Foam core fin build

I am starting this thread from my office, but hope to post a few pictures as I get started this evening.

We have been using small cupped C-5 fins and also Scimitar 451's. The Scimitar's are only available in a natural composite and it feels like those are milled fiberglass suspended in epoxy or polyester. They are exceptionally heavy.

I want to lighten the fin up, without decreasing the stiffness, so I intend to use a relatively hard epoxy and mix in some Cabosil with the milled fibers, to lighten the total mass up.

Cabosil is fumed silicon dioxide. It's my understanding that due to its inert nature, it has been used in food such as kethcup as well as in shampoo and some cosmetics. When mixed into resin, Cabosil functions as a resin thickener amongst other things. For my purposes, Cabosil, by volume, is about 1/20th the weight of the milled fibers...it's my hope that I can reduce weight without sacrificing stiffness, by reducing some of the milled fiber content and replacing it with Cabosil.

Getting back to the build, the first part will be to create a RTV Urethane mold - as first mentioned by Lakewake Mark. The result, after pour, will be an exact duplicate of the original fin, only using different materials.

RTV in the RTV Urethane stands for Room Temperature Vulcanizing. I will make a two part pour mold, rather than a glove mold (brushed on) as I did when making the high density rail foam mold.

A quick overview of the project:

1) Build a small box to hold the RTV Urethane
2) Build up the bottom of the box 1/2 way with Clay. This is important as we will remove the clay and turn the box over, to pour the second half of the mold.
3) Inset the fin to be molded in the clay, sealing the underside by pressing it into the clay.
4) Liberally apply mold release to all surfaces.
5) Mix and pour the RTV Urethane into the top of the open box and completely cover the fin being molded.
6) Wait The RTV Urethane takes 48 hours to cure well enough to handle. Handling it too soon will distort the mold.
7) Turn the box over and remove the clay. The open cavity is now ready for pouring the second half of the mold.
8) Liberally apply mold release
9) Mix and pour the RTV Urethane into the top of the open box (which is the bottom turned upside down) and completely cover the fin being molded.
10) Wait some more.

Once the RTV Urethane has completely cured, it can be removed from the box and the fin taken out.

Clamping the two mold half's together will then form a cavity that can be filled with the resin/ cabosil/milled fiber mixture. Also, I may fill some of the core of the fin with a slightly shaped piece of corecell, which will be encased within the epoxy/fiber/cabosil mixture.

Pictures at 11 Ok, I won't stay awake that late, BUT later.

The first thing I did was to build a box to hold the mold material. I used the fin as a general guideline for dimensions. Take careful note of the placement of the fin base up against the one side.

The box will remain around the RTVU but will be removed when everthing is cured. To make it easy to disassemble, I used a hot glue gun to attach the pieces.

The box will hold the RTVU, but you need an idea of how much that will be. You'll want at least 1 inch of base on each side. Mark a line at 1 inch up and the another at two inches up. Next cook up a bunch of white rice.

Too funny. No, use the rice to measure how much RTVU you'll need. Pour it in to your box up to the 1 inch mark and dump it into a graduated measure that you'll use for mixing the RTVU. Note the amount of rice that the 1 inch mark takes. In my situation it was 26 oz.

Use a BUNCH of clay to fill up the bottom of the box to the first 1" mark. This will be removed later and that void will then be filled with RTVU to form the second half of the mold.

What it looks like mid-application.

After building up a 1 inch layer of clay, I placed the fin to be duplicated INTO the clay. Next, I smoothed the clay all around the fin so that I had a good crisp part line.

You'll notice the base of the fin is up against the wall of the box. This will be the opening to the mold. After we pull the two pieces apart, there will be a gap where the base is up against the wood, that is where we will pour or resin/cabosil/milled fiber mixture later.

It's not necessary that clay be smooth or flat, the two pieces of the mold will mate and in fact a little irregularity will aid in mating the two surfaces together.

The RTVU is a two part mixture. The Part B has a tendency to seperate, so that needs to be mixed well BEFORE mixing with Part A.

Using the volume measurement you got with the instant rice, pour 1/2 of that into a container of part a and part b. Strap on your iPod and MIX MIX MIX! Then, pour into the mold, making sure that your liguid reaches the 2" mark you made earlier.

Oh shoot!

Before pouring - spray everything down with mold release.

The lastly, if you are like me doing all your work outdoors, make sure that everything is level. The RTVU is a liquid at this point and will self-level.

You can see the brown mixture in the box. It is 1 inch thick. You'll notice that I sealed up the corners of the box using hot glue, this will prevent the mixture from leaking out.

The RTVU is a very slow cure, taking a full 7 days to fully cure, but it can be handle after 48 hours. It is tempting to mess with the goop before it's cured enough. As I mentioned above, if you do the RTVU will distort and your mold will be useless.

Check back in two days.

He he I like all that EPS mess on your backyard Jeff. Same here at my home.

Do you see the small sheets strew all over? Sheesh, I am heading to the dump today to get rid of all that stuff in the background and WHEN does the wind pick up? It'll take me six hours to chase all that stuff down.

Except I don`t have backyard but balcony so everything fall down and fly with the wind

Too funny Petr.

The box flipped over and the majority of the clay removed. I'll go over it with a wet cloth to remove the last traces of clay.

Fron this picture it's pretty easy to see how the clay created a void that I'll now fill with RTVU to create the senond half of the mold. The part as it's removed from the mold will still require some finishing, as there will be some flashing along the part line, but it'll mostly be an accurate reproduction.

Hmmmm. It's starting to get interesting now....

Jeff my mold making friend said to keep air bubbles from showing up you would vacuum the mixture to remove the air bubbles, he uses a big pyrex bowl with a homemade base with a vacuum line going thru base . pace the bowl on top of base and vac it down

Yeah, the directions for the RTVU state the same thing, but also stated that unless the accelerator was used, the bubbles would float up and away from the mold surface. If you look at the way that I molded this, the "part" was below the RTVU and in it's liquid state the bubbles traveled up and away from what will become the mold surface.

I can imagine in a production environment the vac would be a wise choice.

Top of mold poured - this will separate into two pieces, allowing easy demolding. I hate birds.

ok you hate wood, birds, ... what else?

Epoxy now

I took the bracing off to get the mold out this evening. The mold surface is perfect.

Next I cleaned up the residual mold release and applied NEW mold release. I also cut external braces for holding the mold together while the epoxy kicked.

Next I held the two mold half's together with rubber bands. I added milled fibers and cabosil until my mixture looked like I wanted it to. I got it a bit too thick on this first pass.

My epoxy started smoking about 5 minutes into this! It's totally cooked before I could get much of it into the mold. I mixed a second batch, but I'm afraid this piece is junk.

Yeah - the first part was junk. But it did demold like I had hoped, so testing the ingedients, I reduced the cabosil to 40 grams, milled fiber to 20 grams and mixed that with 5 oz of epoxy. It was thick but still poured into the mold.

Results of the test

That second mixture gave me a full part from the mold, but I have quite a few trapped bubbles that migrated to the surface of the part. I'll have to paint onto the mold surfaces before assembly and then pour to fill the remainder. I'm thinking that I will have to do it in two batches (one to coat the mold surface, two to fill the core), but possibly not if I can keep the epoxy from going off too soon. If I choose to do it that way, I may layup some carbon on the mold surface like the futures black stixx.

Getting closer!

in trying to rid myself of the bubble issue I came across some interesting products to eliminate them - might be worth a try for glos coats - look up eat-a-bubble from klockit.

My issue was more about mixing in the cabosil and milled 'glass. That created TONS of bubbles. Today I mixed the two additives in the resin and then let it stand for about 40 minutes in the 105 degree heat. That eliminated virtually all of the bubbls from the epoxy. Then I slowly mixed the hardener into the resin and only created a limited number of bubbles.

I also am attempting to create as smooth a surface as I can. I started by shooting several coats of clear resin spray on the mold surface. Hopefully that resin will cure and the affix to the gelling epoxy, laving me with a relatively smooth and shiny surface on the part.

My picture didn't come out, so just the description. Next I painted the surface of the mold with some epoxy. In brushing on the epoxy, I was certain that I had no surface bubbles.

Next I drew the epoxy from the bottom of the cup, where there were no bubbles present using a large horse sized syringe. injcted the mixture into the mold to ake certain I had no trapped air bubbles.

Right now, it's cooking. Hopefully it will come out almost perfect.



Off to Nationals in the morning!

These are crappy pictures, but it's a perfect reproduction. No bubbles and a smooth semi-glossy finish.

Now that I have a handle on the construction, I'm going to mov to the next step. I REALLY liked the FCS H2 fins, which have a divinycell core and are wrapped in Tex. I want to suspend a small section of corecell into the base of the fin and also into the fin itself, mostly down by the base. This will displace some of the epoxy mix and will hopefully reduce the weight of the fin.

This is a rendering of the placement of the d-cell.

We are off to Nationals so I won't be able to get back to this for a few days.

Now that I have the construction down, it's time to build the mold for the other fin.

I got a wild hair and decided to layup some 8 oz carbon on the mold surface...we'll see what it comes out like.

hey jeff you wanna build me a set of fins for my surfboard once you get it down? ill pay you.

Hey Johnny, long time no hear! Sure 'bro, if I ever get the process working, I'll make you a set.

The carbon fin came out pretty interesting, but I can't seem to transfer the picture. I'm going to try and reboot.

I finally got the picture to transfer. I'm not sure if the orientation of the fabric would make a difference, I would think that it should, but I'm not sure.

yea im back in school and just tryin to keep buisy. yea that fin looks sick! so your gunna try to put a foam core in it like honeycomb?

Not the honeycomb like the futures, no doubt I'll try d-cell and corecell and see which one comes out stiffer and lighter.

I did a bit of research and talking with some fin gurus and they opt for hand lamination of fins, rather the mold.

The vector fins, have a concave on the inside. Duplicating that requires make a convex form to laminate over. The production folks use a curved piece of plexi on a glass surface. Then laminate about 4 to 5 layers of glass at a time over the curve.

The next step is to cut the fin shape out of the sheet and then foil that with a grinder.

I wanted to fool a little with the amount of concave so I decided to create a buck from the mold of the fin. It's more curved than I want or need, but it'll work.

In a multi fin pod, the cant of the fins affect drive and release. Less cant = more drive, more cant = easier release. I also want to mess with the fin cant, straight out of the box. It's easy enough to heat the fin up and bend it, but I'd like to have a solid starting point and the ability to change it during build.

To create the buck, I bought some Magic Scuplt. It's a two part putty that when mixed is like epoxy and will harden overnight.

The mixed components are like play-doh

Spray the mold with mold release and after that has dried, pack it with the magic sculpt.

The buck after release and a bit of trim. I will lay this on a flat non-tick surface and then overlay it with several layers of 'glass. After I have built up - I'm estimating 35 or so layers, I'll cut the shape and start foiling.

Hmmm its getting interesting

btw There is some serious dirt on your lens Jeff.

There is? I think maybe it's scratches and actual epoxy

yep check that haze on the right from the centre of last 4 pictures

yep epoxy - part of my body is now covered with epoxy

I see it! Thanks Petr! You are a true shaper when you start peeling blobs of epoxy off your forearms along with skin and hair.

he he Jeff tell me about it, I did my first (ever) glassing of bottom last night and I must cut off hairs with epoxy from my forearm. Now while sitting at work I rub rest of resin from skin

That's funny - welcome to the club. I've laminated my shorts to my butt before. After poly kicks, there isn't much you can do but cut the stuff off. I had a slight reminder to use a rag for wiping my hands off, for about a month.

Surfdad, your killing me!

LMAO

Now I know I only want to be shaper and NOT glasser what a mess