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 Post subject: Building fins
PostPosted: Mon Apr 16, 2012 4:31 am 
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Hey guys looking for a little hands on know how.

I've done about 4-5 boards now, that I like and am abusing the crap out of, as a result I bang through a number of fins and find the cost of these to be rather high. In addition after seeing a few racing vids I was pretty impressed with the speed of the race boards and would like to build one of these to add to my quiver, however the cost of racing fins is RIDICULOUS. for something I'm probably just going to use as a high speed lawn mower.

I've researched mold making a little bit and have convinced myself that stealing a mold from an existing fin design or prototyping and then casting identical fins of my own design will be the easiest method.

I get how to make a 2 piece mold for some 3d shapes. But it appears I have a number of choices from what , I can make the molds out of

a) fiberglass
b) clay
c) silicone
d) pure epoxy.
e) poured epoxy with some type of filler.
f) Aluminum (Not an option no access to a cnc machine)

I've experimented with fiberglass and clay. Both created a decent mold of my part.
However I'm not having much luck getting a casted epoxy based part to release easily. It destroyed my clay mold and I expect my fiberglass mold will require signiciant bending and possible buckling to do the same. A pure epoxy mold I suspect I'd run into the same problem and not get the part out when I cannot bend the mold.
n addition the surface finish of the glass mold I popped off a smooth fin is lacking with visible and has significant visible weave despite my use of excessive amounts of resin.
If I could do it out of aluminum I would simply add threads to pull the mold apart

I'm currently using TR 104 hgh temp mold release wax. Applied in 5-6 coats and allowed to "set" inbetween coats as instructed. This is horribly time consuming and the end result is still a shitty release.

Are there any spay on mold releases that will work with epoxy and give me better results such that a clay/fiberglass mold in more plausible.

Is a tough release just par for the course and I should use a flexible silicone mold
I understand epoxy fins will not have a lot of structural strength and So I plan on adding reinforcments.
Thinking short strands of carbon with some thickener for the epoxy to try to keep it from becoming a unworkable, black hairball in goo. Anyone have any experience?
Better suggestions than epoxy?
Thanks for your help


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 Post subject: Re: Building fins
PostPosted: Mon Apr 16, 2012 5:12 am 
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Silicon RTV mold with a mother mold. No release would be needed. I have thought about experimenting with making some fins of my own. I would how ever go with making a single bar of pressed fiberglass and then cut and sand the individual fins out of it.


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 Post subject: Re: Building fins
PostPosted: Mon Apr 16, 2012 11:19 am 
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Joined: Tue Feb 07, 2012 8:56 pm
Posts: 26
Hi,
check your wax expiration date... I have faced the same problem in the past and lost one mold.
Best results can be achieved using wax systems. There is no need to wax the mold every cycle. 2-5 parts can be made in one application.
Follow the mold preparation procedure: http://www.finishkare.com/dynamic_image/1248797021.pdf

Casting mold for epoxy fins:
Take a thin sheet of aluminum cut a precise hole and press the fin model (sealed screw threads) into it. This will be your mold separation plane. Fin surface must be perfect. Follow the new mold preparation steps. Apply a mold gelcoat (epoxy based) 0,5 - 1 mm thick. Wait 5 hours at ~30 C. Apply glass fiber and epoxy layers.
Drill two holes to pour the epoxy in and four more for alignment pins in the corners. Separate two sides. Take out the fin model and aluminum sheet. Install aligment pins. Wax two pieces of female mold carefully then clamp them together again. The mold is done.
The third piece of mold could be done to form screw treads but its to complicated.
Try to cast one make screw treads and you will see why they are expensive.

Racing fins are usually CNC milled from a solid sheet of G-10/FR4 (Rista fins). Low quantities = high price.


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 Post subject: Re: Building fins
PostPosted: Mon Apr 16, 2012 11:34 am 
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You can also try PVA spray over wax layer for better release.


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 Post subject: Re: Building fins
PostPosted: Mon Apr 16, 2012 12:45 pm 
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Location: rhode island
epoxy is pretty agressive compared to many mold releases.
does the TR 104 labeling say that it is suitable for epoxy?
PTFE based work well.
silicones work well, but can easily contaminate surface that you may want bonded.
obviously, geometry of the part can alway make things difficuly.

in the end i would be willing to try just about any mold release specifically marketed for epoxy use.
(and follow their directions)
if that's where you are now, i stand corrected.




however,
when comparing cost and time, it's tough to compete with these guys

http://www.omnitech-engineering.com/fins/


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 Post subject: Re: Building fins
PostPosted: Wed Apr 18, 2012 3:22 pm 
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Location: Teahupo'o, Tahiti
Do you think any material other than G10 will hold up to the stresses put on fins this length? Lamborghini and Callaway are pioneering a new carbon moulding teqnuqie called "Forged Carbon" which is basically chopped up carbon material that is pressed into a mould. It allows for much more complex shapes and is cheaper to produce. Similar to your black hairball goo. I don't see this process being much stronger the traditional carbon layup method though.


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 Post subject: Re: Building fins
PostPosted: Wed Apr 18, 2012 4:57 pm 
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Beware the exothermy and thixotropy escalation of carbon kitty hair for molding.
Personally I found CF made normal room temp laminating resins (west 105, RR 2000 etc) unmanageable for pouring into one part kite fin molds before it made them strong.
Your mileage may vary, good luck!


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 Post subject: Re: Building fins
PostPosted: Wed Apr 18, 2012 6:12 pm 
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Joined: Tue Jan 02, 2007 11:50 pm
Posts: 274
Location: Ottawa, Canada
I know a guy who took a mold from some surfboard fins and cast some fins with it. Sorry no details on how he was doing the release, but I can tell you that the first fins he made with a couple layers of glass and mostly resin broke pretty quickly. If you do go with the mold method, make sure you get a good amount of long fiber into there, aligned in the right direction.

I'll be interested to hear how that goes. I've also been hearing about boards like the Airush Sector, and have considered making myself a "Lawn Boy" for days that otherwise are too light to ride. I'll hopefully try a couple this summer before i worry about making one.

Another option would be to start with a thin corecell or other foam in the core, and carbon fibre outside. I have a couple windsurfer fins made like that, not big blades, but maybe 24 cm wave fins, and they're great and also very light. The outside might have come from a mold though, I've no idea. http://www.recreationalcomposites.com/Lightningfins.htm

Peter


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 Post subject: Re: Building fins
PostPosted: Wed Apr 18, 2012 6:27 pm 
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Chopped strand mat is not an option for demanding applications however most twin tip fins are made with chopped fibers and resin.
Exoterm is not an issue for small fin casting. You can use infusion epoxy resin if viscosity is an issue.

Racing fins are totaly different world. Autoclave cured unidirectional carbon fiber parts are far more advanced then G10 sheet ( plain fabrics of glass fiber pressed together ). Almost half of fibers in G10 huge fins are not loaded seriously (transverse direction fibers).


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 Post subject: Re: Building fins
PostPosted: Wed Apr 18, 2012 6:58 pm 
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JMF wrote:
Do you think any material other than G10 will hold up to the stresses put on fins this length? Lamborghini and Callaway are pioneering a new carbon moulding teqnuqie called "Forged Carbon" which is basically chopped up carbon material that is pressed into a mould. It allows for much more complex shapes and is cheaper to produce. Similar to your black hairball goo. I don't see this process being much stronger the traditional carbon layup method though.



Not sure about "forged carbon" (patented in the games and toys..... not in materials 8) but Short strand injection molded composites aren't anything new, However they require equipment that will NEVER be present in my garage. And it's known they are NOT stonger than a lay up due to the discontinuities in the fiber and tendency for pull out if the length is not long enough.
However when you say "fins this length", figure your talking racing fins. If I were to tackle these I'd certainly overlay whatever I made with unidirectional carbon for structural stiffness. I like the idea of using a mold cause I don't trust myself sanding anything by hand well or consistently let alone aerofoils.


BWD wrote:
Beware the exothermy and thixotropy escalation of carbon kitty hair for molding.
Personally I found CF made normal room temp laminating resins (west 105, RR 2000 etc) unmanageable for pouring into one part kite fin molds before it made them strong.
Your mileage may vary, good luck!


Re Exotherm, thank you VERY good reminder.... I needed it.... :oops: .
Was hoping some cabonsil (or like) could help keep the kitty hair suspended as it warms up and gets runny.
Played with hair and epoxy when podding inserts and know it'll never pour it. Was thinking the "kitty hair" for flat sided outside surf fins where I can "scrape" it into the mold and squeeze out excess. For 2 piece molds was gunna try making it nice and thick scrape it in each side add a bit extra, slap the two molds together and squeeze out any excess into the pour holes. but at this point I still gotta work on the mold release.

Just playin right now. see if something works.
More of a learning exercise than a real time/cost savings for now.
but might make my money back if I ever make a set of race fins.

I can't imagine these big knives lasting long the way I ride, I COULD be more careful but that would cut into my fun.


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