Hollow Carbon and Balsa Skin construction

Matt S talked about issuing a challenge to build an Aviso style hollow carbon board. The Aviso uses a mold and an inflatable bladder. I just can't justify a mold for a single board, so we talked about shaping a core eps or cornstarch or something that could be dissolved once the lamination had cured.

My original tests on disolving EPS weren't promising. Acetone and Gasoline left a residual goo that wasn't a viable option. We had lots of suggestions and finally found a study that showed Xylene was the most effective. In a test this morning, I found that it liquified the EPS, so...we have some promise.

The construction I was contemplating would be a schedule of:

Carbon
Balsa
Carbon
EPS to be dissolved

In my stidies I have found that any foam whatsoever deadened the responsiveness of a Hollow Carbon board. What I'd like to achieve is a complete elimination of the EPS used for the core that is shaped and laminated. Then drill a hole for a vent, pour in the Xylene and hopefully dissolve the EPS.

That also meant that I had to seal the epoxy with spackle to avoid epoxy seeping into the EPS. I decided to set up a test panel to see if I could completely dissolve the EPS.

I started with a chunk of EPS and I sanded it to replicate a shaped blank. Then I coated it liberally with a lightweight spackle.

Next I cut 4 sections of carbon so that I could laminate them on either side of the balsa.

I also cut 2 srape pieces of e.balsa that are laid on top of the saturated carbon.

Look! Smores!

Finally I laid the last two pieces of carbon on top of the balsa on either side.

I made up a "bag" from a 1 gallon ziplock bag. The vacuum takeoff is already inserted.

Next I stuffed the whole mess into the bag, and taped a bit of breather over the vacuum takeoff to be sure it didn't seal itself to the opposite wall of the bag.

And the vacuum pulled.

This looks to be a very promising approach. I've been thinking of similar approaches but I'm still working on the recycle board and I have two long boards on the way.

I’m glad to see you’re still using w.balsa

It's under the electric blanket now, so hopefully in the morning I will have a bonded structure. Then I can attempt to liquify the eps inside. I'll drill a hole through the carbon and gradually pour Xylene through the opening. The goal will be to see if we can wind up with two separate skins without any trace of EPS.

We can't get w.balsa on the west coast, just the e.balsa. You'll have to share your approaches as you build.

That's funny, out west you get e.balsa and if you call Ohio east we get w.balsa

No Ed, it's e.balsa everywhere. You just don't know foam.

Test panel out of the bag and ready for...testing

very cool jeff! even if it doesn't work - you win! but looks very promising...

did you lap the carbon around one side to simulate a rail structure? just curious. when the foam melts will the sandwich collapse on itself?

Hey Matt,

I win! No, I didn't lap a rail at all. What I wanted to be able to do, with this test is to pull it apart or more acurately have it fall apart and examine the two sides, but also to watch the dissolution process as it is happening. I am afraid that the spackle won't keep the epoxy out once wetted. Further, I was afraid that I couldn't cut into the shell if I lapped the rail to examine "what went wrong" should that happen.

If this works, my next test will be to completely seal a cube of EPS, then see if I can liquify the carbon through a small hole. I know that I can liquify the EPS when I bath it in Xylene, but I can't imagine that Judy would be thrilled if I came home with 100 gallons of Xylene and an above the ground pool I'm interested to see how the EPS will react when it is completely enclosed...also how it reacts with multiple "passes" of the solvent. Maybe it does require being submerged in the stuff - so building a "pool" and submerging the finished board is necessary?

So...it still looks promising, have to see what the dissolution test produces this evening.

jeff, your gettin yourslef in trouble with all these ideas cuz now i want that!

Johnny you're funny. With your skills you can ride anything and make it look like it's the best board around.

I'm not a big fan of Carbon - second on my list of things I don't like to work with just below wood.

What I found interesting is that I had this misconception that Carbon was stiff. I always have the vision of those 1/8" thick laminations on bike frames, but single or double layers are still pretty flexible...10 layers is a different story. Combined with e.balsa though, it's a interesting combination.

I'm not sure I fully understand what makes fiberglass or carbon fiber materials strong. I know that the tensile strength of the fiber is quite high. Setting up a fiber material in a structure so that the tensile strength works for you must be key.

Imagine that a thread surrounds the perimeter of an object. Pulling on the thread from any side should be like pulling on all sides, if that makes sense. I imagine that the resulting force on all sides takes advantage of the tensile strength of a fiber.

On a beam the shear strength is a kind of comparison of the stress on the top and bottom layers. If you’re applying downward pressure the beam bends a little. The arc on the out side of the bend is larger than the inside of the bend. The top layer must be under tension, so good tensile strength must beneficial.

So is there such a thing as e.carbon in the Walker dictionary?

I'm sure given enough time, there will be e.carbon.

Back to the test results.

I drilled a small hole, about 5/16" in the surface of the "deck", I had to double it as it just wasn't large enough to flow the Xylene.

The top and bottom separated fairly quickly, but you'll notice that a hole was created as the Xylene was poured in the top. This is a picture looking up at the bottom of the top skin.

The bottom layer was more fully dissolved as the EPS in that area had more exposure to the xylene.

It took a considerable amount of Xylene to dissolve the EPS - I'm estimating the same amount by volume. I'm think that would be several gallons to dissolve an EPS core. With continued exposure the dissolution process slowed rapidly. There was an inital reaction and then a very slow continuation. In the pool of left of Xylene is some undissolved EPS. I believe most of this had some resin contamination from the sides.

The good news is that with additional exposure and some massaging all of the eps was removed.

The white residual is the spackle. That seemed to be coated in the epoxy and once wet did leave a white residue in the Xylene.

I am thinking that if the board could be suitable filled with Xylene and the gently agitated perhaps with marbles? or something inside the board that could also be removed after the foam was dissolved/disturbed.

So...first two tests, I believe are passed.

Hey Ed,

I didn't mean to ignore your tensile strength post, but wanted to try and grab a picture first. I do believe that most agree, the force on the top of the board is compression (bottom turns or just placing weight on the deck) and the bottom is in tension.

Hopefully this graphic will upload.



Fiberglass is twice as strong in tension as it is in compression and the typical build of a board will attempt to balance this with extra layers of glass on the deck. Not uncommon to see 6 oz bottom and a 6/6 on top.

My understanding is that tensile strength is not real important in surfboards. Actually not in most composites and yet this is the standard most fabric companies use in gauging strength. Compression strength is what matters and this is difficult because of two things. In glass composites, compression is usually only half of the tensile strength. This leads to difficult imbalance in the strength between the side of the board that is in compression and the side that is in tension.

Carbon fiber raises compression significantly and carbon composites are more equal. This is the reason carbon composites can be made significantly lighter. Most fabric companies won't give compression stats because the cloth has to be laminated before you can compression test. This tests the resin as much as the fabric. Stiff resins should be used with stiff fabrics. Flexible resins should be used with flexible fabrics.

I pulled the trigger on this project. I just finished ordering 20 linear feet of 8 oz 3k carbon and about 10 linear feet of 3/16" end grain balsa mat.

For those that are familiar with the balsa - it's a mat that is comprised of small rectangles of the end cuts of balsa boards. The mat is then sealed and laminated.

A picture I grabbed off the 'net:

I am thinking through the build sequence. I will shape the blank, seal the whole thing with spackle and then I want to apply a mold release wax - very soft - over the spackle. Then I want to completely enclose the blank in carbon. I believe that I will do the rails first with tape, then bag the top and bottom skin which will be carbon on the 3/16 EGB. Next I will adhere the e.balsa rails and then I will sand and shape the EGB or is that EGe.B. .

Once all the e.balsa is shaped and sealed I'll bag the external layers of carbon.

With the carbon all cured, it's drilling time! I drill a hole for a vent and start pouring in the Xylene. I believe that it will take quite a bit of time and some manuevering of the shell to get all the EPS out.

Stayed tuned!

jeff, have you thought about making a mold out of fiberglass, then creating each half of the carbon fiber board seperatly then puting the board together using carbon tape along the rail?

Yeah Johnny, that's the standard method like Hydro Epic used. Making a mold for a single board is pretty time consuming. Gel coating a board, 10 or so layers of chopped strand mat, building support for the forms and then the seams become the weak link, you have to build in an overlap of some kind. My understanding of the hollow construction is that typically when they fail it's along the seam.

If I intended to make a bunch of these, rather than just one, I'd build a two piece mold, like you suggest. The one serious advantage of the female mold process, is that the parts come out finished. Most manufacturers spray a clear coat on the mold surface - polyester or acrylic before laying up the fabric, so when they pull the parts out the outer surface is clear and shiny!

hmm sounds interesting. so i cant wait to see this carbon board. i still think the carbon wraped balsa rail is the ticket!

You and your carbon rails! I've still got that Firewire knock off that I'm going to finish. I didn't like the rocker on that prototype, once I get that figured out I'll have those carbon wraped rails.

Sssssooooooo...I have a bunch of carbon. No end grain as yet, but that is to be expect as it is e.balsa.

I want to try building two boards. The hollow balsa carbon board discussed here, but also a very stiff carbon sandwich board.

I am in new territory here with the carbon. The research that I have done shows that Carbon is anywhere from twice as stiff to 5 times as stiff as comparable weight e-glass, everything else being equal. So, if I had 4oz e-glass and 4oz carbon and laminated them with the same epoxy matrix, the carbon should be between 2 times and 5 times stiffer than the e-glass. 200 to 500% seems like a huge range, so I am leary of the final results.

I have had trouble locating carbon in anything but some selected weights. I could get 5.7 oz carbon just about anyplace, but 8 oz proved to be more of a challenge. Although I did finally locate it at Soller composites.

I want the hollow board to have a barrier layer of carbon that will completely isolate the endgrain balsa. My concern is when I attempt to dissolve the EPS core I don't want the Xylene to migrate into the balsa.

I am not confident that I can bag the bottom layer of Carbon and endgrain balsa as I did with the corecell and divinycell composites. So, I think that I will attempt to bag the carbon to the bottom without the endgrain layer and then return and bag the endgrain later. The downside to this is that I won't have a chemical bond between the carbon and balsa, it will be strictly mechanical with a separate layer of epoxy attaching to the cured epoxy on the carbon.

What it will allow me to do is piece together the carbon inside layer to make sure that I have a complete barrier such that no Xylene will migrate to the balsa. I'm not sure how meaningful this will be in the end.

Anyway...hollow carbon is up next.

The 5.7 oz Carbon. This seems to be readily available, I even saw it at TAP on the rollers.




This is the 8 oz Carbon.

If I take a conservative estimate and use a "twice as stiff as e-glass" estimate, the hollow carbon board will have 2 layers of 8 oz per side, which would be close to almost 6 layers of 6 oz e-glass or 8 layers of 4 oz. The other extreme would equate it to around 16 layers of 4 oz e-glass and a shade over 10 layers of 6 oz. All of those numbers are per side, so it would seem pretty substantial when they sandwich the end grain. Time will tell!

Finally got a chance to start work on the hollow carbon balsa board. My plan is to

1) shape the core/blank.
2) seal it
3) laminate to inside layer
4) epoxy the balsa
5) shape the balsa
6) laminate the outside layer
7) drill a hole
8) disolve the core with xylene

Towards that end I shaped the core this evening. This part was a bit tough for me. I had to calculate the dimensions and subtract the thickness of the Balsa rails and skin that are between the layers of carbon. Further I had to square up the rails and bring the deck down to the thickness of the inside of the rails. This is almost 180 degrees different to the way I am accustomed to shaping.

I decided on the outline from Compsand #5. I really like the way that outline rides. I am using the rocker from a TWP Bullet, which I had already hotwired into a core.

Most shapers have template guides cut from masonite. They can then use those templates to improves various shapes. Matt S and Ed S used computer generated shapes that are transferred to paper. I free hand most of my shapes and then make them symmetrical on a centerline. Once I have it, I cut it out of foam and then use that to trace on to OTHER foam that I will shape. This is my outline for compsand #5:

In the picture above you'll see the center line. I wrap the CL on the tail also. Next I draw a centerline on my blank/core. Match the centerline on the blank/core and the trace the outline.



The blank/core with the outline traced.

Oops I didn't upload that picture. The blank/core with the outline traced:

In Ed S's longboard thread Ed is using a handsaw to cut his outline. I'm lazy I use a jigsaw:

I jiggy very close so that I don't need to use the surfoam or a planner to trim touch the outline up. I cut very slow and carfully.

Outline cut:

I had to mow a TON of foam, the tail had to be reduced down to 5/8" and the thickest part had to come down to 1". I marked the references on the rails and then broke out the planer.

1,000 passes later

The bottom has a slight concave and the board is exceptionally thin. Towards the end of shaping the core would flex, so I had to place it on a piece of 3/4 plywood and rotate the section being shaped so that it was "over" a hard section of the plywood. Not easy or accurate!

The final core/blank shaped and ready for step #2 sealing:

The blank/core weighs just 8 oz so any performance gains will NOT come from elimination of weight from the core with Xylene. We will need the Balsa and Carbon combination, with the void between the deck and hull skin, and the interaction of all that to give us the performace enhancements.

The blank sealed and baked. I just stashed the blank / core in the hotbox to maintain the temperature while the spackle dries. As mentioned above I sealed the blank with lightweight spackle to prevent the epoxy used in laminating the inside layer of carbon from seeping into the core. When I dissolve the core using the xylene, anything covered in epoxy will remain. Further, we want to eliminate as MUCH of the foam as possible.

The most popular spackle is DAP's Fast and Final but I have the hardest time finding that here locally, even at the big box stores. ACE Hardware has a similar product and the big box stores locally carry a DAP product called Paint N Patch.

I use a 4" blade to apply and smooth - cheapest plastic one I can find. The spackle cleans off tools with water. When doing this core / blank, do the rails first. The excess will tend to accumulate on the deck and hull, where you can spread it out, pulling towards the center.

I was able to do the entire board in a single session. The spackle dries to the touch pretty quickly, but I'll allow it to stay at room temperature overnight.

The next step will be to laminate the internal layer of Carbon. I principally just want to completely seal the core with carbon. I'm not overly concerned with lapping the rails or even an overly neat lamination. I will however, attempt to lay the carbon such that the layers are 45 degrees opposed to the layer above.

I plan to do the carbon as a hand layup. Bottom first, then the deck and patches to insure a total seal.

After the inside layer of carbon has cured, I plan to bag the endgrain balsa that will be sandwiched between two layers of carbon.

Bottom, inside carbon fiber laminated and curing.

The rail lap will have to be done with a bag. The 8 oz carbon, when wetted out, was like trying to lap carpetting over the rail!!!!

Keep your fingers crossed that we keep the electricty going. We've had nasty storms and were without electricity for almost 12 hours. It's on now and hopefully for 8 more hours!

Nice work. Is that just the top, the bottom, or both at the same time?

We're going to get above freezing this weekend. I hope to lay up a layer or two of glass before it gets cold again.

Thanks Ed,

It's just the bottom inside layer. There will be a layer of endgrain balsa on TOP of this and then another layer of 8 oz carbon on top of that.

In all my research on the e.balsa and having ridden it, the one thing that I hear and have felt, is that e.balsa transmits feel incredibly well. It's mesurably better than foam - pu or eps, corecell or d-cell. I am still waiting on the Rohacell. It'll be interesting to see what the stiffness of the carbon combined with the sensitivity of the e.balsa combination rides like.

Crazy weather everywhere! I'll hope for laminating weather for you!

Got rocker?

5 inch nose and 3 inch tail.

I honestly don't know what happened. I am theorizing that the weight of the carbon and epoxy was so much more than the underlying foam that it weighed it down! I was afraid to bag it, but in retrospect should have.

I believe that I can save it by bagging the e.balsa rails and top using the rockerbed. However, this skin will be in a constant state of tension once complete.

Carbon on the top and rails, rocker relaxed a bit.

It's a little hard to see...but e.balsa rails and 3/16" endgrain balsa sheeting being epoxied to the bottom carbon.

Out of the bag this morning, bottom endgrain balsa sheeting, carbon covered core.

I don't understand the last picture. Is that the back side of the one above it?

Yes, the underside of the board above, if you look closely on the left side of the picture you can see the rail outline in the bleed thru.

I kinda like the look of the endgrain. I almost hate to cover it with Carbon!

Glass? No you should stick with the plan. The next board could be Carbon, end grain w.balsa, and glass

I can't tell you how tempted I was to glass it. I think it's a viable alternative to the e.balsa sheets. This board is going to wind up too heavy to be a great performer. It's currently at 5 pounds and still needs another layer of carbon. Removing the core will save 1/2 pound...maybe. I'm guessing it will have a ride weight in the 7 pound range - 2 pounds too heavy. Although it's stiff as HECK!

The e.endgrain balsa does NOT bend to comform to anything. Under the pressure of the vacuum, the matt split and created sharp edges, destroying my bag and creating a mess to sand and fill with wood putty.

Hopefully I now have all the holes and cracks filled in so that I can bag the exterior carbon starting tomorrow.

The hollow carbon will require a vent, so I picked up two goretex vents from Casica Engineering. casica@earthlink.net

These vents allow air to escape the board and don't allow water in. The carbon, being black, will get HOT and expand, without a way for the air to escape it would just delam.

Pleats!!!!!!! The bag is 36" in diameter and the board is about 20" wide. In order to lap the rails with the bag, I pulled the excess bag material up and wadded it in the middle. I am bagging the bottom of the board first, rails lapped. I'll lap the rails again when I do the deck.

The bottom of the board

I'm going to have some resin veins to deal with especially on the rails - thank goodness for the hotcoat! Also, I had one section on the tail where the e.endgrain balsa mat snapped close to the edge. I couldn't get a piece to fit fully and so filled in the gap with wood putty. The vaccuum crushed that putty and there will be a dent in the carbon conforming to the void.

Out of the bag. You can see one of the resin veins towarsd the top and off to the left. That one looks bad in the picture but is barely perceptible with your hand, it would polish out without problem. Along the rails at the nose I have some buildup that approximates 1/32 of an inch. I'll need to address that before attaching the top skin.

Huh, that's interesting. Has anyone ever read the Google ads at the bottom? It popped up with Carbon Fiber Surfboards and Foam boards

very cool jeff! looks bulletproof!

wow! that is cool... your balsa project was timeless looking while this looks indestructable and sleek/fast

Hey Matt, thanks...it's certainly stiff!

Thanks Chris for both compliments. I was talking with Dennis about the construction, this has 4 layers of 8 oz carbon. The carbon when cut just barely was short of 8 oz when weighed and the resin I applied weighed just a touch over 8 oz so the combined weight came in around 1 pound per layer of carbon.

I have heard references that carbon is equivalent to 3 to 10 layers of the same weight of glass. So this has two layers of 8 oz on each side, which would at a minimum be equivalent to 8 layers of 6 oz or 12 layers of 4 oz. It's seriously stiff.

Out of the bag this morning it's completely encased in carbon now.

I cut a 1" hole that will eventually be used for the goretex vent described above. Then used the hole to fill the interior of the board with Xylene.

It's going to require another couple of gallons and some agitation, but it'll be hollow one day!

Jeff , could you had deleated the balsa sheet in this board , and just make it out of carbon. and could you have used 3+oz. instead of the 5 or 8 oz mat

In retrospect Salty, yeah, you're right. I believe that several layers of CF instead of the Balsa would have been possible. CF isn't as stiff as I presumed...I have visions of those bike frames that are like a brick, but the wall of the that carbon build up is like 1/8". I believe the 8 oz was overkill, no doubt 3 layers of 6 oz would have given me the stiffness I needed, without the balsa nonsense. I also am of the opinion that 3 layers of 5.7 oz is stiffer han 2 layers of 8 oz. although the combined weight is about the same.

One issue I had was in finding available CF. The most common weight is 5.7 oz...but other weights I had difficultly locating.

I guess all part of the experiment!

Thanks for following along, Salty.

Your welcome , i would like to plan to build my own this summer when i move to the lake and get blacks on the east coast from green board