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First (wake)board build

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skriever
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First (wake)board build

Postby skriever » Wed Nov 09, 2016 11:36 pm

Hi there,

Some advice along the way is more than welcome since this is my first board building attempt :). I am not a regularly kiteboarder so I decided to build a wakeboard (for a cable park) instead since I do that more often these days. I want it to be strong and durable so I can have lots of fun with it for a decent amount of time. This means that I will have to add a p-tex base and use a proper amount of glass in the layup. I intend to use ABS sidewalls as well. I am somewhat familiar with fiber reinforced plastics so I am up for the challenge.

I have some wooden poplar strips laying around which I like to use as the core material. Already sorted out some nice ones without to many knots. I aligned the strips such that the inserts are not placed exactly on the glue bond between 2 strips.

Image

Sooo... I started to do some calculations regarding the bending stiffness (flex) of the board. I found the following (relevant) properties of poplar wood on http://www.wood-database.com. I also added properties of Paulownia and Balsa.

Image

I verified the young’s modulus of my poplar strips using a simple three-point-bending test. The values were close to 10000 MPa. This means that the wood core will become approximately twice as stiff compared to a more frequently used Paulownia core. I see some troubles at the horizon. Wakeboards generally have a rocker of about 60-80 mm which will become a challenge to achieve with such a stiff core (large spring back). I hope that pre-bending the wood core will help.

I used my current wakeboard as a reference for the bending stiffness. A little side note; I noticed that most board builders use the term 'flex' which is, in my opinion, a bad unit. A better unit is the bending stiffness per unit width of the board (since boards have different widths). I used a three point bending test and the corresponding flexural formula (d=FL^3/48D) to determine the bending stiffness of my current wakeboard. F is the force per unit width, L is the span length and D is the bending stiffness per unit width. I applied my own weight on the board and measured the deflection using a span length at which the width has a fairly constant value of 445 mm. The board deflected 17 mm over a span length of 0,9 meter under a force of 2245 N/m. So the bending stiffness per unit width of the board is D = FL^3/48d = 2245*0,9^3/48*0.017 = 1826 Nm.

Now I have some reference for the poplar wood board build. I will be using a tri-axial 830 g/m2 E-glass fabric 0/+45 °/-45 ° (425 g/201 g/201 g) and epoxy resin. I am aiming for a fiber volume fraction (Vf) of 40% using a vacuum bagging technique. Thickness of the cured layup is approximately 0.8 mm and weights 1250 gr/m^2 when assuming a Vf of 40%.

Now I can determine the desired thickness of my core which gives me a bending stiffness (per unit width) in the desired range of 1826 Nm. I use the classical lamination theory for this matter using the following layup as an input [0/45/-45/core/-45/45/0]. For people who are into this, the D11 term from the ABD matrix is equivalent to the bending stiffness per unit width. I will spare further details.

Core thickness (Bending stiffness)
7 mm (852 Nm)
8 mm (1147 Nm)
9 mm (1501 Nm)
10 mm (1919 Nm)
11 mm (2408 Nm)

I go for a core thickness of 10 mm as the initial assumption of 0.8 mm for the faces may be slightly less in practice when vacuum bagging. I started to make the following design:

Image

The weight of the board is determined using the volumes and surface areas from the CAD model together with the density’s and areal weights of the core, rails, faces and base material. The total weight of the board will be 4050 gram (without the bindings). Which I think is acceptable considering my current wakeboard weights 3510 gram. Not sure yet if I will add a top sheet which will add 150 gram or so. Another thing that may add some weight is the absorption of resin by the poplar wood. I have no clue how much this will be. A test sample hopefully gives some insight into this. The alert reader may already noticed in the calculations above that I am not that lightweight (93 kg), so the wakeboard will experience relative high loads. That is why I chose 830 gr/m^2 glass fabric for the faces.

I ordered some base material (ptex), topsheet, sidewalls and inserts from junksupply. I kinda know that I will like building boards so I also ordered a paulownia core for a future project. Besides, it was financially more interesting to buy material for 3 boards since everything is sold per meter. I bought the fiber glass and epoxy at http://www.carbonwinkel.nl amongst some other stuff for the vacuum bagging stage.

I glued the strips together with some water resistant wood glue (water resistant for the pre bending stage). I also clamped to top of the strips (not in picture) to prevent them from buckling outwards. I then used the cad model to make a true scale print of the outline for the sidewall channel. I transferred the print to a mdf board and used it as a guide for the router. I shaped the ABS strips in the desired shape using a heat gun. As you can see in the picture I cutted the strips under an angle and placed the bond off centre; it just felt like a smart thing to do. After proper sanding I cleaned the ABS sidewall with alcohol and gave it a flame treatment right before gluing it onto the core with the same epoxy as I will be using for the vacuum bagging stage. After letting the board+sidewall cure I called in a favour at a local woodworker who has a machine to trim down wide wood panels to a desired thickness, which is 10 mm in my case.

Image

It is stressed everywhere on the web that the flame treatment of ABS is very important, so I decided to do a small test with 2 small left over pieces ABS. Both samples were sanded and cleaned with alcohol. But 1 sample was glued to the poplar wood without flame treatment and one with the flame treatment. After curing I pulled the sample without the flame treatment off pretty easily while the other has a very strong bond with the poplar. I found it nice to actually witness this in practice.

At this stage I checked the bending stiffness of my core using the 3 point bending setup below. I used a 15 kg dumbbell for the load. The deflection over a span length of 1,0 meter was 10,0 mm. The bending stiffness (per unit width) is D=748 Nm. Note that D is defined as: D=(1/12)*h^3*E. In which E is the young modulus and equals 8973 MPa. The bending stifness of 738 Nm for the core seems pretty good. Because, adding 2 fiberglass+epoxy faces with a fiber volume fraction of 40% to this bending stiffness gives a stiffness for the board of 1853 Nm... Right on target :). (Side note; the correct term for E in this case would be the flexural modulus instead of the young’s modulus. The young’s modulus has different values for tension and compression. In the case of bending both tension and compression occur so the E hidden in the bending stiffness is sort of a combination of the bending and compression modulus. When the material behaves very different in tension and compression the flexural modulus may be a better thing to use or say. )

Image

The final shape will be achieved using a belt sander and some elbow grease. The desired tips of 5mm are simply achieved by checking it with a caliper during sanding. I did not find it worthwhile to build a router table for this purpose. An even slope is achieved using some colouring wax and a straight wooden strips to indicate the high spots.

Image

The result is the picture below. btw, the inserts are epoxied in from the bottom since I felt this would be a little bit stronger.

Image

I let water soak into my core overnight and fixed it onto a rocker table having 8 cm rocker. This wasn't a success. Only 1 cm rocker was left and the board now has a slight concave in the wrong direction :/

Image

I am currently working on my vacuum bagging setup. I used an old refrigerator compressor. I know it doesn't look that fancy with all the rust but it is the inside that matters :rollgrin: . The setup right now can pull a gauge pressure of -96 kPa. I am using an automatic switch to control the on/off switching of the compressor. Otherwise the pump gets way to hot. Furthermore, I use an old fire extinguisher as a vacuum reservoir. The pressure drops with 2 kPa every 15 minutes when the pump is switched off (I did however used a small vacuum bag ). I would like to install a one-way valve between the pump and bag to rule out leaks from the pump. The outlet of the pump is connected to a jar with some foam to catch the oil vapours.

Image

I bought a vacuum bag for storing clothes. But the bag had a poor connector so I build my own bag using the sealing of the commercial vacuum bag together with a sheet of vinyl. I sealed the edges with tacky tape.

Image

I made a test sample and cut it open. I am not too happy about it since it seems that there is some air trapped under the epoxy. Also, I could pull out some dry fiber yarns.

Image

I did not use a breather fabric for the test sample, so maybe I should get some peelply+breather fabric in order to pull an even vacuum throughout the product.

I am trying to figure out what the best plan of approach is… But first I definitely want to make more test samples.
Last edited by skriever on Wed Nov 23, 2016 10:21 pm, edited 4 times in total.

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rynhardt
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Re: First (wake)board build

Postby rynhardt » Thu Nov 10, 2016 9:09 am

Wow.That's a big post, dude. :)

From my limited experience, it looks like you're on the right track. However I don't know what the best way is to get rocker into wood. From what I've read you can either try the wet soak and form technique, or build a rocker table with exaggerated rocker and expect some spring-back once the board comes off the table.
I always thought kerfing might be a viable option, but haven't seen anyone do this.

As for vacuum bagging - it's a great method to use but I suspect you need to quite a few more test samples to get the finer points of the technique down. Bubbles are a fact of life with vacuum, so you need to find ways to minimize them with careful resin preparation , i.e. spargeing, careful mixing, degassing.

Also it may be useful to use fabrics with good wet-out characteristics. Too fine a weave and you will trap bubbles.

As for the P-Tex base, I have just ordered my first batch from junksupply for my next board. I intend to heat cure the whole board at 50 deg C, which should also help bonding the P-Tex. So consider some way of doing a heated post cure on your board. We just use a heat blanket and lots of insulation.

I use foam which is a bit easier to work with.

Good luck.
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downunder
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Re: First (wake)board build

Postby downunder » Fri Nov 11, 2016 3:22 am

Excellent post mate, and so proud the Junksupply in the EU is doing fantastic work, since the owner is a personal friend of mine :)

Re the peelply, breather and bleeder, it is a must unfortunately. I am getting great results only with using all three. Not two,or one.

Re a fine FG, ie 120grams or less, sorry to say but no, it is absolutely great product to work with, with no bubbles at all. It becomes invisible, and completely dissolved bu epoxy. I would rather use 2x120grams (ie 90° and 45°) than any other FG (and I might).

The bending thingy :) I stay away from soaking. It is so wrong on many levels, first one is you never know how your board will bend after drying. It really depends on a grain, do you know how the wood grain goes?

Are the wood strips positioned like this:
))
)) - this is a wood grain looked from a top side of the board, indicating the wood centre was on the left side of all wood strips. That will make a huge concave on this side (left), if not mistaking.
))
))

Or like this:

))
((
)) - the wood grain goes opposite with each strip. Can we predict the wood bending after drying?
((
))

It is worth to say that it is impossible to bend 12mm Paulownia wood plank 1400x420mm with a bare hands (well, I can't do it). However, it is possible to bend wood strips easily. Very, very easily.

And that might be the solution. NO need to glue strips in the plank all the way through. That way, they bend easily, and stay bent easily on the rocker table (which is just a 5mm perspex sheet I'm using).

In your calcs, you did not include FG at all as I see it. Using a 830 gr/m^2 completely changes flex figures, and is a way, way to much and you might have a great difficulties wetting this out, unfortunately. Even 700 is a fcuk to work with.

Hope this helps. Good luck.

D.

PS

Also, it is more easier to work with if some excess wood is left around the rails, duno, 5-10mm. It's a personal preference btw, but I do cut excess with a jig saw and sand it. With no excess it is extremely difficult to cut a FG and might ruin the rails. On that topic not sure how to cut 830g triax at all :( But this is a last worry atm.

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Re: First (wake)board build

Postby plummet » Fri Nov 11, 2016 8:37 am

Looking good. Tones of rocker.

skriever
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Re: First (wake)board build

Postby skriever » Fri Nov 11, 2016 10:40 am

Thanks for the feedback guys :D. I did not really payed much attention to the allignmend of wood grains to be honest. They all looked pretty similar so I could not really figure out the best way to place them. I wish I have read your comment before I started soaking the board downunder. I kinda regret it since this is how my core looks after drying. But I think it will stay in the desired shape after glassing.

Image

I think it is not a bad idea to heat cure the board, thanks for the tip Ryan. Also, it might prevent outgassing of the wood if the board is PRE-heated! Air is trapped in the wood. So if the board heats up during curing (maybe heat from the chemical reaction or by adding heat after the layup) the air in the wood will expand and "outgas" while the wood temperature is rising. This will cause some additional air bubbles... I think this can be prevented when the wood is heated up before adding the epoxy and making sure the temperature get slightly lower when curing. This way the air will contract and draws some resin in the wood.

I came across some people who treated the layup with some heat from either a propane torch or heatgun to draw air bubbles to the surface. I also found another interesting technique for drawing air bubbles to the surface, this guy on youtube uses acetone. Check it out; https://www.youtube.com/watch?v=2PKn-59pHNM.

The FG actually is in my calc downunder. The bending stiffness of 1919 Nm corresponds to a board with a core of 10mm and a bottom and top face of the 830 g/m^2 [0/+45/-45] FG. The excess wood thingy was probalby a smart thing to do, however my inital wood strips were to thin to keep the excess wood attached to the core after trimming it down to 10 mm :(.

I think you are right about the need for peelply, breather and bleeder. I intend to get my hands on those in the nearby future. The reason I though I might get away with only using a vinyl bag was the following video https://www.youtube.com/watch?v=QwKzI-oIC_o . This guy doesn't use peelply, breather and bleeder. But maybe his cotton sheet kinda worked like a breather fabric.
Last edited by skriever on Tue Nov 22, 2016 4:31 pm, edited 1 time in total.

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Re: First (wake)board build

Postby rynhardt » Fri Nov 11, 2016 11:12 am

The air in the wood will probably be taken care of by the vacuum, I think, but that's not the main problem.
The one problem is any bubbles in the resin, e.g. from mixing, expand under vacuum. This is where careful resin preparation will help.
The other problem is bubbles form due to the chemical reaction in the epoxy. You may have to look for a different resin system if this problem rears its head.

The heat and acetone approaches work with wet layup. But once it's in the bag, not much you can do about it except have breather and bleeder fabrics.

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Re: First (wake)board build

Postby zob » Fri Nov 11, 2016 12:31 pm

Hi!
One approximate absolute wood density (I don't know the exact phrase- wood material without moisture and without empty spaces) is like 1500 kg/m2. Considering this and poplar density at specified moisture, I think there is no need to calculate, to see drawing out air is unfeasible. And if not, stil if "internal vacum" would draw in more resin, this would just leave you with more dry spots and/or heavier board.

Preheating the wood core can leave you with additional warping and the epoxy will start to gel earlier, so you might not succeed to completely wet out the FG.

p.s.: Don't forget to flame the edges again to bond to FG.

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Re: First (wake)board build

Postby downunder » Fri Nov 11, 2016 1:39 pm

I agree with Zob, with heating the wood might warp. If we look at how the "big boys" are doing it, they heat both sides equally in a press with a silicone blankets. Mind you, your triax will develop a significant heat when curing, hence a wood warp might happen if you glass one side only. Epoxy infusion might be difficult with 830 triax as well, so I would rethink the glassing schedule. If you look at my builds, which are similar wood projects to yours, this boards are super strong. And half weight or even less. For sure if you're hitting a park, you need a bulldozer, but for that the topsheet is needed on the water line. And a super strong topsheet, more like a P-tex RY is using.
Hmm...

A pro tip would be to seal the core with epoxy sealer for any air bubbles before glassing.
Your pump needs a filter on vac line, and a good one. I use a 5" PC fan to 'cool' the pump even if is not getting hot (not having a tank). Coz, the vac might leak in 10 hours of working, and than a pump goes to flames burning my house down:) The pressure line is connected to a long tube, and goes out through the window.

There you go, a lot of info:) Hope it helps.

D.

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Re: First (wake)board build

Postby zob » Fri Nov 11, 2016 2:11 pm

Ah and one more fast test you shouldn't skip in your situation is dry vacuum test. It's crucial to know, if your vacuum setup is capable of forming the core to the rocker table.

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Re: First (wake)board build

Postby rynhardt » Fri Nov 11, 2016 8:13 pm

downunder wrote:.

And that might be the solution. NO need to glue strips in the plank all the way through. That way, they bend easily, and stay bent easily on the rocker table (which is just a 5mm perspex sheet I'm using).
.
Took me a second reading to realise just how brilliant this one piece of advice is. :thumb:
I might just try a wood core next. :D


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