So there I was, staring down at my precision designed, poorly executed, slick looking wobble board. To coffee table it or not?
Nah!
Attempts to repair it
Things left out were footpad reinforcement patches and the unidirectional carbon cross straps (65mm). Easy enough to add on and it had been less that 24 hours since the last layer wet down very little surface prep needed. All good.
So why not add the graphics too. Hey, sure. They needed a white background to cover over the carbon so I'll give it a white back ground too. Hey sure.
The plan was to wet out a layer of 4oz glass with clear resin and place graphics down, let that cure a bit and then put a coating of resin with white pigment. So I wet out a layer cloth for the bottom, lay down the carbon cross straps and placed the board on top of it. With the carbon straps being about 0.25 mm thick, this was about 0.5mm where the crossed. With the board laid on top and the perspex underneath, this left the board standing off the surface of the rocker table in a large area around where they crossed. As a result the resin pooled and the vacuum was of no use in removing any air bubbles in the vicinity of the carbon.
Here's the result which gets uglier the closer you get.
You can see the white resin that's pooled around the edges of the carbon giving it the halo effect. This resin has lots of voids in it.
So then I wet out the top layer of glass on a piece of release film and laid the graphics down. This went well and the tissue paper around the graphics vanished and there was no visible diffusion of the ink (standard ink jet printer ink from Coles supermarket). I had visions of letting it mostly cure and then putting a coat of white resin over the back but I realised I had already put the bottom skin down so I didn't have long enough to wait for it to go off and so I gave it about 1/2 an hour before I added the white resin. This wasn't nearly long enough and as soon as this piece got transferred over and laid down on the top surface of the board, the white resin started to diffuse into the graphics, almost completely obscuring them.
The the final screw up was to use felt carpet underlay as the breather material once I had put everything under the vacuum. Turns out the felt underlays density varies a lot and as a result the denser parts pushed the white resin away and it pooled under the leas dense areas. As a result the board came out with what looks like leprosy with the carbon plainly visible through the blotchy white resin.
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| Close up of blotchy resin showing the diffusion of white resin into the graphics. |
You can see in the above photo the behaviour of the colored resin. It gathered in the foodpad support areas but was squeezed out of the layer between the partially cured topsheet and the previous top surface of the board.
A closer inspection also shows lots of voids on the top surface. While I worked on the topsheet to get air bubbles out it seems that the lack of bleeder material on top of the glass gave it zero tolerance. The bumpy breather material then seemed to force all the air bubbles into larger voids areas from which the air could escape.
The Don'ts
- Don't skimp on your planning. Might be worth listing all the steps and checking everything is ready prior to pouring the resin. Once the clock starts ticking its easy to get distracted and forget a step or two. This is especially true when there is more than just putting the laminate on.
- Don't rely on non-cured resin to make carbon invisible
- When creating a top sheet on a smooth surface (and especially if vac'ing it) then there seems needs to be breather /bleeder material on one side to give the air bubbles an way to exit. My only concern here in creating pre-cured topsheets is that the breather material may wick too much resin out and leave the top sheet dry. This will need a bit of experimenting.
- Given the time and pressure in these process, colored resin will diffuse into resin that is not fully cured so don't apply a fill color over your graphics until the resin they are encapsulated in is fully cured.
- Where the thickness of reinforcement materials is not uniform across the entire surface of the board then you need to make sure the table surface or the vac bag can conform to the shape so that resin doesn't pool. On the underside of the board there will need to be something that can, in my case, accommodate a 1/2 mm bump. Maybe some dense rubber matting.
- Don't use breather material that is not uniform in density, it will give the board leprosy.
You can understand why they use the pbt material for the graphics rather than the steps to colour the board in a seperate operation.
ReplyDeleteOne question though about the fill coat of white over the graphics.
Wouldnt it cover the graphics anyway if it was over them?
Hey Peter
ReplyDeleteYes I now get PBT. Also, some of these topsheet materials have a 'fur' on the back called remay which apparently helps with getting a good bond. I suspect this helps cover over the uneven surface too.
The set up for graphics was to wet out the 4oz glass with clear resin and then press the graphic ( printed on tissue paper on an ink jet printer) into the wet resin. This way they are encapsulated in clear resin before the fill coat goes on over the back. If the clear resin has cured then no coloured resin will diffuse into it. You can see parts of the graphics on this board that didn't have white push through it. This was after about 1/2- 1 hr of curing. An other few hours and it would have been fine.
Now that I'm set up for this specific board, I'm going to re-try teh same board with a few tweaks to the process.
Nice work on the rocker table!
Cheers
Matt
I see what you did now, sorta couldnt picture it.
ReplyDeleteI am hopefully going to try a test sheet with the vac pump on the weekend on an old top for a glass table i have with the core flute on it to hel keep some resin in.
I'll lay cotton down and see how it turns out with the glass over it.
I'm also chasing down decent pourable liquid polyurethane for the rails (it goes off in 5-10 minutes supposedly). Companies sure dont like to respond these days with prices.
Thanks on the table.
I should have the adjusters done this weekend , not sure about the table top though. Im More deciding on the thickness i need that will enable me to make the concave easy.
Matt,
ReplyDeleteon the 2nd photo, I see something like a second layer, just a bit raised and goes all the length.
What is this?
D.
Hey D
ReplyDeleteDo you mean the lines that run from end to end? These are step downs from the 6mm core to the 4.5mm rails. Next time I'm going to make these smoother curves rather than steps or alternatively glue the rails strips vertically ( 4.5 wide and 10mm tall) to the edges and then trim the plastic off rather than the wood.
Hoepfully drilling it tonight and try it this weekend.
Cheers
Matt
Hi Matt,
ReplyDeleteI'm starting a board project and have spent most of the weekend on the internet gathering information. I came across your site and it's got the best info I've come across! In fact, your light-wind board is very similar to the board I had in mind. I have a couple of basic question that I have not been able to work out answers to - I hope this is an appropriate place to post them:
- Considering that you are using multiple layers of fibreglass, why would one choose a wooden core over a foam core? A wooden core with maybe one layer of glass would give you a flexible board, but as far as I can tell an appropriately thin foam core board should give a similar flex and save a good couple of kilos.
- If using a wooden core, are synthetic rails really necessary? I would have thought that the wood would be stable enough to withstand dings, as long as they were patched promptly enough to stop water soaking into the wood.
- I was planning, if using a wooden core, to soak the wood and pre-form the rocker/concave, then apply fiberglass (after the core is dry) to the core without a jig. I see you apply the glass in a jig to form the rocker/concave. Any comment on the difference between these two approaches?
Any experienced information from you more than welcome!
- Jeff
Hi Jeff, Glad you stumbled across the site and very happy that you're finding it useful!
DeleteThis light wind board has turned out to be one of my favourite rides. I get probably 20% more time on the water than the other locals here in the Northern Beaches of Sydney with it.
To answer your questions:
i) Flex of a board is ridiculously sensitive to board thickness. The cores contribution to the flex is proportional to its thickness cubed and the laminates contribution is proportional to the thickness of the core squared. This means that 6mm using just foam would have required a lot more fibreglass to make it durable.
Some rough modelling suggests that the paulownia wood core in this board contributes about 50% of the stiffness and strength. This would have meant another 2 layers of glass if foam was used. The challenge with using lots of fibreglass is that its easy to end up with a very rigid board and rigid with foam cores is much more prone to delaminating and then snapping.
If you look at the Brokite videos where they use 6mm pvc foam they have 2 layers of 11 oz carbon with carbon stringers in it to get their boards solid.
That said I think the board was a bit too thin. As the wind picks a bit it bend between the feet so if I were to remake this I'd probably but another deck of maybe 2mm wood (33% increase in thickness which translates to probably 50-60% increase stiffness overall.
If you use thicker cores then foam would be less of an issue. The wood I used meant that the core was note much more than a kilo ( I think - need to check). Foam would come out lighter but when you add in the extra glass and resin the saving isn't great.
ii) Possibly yes. I have seen boards where they wrap the rails like in a traditional surfboard lay-up and they look good. There's a couple of reason I use rails though. One is that the profile of the rail can have a reasonable impact on edging. Very square rails really add to the grip and upwind performance while even a slight radius on them loosens the board up and I find helps in waves and big shop. Having rail material lets you experiment with this and also helps you clean up the outline of the board when you're finishing it up. Pouring rails is by far the easiest way to do them rather than spend on plastic and do all the preparation work to get a good bond. I've never cracked a poured rail and I've seen other homemade boards 6+ years old with poured rails that have never cracked.
iii) That is an interesting idea and one that I've only recently heard of. It sounds like a very good solution to avoiding spring back in the cores which as they get thicker can be significant ( I've lost 20% of rocker and concave in a 11mm wood core due to it springing back). For thin core ( 6mm) I don't think the cores will be stiff enough to suffer much spring back.
If you're cores are going to be thicker, my guess is that vacuuming it onto a jig would still be a good idea to ensure that the core doesn't warp as it cures. Things that may cause the core to warp are the resin soaking into the core material at different rates and to different depths due to variations in the material and the heat generated during curing which can be quite high. Vacuuming is the best way to clamp it because it also removes excess resin and lead to a stronger board.
Vacuuming does give excellent results. I made a vacuum pump out of an old fridge compressor (there are many many examples and how-tos on the web) and the results are excellent.
iv) on the board itself the only thing I found with it was that the very parallel sides both give it excellent up wind performance but because the board is quite long it really requires a lot of back foot pressure to overcome the boards very strong desire to point down wind. Your quads will really get a work out. I think the best bet here might have been to put the fins in a bit closer the center ( closer to the foot pads) so they don't force the nose downwind so hard.
Matt