Ok. I know it's been a while, but I've been hard at work making the RTV mold for the main hull. It's been a slow and tedious process, but I want to make sure I get it right. The material for this boat's mold is so expensive, I don't want to have to do it again.
From last time, I redid the second side of the mold for the turtleback out of epoxy-glass instead of the RTV so it would have a more definite and reproducible affect on creating the edges of the turtleback where it meets up against the hull. Here is that mold now pulled apart showing the rubber mold of the top of the deck next to the hardback.
This next picture shows a close-up of the key indents in the hardback. I show this to make a point on a mistake that I made.
Now of you remember, or look back, at the little nubs I made on the RTV part of the mold, I used little pieces of straws to create them. This made very tall, round nubs that were flat on the ends. You can see here that the resulting holes are very deep. When I tried to put this rubber part back into the hardback, it was near impossible to get all the nubs all the way back into these holes. The reason was because they were so deep and straight that air was trapped in the holes and wouldn't let the rubber go all the way in because the rubber sealed it off in there. There was no where for the air to go.
I ended up cutting the nubs down to be much shorter, but I think I'll have to drill holes though each nub hole to allow the air to escape once the nubs are put into them so they seat down completely.
What I learned from this is to make my keys more shallow. And I think the ideal shape would be a half sphere which will give the best chance for air to get around the key and let it seat in a hardback properly.
That leads me to something I saw on a How To video on line about making keys. It said to poor some of the RTV material, or whatever you're using to make your soft molds, into a flat dish/container of some sort to a depth of between 1/8 and 1/4 inch. Then once cured, take the slab of RTV out and cut it into pieces to use as keys.
Once you've got all the layers of RTV built up to where you want it, in my case the manufacturer recommends 1/2 inch thick, while the last layer is still tacky place the keys onto the surface. Use some thinned material and brush in over the keys to seal them on and soften the shapes a bit. Here they are on my mold.
Now what's taken so long was building up this RTV. I brushed on the first coat to make sure i got all the detail captured. Then i spread thickened TRV on with a putty knife for two more layers. This built up a little more than the brushed on layer, but each coat ended up so rough, because it was so sticky to the putty knife, that it was surely going to have too many sticking up pieces and undercuts it would stick to the hardback that would go over it. So I put the remainder of coats on using the brush. These coats were no more than 1/8 inch thick, and since it takes 4 hours for each layer to cure, it took many nights to get it all built up enough.
After it was all cured, I decided to make the hardback a two-piece mold just to make sure there would be no undercuts that would get caught to it. So I had to build a dividing wall down the center to separate the two halves.
Then for the hardback, I decided to try the Smooth-On brand Plasti-paste that Big Dave recommended. Mostly because the cost compared to all the epoxy-glass I would need to use was so much cheaper. One gallon of this Plasti-Paste is $55. One gallon of epoxy resin is around $120; plus all the glass cloth. Although I know it won't take a whole gallon of epoxy to do this, it does take a long time to cure. This Plasti-Paste sets up fast, and says it can be demolded in 90 minutes—wow!
This stuff worked pretty good, and it goes on very easy, and does cure very fast.
I know it is full of fiber shreds, which gives it good strength, but i am still worried about a mold this long holding together, so I covered the whole thing with one layer of glass.
So one half was done, but it took the whole gallon of Plast-Paste to do it.
Before I could add the second half, I needed to make some keys in the first half. I used a recessed screw bit to make female keys in it.
It was July 3rd, and I had the long three-day weekend ahead of me to get some good work done on this, but through mail-order I won't be able to get more Plast-Paste until the middle to end of next week. I'll never make it to have a completed kit by regatta time if I wait for that, so I went back to making the other half from epoxy-glass.
This time, to build up the angles and sharp edges where it goes over the keys and the edge next to the board, I added some Cab-o-sil, silicon filler, to the resin, and built and filled these areas to make them softer shapes so the glass would drape better over them. THis will keep air bubbles out and keep it stronger. I used 4 layers of glass, with two of them place on a bias. That is the weave of the glass turned 45 degrees. That is supposed to make it more rigid. Stable in 4 directions instead of just 2. Here it is complete.
Once cured I can turn it over and start on the other side. While I wait, I may cast some of the other parts.
I don't know if I'll post any more before the Sub Regatta because I want to use all my spare time to work on the model, instead of resizing and uploading pictures of it. But even though you may not hear form me until after August 4, I will be working hard.