Time to capture some learnings from the project what worked and what didn't and what I'll do differently next time. Here's the key
+ are what worked
- what didn't
* neutral feeling about it
!! idea for next time.
I used Paulownia wood planks 1400x100x6 mm. The deck was built up from 2 layers.
++ cutting a template first so that the core shape could be routed out was perfect. Very quick and efficient. Its important to get the hole spacing for your fin bolts exact because when the screws are off center even the smallest amount its screamingly obvious when you don't have an opaque surface.
+ the wood is stiff. The elastic modulus of the wood is quoted at around 5-7 GPa compared to Klegecel which is effectively zero. e-glass in epoxy comes in at around 15 GPa (typical) but the core is 40 times thicker than the layer of laminate to its contribution to stiffness is huge. This means less layers. I used x 200 g each side but have heard of someone using 2x 160g.
+ using double sided tape to hold the core in place while the shape is routed out.
* exposed wood looks great but the 2 distinct decks don't look so good together - this is obviously a personal preference.
* I left the planks at 100mm wide rather than trimming to 50mm and reassembling them. !!Thin planks look better and apparently help break up weakness in the wood.
- I really needed to thin it down to less that 10 mm as this stuff is really stiff. Using an electric plane and 40 and 60 grit sand paper was time consuming and left an uneven surface as the electric planer leaves tracks on large areas like this and they need to be sanded out. !! Use a router and jigs to do the bulk of the shaping on the core and then an orbital sander to take unwanted sharp edges off.
- Sealing the core before glassing. I think sealing is the right thing to do but I did not let it cure long enough and so it has not sufficiently gelled by the time I placed it on the table. The result was that the white pigment in the underside sealing coat diffused into the graphic inlay and obscured parts of the image. !! Let it cure for at least a few hours so that it is tacky but notquite set yet.
- I left the tips at 6mm and I think that this is too thick to get enough flex. Will have to wait and see.
- building the core up from 2 layers, while straight forwarded, added lots of time to the process. (gluing the planks, routing, sanding and then gluing the layers together. A single piece carved into shape is much easier if you can carve it efficiently.
--- the stiffness of the wood when glued into the 400mm width was too much for the perspex (plastic) sheet used on the rocker table surface and it deformed. This resulting in concave vanishing and rocker reducing significantly. !! Either change to using Formica for the rocker surface of brokite style 2 layer mould to encase the core pieces while they set.
I poured the rails with the same epoxy I used for laminating. I didn't add any thickener to it because I wanted to keep them transparent so the graphics showed through and didn't want to run the risk of reducing the strength. Given they were being poured into a channel I didn't see any reason to use q-cell etc to improve adhesion. It used around 250 g of resin which is actually slightly more expensive than using ABS plastic strips (hence the reason to use q-cell in them). Alternative pouring materials I've heard of are liquid polyurethane (can't remember the exact name) but think that is had a shore hardness rating of 70D
++ clear rails look great. Graphics shine through when sun is behind it.
++ using the router to create the channel gave a good uniform width rail and allowed the laminate to be laid past the rail so no steps that might have accumulated resin or resulted in the glass bridging.
++ mixing the resin very slowly and pouring the resin in really slowly into the channel meant that I had no air bubbles trapped in the resin. I used a plastic tomato sauce squeeze bottle with a pointy nozzle to direct the resin.
+ cutting the slot for the rail all the way through the material and using tape on the underside to create the channel work OK. Only hiccup was that the tape had a couple of wrinkles which of course meant wrinkles on the surface. This can be overcome with better technique, I'm sure. Maybe put tape down first with adhesive side up and then place the core pieces on top.
* I'm still be be convinced that pouring the rails is the best approach. It was critical to get the table level and the core an even thickness to ensure that resin didn't pool and overflow while draining away from another place. I didn't manage to do this and so a lot of work was needed to sand it back and refill.
- There is no way around the waiting time for curing. And because I had to top it up I had to wait again. This puts a lot of delay in the process.
+++ Inlays printed on 'silk' ( I haven't heard from the supplier about the actual material. It feels like a very thin peel ply). Looks brilliant, super easy to work with and cheap.
-- no scratch protection or UV protection and requires you to do all the surface finishing that wouldn't be necessary if protective bottom and top sheets were used.
!! would still like to get PBT bottom sheet to avoid have to do all the surface finishing that was royally screwed up.
Laminate and vac bagging
++ as much as I hate working with it and its expensive (cause it has to be thrown away) tacky tape seals like a champion. The only viable alternative I can see is to put the entire table/mould inside a the bag so that tape is not necessary. I'm keen to have a go at this for next time. !!Maybe creating a fibreglass mould for next time.
++ doing the laminating in one go rather than top then bottom as I did previously. Saved an entire curing cycle.
+ using paper towels as breather/bleeder material seems to work fine. Despite the surface coming out a bit dry (and I think that this may be due to the wood absorbing resin) the finish was nice and uniform.
* Peel ply is very effecting in letting excess resin flow through it. However, I think that may too much as able to flow out and so !! next time I'd like to try using perforated release film instead of peel ply. The release film lets less resin flow across it and so holds more in the laminate. Because of the wood soaking up lots of resin, holding it in there might be a better idea. It also means less is required to finish the surface as its already quite shinny.
* 27 mmHg may have been too much pressure and resulting in the dry laminate on top. Not sure, I need to explore more. It took about 20 mmHg to get the board to conform. If the planks are being glued together (along the length of the board) at the same time as laminating then the problem would go away.
? Resin. The top layer of laminate appeared too dry after it came off the table and there are a few different theories going around in my head. One being that the resin soaked into the wood core because of the long cure time ( I used slow hardener which has a 45 min pot life and multiples of that when its spread out). Maybe this gives too much time and allows the resin to be absorbed into the core. Although, this was not the case on the underside which had a good quantity retained in it. Maybe I need to use a more viscous rather than less viscous resin to prevent too much flowing out when its under vacuum. I followed up the lead from a discussion about the viscosities of FGI and west system 'entry level' resins but the data sheets for the FGI resin quote a huge range for the viscosity 110 -1500 CPS. The West System'2 105/205 is quoted at 580 CPS. I don't understand why the range is so big. A question for FGI.
+++ The matching metal frames, the jig setup, working height very low, the ratchet straps to apply lots of clamping pressure to the jigs.
--- The rocker table surface for wood cores was a disaster and changed the rocker and concave to flat and flat.
!! add 400w workshop lights ($14.95 each at bunnings hardware) to be able elevate the temperature for post curing. Needs a temperature controller for switching on and off.
!! change the surface to either Formica laminated to the ply or use a brokite style construction where a bottom and an upper layer of perspex is laid over the core and the whole thing placed inside a vac bag. The 2 layers together and the symmetrical pressure on the mould should be enough to prevent the distortion. It also works to hold all the pieces together and so makes it possible to do just one gluing and laminating pass instead of the 4 needed to do the rails, 2x cores + 2 cores together, laminating and finishing.
Really not sure how to turn this around an would like to avoid it as much as possible by using PBT bottom sheets top and bottom ideally. Reality is that probably will only use it on the bottom if I can get hold of it (due to cost).
++ Perspex sheet on the bottom results is a great finish on the board straight off the table.
--the sanding I did made the surface murky. !! do another sanding pass with greater than 600 grit paper and maybe buff it.
-- using epoxy to do a final coat left the surface uneven, 'wavey' as it did not self level very well. !! Maybe try thinning the epoxy of finding good advice on UV stabilised varnish.
+2.3 kg without accessories. 0.4 kg heavier than foam core board. Can't complain about that.
+++ Antig99 footpads and fins. Hard to be on price and work fine. I've give the fins a good flogging in the sand and hit a few rocks and they handle as well as the fibreglass fins do maybe even better without the screen printed logos to show wear.