infusing with contour balsa

I have a large swim platform to build and plan on using 1" contour balsa as the core even though i will be making a flat part on a mold table because i have it in inventory and want to use it up. This will be my first infusion and of course i have a few areas im unsure of which i will list below.
1/ since contour balsa is just glued to scrim on one side there are all the spaces between the blocks for the resin to bleed through to wet the bottom(outside) skin, is this enough? I think with foam core sheets for infusion have holes drilled on about 20mm centers which is a lot closer than the blocks in the contour.
2/ The platform is a fairly typical one with a 100mm turned down edge so i will be making a buildup on the table to create this, so im trying to decide if i should run the core all the way to the edge or if i should hold it back and taper it. If i were hand laying it, for this part i would run it to the edge but for infusing it im wondering if i would get the resin to the corner of the bottom skin.
3/ I will need to lay a lot of extra layers to build up the thickness of the turned down edge which will just be strips, should i include these in the dry stack or maybe being my first try i should wet lay them later just to keep it simple?
4/ The platform will be about 13ft x 6ft+ and i was planning on 2 feed lines running lengthwise, about 2ft in from each edge, or should they be closer to the edges since the 2 vacuum lines will be on each long edge on the table and the resin will flow out toward the edges.
Ok, as i was writing #4 i confused myself, im now thinking i should have a feed line in the middle and one each side of that.

Steve.

 

Whilst ive never infused balsa, i can imagine that the core will soak up HEAPS of resin as the vacuum will draw it deep into the balsa...

1. With a good low viscosity resin (under 200cps), you can space the perforations at 2inch offset centers without problems. Beyond this, is asking for little dry spots centered on each block.

2. I would stop the core just before the turndown with a 45deg chamfered edge. Bring the top and bottom lamiantes together and take the turndown in solid glass - infuse it all in 1 shot of course. You could put another strip of core near the bottom of the turndown if you like, it adds rigidity.

3. Infuse it in 1 shot.

4. Totally depends on your resin viscosity, flow media, and temperature. You should know how far your resin will travel and at what speed so that you can plan your strategy around your available pot life. I see no reason this couldnt be done from a single feed, and a single bucket of resin - its not that large. But this is totally dependant on being able to flow 3 feet each side from the center, before the resin in the bucket exotherms and tries to catch fire - yes its happened to me before

So you need to a small test peice with the exact same core and laminates to determine how long it will take- then determine if your resin has enough pot life to make the distance. If you cant make it, then you can do a fish bone pattern which will fill more quickly, or a sequential feed like you suggested.

Example of fishbone pattern,


Or sequential feed;

 

Thanks Groper, I will be doing a couple of test panels first which, assuming they workout, will be part of the boat, they will be about 2m x 1m with the same core but a little different laminate but close enough I think. I will be using VE infusion resin and will look up the viscosity. My thinking with using 3 parallel feedlines is that it breaks it up into 4 x 20" panels which should wet out fairly easily. I was thinking that one line in the middle would have to draw the resin about 40" out to each edge of the platform including the turndowns which may be too far,i don't know, so I thought it would be safer to break it up into narrower panels. Actually we havn't settled on the width and it may end up wider as it will be carrying a good sized RIB. Is there any disadvantage to more feed lines?
Another question is that this thing is going to be raised and lowered with hydraulic arms so I plan on putting in Coosa board inserts in place of the balsa where the arms attatch so I will need to drill those as it doesn't come infusion ready, would holes on 1" centers be ok? and should I join them with a shallow groove on the bottom side? Lots of questions when you've never done it.

Steve.

 

The gaps are the most weight inducing feature -

"but he adds that in tests where core with grooves on both sides was infused between sheets of plate glass, the core’s weight was increased significantly. He estimates that even knife-cut core (thinner grooves) increases in weight by 10 to 15 percent "

from

http://www.compositesworld.com/articles/aiming-infusion-at-the-application


Just for reference, a lot of people assume that Vynilester is as good to use as Epoxy for infusion, but since you need low viscosity for successful infusion -

'Cocquyt, , offers a caveat: “Vinyl ester and unsaturated polyester resins do become brittle when you add large percentages of styrene.” He says that adding diluents to epoxies does not lower elongation at break very much, but it does reduce their heat deflection temperature.'

 

Steve, I think the tests are a really good plan, I'm not skilled or experienced at infusion although I did a balsa panel at the FGI lab with one of their techs.... where everything was a bit wrong for the shot including out to in resin supply, coupla leaks, resin kicked b4 full wetout, very informative at the time.
One thing & remember my caveat on infusion experience... I'm not sure with contour balsa that the contour cuts can be relied upon for resin spread especially on a flat panel, the contour balsa I have the end grain is simply split & a near perfect fit when laid flat, some one with experience will know, maybe another strategy like side to side might be better?
Jeff.

 

Jeff, see, this is exactly why I started this thread, of all the threads ive followed everyone is using foam, don't get me wrong, I would rather use foam but I have all this balsa that needs to be used up. I know lots of production boats use contour balsa with infusion but it is always curved so the cuts are open. I plan on doing a test panel without gelcoat on a sheet of glass first so I can see how the resin flows on the mold side before proceding with an actual part.

Steve.

 

If the gaps between blocks close right up when layed flat, then you simply need flow media over the top to do the same job of distributing the resin. You will still need some perforations to allow resin flow to the tools side however, this may also be a problem if the gaps close right up.

The test peice only needs to be very narrow, but it does need to be the same flow distance in order to guage the flow speed properly and determine you feed line strategy.

I used test strips only 100mm wide x 1000mm long. I found with 200cps resin, i could reliably flow 900mm within my pot life time, in my ambient conditions. This could be extended by changing the flow media to a faster type however. You need to remember that the pot life doesnt scale linearly either, as a large amount of resin will kick sooner than a small batch - something to be aware of...

 

[

Just for reference, a lot of people assume that Vynilester is as good to use as Epoxy for infusion, but since you need low viscosity for successful infusion -

'Cocquyt, , offers a caveat: “Vinyl ester and unsaturated polyester resins do become brittle when you add large percentages of styrene.” He says that adding diluents to epoxies does not lower elongation at break very much, but it does reduce their heat deflection temperature.'[/QUOTE]



Adding diluents to epoxy reduces its physical properties rather quickly, and to reduce it to viscosities that are good for infusion can about destroy it.
VE’s and polyesters tend to do much better in retaining physical properties because not much, if any additional styrene is added. When the proper amount of styrene is added to VE’s and polyesters for them to cure correctly, the viscosities can be rather low, this is why thixitropes are added to bring the visc up to workable levels for hand lamination. Without these added thixitropes the visc is in the range of what’s needed for infusion.

If you started with a hand lamination VE and needed to reduce the visc for infusion, then yes the physical properties could be affected.

 

This is my fear and why I was asking about needing to drill some holes and not relying solely on the gaps between the blocks. The resin I will be using has a 25 minute gel time and 275 cps at 77degrees F. It will be a challenge to get the temperature but fortunately with VE i don't need to maintain it for as long as epoxy. I think that doing the test on glass will tell me if the resin will flow through to the tool side ok.
My finished part will be about 4m x 2m, what size would you do the test panel?

Steve.

 

What glass do you plan to use, some flow very well and will need no additional help to wet out under the balsa, others will restrict the flow.

 

I will be using a 3/4 oz mat skin coat and then 4 layers of 11oz uni e glass each side of the core at 0 - 90 orientation.

Steve.

 

Well if you wanted to put a single feed down the center of the real part, then you need the resin to flow 1m. So your test piece needs to be 1m long but fed from 1end rather than the middle to keep the same flow distance. The width doesn't matter unless you need to test how the contour balsa will wet out and pass resin to tool side - which is a good idea in this case. So 1m x 1m will suffice, but fed from 1 edge not the center- vac line on opposite edge. Make sure you chamfer all edges of the core at 45deg so it won't racetrack around the edge of the panel.

 

Thanks Groper, I think I will have to do the test panel with 20mm balsa rather than the 25mm I will be using for the part just because of having enough materials, im sure this will make some difference but it should still tell me how the resin flows through the core. I do plan on 3 feed lines on the part from 3 separate buckets just to be safe. Is there any disadvantage to this approach that im not seeing?

Steve.

 

20mm or 25mm balsa wont make a difference...

3 feeds and 3 buckets just adds complexity... on a small part like this, it really isnt nessesary. The more complexity you have increases the chances of stuffing something up, like allowing the resin hose to come out of the bucket and sucking in air, forgetting to clamp it off in time as your preoccupied watching some other detail, knocking a bucket of resin over spilling it everywhere - im trying to think of something else ive done but i think thats it for now

 

Balsa will not soak up lots of resin even under full vacuum, but the gaps between the blocks will add weight. Have only infused balsa a couple of times, when using perforated only core I allow for 600gsm of resin for each side. A lot of this goes in filling cracks and imperfections rather than surface fill.