I’ve been wondering why I’ve been having such good luck preforming the skins while others have been reporting pinholes and such. I always thought that the high pressure I use (22" mg) forced glass to lie flat, but now I think that there might be more going on. In one of our meetings at school we were arguing whether we should teach the kids about the Ideal Gas Laws (I know, it was a very boring agument). The laws talk about how pressure, volume, and temperature are related. It occured to me that when you bag on glass the resin is also affected by these laws. As you increase pressures the resin must either flow (increase its volume) or get hotter (increase its temperature). Both of these boards are very similiar (I build them to be as exactly the same as I could get them)
The only thing that I did different was to the skin on the board on the left was to go with formica-glass-balsa-plastic-shadecloth in the bag while all my other skins I went formica-glass-balsa-shadecloth. Both pulls went equally smoothly and the same pressure. Both skins are equally flat and smooth, but the board on the left has all those pinholes (by the way, its not as bad as it looks - I took the picture at the absolute worst camera angle). Here’s what I think happened - as you increase the amount of pressure you either change the resins volume or temp.
On my good skins the only way to draw a vacuum is over (and through) the balsa. At the edges of the skins the bag is stuck to the formica so that neither resin or air can flow through it. This means that the resin flows through the glass until it reaches the microholes in the balsa and then it is drawn into the balsa. This means that dry spots are eliminated and pinholes are filled in. The greater the pressure the greater the effect. By the way, if you also did the skin in a hotboat you would effectively be increasing the pressure or volume (more impregnation)
On the bad skin the plasic acted as a barrier that limited airflow
through the balsa. It was still able to draw a full vacuum
because of the shadecloth but the resin wasn’t encouraged to move
because of the added layer of plastic. This caused the skin to
get loads of pinholes and a few air bubbles in the cloth (which I’ve
never had in my previouse 5 skins) which proves that the trapped air
couldn’t escape. Since the resin couldn’t flow all it could do
was to increase its tempurature which worsened the problem because it
How much pressure did it take to get the flat panels to assume the concave deck contours? I’m very interested in what you guys come up with as I’m about ready to do some bagging myself.
didnt follow all the details but here are a couple of general thoughts…
epoxy resin hardens thru exotherm…it gets warm/hot no matter what, just differnet degrees thereof
when you pull a vac, liquids will boil at much lower temps, thats why your car’s radiator is pressurized…so that coolant doesnt boil as easily…with enough vac, water will boil at 100F
you proly know all this stuff anyway…nice work on them boards…i like your two board experiment…a worthy effort
Very cool! Write back with whatever you find out (good or bad). I think on on to something, but I’d rather find out sooner than later that I need to look into other causes.
John,
It was at about 5" or less that it fully followed the concave. Its really all about the orientation of the wood at the point of the conture. If the diagonal wood was to run over the hump the concave it would take more pressure to make the bend.
Meecrafty,
I thought that you would like the 2 board challenge. I tried to make both boards the best I could so that I could compare the two approaches (springer/nonspringer). I even went with the 4-way fin system so that in case I jacked up the fin installation I could rectify it.
Nice experiment. Don't know that the ideal gas law really applies in the manner that you've discussed it in this case. Remember, it only applies to the gas bubbles trapped in the resin and the gas in the balsa, not the resin. I think the big difference here is HOW you've applied the vacuum pressure. If on your non-glass side you had the plastic, then you've applied a uniform pressure across the part, but you've provided no direct pressure THROUGH the wood...the only way for the gas to move was towards the edges. By removing the plastic, you've allowed the interstitial space between the balsa fibers to act as channels for gases to pass through.
This means that all the little bubbles that you'd have otherwise up against the balsa, but underneath the glass, are pulled through to the back side of the wood. When you have the plastic there, you stop this from happening; the only place where you can have gas flow THEN, is along the formica...basically, in the resin flow from the bottom, along the formica, to the side...which is why you end up with all of the voids along the formica faced surface.
Ideal gas law is great for gases. Best be using Law of Partial Pressure when thinking about epoxy (2nd year physical chemistry). I can assure you; putting the epoxy under vacuum doesn't decrease it's viscosity, doesn't make it generate more heat and doesn't change the way the chemical reaction happens. It DOES change it's ability to penetrate surfaces and it's ability to release gases as the liquid doesn't expand under pressure, thus the trapped gases are subjected to the decreased pressure and as they're removed, the voids that are created suck the epoxy in. Don't know why you'd have ever put the plastic on the back side to begin with. Will say, that If I had the space though, I'd set up to do it exactly as you are...pre-formed skins, all the way. Just like making your own pre-pregs. Nice work.
Hey DanB, Great stuff!!! Just curious as I hadn’t heard of shadecloth being used before (didn’t find it on a search either), as a breather I understand but does shadecloth release? I see that you are not using it on the same side as the cloth but I am sure some resin gets through. Oh, and what type of shade cloth are you using?
Hhhmmm… The vac isn’t gonna pull air away through the balsa if plastic is in the way. It’s gonna suck around the plastic and side (maybe) and draw air from the balsa, THROUGH the glass. That’d cause bubbles.
Anyone see a flaw in that logic? If it’s right it means we always want to pull through the balsa pre-form, not around it.
While we are on a related topic… Has anyone used silicon spray for a mold release? And yes, I have read the Mold Release thread. Main thing I was wondering about the products mentioned in there (hell, and silicon spray too) is how well they come off at the blush wash stage.
Just curious as I hadn’t heard of shadecloth being used before (didn’t find it on a search either), as a breather I understand but does shadecloth release?
If it can be used in the VARTM process which will imbed it pretty good, it better release. Just make sure it is made from a plastic that resin will not adhere to.
Just make sure it is made from a plastic that resin will not adhere to.
Spray your mold release agent onto your shadecloth.
Absolutely! Unless, of course, you wanna keep your balsa dry…
Isn’t this why we figure out epoxy:glass ratios by weight (like 1:1)? I suspect we’ll never keep our balsa 100% dry and achieve a strong bond… It’s against the principles of mechnical bond adhesion.
Shade cloth is used to protect plants. They sell it at Home Depot (or anywhere). Heres what it looks like.
Formica is the stuff that counters are made of (on top of the particle board). I cover it with lots of mold release and it really made it nice. As far as whether to bag or not - I just got through hand glassing the rails and I spent the whole time trying to figure a way to get the board into a bag. It turns out sooooooooooooooooooooooooooooooo much better that once you try bagging the glass you’ll never go back. It took more resin to do 2 6" rail bands than it took to do the whole top skin and both layers of inner glass.
GreatWhiteNorth - I want to get back to you on the Pressure Law thing (I need to read up on it) but I’m short on time right now. I agree with everything you said - in fact I wanted to say it that way - but I’m not as articulate.
I tried the plastic layer because my shade cloth kept on getting mucked up with resin so I thought that I would isolate it with a layer of plastic (at the time is was under the impression that the vacuum just acted as a heavy press that basically forced the glass and resin into the balsa. After today my whole view has changed.
I don’t think that we actually want to keep our balsa dry. I think that a resing filled balsa adds very little in the way of weight but adds loads of impact resistance and eliminates those pesky resin bubbles. I was thinking that one way to strengthen the balsa would be to use a solvent/epoxy mix and paint it on the balsa, then put the glass on, then flip it over onto the formica then pull it to 22". I bet that jjp’s heal dents would be a thing of the past.
You’re on the right track with the plastic to keep your breather from sticking. What you can do is to use perforated plastic sheet instead. Just poke a bunch of pinholes in your sheet every 1" or so. This will allow the air to pass from the laminate into the breather, but will keep most of the epoxy in the lam, and keep your breather from getting stuck.
You can also use smaller strips of perf.plastic and breather material instead of a full breather to save time & materials.
When bagging some core materials (like PVC foam)to a table, it’s also necessary to perforate the core to prevent air bubbles. Shouldn’t be a problem with balsa though.
" I don’t think that we actually want to keep our balsa dry. I think that a resing filled balsa adds very little in the way of weight but adds loads of impact resistance and eliminates those pesky resin bubbles. I was thinking that one way to strengthen the balsa would be to use a solvent/epoxy mix and paint it on the balsa, then put the glass on, then flip it over onto the formica then pull it to 22". I bet that jjp’s heal dents would be a thing of the past."
That’s exactly it Dan. Precisely the point. You wouldn’t build a concrete structure by putting a layer of rebar and concrete, then just foam, then rebar and concrete again unless you were building it to withstand nothing but static loads. You wanna have your matrix going all the way through. With the greater impact resistance gained in saturating the balsa, you can for sure get away with lower density cores (.75# or less) as the surface will spread the load better. What you add in weight in the skin is easily removed from the core…and I’m sure you can think of ways OTHER than just going with lower overall foam density… You can also go with thinner wood and still obtain the same degree of stiffness thus saving some weight. I know that you have the “Burger Cut-Away” photo. Look closely at the thickness of the skins. I know you’ve examined it in regards to the springer, but look at how thin those skins are…WAY thin.
Just a couple of days ago Bert mentioned something about investing in lots of expensive equipment or using some solvent to help…that comment was directed exactly at wood penetration; an infusion process of sorts, with the solvent being there simply to reduce viscosity and enhance penetration. And who cares if you diminish the physical proerpties of the epoxy a little bit by adding a solvent? If it’s hard getting a jiggy through the balsa, I bet it’s strong enough to withstand a couple heel dings…
I’m not interested in building the next Sunova, but big thanks to Bert to helping us all along the way to building better equipment. You’re doing great stuff; wish I had the shop and resources you do. And those twin boards looks real nice…
I do love the toys. Its funny, but at the time I didn’t feel like I had that many tools. However, as the projects multiplied so did the tool requirements.
There must be other similar stuff used in the wood industry…
Quote:
Pentacryl Wood Stabilizer
Carving or turning green wood is certainly faster and easier than working with dry stock. But sometimes the extra work involved with slowly and carefully drying the wood to avoid checking hardly seems worth the trouble. Not any more! Pentacryl is a revolutionary new product that combats the negative effects of drying by displacing water and moisture in the wood fibers. Your wood will dry quicker and more evenly, and the Pentacryl residue even lubricates your tools as you work. This non-toxic solution doesn’t stain the wood or affect finishing in any manner. The recommended method of application is total immersion, but the product can also be brushed or sprayed. Compared to the traditional wax coating method, you’ll find the drying time reduced by as much as 90% with Pentacryl! On average, a green 6" x 6" x 2" bowl blank would absorb 2 ounces of solution, be fully saturated in a week, and, if turned thin, dry in about three weeks.
Also what if you “cooked” your balsa sheet on a heat blanket just prior to glassing it and also thinned out your epoxy using more Additive F and preheating? I know of some guys that glass with sanding UV.
Along with using perf release and blotters under your shade cloth I think that would do alot to increase penetration and flow through the wood.
Would an acrylic sheet or sheet of glass on ply work better than the formica since the formica is so thin?
And you doing 1/16" 1/8" or thicker balsa strips?
BTW when you get it, bamboo on the deck only and no probs with pressures just watch the fairing of the rails. You can prefad a deck lam with just a an 8" strip of the bamboo down the middle and balsa sides to make the fairing and blending into the wood rails alot easier but sand the finish off the bamboo first.
When you bag the skins this way do you get resin coming through where the pieces of balsa butt together? Or does how you join them previously prevent this? Also what thickness balsa did you use on the test boards as well as glass schedules and overall weight?
A little seaps through, but not much. The key is to get nice, straight edges - tape it together on the back - flip it over and glass the exterior - then flip it back over and remove the tape. I’m using 1/16" balsa with 2oz-balsa-2oz on the bottom and 2oz-balsa-4oz on the deck. I’m a little nervous about going so lite but at least I’ll have an idea about how lite is possible. I don’t know the weight yet.
