I want to make a head mould and start re-heading some fins. Does anyone have any opinion on the mest medium to cast the mould in? I was thinking of using silicone mould making stuff like this. www.barnes.com.au/single-product/pinkysil-fast-set-silicone/
All opinions and info greatly valued.

Normally I use a split F/glass mold.

I tried using silicone but had problems with aeration, also found that expansion of the epoxy when curing would compress the silicone so the fin came out oversize.

For my next split molds (mine have been used and abused severely) I'll probably use Glass reinforced urethane
www.barnes.com.au/single-product/procast-polyurethane-resin/

The stuff goes of quick (5 mins) and you need to pour it down onto the item you are molding (so the air bubbles rise away from the mold face) quite nerve wracking when doing (because of the quick cure) but the results are great.

Fibre glass with gel coats starts chipping on corners after a while, but that's minor if you have air pockets in corners.

Have fun

Cheers!
I should have said I want to cast powerbox heads. Dunno if that makes it easier.

Possibly would do as the taper will just lift the fin above the box height if it swells in the mold. this can be filed back easily

If working with silicone cast it in something like a tupperwear tub (make sure there's a bit of a taper so the silicone will come out) that way it helps hold the shape when you are doing the fins.

If you have access to a vacuum pump try to de aerate the resin before you pour

Mine is really simple. Two wooden blocks with casting resin to make up the exact shape. Rub on the mould release, set fin and pour in the epoxy. Good for 5 fins so far. Nothing like Pepe's work of art ones but very cheap

Pepe's, like all of his work are F'ken awesome.

The casting resin is what I use for my fin molds

What filler material should I mix in with the epoxy to make the head durable? Microfibres? Microbaloons? Chopped glass?

Don't you normally use fish heads?

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Shifu said..
What filler material should I mix in with the epoxy to make the head durable? Microfibres? Microbaloons? Chopped glass?

Microfibres makes the resin bloody hard , not sure about how tough . Microbaloons will crumble and explode . Chopped glass would be strongest .
How strong do you need it ?
I think a big part of it is , how deep into the head you go . Deeper the better , I've seen a few fail because there only set 15 to 20 mm deep . Good fins go down enough to hold the barrel nut .
Id go microfibres.

Also interested in this to put power box heads on a few trim box fins. I had read you can buy a fin box and use that, has anyone done it that way? Also thought of just making an ever so slightly oversized wooden mould and just waxing it. I think power box would be a pretty basic shape to recreate. I'm keen to see some pictures of peoples moulds, especially how they have centred the fin in them.

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Shifu said..
What filler material should I mix in with the epoxy to make the head durable? Microfibres? Microbaloons? Chopped glass?

I generally use a couple of layers of 4oz f/g clothe
Resin I have some very fine glass fibre from our old gasket manufacturing plant, but realistically it's not needed.

With the glass cloth I put half the resin in the bottom of the box then put the cloth in and wet it out so that it sticks to the sides making sure it's seated well on the bottom, this gets rid of any air pockets which can show up on the outside of the casting.

For the last 5 years I've been using the polyurethane resin I mentioned earlier to do the fin bases, I had a bit of play in my alignment jig which I wouldn't find out till the next day it had f'ked up by that time the resin was hard so had to be ground off. The PU sets in 5 mins go off so you can see quickly if there's misalignment but it is soft enough to peel of with a chisel. The added benefit is it doesn't chip like normal epoxy.

This is my setup for making the mould. I made this some time ago then lost interest. I was going to wax (and smooth over anything that could prevent release) the fin head and make the mould directly off it.

That is soooo sexy

Ahhh, Master SHIFU !

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elmo said..

Shifu said..
What filler material should I mix in with the epoxy to make the head durable? Microfibres? Microbaloons? Chopped glass?

I generally use a couple of layers of 4oz f/g clothe
Resin I have some very fine glass fibre from our old gasket manufacturing plant, but realistically it's not needed.

With the glass cloth I put half the resin in the bottom of the box then put the cloth in and wet it out so that it sticks to the sides making sure it's seated well on the bottom, this gets rid of any air pockets which can show up on the outside of the casting.

For the last 5 years I've been using the polyurethane resin I mentioned earlier to do the fin bases, I had a bit of play in my alignment jig which I wouldn't find out till the next day it had f'ked up by that time the resin was hard so had to be ground off. The PU sets in 5 mins go off so you can see quickly if there's misalignment but it is soft enough to peel of with a chisel. The added benefit is it doesn't chip like normal epoxy.

Does polyurethane set like skate board wheels? Those things are tough as buggery.
The only time I have done this I used polyester resin with tons of short strand glass. It worked ok.

Just found this: resin-expert.com/en/guide/polyurethane-resin
It sounds pretty user friendly but not a great adhesive.

For fin head molding
Smooth-On '300' sets in 3-7 minutes with no air bubbles

Have tried 305, 370 and task 2 but these were not as good and more affected by humidity and the consequent air bubbles.
As used by fin manufacturers.

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Shifu said..
Cheers!
I should have said I want to cast powerbox heads. Dunno if that makes it easier.

I was successfully using this to convert fins to power box.
4boards.co.uk/product/spares-repair-shop/fin-box-adapter-head/