Making a Fin Mould - Stage 1

Making the Parting Plane & Waxing the Fin.

Please view in full before starting as you may need to complete some of

the later slides such as fixing nicks in the fin first.I hold no responsibility for you damaging a perfectly good fin!

You will need some car filler(bog) for the mould and some metho in a squeeze bottle for cleanup.

Make some spatulas and filleting tools with rounded and chiselled ends.

Start by tracing the outline of the fin and head onto your melamine coated board. Try to make the line neat and offset it about 1mm or so. Cut out the inner section by drilling 10mm holes in the corners(but not outside of the outline).

Use a jigsaw to cut the inner section using the 10mm holes to start and finish each cut. Once the section is cutout clean the edge up to the ouline using a half round file. Place the fin in the cavity and file any edges that catch on the fin.

Now trace another outline of the fin and head onto a second melamine coated board. This time offset it 15mm from the fin outline. Cutout the inner section using the same method as the first sheet. I did not offset the board enough in this image and recut it at a 15mm offset afterwards.

Dont worry about the board chipping - these will be filled later with bog.

Place 3 strips of 20mm masking tape on the underside of the fin as shown. You can use more tape if you are not confident with your use of the bog and its placement.

Be sure to wrap to tape over the leading and trailing edges slightly.

Remove enough modelling clay to make 3 x 15mm balls. Work it in your hands until it is warm and soft. Place a ball on each end of the fin head. Push them down until they are stuck to the tape.

Place a ball on the tip of the fin. Push it down until it is stuck to the tape. The reason 3 balls are used and not more is so that the fin doesn't have any twist or flex added and turn out assymetrical.

Turn the fin over and place it in the board with the close fitting outline(base board). Make sure that the modelling clay does not fall off. Gently push down above each of the 3 blobs of clay until the leading and trailing edges of the fin are on the same pane as the top of the board. The top of the board will be the parting plane - the plane at which the halves of the mould part or seperate. The fin head may still have have a small ridge visible which may be the part line when the head is moulded. This can be used as a secondary check to the leading and trailing edges.

Use a ruler to extend the parting plane. It should line up with the mid point of the trailing edge. The trailing "Face" could be up to .5mm wide.

Once the fin is correctly positioned you need to fix the fin to the base board. At the moment the board could be removed and the fin would be supported by the 3 balls. Warm up some more clay and make at least 7 tags which will hold the fin to the board. Make sure that you do not press down enough to move the fin. If you do you will have to start the step with the balls again.

Now slide the base board on the work surface so that the balls unstick. Stand the base board on its side and look down the parting surface to ensure that the fin has not moved in relation to the board.

Don't worry if the balls stay on the work surface, you won't need them again after this step.

Mix up a small amount of bog, about the size a heaped desert spoon. Use the rounded spatula to do this. You can use the offcut from the base board as a bog board.

Place a bog fillet along the head of the fin.

Place a bog fillet at the tip.

Place some bog tags on the middle tape strip.

Allow the bog to cure to stage B, when it is rubbery but workable.This should only take a few minutes max.

Once the bog has cured enough to support the fin scrape of any excess that is on the bottom of the board. This will allow the board to sit flat on the work surface. Use the square or chisel end scraper.

Turn the base board over again and check with a ruler on the parting surface that the fin sits as intended. Once you are satisfied pop the fin off of the bog tags by gently pressing the fin on the masking tape strips. Remove the masking tape from the fin.

If the fin head(tuttle in this case) has been sanded or modified it is a good idea to build it back up to its original shape as you are going to make copies. Use a head mould(I will cover this in a DIY article sometime) to re-mould the areas that have been removed from the head. Alternatively, build up the head with bog and sand using a fin with a good head as a guide.

As you have seen in the previous slides, I only decided to fix the head after I had moulded the support tags. I will need to sand the ends of the tags down to ensure the fin will sit flush now that I have modified the geometry of the head.

You can see where the ends of the head have been sanded roughly. You want to build up these areas only. The select fins have a thermoplastic head, I think it is nylon. The bog will bond very well to this but it is a good idea to roughen up the areas that need to be built up.

To build up the ends I have waxed the mould and fitted the head of the fin. Only place bog on the areas to build up as too much will stop you from closing the mould correctly.As you can see at the top of the fin this is a Deep Tuttle mould.

The head has been repaired and the bog sanded with 240 grit paper. If there are any large nicks out of the fin it is a good idea to repair these with the spare bog you have. Any small scratches can be sanded down more easily in the mould haves as they will be the negative of the scratch ie a small ridge.

Once you have popped the fin out of the base plate and cleaned any splatterings of bog(!) it is time to wax up the fin ready for stage 2. You can get mould release from moulding or fibreglass supply stores. Most floor waxes from a supermarket work equally as well.

Work the wax onto the surface of the fin using a soft cloth or foam applicator(this may come with the mould release). Use large circular strokes and be sure to cover the entire fin. Allow the wax to dry or harden, it will turn cloudy when this has occured.

Once the wax has hardened buff it off with a soft cloth. You may need to keep using a clean area of cloth as the excess wax is picked up. You will know when this happens as the cloth starts to grip. Re-apply the wax and repeat the cycle at least 4 times.

That is the end of Making a Fin Mould - Stage 1Stage 2 will be coming soon...