X file mystery ...

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#1

I Recently made a test part comprising some foam core, and we made the mistake of not bevelling the edges of the foam enough so the fiberglass bridged a little bit, what I can not understand is how come the void due to the bridging of the fibers in the corner did NOT get filled with resin and stayed void …we pulled full vacuum and the bag kept it overnight we infused the part and when we removed the peelply and made a sectional cut discovered this mysterious long void that IMO defies the laws of physics, can anyone tell how could this happen ?? how come that the void did not ended filled with resin ? the vacuum was there …

I will take a pic tomorrow when I go to the shop and post it here only have a bad picture now that shows the void from outside, I can post a better one tomorrow (but curiosity is killing me ¡¡¡¡)

#2

Try to fill that void with resin and see how much it takes. I would say that the infusion process by nature will leave voids if there is bridging, because the vacuum and hence resin content will be equal over the whole part (all things being perfect that is, obviously flow issues are another matter).

You won’t be able to fill a void with resin unless you have that same resin content over the rest of the part.

#3

Magic of composites, lad. Any bridging will not fill with resin all the time. Sometimes it fills, sometimes it is a void. Depends on tons of things. resin degassing, racetracking, exotherming, leaks, fast resin flow, etc.

it’s like the panel I made last week. 99% of it was perfect. Along the tool side, there is one square inch that has weave voids. Why? No one will ever know!

#4

Hi, thanks Hanaldo and Riff for your answers, but then the mystery remains, why is it a mystery? i think it defies the laws of physics, and Hanaldo, how can you accept what you say that “the infusion process by nature will leave voids if there is bridging” ? during the infusión process we are constantly seeing the resin filling voids, voids between fibres and the void inside the resin hoses so it is filling voids of any kind and then when the void is caused by bridging, the resin resist the atmospheric pressure and refuses to enter those particular voids ONLY ???, are we to believe that the resin does not enter a particular void arbitrarily ?? then we would be forced to believe in Riff´s words “magic nature of composites”, very very strange there has to be a better explanation to this …

#5

sure, if you have perfect full vacuum (0mbar pressure(space)), perfectly sealed bag and film, no leaking tool, no moisture or boil off, no air upon injection of resin (from the resin inlet line) then the void will fill in with resin. But the fact is, if you have a void, and air is present for whatever reason it will always go to the corner bridge area since its an area of low pressure. This is the natural place for it to sit. If you want to avoid this in the future, first make sure the materials including mesh and bag film are not bridging, second, make sure all your variables I mentioned above are perfect, or as close to perfect as possible. Otherwise and residual air present within the bag/laminate, or induced during infusion will end up in that corner. Not much of a mystery.

#6

Hi Hojo, of your explanation, what I do not see is why is the bridging area an area of low pressure, IMO all places under the bag, when time for equalization is given are at the same pressure, everywhere under the bag is connected by channels that allow the resin to go everywhere, those very same channels “convert” the space under the bag in a volume (or plenum) that, due to the laws of pysics should be at the same pressure, not a lower pressure, so IMO the mystery is still there

The only “explanation” I can accept is more inline with Riff´s “magic nature of the resin” or as I often say to my working buddys “the devil himself lives in the resin” off course not a satisfactory explanation

#7

Under the materials that bridge, there is no atmospheric pressure acting on this area. When you pull “vacuum”, you are removing atmospheric pressure from the bag/laminate, which in return applies the atmospheric pressure to the laminate. So, if the atmospheric pressure of the day in your area and elevation is for example 1000mbar, and you are able to reduce your inner bag pressure down to 20mbar, then you will have 980mbar of pressure on the laminate, with a remainder of 20mbar inside the laminate. However the areas underneath the bridge do not “see” the atmospheric pressure because the layers above are bridging opposing that force. When you infuse, typically the majority of remaining air is flush out in the flow front, however at bridges, the resin will overflow these areas and trap air, leaving voids. Thats why I said, if you had a “perfect” vacuum and infusion, bridged voids would indeed fill. But in your situation, it was less than perfect leaving voids unfilled. This is why I always suggest putting an absolute pressure gauge at the infusion inlet before infusing to confirm the actual pressure level.

#8

Foam is not completely closed cell, in datasheet always indicate some % water absorption after time. In a long time process like vacuum infusion some cell could break and if there is a bridging the air inside the cell will move there. Time ago my company made foam cores for a technical production process and we were asked to produce it in closed mould, with that process a surface skin is created and the possible problem of break cells is overtaking.

#9

As hojo somewhat said, vacuum bagging is a rough vacuum. There is still a lot of air in the bag. You can get higher vacuum pumps, and even 90k$ pumps that will suck out to near perfect vacuum.

Another point is, racetracking. If the resin takes the path of least resistance, it can enclose an area, such as most of the bridging area next to the foam. Gravity will dictate that the resin flows into the void on the bottom. Once resin surrounds that area completely, the air WILL NOT MOVE, because the pressure of the resin is not full vacuum (it can not compress), so the air bubble just kind of stays there, as the resin flows past. We are doing MAJOR research here about how to get voids and air out of laminates. There is a lot fo magic going on inside a part, with flowing resin, void filled tows, interactions between everything, etc.

#10

I truly appreciate the explanatory efforts made, but some explnations could have some “mysterys” in themselves, Hojo,what you said “However the areas underneath the bridge do not “see” the atmospheric pressure because the layers above are bridging opposing that force” it sounds reasonable untill you realize that due to the permeability of the materials under the bag, they can not trap an area with a different pressure than its surroundings, the permeability needed to allow the resin to move and impregnate the fibers, should also allow for pressure equalization so given time the pressure under the bage should be equal everywhere.

And Steyrn, IMO, if there is some gas inside the foam core cells, and the cells break and consequemtly liberate the gas, this gas would do the same as the rest of gases that whre once inside the bag, wich is off course being sucked out leaving then a void or better said partial void, 99% void, and then this void shoul “behave” like the rest of voids under the bag, “who´s” duty is to get filled with whatever present fluid in our case off course the fluid is the resin, by now, the theory I find more credible is riff´s about bubbles of gas trapped, maybe (very likely) they are bubbles coming from the boiling resin front that are getting trapped there …

#11

Sure, the entire laminate and voided areas have the same vacuum pressure level (more or less), but physical atmospheric pressure on the laminate will not be the same because the bridged materials appose the force to the under lying laminate layers.

#12

Completely agree, but this would explain why the bridge is there, but not why it has not filled with resin, I believe the only reason for a void not to fill with resin is that it is filled with something else (so it was not as void as it seemed to be), maybe the gases coming from the boiling resin front do not have time to get evacuated from the voids once entered, because the progression of the resin front does not give time for the gases to get sucked thorough the tight compacted fibers, so the resin goes around and traps those gases in the corner of the briging not allowing the resin to enter the void …

I will sleep much better tonight … and would have not been able to get to this reasonings without the help given here

#13

The void didnt fill with resin because air or other outgassing filled the vacuum rather than the resin. If the resin was traveling slower, it could have been possible for this gas to escape, and be replaced with resin, however the resin probably flew over the void before it gave the air enough time or ability to be flushed out and replaced with the resin. So this goes back to what I was originally saying, make sure you have a very high vacuum, no leaks and hold this vacuum long enough to boil of the moisture prior to infusing. Get a digital absolute pressure gauge and observe the actual pressure at the part resin inlet, you will learn a lot about whats really going on, and how good of vacuum, and vacuum integrity you actually have.

#14

I will follow your advise in regards to the absolute pressure gauge … I need to fully understand everything I am doing …

#15

yes, highly suggest it. I would recommend the GDH200-14 gauge

#16

Don’t forget race tracking. if the flow media caused the resin to flow over the laminate, and some went along the bottom or sides, if the void did not fill with resin in the first place (speed, etc), once ALL sides of the void are surrounded, that pocket of air will stay there. It will not keep sucking through the part as the resin is flowing. That is why it’s hard to get a perfectly filled carbon fabric…resin flows around the tows, cutting off the void on the inside. If you infuse slower, the resin has more time to get all the way into the tows before it’s cut off.

#17

So now, just to avoid boredom we have to find another mystery to solve …, but no problem since “the sherlock holmes of composites” are here in composites central forum to give hand …