Casting hybrid lamination fins

I’m working on ways to (re-)produce fins.

I’m going for the holy grail of making a complex shape centre fin so that it can be accurately reproduced with high strength, low weight, and minimal waste of materials. Maybe even good looks…

I started my first fin project a few days ago, by laminating marine ply wood and carbon/kevlar cloth with epoxy resin. I don’t think I’ll ever finish sanding it, because it seems such a waste of material, an enormous amount of dust is being created, and every fin will be different and potentially poor quality if the sanding goes wrong.

So I put my thinking cap on and watched youtube videos about casting moulds with various techniques.

The general plan that resulted from several days of meditating on the subject is to make half fins, then bond the two parts together before the epoxy has cured, to achieve a primary bond.

Having two half fin moulds will allow all sorts of manufacturing techniques that do not work with other moulding approaches.

I want to use solar curing Zerovoc resin as the first layer in each half-mould. This will leave a UV resistant outermost layer without too many bubbles, and ensure excellent surface detail reproduction. Next, apply one or more layers of fibres and epoxy resin, probably carbon/kevlar composit cloth and two component epoxy resin. Then, over-fill the void with styrofoam balls covered with more epoxy resin, cover with a layer of resin soaked cloth, and put a glass plate on it until half-cured. The two halves of the fin need to be made at the same time. When the resin is gelled but not fully set, remove glass plates, put the two sides of the mould together (with a bit more fresh epoxy) and press the two parts together until the resin has set. Then open mould and hopefully take out a near perfect fin. I imagine it will only need a light sanding along the seam line and a touch-up with Zerovoc along the seam line. And drilling of two holes.

The problem is how to make the moulds that can be used for the above production process.

I’ll try to document the progress (or failure) of my little project in this thread. It’s a work in progress and constructive criticism is most welcome.

I bought 4 glass plates 350 x 350 x 10mm with polished edges, the purpose will become clearer later on.

First step is to make a half model of the fin with a near perfectly flat side, for both the cast and the surrounding mould material. That is so that the glass plate will eventually sit flush on the mould and cast when the actual fin halves are being made.

Step one is to create a level working platform and mount the fin on it so that it is perfectly level. I used a self levelling laser level for the purpose. Note the rubber O-Rings under the first of two glass plates, to level the glass plate. Having two glasss plates on top of each other allows you to move the top one around without messin up the horizontal levelling of the bottom plate. Non-drying modelling clay is used to mount the fin perfectly horizontally:

Eventually I had a perfectly level and firmly stabilised fin on top of a layer of modelling clay:




The underside:




This is the little marvel that creates the red lines on the fin edges:



The purpose of this step is to allow the use of a self-levelling liquid to create a flat surface area exactly to the hight of the fin’s centre line. I hope Plaster of Paris will do the job, but that will have to wait until tomorrow.

very interesting

Casting with Plaster of Paris does not allow self-levelling as I hoped. I used square 19mm aluminium tubing to build the form:


The first pour had to be scraped out again before it set fully. There were several problems, including not quite enough PoP mixed, too much water in the mix (300g water and 300g PoP) and the general way that PoP seems to work. Had it been silicone, it would have been fine, because it looked like I poured just the right amount to reach the correct level. But the PoP then turns into a settling layer of solids, and a watery layer of bubbles and water on top.


So I removed the PoP before it set completely, and started again. The next time, I aimed at the mid-range of the mixture range recommended on the bag: 85ml water for 100g PoP. It says 70ml water for 100g Pop makes the strongest mix, and that you should not use more than 100ml water for 100g (that’s what I used in the first mix). Maybe I would use the 70:100 mix the next time, not sure. My reasoning was that I wanted it to be runny enough to achieve self-levelling. Well, I don’t think any longer that good self levelling can be achieved with any PoP mix ratio. A water layer build on the surface and it has a lot of bubbles that will not go away easily.

Using the 85:100  PoP mix I over-filled the form, expecting that water would again pool on the surface, and it did. But the top layer of the actual PoP was too rough and too bubbly anyway. And it was too high.



It turned out that the PoP can be modelled very nicely for an extended period of time (at least roughly 4-8 hrs after pouring), so that a flattish surface can be made by use of a scraper. The laser level was very useful again:







I hope the result is flat enough to allow the making of good half moulds. Time will tell…

Printing the two halves of the mould would be the smart way to do it. Because it does not need to be very strong, many materials could be used. I’ll have to look into that again one day.

Here is an interesting little article about Plaster of Paris, if you are in arush to get it to dry:  Short version: If you heat it to fast, it will crack or explode, but gentle heating to 65degC (150degF) is the standard method. The chemical reaction seems to finish quite fast (unlike concrete which should be kept moist for at least several days to set properly).

Here is an excellent write-up about working with plaster:


Don’t put the PoP mould in the sun to dry it.

It cause the fin to heat up more un the underside than on the top, expanding the underside until the fin tip lifted out of the PoP. 

It did settle back down after cooling under a sand bag in the shade, but there is now a risk that the silicone will creep underneath in the next casting step.

Having done a few thousand pop pours to make dental prosthetics,not to mention a few fins here and there,  it’s easier to just embed your object, build up the sides around the item, let it dry and then trim the block to be level later.

You should add the powder to the water, mix very comprehensively with a wide hand spatula to avoid the water/ solids separation or dry spots. Pour the mix gently and vibrate it to raise air bubbles before you invest/ embed the item.

Use warm water to get a quicker uniform set  rather than heat the block,  it sets via a chemical reaction and gets hot anyway, apply several layers of separating medium before you put anything into pop. For anything you’ll need a perimeter and base thickness of at least 1 inch for strength because it can crack easily.  There’s another similar product known as ‘Stone’ that’s yellow and twice as strong and with a finer grain.

Of course the Holy Grail is to glass a whole fin, both sides having a contoured surface. You’ll need a split Mold and be able to vacbag it to compress the 2 Mold halves together and extract the excess resin completely.

Thanks, Surffoils!

If I understand you correctly, than that’s kind of what I have done - level the block later.

PoP seems very workable before it is fully dry.

I made the PoP layer a lot thinner than you recommend, but all it has to do for this application is sit flat on a plate of glass for a couple of days. After that, I prefer it to break easily.

Before pouring silicone over it, I very carefully filled any small gaps around the fin contour with modelling clay, and made the fin tab a bit longer, too.


Are you copying a 3-D printed Roy Fin ?

What gave it away?

A gazillion bubbles…

I’m not so sure wether the vibrating table did anything other than scare me. I did not notice a massive increase in the imperceptivly slow rate of ascent of the bubbles. Hard to see if there are any left on the bottom, where it counts.

I used symmetrical objects as keys for the two halves of the mould, rather than using techniques that leave ‘male’ keys on one or both sides of the mould. This way, all key parts in the mould will be ‘female’ when the objects are removed. That way, a glass plate will be able to lay flush on each half-mould to compress a half-fin.

I used vaseline on the clay as release agent, and nothing on the fin itself. I hope the fin will remain stuck when I remove the glass and the clay tomorrow.

The vibration needs to be in a vertical direction rather that having a cordless drill with a hex key spinning around. That’s where the experience of mixing the pop without incorporating air in the mixture helps. But you’ve got a great new angle of  making brilliant fins, great work. Looking forward to seeing it all happen.

You are both clever and persistent. I wish you every success. 

All the best

I think you may be right about the direction of the vibratios needing to be vertical to make the bubles rise.

Maybe I can mount the eccentric Allen key so that it is orientated like the propeller on an aeroplane, rather than the rotor on a helicopter. But that will make the workbench jump even more than the previous setup. I need to mount it on something flexible, like foam or an air mattress.

However, this morning the cast looks like it may be quite good, majority of bubbles at the surface, which does not matter much, except that it will reduce transparency during the eventual fin-making process.


Now I need to figure out when to de-mould it.

The data sheet says demould after 12-24hrs at 25degC, but my garage is around 17degC at the moment. I did heat the two components (in a little heater/cooler eski) to about 33degC before mixing them, hoping it would make it easier for bubbles to rise when it is warmer and presumably more viscous.

Can de-moulding much later than at 24hrs cause any issues? Say a full day or several days later?

I’ve been running this one in parallel when the sun is shining. Purpose is also to make a negative 1/2-mould of a fin.

Steps so far:

1: Attach a handle to one side of the fin with hot melt glue.

2: Cover other side in vaseline.

3: Apply Zerovoc UV curing resin, set it, then apply some fibreglass to the still tacky surface, then apply another layer of Zerovoc and cure it, leaving the surface tacky.

4: Build a dam around the fin using modelling clay. Mix a small amount of out-of-date epoxy resin (60ml part A +20ml part B), apply some bracing and then cutoffs left over from the Kevlar/carbon/Marine plywood fin lamination.

5: After cleaning up the clay (major mess involving scraping, lots of soapy water, nylon brush and then a wire brush): Apply a Zerovoc + chopped glas-strands mix around the rim, to further strengthen the edge before de-moulding. Put in sun.

I’ll try to get the fin back out of this blob later today.

If it’s too cold in the garage take it into the house, if you can.

I made these fin-lets years ago with scraps or glass and excess resin. But I think I should do a quick tutorial for you using a double foiled Nubster fin.

I have got any plaster so I’ll use concrete.

Start with a takeaway container or whatever so you get an inch of support around the fin.

Here’s the hardware you’ll need.

Clockwise from top left…

Container for casting the fin. 

Plaster ( or concrete) mixing bowl. Fill the casting container with water and pour into bowl to mark an approx level so you don’t waste material. There’s nothing worse than mixing up 6 times more than you need and then throwing it out.

Spatula for mixing. If you whip the mix really fast you’ll incorporate air in the mix and that’s not what you want.  Fold it together gently.

A small but long blade to do the fine adjustments before it sets.

 Get some surf wax and fill in all the minor holes and routing. These shallow areas won’t cast very well anyway and they are the areas that crack first and also are a pain when you’re getting your fin out cleanly.

Plus, you’re copying someone else’s product and probably poorly,  so don’t leave their name on it…

Take thin slivers of surf wax, let each one soften in your fingers then push them into the recesses and routing. When it cools and hardens it’s easier to scrape it level to fit with the rest of the contour.

If you have a scalpel blade and know how to use it, even better.

We will polish it up to make it smooth later.


Molded fins have a join line where the two halves of the Mold  meet and along the leading edge it can be quite sharp and make It harder to remove the fin from your mold…

It’s not a part of the design so smooth it out all around the fin with an edge or blade.