“Emanis believes double bagging overcomes this”
You can believe in a lot of things, but that does not mean that it is true.
I’ve read that article which inspired me to experiment. It’s full of false assumptions though like this one:
“Emanis believes it is essential to split the two main vacuum functions, assigning volatiles extraction (removal of entrapped air, ambient moisture and/or solvents) to the inner bag and then using the outer bag for compaction.”
The outer bag cannot provide any compaction. Applying vacuum to the outer bag will only determine the compaction of the breather between the two the bags. It cannot apply pressure to the laminate via atmospheric pressure. To say that the outer bag can provide compaction to the laminate is like saying that an envelope bag sitting on your bench can provide pressure to the bench.
The only feasible explanation I can comprehend is that the inner bag applies the initial compaction of the laminate then vacuum is applied to the outer bag. As the vacuum reduces in the inner bag during infusion the inner and outer bags pressed together hold their form and reduce SOME of the fiber rebound in the laminate.
I still think the inflatable bladder would overcome this. I think if the bladder were inflated above the mold flange, the second bag would use leverage to pull the bladder down towards the mold. Just like the sponge has pressure exerted on the mold surface.
Am I wrong? I think you could get some decent pressures out of it too.
I can’t see how a bladder would help. The bladder would just push the two bags apart.
Exactly!
The fulcrum for the second bag of this setup is the mold flange. The first bag is pulled against the mold. The second bag is pulling the inflated bladder into the mold.
These are the exact same forces being exerted when a single bag is compressing a sponge or stack of fibers.
Yes, the bags are being pushed apart, but the first bag has nowhere to go but down, farther into the mold. The second bag is being pushed away from the mold, with the bladder trying to separate the vacuum between the bags, not going to happen.
Forgive my lack of technical writing skills.
On a typical setup, the bag “pulls” the layers of fiber into the mold, compressing them, correct?
On a typical setup, if you replace the layers of fiber with an inflatable bladder the bag is pulling the bladder into the mold. If you inflate the bladder the pressure increases exponentially per psi.
The forces exerted in an atmosphere with lower than atmospheric pressure are astounding.
[ame=“http://www.youtube.com/watch?v=E_hci9vrvfw”]Tank car implosion d’un wagon citerne - YouTube[/ame]
The vacuum on a mold is no different. So, pull a vac on a bladder setup and you then have a tremendous amount of force contained under the bag. It would probably warp even a strong mold.
When adding the bladder, the first bag now becomes the mold surface so to speak. The second bag is trying to compress all of the air within the bladder, just like the sponge, forcing it into the mold. After the vacuum pump runs out of lungs you could crank up the bladder psi. However I’m not sure what kind of pressures would be in the bladder after a vac was pulled, if you started at say 5psi. 10? 100? 5.1?
If my theory is correct you could then vent the resin lines and set your vf to whatever you like using the bladder and weighing the excess.
Of course I could also be totally, 100% wrong. Which is usually the case, or so she says. 
Here, I baked you a pie.
In the above engineering masterpiece you can see the “pie”. Black is mold, yellow is the fibers, red is bag one, green is bladder and blue is second bag.
I think the key to success with this setup would be to have air pockets (purple dots) to give the bag something to pull down on, otherwise maybe the bag and bladder are pulling themselves together like on a typical 2 bag setup.
I bet if you did this setup on a piece of glass it would explode when air is added to the bladder.
It will not explode. There are no forces working on the Mould. If you want to create forces you have to pump the bladder with more than the atmospheric pressure, and that will not work with a bag.
The other problem by reducing the first vacuum is that you raise your maximum porosity!
Than smaller your absolute pressure, than smaller the maximum mistake.
The maximum compression you can reach is the difference from absolute vacuum to atmospheric pressure, not more.
And your Vf depends on the ammount of resin in your laminate and to the pressure your laminate is compacted.
And than smaller the absolute pressure in the infusion chamber/ bag1 than smaller the voids by enclosed air.
I baked you a pie.
And it exploded!
Hence the reason I am a forum troll not an engineer. lol
Thanks for entertaining my stupidity.
I use bladders for mold work every work day of my life. I’ve made them with bagging film before. The bladders by themselves will fail at 3 or 4 psi. They can hold 100 psi in the mold because the mold supports the bladder and the bladder then only functions as membrane. In the above bladder the process the strength of the outer bag will determine maximum pressure that the badder can exert…which will be minimal.
Now, if the outer bag was replaced with a steel plate (or other strong/stiff material) and the mold can tolerate the pressure without distorting you can infuse through the inner bag and then boost the compaction with the bladder…been there done that. It’s the same as infusing through the inner bag and then boosting the pressure in an autoclave. If the vent lines are exposed to only an atmosphere you can increase the compaction and drive excess resin out multiple vent lines. You can also apply vacuum to the autoclave during infusion so the fibers aren’t as compacted making them easier to infuse then pressurize the chamber for compaction and vent the feed and vacuum lines outside the chamber. You need to use a really slow curing resin to give the layup lots of time to debulk before gelling takes place. Many people think a double bag works the same way. The big difference is the environment in a autoclave can be isolated from the atmopsheric pressure due to the walls. A bag cannot.
So I’m not stupid, I just need to change my bill of materials from plastic bags to steel plate.
The theory is correct and that’s all I was trying to get at.
I even made a 2 layer cake for you to show what I was getting at but it’s not needed now. I’ll give it to the old lady.
@dallas: What do you want to tell with your picture?
Lets go detailed:
You have an atmospheric pressure of about 101300 Pa
Pa=Pascal= 1N/m²
and if you have maximum vacuum, means absolute pressure=0 you have a load on your laminate of
101300N/m²
If you have a absolute pressure of 0.6 inHg = 20 mbar = 2000 Pa in your first bag the load is the difference between the ambient pressure and the absolute pressure in bag 1
101300PA - 2000PA = 99300PA = 99300N/m²
You say you want to reduce the vacuum in your first bag
So lets take you work at 18 inHG = 609mbar = 60900PA
Than your load with one bag would be
101300PA - 60900PA = 40400PA = 40400N/m² (that will never work well)
Now you add a second layer with breather material (makes no difference) and pull full vacuum after infusion:
lets take the 0.6 inHg = 20mbar. Than you have a load of 99300N/m² working on the second bag. The second baggig film compacts the breather with a load of 99300N/m².
The breather is pushed to the first bagging film with this load.
Inside the first bag is a absolute pressure of 18inHg = 609mbar.
60900Pa - 2000Pa = 58900PA = 58900N/m²
Its working against the direction of the first load
So we have
99300N/m² - 58900N/m² 0 40400N/m²
Oh thats the same load than with one bag and 18inHg :nuts:
Is that funny? No, it is simple physik.
Have you got a link for that study?
What do I want to tell with a picture? Exploding pie… Duh.
And the simple “physik” gets less simple.
The reason why double bagging works.
Navier/stokes
Single bag loses pressure value at Infusion. Two bag does not.
And dc if you want to show calculus and go into computational fluid dynamics on a forum be my guest… I’ll keep smirking and let you.
You can lead a horse to water but you can’t make him drink.
I am going out on a limb by saying this ddcompound and might be completely wrong. But you are selling and marketing a great product that works for VIP. Is this cause for your bias against double bag infusion? Simply stating pressure values is not physics. Nor simple physics as you call it. In either infusion process we can only achieve atmospheric pressure exerted on our laminates as a maximum value. It is proven that VIP loses compaction at moment of infusion because… The laminate itself decompresses with saturation of the resin. Vacuum is no longer keeping the laminate compressed. Double bag infusion is a vehicle for which vacuum can constantly work to keep a compression force on the laminate before during and after saturation has taken place. VIP does not allow this.
It is complete independent from our product! Double bag has nothing in common with our product! NOTHING!
Yes, laminate looses compaction after saturating because the resin is pressed in by atmospheric pressure! The same pressure YOU want to take to compress the laminate! So two, three or 100 bags does not change the forces.
The only thing you can change that is using an VIP Autoclave Process or mechanical forces for compaction.
A second bag is useless for compaction, and this is independent from the MTI hose. That works complete different.
The double bag DOES loss pressure during infusion. I just doesn’t loose as much because the inner and outer bag are trying to hold their form. It is the stiffness of the two bags being sucked together the holds the form. It is a complete myth that the atmospheric pressure can be maintained throughout the complete process. It is NOT atmospheric press that is maintaining the pressure.
Here are some quotes from page 6 of the double bag patent by Boeing: (Bold italics are from me)
High vacuum integrity with a double bag system of our design helps to yield high quality composites consistently with low void content, minimal surface porosity, excellent thickness control, and high fiber volume fractions. The double bag improves the stiffness of the bagging material to avoid relaxation behind the wavefront…
And later,
The inner bag vacuum level should equal or exceed the vacuum level between the inner and outer bags so that a pressure is exerted on the inner bag from the camber defined by the inner and outer bags. This situation occurs naturally when both the inner and the outer bags are connected to the same vacuum source. If the outer bag level exceeds the inner bag vacuum level, the inner bag can be slightly displaced with less effective compaction of the preform.
And later when talking about balancing the feed and pull-off rates:
Using this approach approach, the inner bag vacuum level is typically dropped from 29+ inches Hg to 22-27 inches Hg. Shortly after dropping the vacuum level, the bubbling will stop as with throttling devices. The problem with this approach is that the inner bag may move toward the outer bag because of the reduced vacuum. Movement reduces the preform compaction and ultimately produces lower fiber volume composites.
Boeing fully understood that preform compaction was ultimately determined by the inner bag and that the vacuum between the inner and outer bag created a somewhat stiff shell that reduced inner bag relaxation during infusion. Note that Boeing is using a breather that .20 thick. The compacted thickness of the breather between the inner and outer bags determines the stiffness of the inner/outer bag form that helps reduce bag relaxation. The thicker the breather the stiffer the form.
Reducing inner bag vacuum will reduce compaction that may or may not be able to be regained. The outer bag cannot provide compaction via atmospheric pressure nor can atmospheric compaction be maintained on the laminate through the whole process.
On the contrary explain why laminates using single bag VIP can have different thicknesses and not maintain consistent vf while laminates using advanced double bag infusion can and do on a continuous production basis.
Oh probably because the second bag does nothing because of… “simple physiks”. Right?
And your product will work and works well with double bag infusion… FYI. Its just not an all out can be used for all situations like with a single bag setup. I’m not at all bashing Mti hose because it is a great product.