Moulding fins

General Discussion
1

Thought I’d share my recent experience trying to mould some kiteboard fins. Its going to take several posts to get the photos up, but bear with me. Also, due to the lack of raked tips, you can use a one-part mould for these, as you could for keels. Most any other fin shape would take a 2-part mould. These are foiled a little thick on purpose, as on a kiteboard while it gives up speed, the grab helps with going up wind, something especially challenging for beginners like me :slight_smile:

Thanks mainly to Geoff - GreatWhiteNorth - for all his offline advice in fin shape, placement, mould making, compound choices, and on & on.

Thought this was a good time for this thread, since there’s been so much talk of fin making recently. I’ve tried several different plastics, all with unfortunate results. As a longboarder, I need a drilled-out hole for a screw. I’ve shaped beautiful fins from Polycarb, UHMW polyethylene, and FRP, only to have the hole crack out. But it sure does foil easier than glass & resin. I hate making glass & resin fins, exactly because of all the grinding & dust. And I can never seem to get clear enough resin over wood fins, plus I’ve had the same hole-cracking issue.

So some general notes here - mix the best of everything. Foil a plastic fin, use it as a master for a mould, and make 4 more out of epoxy & chopped glass. Mould the threads right in (works great, btw) and paint them to cover cosmetic issues of chopped glass & some bubbles. Plus, with a little heater & hotbox, its something I can do in my shop in the winter without having to heat the whole space. I’ve finished 2 so far - its so easy once you have the mould. And only 1.5oz of resin per fin isn’t going to break anyone’s bank.

And best of all - I built the little hotbox just big enough to fit the mould…and my booties :slight_smile: Warm, dry booties every day doesn’t hurt my feelings at all!

So, with apologies for the rembling preamble…and on to the photos.

Ben

So first thing is to cut the outline from a piece of 3/8" polycarbonate. KB fin screws are 1/4"-20 so you need 3/8" to fit them in. Kiteboards are drilled with 2 1/4" holes, 3/4" apart, so fins are usually made with 3 holes all 3/4" apart, so you can go back & forth a notch.

Drill & tap the holes before foiling the master, because its easier to clamp when its flat.

Drill:

Tap:

And since that worked, I used the rest of the polycarb to knock together the moulding box. 5 minute epoxy puts it all together if you rough up the plastic a little.



2

With the master tapped, its ready to foil.

I believe in clamping down my sander & holding the fin to foil it…

All done. I left it just a little thicker than necessary on the ends, so I can lightly sand the finished glass ones for a nice, paintable surface.



3

Fit it into the box with a little lid to hold the screws

Pour water in to measure the volume of moulding material I’ll need

Looks like 150 ml.



4

Mix up 150 ml or so of RTV silicon (Room Temperature Vulcanizing)

Fill the box & push in the fin

Into the hotbox for a couple hours



5

All set up, cap & screws removed

Pull out the master

Again, pour in water to measure the volume of epoxy I’ll need

Notice I moved the screws forward a little for the glass ones…



6

Its going to be 1.5 oz of epoxy to fill the mould

Nothing will stick to the silicon, but I still have to wax the sides of the box, the lid, and the threads of my screws

I’m just mixing in chopped glass with the epoxy for some strength



7

Fill the mould, tape down the cap, tap out the bubbles with a hammer. If I were doing clear fins, I’d subject the resin to vacuum a few times before pouring it in - that would take out all the bubbles perfectly.

Back into the hotbox

And bake until done :slight_smile:



8

Next morning, extracting the bolts is easy and the epoxy female threads are very, very good.

Give the fin a little sanding & re-foil the back end for a smooth release. No need for power tools here, just a few runs along a block.

And a shot of paint

Easy peasy. Only 3 more to go :slight_smile:



9

Ben, those turned out really nicely. Where did you get the molding compound?

10

nice one benny

i just want to add that the mold itself could be cast into a metal mold for brass infusion molds

i think Rio Grande sell rtv in the states

i get it from jewellery suppliers

i havent molded fins before but have played with casting and rtv and vulcanizing rubbber for many years.

this is a great thread,thanks again benny

now can we just design a simple home made plug as well!

12

Yeah, Spuuut, part of the reason I bothered to take photos along the way was because of all your recent stuff about fins. Between you, Roy, blakestah, finfektion, wal2, tomatdaum, etc. we’ve covered so much cool new territory, that I just had to try something. I’m no pro, but I’m willing to tinker with just about anything as long as its cheap :slight_smile:

Silly, I like the silicon because its soft enough to wobble the cured fin right out. I even think I could cast big stuff like upright pivot fins or D fins, as long as there wasn’t too much rake to the tip.

Dan, the RTV silicon is from TAP Plastics. They actually have a pretty good little video section on their website on doing this stuff. Between what Geoff emailed me and what I saw there, I was totally confident to jump right in. Its kind of fun and makes me feel productive. And its seriously cheap, compared to retail kiteboard fins at $20/each!

Silicon RTV:

http://www.tapplastics.com/shop/product.php?pid=61&

Make a mould box:

http://www.tapplastics.com/info/video_detail.php?vid=2&format=windowsmedia&

Using silicon RTV part 1

http://www.tapplastics.com/info/video_detail.php?vid=8&format=windowsmedia&

Using silicon RTV part 2

http://www.tapplastics.com/info/video_detail.php?vid=9&format=windowsmedia&

14

That’s funny about the dental stuff…my wife’s dad was a dentist and she used to help out sometimes. She saw my air bubbles in fin #1 and said I needed some little shaker tray that they used to vibrate out the air bubbles. Any idea what search terms I’d use on eBay to find such a thing?

I’ve certainly thought about a clamshell mould for bigger stuff now. Top (base) open just like these, so I could pour in the resin with it already clamped shut - just have the mould in 2 pieces for extracting the finished pieces. Maybe later :slight_smile:

Double-foiled center fins would be the main goal, but my fins tend to be aroud 9-10" long and that’s a lot of work (no wonder I hate foiling panels!). Of course…I have a couple beautifully, tenderly, lovingly made plastic fins with f$#^&*g broken off screw tabs…they’d make pretty good masters…

Thanks for the kind words on my gear…but there’s a reason I post such small photos. :stuck_out_tongue:

15

Benny1,

I use an old palm sander to vibrate air bubbles out of small concrete pours(table tops etc). I run the factory foam base of the sander along the sides of my form and it works very well. Smaller projects like this may benefit by adding an additional softer foam pad to the base.

Anyway, NICE JOB!

7/10

edit to add; actually any small motor with an eccentric weight on the shaft could be mounted to a tray to make a vibrating platform for such uses I tink

16

Nice work benny1,

                       Do u get any bubbles around the thread zone

even milk bubbles[tiny]?

                        Do u think a fin that is 100%finished would be better than 90%finished? 

                        Tint your resin before u pore than no need to paint. 

                        The silcon your using is top of the tree down under try white general purpose just as good but cheaper. 

                         To get  the bubbles out buy a paint shaker

or stick to vibrators to a dinner plate. [lol]

Regards Rod

finfektion

17

“I’m no pro, but I’m willing to tinker with just about anything as long as its cheap :)”

…I can relate !

thanks HEAPS for this thread , Ben …ESPECIALLY the photos !!

I can tell that i will have to try this one day …sure as heck beats laying up multiple layers of cloth and resin , and grinding them , and getting itchy from fibreglass dust !

…keep 'em coming , mate !

cheers

ben

18

Rod & 7/10…you gotta know its every fin guy’s dream to solicit a post from the guys who don’t post all that much, but really know their stuff. Thanks!

7/10 - funny… I have a homemade concrete vibrator that works as well as any small stinger I’ve ever seen - its the 16" tree cutting Sawzall blade with a 1/2" nut & bolt drilled through the end. Put it in a Sawzall, drop it in the mud, perfect vibration. The palm sander idea is great.

Rod, too many bubbles on these to tint the resin & hope for the best. But that’s definitely down the road. Maybe someday, even layered tints - base to tip - like Lokbox & Austin have showed on here. For now, thanks so much for the encouragement.

Chipper…oh hell - I knew this one would get your attention. :slight_smile: Cheap bits, no waste, something you can do inside an apartment in all seasons, photos, fins… Its got Chipfish written all over it!

19

very, very cool benny. a fascinating thread for someone like me who’s only ever played with fibreglass, resin & wood.

20

I build trolling lures using a lot of the methods you described in your fin reproduction process and one trick I learned is to gloss and polish the master lure head or fin in your case then paint it with a thin layer of the mold material (making sure there are no bubbles), let it dry then pour the rest of the silicone to fill the mold. The thin layer will prevent bubbles from sticking to the master and it will come out of the mold almost perfect (once out of the molds I wash the lure heads in acetone, sand them with 400, then polish them on the wheel).

Great idea with the plastic box, I’m going to put some together in the near future.