FangyFin MkII Theory for insomniacs

Successful Casting Trial Run #1

A FangyFin Hollow 20cm has arrived fresh from the oven. They are on their way to me and hopefully I will get to see them for real, next week. I don't know the final weight. Three Hollow 20cm were cast. They are destined to be demo's, one in Powerbox and two in Tuttlebox, and we will set about trying to destroy them in the weed here, and see how they hold up.

The picture below shows the 3D printed pattern, the cast fin, and to the right the core box and its split pattern. The angled 'tongue' in the split pattern fits down inside the fin to create the hollow. The split cast core box pattern can be modified to make the hollow section much larger in future castings = less weight.




The next pic shows the results of various trial castings with different set ups and temperatures. The 20 cm fins are hollow, but both the 24 and 28 fins are solid. I will check them more closely when they arrive and set about finishing the 28cm's to be demo's as well. If the 24's are OK, I will go through my list and offer them at cost. They will need to be tapped, finished and the hollow manually drilled out. You are under no obligation to to take one because, I have been 'promised' there will be the real deal hollow versions of the 24 and 28 available in early November.

Thanks Swindy. I will need to get busy on the 28cm and drilling out the guts - being solid, I am sure it weighs more than your average bag of doughnuts!

EDIT for my post above; The first production batch of Hollow versions of 20, 24, 28 cm will be available early November. I am just hoping he meant 2017

Select to expand quote

nebbian said..
That's amazing!!! I take it they are sand cast? And you're using the 3D printed patterns?

So great to see some real prototypes.

The 3D patterns were quickly copied into a urethane resin, so Fred could try things and not be concerned about damaging the 3D prints. These were trials runs, mucking around with temperature, sprues and vents. So yes they are split sand castings. My understanding is the next few weeks will be consumed making dies for the fins as well as the core boxes, and then Fred can easily pour several fins per day. If his efforts with finish in the sand casting are anything to go by, the results should be really nice.

Select to expand quote

keef said..Your lucky to find fred fangman ,it looks like hes getting into the project, By copying the 3D prints do you mean he took a mold

I have to admit Keef, I got a bit confused as to what Fred was doing, but now I have a handle on it...sort of. Fred is a perfectionist who can't be hurried and there is a lot more to setting up a pour than I realised. The size of the vents and sprues has to calculated so that the metal goes in at the right temperature and cools evenly without porosity. So Fred took a mould of the my 3D prints and cast them in urethane. He then used these urethane copies to design the casting set up and change things where needed without the fear of damage to my original 3D prints. He then used a split cast sand technique to do his trial runs. In the picture are the fins immediately after cooling and knocked from their sand mould ( you can see the sand on some of them - top left especially).



Compare the difference with the first set of fins I had cast at the previous foundry in 2016 !
Fred ain't cheap or fast, but there is a lot less finishing required by me - happy days :-)




IF I understand Fred correctly he is know making the entire core boxes set ups, for the 24 and 28 . All of the core boxes will be made larger resulting in a larger hollow and lighter fin. They can be made larger because Fred has the expertise to do so - his original trade qualification was as a Pattern Maker - and there is still an excess of material from a strength point of view. ( I am really keen to pack the hollow with polystyrene foam and see how buoyant the fins are - it would be brilliant if I could get them to be more buoyant than a G10 layup of the same size) Once the core boxes are done I think he is then making dies for all three, but even if he is going to continue sand casting, I will be happy with the result!

Select to expand quote

fangman said..

IF I understand Fred correctly he is know making the entire core boxes set ups, for the 24 and 28 . All of the core boxes will be made larger resulting in a larger hollow and lighter fin. They can be made larger because Fred has the expertise to do so - his original trade qualification was as a Pattern Maker - and there is still an excess of material from a strength point of view. ( I am really keen to pack the hollow with polystyrene foam and see how buoyant the fins are - it would be brilliant if I could get them to be more buoyant than a G10 layup of the same size) Once the core boxes are done I think he is then making dies for all three, but even if he is going to continue sand casting, I will be happy with the result!

Buoyancy is displacement V's weight. If they are filled with air they will be lighter (and therefore more Buoyant) than if they are filled with any foam. But of course, if the foam is just there to keep them filling up with water, that will work!

In the versions I have now, where I drilled the hollow out manually, I have packed the hollow with polystyrene foam to just shy of the top. I then sealed it with a lid of window silastic. I started talking buoyancy because the fin is usually almost entirely submerged in the water, and the internal hollow acts as a buoyancy chamber.
BTW: The FF20 straight from the foundry is almost 700grams in weight. (arrived today :-) By the time I have finished polishing etc it should be around 600' ish I think.

Select to expand quote

fangman said..

decrepit said..
Also filling them with foam, should make it possible to have less alli

Mike, A while back you mentioned a two part foam ( I think?) Does it set to be structurally signficantly strong?

two part polyurethane foam is the same material as polyurethane surfboard core blanks. But in Surfboards the blanks are 'blown' under pressure to control the density.

Remember, unless you want the strength benefit of some sort of foam filling, AIR is lighter!! The most buoyant and lightest fin could be hollow and have air sealed inside it.

Select to expand quote

fangman said..

sailquik said..

two part polyurethane foam is the same material as polyurethane surfboard core blanks. But in Surfboards the blanks are 'blown' under pressure to control the density.

Remember, unless you want the strength benefit of some sort of foam filling, AIR is lighter!! The most buoyant and lightest fin could be hollow and have air sealed inside it.

Thanks for that Daffy. I was just pondering whether the thickness of the aluminium in the foil 'skin' could be lessened if there was a lightweight core that added strength.

I thought so. I recon the polyurethane 2 part pour foam WOULD add quite a bit of support for a thinner casting, if that is achievable.

Select to expand quote

decrepit said..

Yep, if you can seal the entry hole and create compression it can achieve fairly high density. You'd need to do some tests though. There'd be a point where getting it dense enough to support a very thin wall would distort the fin as it expands.

Sounds like more fun than blowing up cans of Baked Beans.

Select to expand quote

fangman said..

decrepit said..

Yep, if you can seal the entry hole and create compression it can achieve fairly high density. You'd need to do some tests though. There'd be a point where getting it dense enough to support a very thin wall would distort the fin as it expands.

Sounds like more fun than blowing up cans of Baked Beans.

Don't EVER use two pack foam in an old sealed water bottle to see how dense you can make the foam.

It becomes a foam grenade.

That stuff goes every where (especially when you are working nearby) and sticks like sh1t on a blanket

Fangman, im no scientist. But surely the weight of foam would almost negate its bouyancy. Air would have more bouyancy n near zero weight?

Can you fill them with helium?

Select to expand quote

fangman said..
. I have no experience with polyurethane foams and have no idea as to how dense the foam needs to be before it becomes structurally relevant. At this point in time, I will KISS principle and simply have an air core and a silastic plug, or a low density foam core to keep the water out.

Ok, so the foam in your boards is definitely "structural" it's probably around 13kg/m3 the two back urethane foam uncompressed is probably heavier. I think there is a data sheet somewhere that tells you all that sort of stuff. I'll have look later.

Select to expand quote

fangman said..

olskool said..
Fangman, im no scientist. But surely the weight of foam would almost negate its bouyancy. Air would have more bouyancy n near zero weight?

Yes olskool, you are correct. I have been experimenting on how to seal the hollow and stop the water getting in. So far, packing the entrance lightly with polystyrene foam for support for a silastic 'plug' over the top has been the best: easy and cheap.
With Fred's work on making the hollow bigger, I was having a bit of a thought bubble - What if I made the hollow so big that all that remained was an aluminium skin? Then use a foam core inside for structural reinforcement. It would then be a question of 'weighing up' whether a thicker aluminium and air core would overall be lighter than an alumiunium skin with a reasonably dense structural foam core. I have no experience with polyurethane foams and have no idea as to how dense the foam needs to be before it becomes structurally relevant. At this point in time, I will KISS principle and simply have an air core and a silastic plug, or a low density foam core to keep the water out.

Ross, taking the aluminium skin concept further, could you press 2 halves of the fin into a female mold from say 4mm ally plate, then weld the 2 halves together and fill with foam? Not sure how you would deal with the box end...

Select to expand quote

Stretchy said.


Ross, taking the aluminium skin concept further, could you press 2 halves of the fin into a female mold from say 4mm ally plate, then weld the 2 halves together and fill with foam? Not sure how you would deal with the box end...

I looked at doing just that really early on Rob. At the time the tooling cost in WA frightened me off. But in retrospect the costs probably weren't so bad...