alright nick, Ive done some pics to show what to do but fi theyre not clear I'll buy some POP and do a physical demo next week.
1. So the first thing is to get or make a box that fits the fin, pick something with at least 2 inches around the outside of the fin and at least 2 in deep. POP (plaster of paris) cracks easily, you can get a stronger type of powder called "stone" or even use cement powder to make molds.
All the pics are side-on.
[IMG]http://i153.photobucket.com/albums/s225/SURFFOILS/SURFFOILS%202011/1.png[/IMG]
Get your box, even cardboard will do because the box is only used until the POP sets.
Prep your fin by painting some thin coats of mold release on your fin, I use straight washing liquid and give it several coats and let it dry. If youre doing FCS fins they go in and come out of the POP OK, but Futures have that complex angles in the base design and you'll need to fill the gaps in with wax or risk cracking the mold when you pull the fin out.
Overfill the box with POP,sloppier is better than dryer because you will have more time to work with it and it will give a smoother shape. Give the box a good vibrate to settle the POP and lift bubbles to the surface and pop them. Youre trying to get Zero bubbles on the surface.
[IMG]http://i153.photobucket.com/albums/s225/SURFFOILS/SURFFOILS%202011/2.png[/IMG]
Once youve got a bubble free surface, push the fin into the POP, put it on lightly and wiggle it down to smoothly submerge it, but only just slightly below the leading/trailing edges all around.
[IMG]http://i153.photobucket.com/albums/s225/SURFFOILS/SURFFOILS%202011/3.png[/IMG]
now flatten out the POP shoulders around the fin and carefully expose the edges of the fin.
[IMG]http://i153.photobucket.com/albums/s225/SURFFOILS/SURFFOILS%202011/4.png[/IMG]
The registration grooves are needed to make sure the 2 halves of the mold go together perfectly later on. The grooves are shaped like an elongated pyramid or an upsidedown roof of a house. You can do the grooves while the POP sets or later.
Make 2 of them on either side of the mold 1" X 2" and about 3/4 in deep but no deeper because then the 'male' part that goes into the groove will be too thin and fragile.
Keep the sides as flat and sharp as you can. If the POP sets before you can make the grooves, you can do the grooves later, it will be harder and take longer but they will be sharp as you are carving them out of set POP.
[IMG]http://i153.photobucket.com/albums/s225/SURFFOILS/SURFFOILS%202011/5.png[/IMG]
So this is a little drawing of what the grooves look like from above, the sharper and neater they are, the better the molding process later on.
[IMG]http://i153.photobucket.com/albums/s225/SURFFOILS/SURFFOILS%202011/6.png[/IMG]
Now once youve got the fin settled and flattened the shoulders around the fin.
Let it set, dont frig around with it, plaster sets in 11mins but gains strength over time so let it set. Come back the next day.
Plaster also shrinks about .6% or similar so if you let it set properly overnight it will shrink away from the fin and be easier to get the fin out later too.
Once youre happy with the condition of the mold surface, apply several layers of seperator to the whole top surface of the mold, inc the fin, the shoulders and the grooves.
Now build up the sides of the original box by 2 inches.
And pour another batch of POP to cover the fin, make it at least 2 in thick for strength. flatten the top and leave to set overnight.
[IMG]http://i153.photobucket.com/albums/s225/SURFFOILS/SURFFOILS%202011/7.png[/IMG]
Thats how to make a mold for a double sided fin.
To take the fin out the next day, soak the complete mold in a warm bath of water and gently lever at the join between the 2 halves with a wide blade , dont stab at it like its an oyster.
The mold halves will open, prise the fin out of the mold, any additional plaster chips that come away with the fin just means that there will be some extra resin added to the fin. Thats no problem.
Get the fin out , scrupulously clean the mold and examine for any bubbles or holes, fix any holes by mixing a tiny amount of POP and smearing it in and over any holes and leave to set.
Then do the seperator routine again over both internal sides of the mold halves.
Its now ready to mix and pour resin and CSM into the mold and compress the halves together or you can tape the 2 halves together, stand them up with the Tabs in the air and pour the resin in thru the tabs. Youll have to drill a hole down to the Tabs for that.
Or you can hand layup mat/ foam and resin into the concaves on each half and then join them.
