are not only better in look for the clear visibility of the texture but they are also much freer of defects.

Another reasoning, but first of all I have to show you my current procedure:
This is the procedure I follow to make the silicone vacuum bag (for brevity "slimer" given the green color)
The gray part is the mold, first of all I make a test shell using hand layup and vacuum bagging (that how I saw that it always comes light if hand laid). I use this test shell as offset to simulate the real shell and then I start to brush the silicone in different coats in order to create the slimer.

This is how I make the infusion: I simply lay 2 jute cloth layers, I add a layer of jute net that I use as flow mesh (but that I'll leave IN the laminate, adding stiffness and reducing waste), I add few pieces of peelply+breather material on the edge to connect the center of the slimer with the vacuum channel.


Now the reasoning: could be the compression difference be caused by the probably slightly different thickness of different slimers? They are created just hand brushing coat after coat and it's likely to have different final thickness even with the same number of coats.
And here the doubt: does a thicker slimer give more or less compressed(>>Dark) laminates?
Unfortunately I find ideas that can support both one thesis and the other...I guess that I only need to try to make an exaggeratedly thick slimer and an exaggeratedly thin slimer and see...the bad part is that those brushable silicone cans are not so cheap

A last doubt after the measures taken today: the test-offset-shell is only 1.6mm thick. So the room left for the laminate (1.6mm)is naturally less than the necessary (2.2mm or 2.5mm) because it does not take into account the jute net.
However, I don't know what to do with this news, since both slimers (both the functional one and the "darkener") were made with the same (wrong) offset.

About temperature and moisture, I did another test that I'll explain the the reply below.