FangyFin MkII Theory for insomniacs

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elmo said..
Fangy,
how is the accuracy on the printer?

I spoke to a printing company about achievable tolerances looking to make fin molds and they advised that they weren't able to get near a 0.1mm tolerance is this still the case?

Elmo, as far as I understand shrinkage in the printing material is offset by the software, but I am not sure to what degree of tolerance. My only slightly educated guess would be that what you were told would be correct and mould manufacture would require a fair bit of careful post-print finishing.
With my fin prints, the current print layers are at 0.2mm thickness, for two reasons:
Whilst a 0.1mm print is possible, it takes twice as long, and as a result doubles the cost of the print. The 28cm fin would take 72 hours to print! The Minister Of Finance here is not keen on that expenditure...
Second, the investment sand used in the casting is not super fine, so even with the nicely sanded and smoothed pattern I gave the foundry for the prototype, the castings come back with a surface rougher than hessian undies. To top it off, add in 5% shrinkage in the ally as it cools, plus all the polishing and the fins end up being a nice facsimile of the CAD drawing at best.

Thank you for the emails people :-)
I want to kill a few birds with one stone here and answer a few questions I have been getting. For those of you who have been masochistically following this thread from the start, you can go back to the funny images thread, there is nothing new coming, just stuff dredged up from way back.

This bit is really important: This is not a fin designed for epic PB speed days.
It is a niche fin. It is a fin designed to sail in weed on lighter days where you need to carry a big sail on a short highly raked fin that doesn't spin out when you run into chop. At lower speeds and lighter winds, you have a lot more board surface in the water. The drag from your fin becomes only a small part of the overall drag equation. The fin is designed to create maximum lift with the smallest amount of drag AND resolve some of the lift vertically. The idea is to lift your draggy big board clear of the water sooner. Yes, there is a form factor drag penalty from the fin, but the idea is that the drag from the board is much greater at lower/moderate speeds than that of the fin. The leading edge is blunt in an effort to create a pressure wall to keep the weed from wearing the leading edge and to start forcing water down(lifting you up). Remember you are on big gear, so you won't be breaking speed PB's, but you will stay planing longer, you will have a more comfy ride, go uphill much better and you won't spin out.
Warning: Unlike Deltas, it is possible to be smacked for being over finned if you are a lightweight. The vertical lift will bite you if you try and carry a fin too big for the speed you are travelling.

Is all that clear as mud?

Below: Printing the 24cm fin up near the tip, the machine has run out of filament and stopped - you can see a remnant hanging down. Also, the surface roughness of the print is evident in this photo - hence the need for further finishing before it goes off to the foundry.

Help please resin gurus :-)

The pic is of the surface finish on the 28, which was such a massive print it brought its own technical problems. The surface thickness of the plastic is thin and fragile in the tip, trailing edge areas, so my normal approach of sanding may be problematic. The pattern has to be strong enough to stand the rigours of multiple pours at the foundry.
I want to 'surface' the foil in an attempt to smooth it and strengthen it. I have just tried a spray polyurethane coat on an existing 3D print. But I was thinking that perhaps a resin gelcoat maybe better, that is stronger and fill the imperfections/roughness better. My question is, should I thin the resin at all( if so, how so?) and because the print plastic is a thermoplastic I need to be careful of curing heat. Or would a spray auto bog filler be better?




Around the base is remnant print plastic to be cleaned off. The box frame can be seen with its built in template guide to grind the Tuttle box to Powerbox.

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pepe47 said..
Just guessing here Fangy, but the surface looks smooth (satiny), and if so it may need some etch primer before the spray bog will stick.
It may be less labour intensive, as the resin will take up a fair bit of your (valuable?) time.

good point Pepe - and happily I have some spray etch primer here. Just wondering whether the acid will actually etch the plastic like it does metal??

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pepe47 said..
Have you got some waste you can test it on?

Yep. I only polyurethaned one side of the old print. So I can use the other side to compare

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keef said..
You can try plastic adhesion primer for plastics and bumper bars, K+H SURFACE TECHNOLOGIES

Nice one thanks Keef!

I found the plastic primer had a strong glue smell like contact adhesive ,the etching your getting on the print is probably thinners in the propellant, you could give the fin a light coat of 2pak maybe

Introducing the all new SAGGYFIN 28,
or What not to do to your 3D prints. The FangyFin 28 has had a tough time of it. The print thickness was too thin in the tip, so there was too much flex for my liking. I sanded, filled, and generally smoothed the print until it was ready for use of a casting mould. I was till a bit worried about the tip, so I added an expoxy gel coat. It was a fairly hot mix so I could get it off to the foundry sooner.
And there was my mistake. 3D prints are done with thermoplastic. There was enough heat generated with my use of too much catalyst (for this job ) for the plastic to soften and sag under its own weight...
Now consigned to the rather large bin of Fangy Fails.
Back to the printers for a new one, but at least this time it will be printed with thicker walls and will be a better pattern.
(I have not passed on the good news to the Minister for Finance and Excessive Windsurfing Expenditure yet ...it might be best to wait until she is asleep before I tell her. )






On the plus side, the 20 and 24 came up really well and now look brilliant. They are on their way to the foundry for casting.

that is gut wrenching ross hope all go well in the foundry

Keef - thanks mate, but I always fall back on; 'He who makes no mistakes, has made nothing at all'
Yoyo - thanks for the acetone tip - I will try it out on the aforementioned trashed fin and see what effect it has on the parts that are not covered in resin.

Got any thoughts on a 'Last Request' ?

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yoyo said..
Ross when I investigated 3D printing fins some years ago the technique for getting a smooth surface was to enclose the part in an airtight box with a cap full of acetone. Letting the vapor smooth out the ridges.
However with the horizontal stream flow grooves you may have got a better result if you just left them as is from a hydrodynamic point of view. Sort of a built in 3M V groove the American Cup boys paid a fortune for. Of course that may not have been possible with requirements of making mold for casting.

Yoyo- unfortunately you cannot do acetone vaporing on PLA prints, which is what my fins are printed in. It does however, work for for ABS prints. Thanks for the tip anyway.

Interesting stuff Elmo^. From what I can find out, the problem is made worse if the nozzle temperature is too high and printing a small object.

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fangman said..
What's more convoluted than a bowl of spaghetti? Getting a FangyFin. I have just been advised that by owners of the foundry I use for casting, that they are retiring and closing by the EOFY. I hope they will have completed a small batch of fins by then, but I don't think it will be the full amount I have requested. I will be given some method of continuation, but as yet I dont know what that entails. So, if anyone knows of a Aluminium foundry ( or metal art casting factory/studio) that is located in Australia that will do small production runs and really cares a lot about their end product, I would be grateful if you could let me know.

Fangy, have you been accumulating bad karma perchance?

Apco engineering in Hobart do non ferrous casting. Do some high quality work in the maritime space. apcoengineering.com/ could be worth a phone call.