infusing with contour balsa

Here is the boat, pump, resin trap, and the end product.

 

This could be moisture boiling off rather than a leak... If it suddenly drops then holds just below the water vapour temp for that pressure, then you still have moisture in the stack and you should keep the vac on for longer before trying again to get rid of the moisture. My conditions usually Need about an hour on vac to get rid of all the moisture accumulated in a fairly clean new materials job. A job like that, will probably take a lot longer for the number of layers involved and some of the existing substrate will have moisture in its porosity...

Nice job, must have been fun getting it all to stick overhead Did you use spray tack to hold it all up there or lay it all on the plastic bag and lift it up as a stack?

 

Hi Steve,
that sort of surface is hard to get the mastic tape to provide a satisfactory vacuum. The way around it is to use a double line of tape say 75mm apart with a spiral down the middle or tree tape. I use tree tape (hessian tape) as it lays flat. The vacuum perimetr is then connected to the vac pump. This means the job edge is connected to the pump and the vac perimeter is connected to the pump so any leak in the outer tape goes to the pump. Try it on a small job and you will be surprised at the better vac it gives. This approach is what a double bag gives you without an entire double bag. I also notice you are using tape to seal all those valves and plugs etc. I recommend you wet seal with resin sealant not tape. I wax fittings and use 5 minute epoxy for all hardware sealing. standard loctite liquid gas sealant is good as well. cheers Peter s

 

wow, that must have been a real ***** getting 25 layers up there! Nice job, that would have tested my patience for sure lol!!

 

Thanks guys, yes it was a challenge getting the stack in place but compared to doing it layer at a time wet, a breeze. The boat owner and crew made a plywood box the size of the repair perimeter and filled it with spray foam to make a rough female mold of the bottom, then carved out space for the plumbing. This was used to tailor the glass stack which was then removed, we then installed the consumables and then the stack reinstalled, we then offered it up to the boat, sealed the bag and pulled a vacuum. We could then remove the mold and then spent a lot of manipulating the stack inside the bag under low vacuum. This was really the biggest problem we had as with that many layers when you think you have it looking good then up the vacuum and compress the stack it all changes, ultimately though we got it pretty good and had no bridging in the sump root area. We did run 2 rows of mastic around the perimeter but not that far apart,and tried to leave a little slack in the bag between but I will have to try your method Peter, it sounds like a good plan. All threaded fittings are sealed with Teflon tape and thread sealant but for the penetrations into the bag we used mastic and in some we kept adding as we chased down leaks. I have some Airtec bag fittings on the order. There are a couple of changes I would like to implement in our process, first will be to use an absolute pressure gauge for setting up the bag and I would like to transition into working in only metric. Could you guys please lead me through exactly how you use the gauge as while ive seen lots of reference to them on forum threads ive never seen anyone explain how its used. German advance composites who i will be buying it from say its only used in the feed line for setting up the bag but I thought vacmobile plumb it into the resin trap manifold but im not sure about this.

Steve.

 

Hi Andrew & Groper - I've been quite on the project for a little bit. The infusion discussion has touched upon the issue of springback quite a bit so I have asked a lab to do some compression tests on dry laminate stacks to establish the internal pressure due to the springback. These results should be in over the next couple of days so I'll update you then. Regards Peter

 

Hi All - the results for the compression tests are in. I sent 3 laminate stacks to a company that has an instron machine. These were 5x, 10x and 20x 1200gsm E quad glass. From testing many types of laminates over the years I know that when we infuse csm and multiaxials we & others get 30% resin by weight very regularly. If I set up what I call an advanced bag I can get 25% resin by weight. The aim of the test was to determine if the mechanical pressure agrees with atmospheric pressure thicknesses and if the stack is compressed multiple times what happens. There were a couple of teething problems with the test because I asked the company owner to do it and he instructed a tech and in the translation I didn't get exactly what I wanted the first time around. So the stacks have been compressed 6 times not 3 but I think it establishes what I expected to occur. In summary the laminate stack acts like a linear spring in the range of 0.5bar to 1.5bar. So if your infusing and you know your typical thicknesses and laminates, their thickness should/will be proportional to the applied pressure. No real news there I think. The laminate clearly gets stiffer at about 1.5bar but we can't apply that sort of pressure with infusion (the test ran to the equivalent of 3bar). After 3 applications of pressure the laminate became consistent in its stiffness. ie the first compression cycle was about 30% less stiff then the 2nd and third. The second and third were very similiar indicating that the mechanical bedding in may be at max. After 3 load cycles if we apply 1bar pressure to the stack it results in a thickness equivalent of 25% resin by weight, which by experience is the lowest resin content acheivable using VIP. So if you either let the vacuum bed the stack down or encourage the stack to get thinner with a roller for instance 30-35% is the usual resin weight average and some form of debulk will get you 25-30% Rw. (by the way your Rw depends on resin density and resins vary from 1000-1200kg/m3 the resin I usualy use is 1000kg/m3) I'll add the spring stiffness in my VIP theory and see what happens. For the less technical out there, since for every action there is a reaction and 1bar is 10T/square metre this means that when you pull the bag down the stack is pushing the bag out at 10T/sm which is quite a big load trying to make your job thicker. Once the resin fills the laminate the pressure in this area is not vacuum but closer to atmospheric so if there is any degassing or leaks it means the volume/thickness changes accordingly. The volume does not change usually as resin is incompressible and as the job fills it creates a hydraulic lock but leaks & microleaks work against us and the bag "inflates". So pay attention to perimeters and make them as big as possible. This improves the vacuum to job size ratio. The perimeter also clamps the bag down stopping membrane action in the bag helping a little bit more. So on a practicale level we are either establishing a ~30%Rw thickness and filling at that thickness or we are establishing say a ~25% thickness and as we fill the job is expanding to ~30% thickness. Cheers Peter S

 

Can i make one correction Peter, you said -

When the resin line opens, there is an open pathway of very low resistance between the resin bucket directly into and under the bag via the feed line. Thus you have full atmospheric pressure under the bag at this time - which is the force pushing the resin in. You ALSO HAVE, the laminate and preform spring pressure acting in the same direction. Therefore, you need to SUM the 2 pressures inside the bag, combine the other dynamic effects, and you will get a NET INCREASE in preform thickness at this time (when the resin front reaches the location we are considering).

After the front passes, i dont profess to know all the dynamics involved, however i often observe voids forming above the flow media after clamping the feed - afterwhich, the dynamics change again. It is at this time, that the reduction in vacuum intensity can reduce the growth of voids... Again, this is real world observations by Andrew K and myself. Ill leave the explanation up to you, should you feel the need...

 

I observe the same thing, when the resin port is closed outgassing starts, if you crack the resin port open and allow flow the outgassing stops. I hope to be able to explain this eventually but its a kind of negative pressure hammer. The inertia of the flow creates a pressure drop when the port is closed. This pressure drop results in the outgassing. The pressure in a flow situation is the sum of the dynamic pressure ansd static pressure, when the flow stops it reverts to the static pressure which is lower then the dynamic pressure. cheers

 

Well some of the great minds postulate that it is also caused by the spring pressure differential. That is, after the resin enters the bag, the dissolved gasses are at atmospheric plus spring pressure. If we assume a spring pressure of 150mbar, then what this means is that the dissolved gasses are at 1150mbar absolute (sea level infusion). The bubbles that are mobile can escape, and will do so during the infusion. There are other bubbles that cannot due to being mechanically trapped, and still other gasses that are still in solution. All of these gasses, are at 1150mbar so long as the feed is open.

After clamping, now things begin to change. If the vac is kept at 100%, The laminate will start to compress and get thinner provided the entire system is still flexible. Resin brakes etc will impede this scavenge of excess resin. But assuming the resin is still mobile, the laminate thins out. As it thins out, the pressure inside the bag changes from 1150mbar down to 1000mbar, and equilibrium is established on both sides of the bag. During this equalisation, the absolute pressure drops, therefore any remaining gasses will expand. Some will come out of solution, and bubbles will grow larger. Provided the resin is not locked, the expanding air pushes the resin further still, but we now have larger voids in our laminate compared to before clamping.

Additionally, Apparently the gas expansion is not linear, but more so, so presents as a rather large problem.

Now back to the real world, a combination of details, can minimise the growth of the voids and is how a practical infusion worker minimises the negative effects. Resin brakes is one way of disabling the mechanic by which the resin is still mobile. Others switch off the vacuum and clamp both ends, yet still others reduce vacuum to ~85% at the time of clamping to eliminate the differential pressure. All these methods achieve the same end, to reduce the growth of voids after clamping.

And As I'm sure you know, the best method is eliminate the air from the resin in the first place... no air in the resin, then no air under the bag, then no voids to grow...

 

On another note, I'm seriously considering infusing a 12m wing mast for a catamaran we just bought... It has no rig at present, but I want to put a rotating stayed mast on it and the aluminium mast suppliers seem rather expensive... For the same money, or less, I could make my own...

I want it to feed very slowly, probably no flow media. Flow distance would be less than 300mm. I'll need to source a good resin for this somewhere too. Long gel time and post cure.

Any tips?

 

This is what I use, up to 2hr. pot life and is also used for big wind turbine blades.

http://www.carbonfiberglass.com/Res...n-Epoxy-System-Slow-Gallon-39oz-Hardener.html

 

Hi Groper - do you want to do it in carbon or glass? ATL can provide epoxy with gel times up to 16hrs or indefinite if you want. When I was doing superyacht booms we used an ATL resin with 24hr gel time at 20deg C. But we put it straight in an oven when filled. cheers Peter s

 

Carbon of course... ATL are the only place I know of where to get this type of resin, is there anywhere else in aus?

When designing it, how close does one get by sizing the wall thickness the same as an aluminium mast, then letting the lower density of carbon/epoxy save the weight?

What about the mast bend? How to work out optimum pre bend ? Or is it not worth it and just build a straight one?

 

Lavender have a long gel time infusion epoxy, and Ironbark Composites have a very, very long gel time epoxy (24hrs plus from memory). For wing masts they get too stiff to prebend unless you mean to mould the shape in? These days a straight mast with a square top is the norm. If you want a rig thats "automatic" you will need to transition to a small top somehow. This is rarely done except with skiffs. Re: Al vs CF depends on what you want. A typical mast laminate can range from equal to AL in stiffness (70GPa) to say 90GPa using std modulus. I'd shot at the 70-80Gpa range. If 80 then size to size you will be 1550/2700= 43% lighter. This will be a 70UD and 30% DB (by weight) combination. But a thought; a wing mast is geometically big and you can build a glass mast that is nearly as good for much cheaper. Since you are doing very good infusions I'd expect you can get 35Gpa from a glass laminate. So size for size 1800/2700= 33% lighter but not as stiff. But since the Al mast is usually designed at SF=3+ but the glass mast is 3x stronger then you can build a glass mast much lighter then the alum mast but not as light as the CF. Depends on what you want cost vs performance vs weight. If you get some S2 glass then you can build a 45Gpa laminate and it will be very light and cheap compared to aluminium, but again not as light as the CF (but much cheaper) So since you are building a mould you could do a glass one as a trial then graduate to carbon as the ultimate? Cheers Peter S