CONSTRUCTION METHOD AND MATERIAL
As this is the first board making project I've had a go at I decided to keep the design 'mid tech' because even though a few people have created some great looking boards just wetting up a foam core by hand and hanging weights off it to pull the rocker and the concave into it, it seems natural that vaccuum bagging will produce lighter stronger boards which, even for a first attempt, is one of the goals.
Some evidence of the weight issue (Trevor reckons) comes from some tips from the extremely helpful people at FGI (Fibre Glass International) in Brookvale here in Sydney who gave me a 'rule of thumb' estimate on the amount of resin needed:
i) if you apply one layer of 6 oz e-glass at a time and let it dry between successive layers then you can expect to use about 800gms of epoxy resin per sq. meter on average across a few layer. The first uses more because it needs to bond to the core;
ii) if you apply wet-on-wet, i.e put all the glass layers on at the same time without any drying time in between, then this comes down to about 500g/sq m. This apparently needs a bit more skill and speed as the resin will start to cure when its spread out and so if you have 6 layers to do then you will already have the early layers going off before you can clamp the work down;
iii) If you then vaccuum bag it this can come down to around 300g/sq.m
iv) if you use peel ply on top of the work (perforated plastic sheet the sits between the glass and then vaccuum bag itself) to allow the excess resin to flow away more easily then this could come down again to 230g/sqm.
When you tally up the difference by vaccuum bagging then you can end up using 50% to 30% of the amount of resin that hand layups might achieve. On a average sized board with 3 layers of 6 oz cloth on top and bottom this adds up to 1.5 kg to 3.5 kg!!! This is a massive difference when it comes to kiteboards where a couple of hundred grams is very noticeable. Reading around various blogs 2kg before accessories seems to be the goal for most board makers. Less and strength appears to get compromised.
On top of this, vaccuum bagging just seems like such a cool idea that I wanted to have a crack at it.
There is a brilliant resource on vacuum bagging available on the West Systems webiste (providers of epoxy resin systems among other things).
So, even though this first attempt may not be the greatest board on the market it is going to cost a couple of hundred bucks and so I wanted to give it the best chance of being half decent.
CURING UNDER ELEVATED TEMPERATURES
On youtube I came across a guy who used this technique but also cured the board under heat lamps. With the standard epoxy resin that I bought at FGI the data sheet says that curing under temps up to 50 degrees c can dramatically improve the strength of the board. The video I saw certainly supported that as the guy put it between 2 stacks of books and bounced up and down on the middle (travelled about 10cm each bounce) without it snapping. In the video he just put the whole rocker table inside a 'tent' of insulating material and put a few heat lamps inside and let it cure away. I also came across a video from Brokites on their construction techniques and they similarly use heat to cure it and get similar strength (althought their boards seem to use a lot of carbon).
I'll be keen to give this a go in the future and compare the results but at this stage its in the high-tech basket.
Core material was the next thing I spent some time researching.
The main options for core material are marine ply, other wood, PVC foam.
i) marine ply wood - I've seen people laminate 2 layers of 3/16" or 1/4" marine ply, one layer of glass and few bolts and away they go.
ii) other types of wood - balsa, western red cedar, poplar and paulownia. In general the stronger the core material the less glass that is needed and fasted you get out on the water. The choice between the different woods is largely about the strength/weight trade off ( look goes without saying).
Balsa - 40 - 340 kg/cu m (huge range)
Paulownia - 275 kg/cubic meter
Poplar - 350 - 500 kg/ cu meter
Western Red Cedar - 450 kg/ cu m
Paulownia seems to be the board making wood of choice at the moment. Its touted as having superior strength to weight compared to the others - can't say. But it does seem that as the heavier density woods get introduced they are often used only as part of the structure.
Check out this video of a crazy many with a circular saw showing the composite construction of a slingshot SX but cutting it open!!!! http://www.youtube.com/watch?v=EtI1AJiiC9o
iii) PVC foam - PVC foam is used extensively in boat building and often goes by the name Divinycell or Klegecell. It light at, typically stocked stuff, at 80 kg/cu m. Its pluses are its very easy to work with - you can use a standard old hobby knife to cut it and it can be sanded and shaped just like a surfboard blank - it comes in large sheets ( 2.4 x 1.2 m ) which can be cut into 5 pieces for for various sized boards. The cons are that its obviously the weakest of the core materials mentioned and so needs the most glass. One blog I read said that you need about 30% more glass on a PVC core compared to balsa core.
The other thing that was attractive about this as a first timer is that its lack of stiffness makes it easier to have the rocker and concave introduced without worrying whether the vaccuum set up I'm going to use has enough rigidity in it to bend the material.
On pure aesthetics I was keen on using balsa. It is very available (though pieces need to be glued together as I could find pieces longer than 1 meter - hobby stores sell the stripes used to make models with and FGI sells sheets 1 m x 75 cm), easy to work with and looks brilliant. However, only big downside is that if the glass skin on a balsa board is ruptured then the standard balsa will absorb the water and it will become waterlogged. High density PVC foam repels it. So this made PVC foam a lower risk option for my first board.
E-GLASS, S-GLASS, CARBON...
The choice between glass and carbon for me was a $$$ issue. The difference is $5 /m for e-glass vs $45+ /m for carbon. Carbon is a hell of a lot stronger than glass so you naturally use less of it (lighter, stronger and potentially stiffer boards) but I'm guesstimating that I'll use about 7m of glass mating in the board (3 layers top and bottom) so the initial outlay for carbon is too much until I know what I'm doing.
S-glass is stronger than e-glass, the chemical structure is different and you can save weight by using s-glass. It is also a little more expensive ( as you'd expect) and so again that is in the 'next time' basket so that I at least have some benchmarks for how strong the standard stuff is.
Laying the glass so that that weave runs at different angles seems to be a common method for improving strength of the board. You need to take this into account when working out the length of cloth to buy as the it can mean that you will end up with quite a bit of off cut cloth to accomodate the off-axis line of the weave.
EPOXY vs Polyester Resin
In a nutshell epoxy has stronger adhesion to the laminating material (4 times the strength in the kevlar example in the above link) and has lower moisture absorption and is more flexible than polyester resins. Epoxy is a bit more expensive but you'll end up with a stronger lighter board less prone to delamination - expect to pay around $75 for 2 kg of resin + hardener vs c. $55 for polyester resin + catalyst (MEKP)(note: the polyester resin hardening process catalyst as the resin apparently has the hardening process already underway when you buy it and the catalyst speeds the process up) http://www.dionchemicals.com/resin.html .
Part of the price difference is the different ratio or resin to hardner that is used. In Epoxy resins the ratios of Resin to Hardner are in the order of 5:1 while with polyester resins you use around 100:1. Hardner accounts for a lot of the price difference.
Having said that also depends on what quantities you are purchasing. The pricing on epoxy is set for the typical large quantities of resin used in boat building the price per Kg drops fair quickly.
Although at 1.5% hardner the curing time for polyester resin is short (5 - 10 mins) it is possible to control cure rate by using less resin. However, 1.5% in the typical volumes being used in board making scales down to partial 'drops' so managing the cure time to allow you time to wet out all the layers and get into clamped/ sucked onto the mold might be tricky. According to the wikipedia page above of the resin comes with the hardening reaction already underway (albeit very slowly). The 'hardner' MEKP is actually a catalyst that speeds the process up.
The resin I've got hold of is R180 Epoxy with H180 hardner. H180 is a slow hardner and give 40-50mins gel time which should be plenty to do the lay up.
There seemed to be 3 main options here again: poured epoxy rails, abs plastic or wrapped.
Poured epoxy rails are made by routing a 1o mm channel around the outside of the blank core and pouring in a mixture of epoxy resin, q-cell ( or microballoons) with chopped strands of glass. Q-cell thickens the epoxy and is a hell of a lot cheaper than epoxy so it saves money and the chopped strands are literally 5-8mm pieces of glass fibre that you put in to increase the strength of the material once it sets. You can then flip the blank over and sand it down till you expose the pour material and there you have your rails. Note that q-cell does weaken the epoxy a bit and 15% q-cell by volume seems to be about the proportions people have reported using.
Q-cell is also useful as a filler to fill the gap between square edges on different layers of the board ( if you're laminating several layer of foam etc). Fibre glass can't conform to the sharp changes and will bridge over them which means it is not in contact with the core material. Q-cell can be used to thicken the resin up so that you can fill these 'steps' so that the glass has a bonding surface to attach to.
ABS plastic seems to be the best choice. Unlike some other plastic material, epoxy will bond to abs plastic. Other plastics ( and #$#% there is a lot of them) may need to have the surfaces of them 'flamed' to get the right chemistry on the surface so that the epoxy will bond to it. Its very easy to work with and very flexible. I got an off cut sheet from a plastic supplier which was about 1.5 x 75cm and it cost $70. Ouch but I used it to repair the rails on another board that had been smashed when I ran over some rocks - it worked like a charm. I have heard of people going to sign makers and getting off-cuts for less.
I've just become aware of another possible approach. I'm waiting for some more details but it involves using liquid ABS plastic rather than the sheet discussed previously. My initial thoughts are that you would lay the core material over wax paper and fix it in place, then squeeze out the liquid ABS and sculpt it (like you would putty) into the rails making sure to make the 5 mm or bigger than needed so that you can trim it back later and get a nice clean edge, let it cure and the sand if back to the thickness of the core. After having spent about 3 hours cutting the ABS sheet to the exact shape I needed this approach seems brilliant.
A bit of searching on liquid ABS revealed that you can make your own by simply disolving ABS shavings in acetone and letting it disolve in a sealed container. If you end up with mixture that is too thin then you can just let the acetone evapourate off or add more shavings. One blogger said that he was able to get a bag of ABS plastic pellets from a local vacuum forming shop. Other just took ABS plastic offcuts or sheet and shaved it with a surform or an electric plane. One guy cut a sheet into 2cm x 2cm pieces and he said it took about 3 days to dissolve - so shavings seem like a better approach.
From all reports, when the liquid ABS cures it is as hard as the original material. Lots of people have used this liquid ABS as a glue for ABS plastic pieces and claim that the glues material broke before the joint itself.
The role of the acetone is to disolve not only the plastic to get the liquid material but also dissolves the material that the liquid ABS is subsequently applied to and hence forms an ABS-ABS bond (this is effectively solvant based plastic welding). A question yet to be answered is whether the acetone will destroy the PVC core material being used for the board. I'll update the blog once I find out more.
One final comment on this approach is that I have seen one guy who mixed up epoxy and Q-cell and chopped fibres to 'peanut butter consistency' and used this same approach to as described for liquid ABS to sculpt the rails. This may be a slightly cheaper way to do it.
WRAPPED RAILS as I understand is just where the glass is wrapped around the rail so that the board is completely encased in glass rather than relying on the rail material to form the waterproof barrier. The one, small, drawback of this approach is that you have to have the rail shape finalised before you do the layup. Not a big issue unless you want to experiment. This approach could be used in conjunction with other rail material as I suspect it would be important to have tougher material on the rails as they do take a real beating and reducing the risk of water logging if the rails are damaged through having a 10mm or so rail before the core would seem to serve this purpose.
As I had the ABS plastic on hand I decided to use this.
So that is all the basic construction and materials selected. The next installment will look at the design of the board itself.