If the vacuum is to high what are the problems ??

What happens when you have the vacuum to high when bagging a core ?
what are the side affects??
what is the down side??

 

You run the risk of starving parts of the layup of resin with too much vacuum. It'll squeeze the stuff right out of the fiberglass. Side affects are weak laminates, delamination etc. Downside is you might have to start again.

It's easy to control the vacuum to optimum levels. Adjustable vacuum switches are easy to get and relatively inexpensive. Pretty cheap insurance to ensure you don't ruin a whole layup.

For small parts & hobbyists I used the vacuum switch setup from joewoodworker.com

http://www.veneersupplies.com/pages/DIY__Vacuum__Press.html

 

That is the problem with too much vaccuum. The laminate will be dry and have adhesion problems.

 

There some really good material on this site , thank you KnottyBuoyz for the post , its well worth a look at

http://www.veneersupplies.com/pages/...um__Press.html

Its interesting to slowly go through that site and look at the basic charts about pressure and the amount that gets exerted as the vac climbs up the scale .Plus there is a lot of other useful info in there as well about making and setting up vacuum systems , its all relevent to glassing as well .

Answers so far touch on resin starvation !There a few issues with this that need answers as well ,
but what about if you are using corebond ? what can be some of the problems with to much vacuum ??

 

Are you infusing resin to a cored laminate or you are just bonding a core to a cured laminate using vacuum to apply pressure to the foam?

 

This is just bonding to a cured laminate ! no infusion being used at all .

 

-20 to -26" of Hg shoud do. Apply peel ply on top for safety measure then use a plastic wire mesh (similar to an expanded wire mesh) Bleeder cloth, then vacuum bag. The plastic wire mesh is an insurance that pressure is evenly distributed. You may omit it for panels of about 2 foot square.

If you are using corebond, make sure the foam is pierced every 2 feet (1/32" to 1/16"holes). This is to check if there are no trapped air underneath. unfilled hole is an indication the core did not bond properly.

 

tunnels, why don't you just tell us.

But in general for core bonding you do not want too much vacuum, usually 20 - 50% depending on adhesive viscosity, type of foam (scrimbacked block cut, double cut, triple cut, thermo formed etc). Too high a vacuum can lead to a too thin bond line, so use just enough to expel the air and fill all of the voids.

 

Why dont i tell you !
I am interested to hear other peoples exsperiances and what they do . For everyone reading this that dosent know much about it they should be able to glean a wealth of info . Each bit of input is contributing to the a store of knowledge that we can all learn from .

 

yeahh, so true...

Well,

I know why!

But this time I will skimp on my nasty comments!

That does not mean I have forgotten this thread!

Regards
Richard

 

On bagging cores:

The aim for the vacuum is to get an even pressure on the core, keeping it in place, and pressing it into the bedding compound.
There is no real need to go lower then, say, 50% vacuum. If the core does not want to lie down at 50%, then it would have been better to preform the core material.
As for bedding material: Some glues have a minimum material thickness, which can be achieved by applying the material with a notched throwel. Use perforated foam and a light vacuum to get it down. This mimimum material thickness is especially true for polyesters, which tend to undercure at thin areas. Epoxy is much more forgiving in that area.

Vacuuming wet laminate (wet bagging). You can easily go to (almost) 100% vacuum. It is always fun to see people claiming it cannot be done, and al kind of problems can occur. How do they explain the use of an autoclave, which generates even more pressure on the laminate. You do need to adjust your vacuum materials for higher pressure, however. This is very true for the perforated film (less holes) and breather (should not collapse under the load). Vacuum film and even sealant tape are less prone to problems (I even have a customer running 4 bar @ 120 degrees C, and uses low temp sealant tape, which I only recommend for 40 degrees C and less...)
Oh, and for those that say only prepreg is used in autoclaves: A lot of times also wet bagged material ends up in the autoclave, and at 80 degrees or more, the prepreg resin is thin as water.

 

I have worked in fibreglass since 1972 and it has been a continuous learning process ,over the years i have worked with a lot of very clever people and aqumilated a lot of exsperiance and knowledge . I donot proffess to be an expert like some , I know what i know first hand from what i have done and the places i have been . I will share with and help any one in any way i can at any time , Travel and working in other countries has taugh me a lot and i have been able to share and hand on my knowledge and help a lot of people , most have become personal friends .
This is to back up my asking about vacuum and how much should be used !
Everthing reguardless of shape or size had a drawing and all the laminate information , stamped and signed by 3 people and we had to build to those Survey standards .
Flat panel construction can be quite interesting and a lot can be learned ! No two panels were ever the same as there were for differant parts of the construction process , Thickness of core and differant densities with differant types of glass being used not always the same both side as well !.
almost everything was sucked into core bond and so we all became very good and quick as time went on at refining the process to the last fine detail .
Because of size everything was cut and laid out totally dry before hand and then stacked on one end of the table including the pipes and vac hoses etc etc ,gauges that were placed at 2 meter intervals along the panels .
one side of the bag would be stuck down and folded back as well . then away we went peel ply laid and wet out then the layers of glass wet and rolled and left to harden . The core bond when left standing for 24 hours began to separate in the pail so had to be stired well and completely . Trowling core bond was my responsability as there was a trick to this . It was thought that using a notched trowl the core bond would squash down the pattern and fill in all the hollows the trowl left but on close observation in some places this doesnt happen completely ,so as the right quantity was trowled out the trowl was laid almost flat to flatten the pattern to a smooth surface . It was also found that the bleed holes in the sheets were not enough so we doubled them , yet another problem was if they were drilled the swarf could block the core bond from coming to the surface so was better to use a thing like a ice pick and force it through to end up with a completely clean hole .Wetting the back of the sheets is crucial and a generous coat of resin should be applied including the edges !!This helps the core bond to slip and slide and not leave any voids anywhere !!
Once bagged down and sucked into place the gauges were placed on top of the bag and sealed over a small cut ,along the panel and a very close eye kept on the process from end to end . There were on occasions 3 vac hoses fitted to maintain a constant equal pressure from end to end . The timers were set for 2 hours and at 10 minute intervals all the pressures checked and all readings written on the bag along side the gauges .
Beacuse of the size of the table there were sometimes had up to 8 differant panels being built at all differant stages and all differant layups all going at the same time . So there was no time to stand and pick your nose or go away for a smoke . Once started it was all stops out untill the job was completely finished ,tea breaks and lunch breaks came at anytime so we worked glide time . All panels were left over night to cure under vacuum and then released the following morning because of the amount of differant jobs going on all at the same time in other parts of the factory we had to be doing and making away in advance so there were no holdups or delays . I saved and collected all off cuts from all the panels and articals we made and progessively tore them apart looking for defects and if any thing was found we looked and tryed to find out what had happened and what caused it .
Every panel had 50 mm core and this is very easy to work with , later 30mm cores started to get made and then even thinner down to 20mm and even 15mm . Our process never changed and it wasnt till we got to make 20mm cored panels the all of a sudden massive problems appeared from nowhere . Between the core and the glass huge bubbles ! as big as a dinner plates all over the place not isloated in one place but everywhere . Mad panic !! what was happening ???? . for 3 days we had every expert in creation beating a path to our end of the factory, engineers from the university , laminate engineers from the differant resin and glass supplyers any one and every one all standing round scratching there heads and offering loads of completely useless information ,samples were taken and we even made completely new test panels with new batchs of everything , still the same problem huge bubbles . Then a quiet spoken chemist arrived and looked and smiled and handed me a plastic suringe filled 1/2 full of resin , he bleed off the air and sucked one very small bubble in then placed his finger over the end and gently pulled the plunger out ,at once the small bubble grew to twice its original size ,with Yet more pull and suck and the bubble became 10 times its origanal size ,Pulled as hard as it was possible it grew really big . I shock my head in disbelief !! a simple test and there was the answer !!
So we had the vacuum to high . i ASKED the question HOW MUCH VACUUM is enough ??so this problem will go away ?? Know one knew !! not one person could tell me all the brains of a nation had no answer !!
So i set up 6 test panels along the bench and did the same lay ups on each one and a gauge on each one ,
i dropped the vac down to 50% on the first panel ,then 25 %on the next and so on , then 10%, then 5 % ,then 2% ,then 1 % of vac once the panels were cured we relased everything and turned then over to reveil the finding first 50% no differance ,big bubbles , 25% bubbles a fraction smaller ,10% noticably smaller, 5% wow now things were really noticeably smaller!! at !% of vac which is frighteningly low ,Not a bubble to be found , i ground the core away to the corebond level still no bubbles I then ground carefully right through the core bond to the glass below and still nothing so this was the magic pressure 1 % . The 2% vacuum panel had very tiny bubbles just here and there , So one last try at 1.5% of vac and perfect !!! good bleed to the surface of the bleed holes in the core ,good bond not a bubble to be seen even with a magnifying glass.
All 20mm cores panels from then on were only pulled at 1.5% of vacuum and we never had a problem . The gauges we had were oil filled and the scale was marked from 100 being maximum down to 0
From then on i have been interested to know how other people get on and what do they do .
Even when using just 450gramm really wet csm and a 20mm thick core i never pull any more than 2 % vac .
One there is a good amount of the resin retained in the glass and cures well and properly ,
two theres always a good bleed of resin to the top surface through the bleed holes in the foam sheets .
Also when using balsa the same ,an excellent bleed up between the blocks to the top surface and cure of the resin is excellent as well .

Not everything was just flat panels ,we made anything and everything that came along .
A real brain teaser is origami fibreglassing But i will save that for another posting !! Its panels that fold in all kinds of shapes but made in one piece ????
WHAT THE HELL IS HE TALKING ABOUT THIS TIME ??
this will really get some people going and raise a few eye brows !

I had made a dvd of everything that went on during the testing with photos and video but it has been packed away along with a huge pile of photos . i am due to head out again very soon to yet another place where i can help and pass on what i know .

 

We all learn but sometimes in a different way.

We were making vacuum bagged prepreg panels with Hcomb before. There were always bubbles. Sometimes the size of a quarter, sometimes 2 feet x 4 feet wide. Not even the main factory could solve it and it was inconsistent. Sometimes its perfect, sometimes not.

We knew that there is trapped air. We suspected the resin is releasing gas.

We were bagging at 27 to 28" of Hg. The top skin has a caul plate of flat steel to have an even surface. There is peel ply between the prepreg and the top plate. Bleeder cloth in between caul plate and vac bag.

We had to suck out the trapped air.

Solution? Drill the top plate with very small holes at 12" interval. Perfect panels everytime at high vacuum setting. It has remained a trade secret until (now).

It became standard. We would pierce the foam when vacuum bagging it.

If laminating cured top laminate over a single sided foam laminate, better to use sandbags to weigh it down if you do not want to pierce holes on the top panel.

 

I have never used Honey comb on anything !! That was the reason for my ignorance to the question . Like all things there are tricks and do and donts and unless we ask we never learn. I have never had the use for honey comb anyway with anything i have worked on .

An add on to what i have already written .
A group of know it all contractors that worked in the factory next to where we were had a major problem on a big curved panel they had made and thought was finished ,the sheets of foam simply fell off when they released the vacuum off to remove the bagging and all the bleeders etc etc . The core bond was soft and could be scooped up in hands full of soft and mushy like sticky porridge consistancy !! First thought was no catalyst so lots a fingers being pointed and doors locked so no one would see what they had done , during the scoop up and cleaning i took one look and said with a smile the vac was to high !! There was a rush for there boss to the front office and a immediate storming of my department and asking what i meant about to much vacuum . I said i would reproduce what had happened so we could all learn a valuable lesson .

I made a envelope with a chunk of foam core ,catalysed core bond trowled on bothsides of the core and sealed inside a bag of peel ply and scrimm with the bag , a pipe and a hose coupled to the system as the vac went on resin began to appear almost immediatly and started to saturate the bleeded and even travel up the pipe and get to the hose after just a few minutes ,4 hours later i released the pressure off with everyone standing watching and there was a soft mushy sticky porridge same as what they had .
There was still a mystry as to how it had happened to them and not with the panels we had been making . As i have said i used to save all th off cuts and bits and tear then appart to look for problems during my findings i had noticed the 50 mm foam panels roind the very edge of the faom was pressed completely against the glass panel and there was almost no core bond at all but the rest of the panel has a half a mil to 1 mil thickness . My conclussion was the foam had sealed its self round all sides and even to the point of where the core bond came through the small bleed holes on the top of the sheet the peel ply would not let the core bond come all the way up and over the top , just a small amount the size of a 20cent coin but the resin would bleed out of the core bond to the size of a 50cent coin . THE way we set up our bag was with peel ply right over the edge out on to the table 50mm past the core and panel line then a bleeder then a scrim with a course weave to allow the vac to get right to the edge but not over the edge then the bag sealed to the table .

What the other guys had done was take there peel ply and scrimm completely out and over the edges of there core and about 75 mm out over onto the mould they had built ,then the bag was sealed .During its cure time i noticed the vac hose had a lot of resin inside it during the time it was under vacuum .
Vac .

Simply the peel ply had acted like a filter allowing the resin only to pass through and leave all the fillers and semi solids behind on the other side where it eventually had so little resin content it was impossible for any kind of cure to take place , so it stayed soft and mushy .

this would never have happened if just wet glass had been used but the resin content in the glass would be low and so under cure could have happened .

I have asked the question many many time how much vacuum do you use ??? The point to all this is to see if other people have had problems and never know what the causes were and hushed them up and said nothing . I have been lucky and when problems happened i took 2 steps back and searched for answers as to WHY it happened . When theres problems there are causes and if you understand the causes of these problems you can build to prevent the problems from happening in the first place tricks of the trade its called ! if we dont talk about them then they are lost and have to be relearned all over again by someone else so progress is very slow .

SO PEOPLE ,SHARE EXPERIANCES AND LETS ALL LEARN FROM EACH OTHER !!
just when we think we have all the answers the rugg gets pulled out from under and we come crashing to earth in a screaming heap .

 

First thing to learn:

let the core **** out, where ever possible. It is not worth to bother with it. (sundeck roofes excepted)

If you MUST use sandwich cores, go for Balsa or Kiri (the latter brings some weight in) and learn your lessons before building the sampan.

A proper cruiser design has NO need, absolutely NO NEED for a sandwich construction. (foam or honeycomb)

A racer must not be designed to last for more than one race, one could even use open cell foams.

A compromise is what it is, the worst of both worlds.

Vac bagged Honeycomb is a brilliant idea............






but far away from being sensible in boatbuilding. period!