Doubts about vacuum levels 90 or 98 % ??

I will soon be doing my first large part infusion, a 33 feet boat hull, we will use vinylester infusion resin and have a big doubt in regards to the best way to do it, so far, and as far as I know, the best advise has been in order to avoid a too dry part has been to infuse at maximum vacuum levels and when part was infused reduce the vacuum level to 90% (or 27 inches of Hg), this can easily be done in small parts with only one resin inlet, and when vacuum is reduced to 90% the stack decompress a bit absorbing some extra resin into the stack making it a bit more resin rich and better looking.

My doubt arise as how this could be done in the case of a sequential strategy in a boat hull because then at the end of the infusion all the closed resin inlet tubes would have to be reopened for the extra resin to enter the stack and maybe this could cause uneven thicknesses, on the other hand if the hull is infused at a constant vacuum level of 90% since the beginning, the thicknesses are more likely to be equal BUT maybe as not all gasses have been removed ( there are still 10% of them )there is more tendency to getting some bubbles in the laminate, so what should be done to achieve best results? simply infuse at constant 90% vacuum and be done with it ?? or infuse at 98.5% and forget about the laminate being slightly dryer than if done at 90% ???????

What should be done ??

 

no one ???

 

Jigger,
The laminate decompress as soon as the resin hit it. This is due to the spring pressure of the laminate, typically around 150mbar for a boat hull type layup. This 150mbar spring pressure combines with the atmospheric pressure pushing the resin in, which allows the stack to increases in volume as the resin front passes. There are studies which have measured this increases in thickness. After the front passes, it slowly compresses again, along with many other dynamic forces interacting together under the bag. How far it re compresses, is rather variable, however the largest effect occurs when you close the resin feed after the hull is infused. When you close the feed, many things start to change, trapped air bubbles start to expand, the resin content gradient in the laminate starts to even out between the inlet and outlet, the front keeps moving if there is volume available for it to do so. It's quite fascinating all the things that are happening at once and how they effect results.

The common practice is to infuse at maximum vacuum. Then once you close the inlet, immediately reduce the guage vacuum pressure by around 150mbar to eliminate the pressure gradient largely caused by the laminate spring pressure. This will minimize void growth and permit best cosmetic results. A slightly higher resin content will also result, however it's only a difference of about 6% fiber to resin weight fraction.

 

Hi Groper, thank you for your time and advise, what you say about "after closing the inlet reducing vacuum levels", sounds and is easy when there are few resin inlets or only one, but my doubt arises as to how to do it in a large piece like a boat 33 feet hull, which has several inlet tubes feeding the several resin difusssion lines (spirals or enka channel ..), I understand that you suggest after all the sequential lines have been opened and closed in sequence, then to reduce vacuum levels by around 150 mb abs,
but if doing so, from where would the resin come to fill the new volume after this slight decompression ???

Shouldn´t then be necessary to reopen all the feed lines again to allow some resin to enter and fill the new volume ??

 

I'm not sure exactly what your asking. So I'll simply add this;

When you raise the pressure by 150mbar abs, this is only acting on the dry areas in front of the resin front. It does not decompress the wet areas, the vacuum is already gone there.

When you close the last inlet pipe, the resin keeps moving inside the bag if the laminate still has dry spots. You can observe this very easily in any infusion you do. So how can this happen if no more resin is entering the bag? Well, the inlet areas are resin rich, and near the outlet it is resin starved, so the excess resin near the inlet continues to migrate towards the outlet. The entire system under the bag is still elastic at this time. This is why air bubbles will grow at this same time. If the job is completely filled, and there is no high permeability areas left to fill, the elastic nature of the layup is somewhat disabled - there is nowhere left for the resin to move into. In reality, there is always some degree of elasticity, resin still moves through resin brake areas, just much more slowly. If you turn off the vacuum, and let it be, completely sealed, then there is no more problem, however this is extemely difficult to do on a large boat hull as there is always some degree of vacuum leakage so it's basically nessesary to keep the vacuum on until the resin is cured.

 

Hello Jiggerpro, if boats have been made from this mold before you may be able to identify the previous builder to determine the best infusion and layup plan. On the other hand, if this is a new hull design it would be wise to get an infusion plan developed for it. Polyworx sells both the software and also offers services. As for their services, if you send them a model of your mold they can provide an infusion plan that will suit it. The software analyzes flow rates based on the shape, materials, resin system and other variables.

http://www.polyworx.com/

Good luck

 

All three options will work so becomes a personal choice.
For me it would be a choice between infusing at max vacuum and leaving or reducing after impregnation. I do not like infusing at reduced vacuum.
Is this a cored structure with thin laminates?

 

Hi Joseph; Yes polyworks can do that for you, only problem is that they will charge a "kidney" for that, it must be the most expensive software services vendor in the galaxy.

Hi Groper, I do not pretend to doubt about you knowledge but IMO infusion is a more complex subject than it seems, In my humble opinion, what you said "When you raise the pressure by 150mbar abs, this is only acting on the dry areas in front of the resin front. It does not decompress the wet areas, the vacuum is already gone there" can not be completely correct, IMHO, when feeding resin inlets are closed increasing or decreasing pressure even when it is being done on the "vacuum side of" the resin front does have an effect of increasing or decreasing pressure on the resin behind the resin front as well because both sides are connected by a fluid which is the resin itself.

If this is so, again IMHO, when vacum is reduced compaction forces are reduced as well and the laminate must increase its thickness, wich should produce a resin absortion from some place to fill the new increased volume of the cavity, if all the resin inlets are closed except the two last resin feed lines of the sequential then all the needed aditional resin to fill the new volume must enter the cavity thorough those last two resin feed lines of the sequence and then IMO this might produce an uneven thickness of the whole laminate stack,

All this is theory, and I never heard of anyone having trouble due to this, so I will assume that waht must be done is:

-Infuse at high vacuum 95-98%

-Before the resin front ( six inches before) reaches the vacuum perimetral line, close the last resin feed pipe.

-Let resin reach and wet all the fibers to the edge and then reduce vacuum to 90 % to allow the increase in pressure compress any gas bubble into liquid form.

-Keep pumps running producing 90 % vacuum untill resin is well cured some 24 hours more.


Anything to do better ???

 

You are correct in that it is a complex subject. Difficult to fully appreciate in a few small paragraphs on a forum... But if you wish to dig deeper, then I'm happy to continue...

When the resin enters the bag, it has ATM pushing it in. And of course, at all times, there is 1 ATM outside the bag, there is an open circuit between the 2 whilst the resin line is open.

Once the resin enters the laminate, you have 1atm of pressure pushing resin in + 150mbar spring pressure combined. As there is only 1atm outside pushing back, the laminate relaxes and increases in volume as there is more pressure inside compared to outside. In this condition, it stands to reason that any air behind the resin front is now at 1150mbar. However Any dry areas connected to the vacuum are at almost zero mbar. Whilst the resin is mobile, so can be the air bubbles, and many will escape via their high mobility, which you can see during the infusion. There is more air than you can see however, such as in solution and also mechanically held between fiber tows, and also inside fiber tows, and also in surface porosity of core materials etc. it's very small and because they can be mechanically trapped, they are not mobile and cannot escape.

Now skip to the end of infusion where the resin line is closed. We then have lost the ATM pushing the resin in. Any entrapped air in the wet laminate, is still at 1150mbar. The resin front keeps moving as the area of higher pressure, nearest the inlet, equalizes. It pushes itself outwards and the laminate thickness reduces in unison. However, there cannot be a reduction in pressure without a change in volume of the now closed system. As liquid cannot be compressed or decompressed , it is the air which expands and increases the volume under the bag at the same time, the bubbles growth pushes the resin towards the vacuum and into the vacuum line until the pressure inside th bubble is the same as the ATM outside. Ie, voids will grow.

If you increase the abs pressure from zero at the vacuum pump, upto 150mbar abs, some air enters the vacuum line through the bleed valve connected between the pump and the bag. This increase in pressure, backs up against the resin front, and equalizes the bubble pressure trying to force the resin into the outlet. Thus, the voids will remain the same size, rather than increase in size.

 

Very interesting ............. so the change in volume when pressure inside the void is increased to 150 mb, does not need any resin, the micro gas bubbles take care of adapting the fluid´s volume to the new size of the container ....

We all involved in marine projects want the same I suppose a not too dry laminate as glaslike as possible and completely voidfree, maybe hard to get 100%, but to have any chance, the process should be thoroughly understood ..... theorys about what the fluids are doing and why is IMO, necessary ...

So Groper go ahead with your thoughts ..

 

they are not just "thoughts" - most of the information i have given thus far comes from reading infusion thesis papers. If you google, "resin infusion thesis" you will find so many papers on the subject it boggles the mind. But if you really want to understand it thouroughly, then you have no choice but to read them yourself - it certainly beats watching the idiot box of an evening.

In either case, you cant reduce the size of the bubbles that enter the bag, only keep them the same, unless of course you have an autoclave big enough for your entire hull. So the best approach in our situation, is to first minimize the quantity of air that enters the bag in the first place. If there is no air in the bag, then there can be no voids, ever.

So, first evacuate the bag 100% - the higher the vacuum, the less air is present in the layup to begin with. Hold it for at least an hour, preferably longer, to boil off moisture absorbed during the setup of the preform - all the glass etc absorbs humidity from the air whilst the mold is open. If you start your infusion at 90% vac, then there is already 10% of the entire volume inside the bag, which is ALL AIR! - Most of this will be pushed out as the resin displaces it, but remember all the little mecahically locked pores and holes? The resin bypasses over them and the air remains.

Once you have no air or moisture in the preform, the only air introduced is via the resin. (or leaks!) So preferably, invest in a resin degassing system that will work with the quantity of resin you need to infuse your hull. For such a large quantity, it would probably involve pumping the resin continously through a smaller vacuum chamber before going to the hull. I am quite certain companies like vacmobiles have systems available to do this.

So if you start with no air in the job, and introduce no air via the resin, you must end up with no voids and a very high quality finished product. Minimizing air ingress in the first place, is where your focus should be for this type of infusion your doing.

 

I will certainly read and google what you suggest, and read everything thoroughly ... studying is the way to "enlightment" ....... meanwhile, and in regards to degassing, as far as I know, no one does any degassing when the amounts of resin to degas are so large around 1400 lbs, what we plan on doing is to treat the resin carefully in regards to not incorporating gases when mixing the catalizer and let the resin " rest" for a few minutes before letting it enter the mould, specifically formulated for infusion vinylester resins are supposed to be ready for this process ...

I have always thought that apart from dissolved air in the resin, the solvents present in the resin where the source of the gases and thus most of the bubbles, which anyway do expand at the resin front and at the difussion medium (green mesh or similar) expanding exploding and ending being sucked by the vacuum pups, during my testing of products infusions is what we believe it has happened, and the resulting pieces where very satisfactory

IMHO the resin we are using which is a water like viscosity vinylester made for infusion, degasses itself when passing thorough the compacted fibers and the liberated gases are carried away thorough the difussion mesh towards the pumps and out of the laminate stack.

Still would be interested in a knowing an effective way to degas the resin in the large amounts used in large pieces like a boat hull ..........

 

To digress slightly,does applying such levels of vacuum cause any of the constituents of the resin to boil off?Or would the resin manufacturers tend to supply an infusion blend that doesn't contain high levels of volatile substances?

 

The foaming at the resin front is very evident. Remember tho, behind the resin front in the fully wet areas, the vacuum is completely gone! The resin is at atmospheric pressure... it is the force which is pushing it in. Therefore you do not get a self degassing mechanic for all of the resin in the job. And there is always the mechanical locking of bubbles in various regions of the layup. It simply cannot be eliminated, only reduced or minimized.