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Hard Back Material Options - What material for large hull?
Hard Back Material Options - What material for large hull?
I've noticed that both Kevin McLeod and Steve Neill used plaster and burlap for thieir hull mould strong backs. I wish to do RTV moulds for a 97" hull and feel that at this size, plaster is just not practical. How about GRP instead? Comments & suggestions?
Steve actually uses Ultracal 30. Which is like plaster but easier to work with. I don't think it is any lighter though, and weight will be your enemy. BD.
sigpic"Eat your pudding Mr Land"
"I ain't sure it's pudden" 20K
Thanks Dave and Andy. I had figured that GRP might be the way to go but I wanted to be sure. As far as price is concerned, well, I've been hearing a lot about various RTVs on the market. Some are not too prohibitively expensive. I figure I might be able to get away with about a gallon and have some left over if I keep the RTV glove relatively thin. I'm a bit "gun shy" as far as a hard shell mould is concerned. Since this is going to be my first GRP lay up, I'd rather err on the side of caution! By the way, what is the minimum thickness of RTV that I can get away with? I've read Kevin's Oscar build thread with interest. His RTV system sounds very attractive for my application.
GRP is best. Takes longer to set up and is toxic. Which is why I don't use it anymore. I developed a reaction to GRP. Wear a mask, and gloves always. Get rid of any exposed material not being used like old cups, sticks and drips. Clean the cans off well before storing.
David Merriman uses it all the time as do many of the pros. It is truly the best way to go.
Thanks U-812. Just had a talk with Kevin. Turns out that the SmoothSil 920 that he used for his Oscar hull mould costs about $250 canadian for a 1 gallon kit. The Rebound 25 costs the same. That means about $500 for RTV alone!!!! EEK!!! I might be forced to go with a hard shell mould after all. I suppose the reason I'm nervous about the process is that I can envision going through all the work only to find that I can't seperate my hull from the mould! I suppose Its' a bit irrational in a way but through hard experience, I have learned that MURPHY never sleeps. I understand about undercuts and all, it's just that the last thing I need is a disaster at the last minute.
I think the last time I got a gallon of GI 1000 it was about 130 dollars.
Their RTV has been around for years and is reliable and bullet proof. Another words not too fussy or over reactive.
I first became familiar with it while working in the film biz and have always used it.
Might help. Sure costs less and you can get fast cat, thickening agents and more like the rest.
I should have used this on the type 17. But I did on my more recent Washington class molds and it turned out great.
You can also make a case mold. This is where you place a layer of clay even over the entire master to simulate the RTV glove in thickness. Then lay up the glass and epoxy. Once that sets up you can remove it and the clay and re key the mother mold back over the master. This is done by drilling holes and screwing the mother mold to the table before you remove it and then the clay. This way it goes back exactly in the same position.
Now once that's put back and screwed down pour the RTV in through the top of the mother mold. All you need it aa 2 inch in diameter mother mold. Do it slow and make sure you don't trap air. So in the end you have a nice case mold. Didn't waste any material and your RTV will go a lot further. Use water base clay and save it in a plastic bag. When you go to mix and pour your RTV you can compress the clay into a container and get a gauge of how much RTV you need to mix.
Sounds great U-812. I'm having difficulty understanding how one can avoid air bubbles using this method. I figure it might work if the RTV was almost as thin as water. I imagine that it must be a heck of a lot thicker though. Sorry for all the dumb questions. I figure that asking questions (even dumb ones) is a heck of a lot better than a very expensive screw-up!!
That's very true. There are no dumb questions in my book.
You can drill holes anywhere you want to relieve trapped air. Once you see the RTV coming through the hole all you got to do is plug it up with clay. You can also put on a splash coat before you put the case back over the master. This way all the tension is released.
You won't get any near the surface this way. Never had much of a problem with trapped in quite a few modls over the years.
The main reason the studios had us do it this way was to conserve on waste, thickness and weight. When you lay on the clay over the master visualize that your really looking at the RTV glove to come. Make sure it's nice and round. As smooth as you can get it so the RTV runs without obstruction. This again eliminates any chance of air bubbles.
Thanks U-812. Have to admit I'm just a bit leery (I'll get over it). However, if it's worked for you, I think it might work for me. Just bear in mind that I'm sort of trying to put all my ducks in a row. I'm not even at the final drawing stage. I'm just trying to develop a clear idea as to which moulding technique to use when the time comes. This is a dream project for me. I've been thinking about building something like this for over a decade!! I cerainly want to do it right.
Take your time. In fact do a small test to get use to the process and material. Make a small piece. You can buy very small amounts of the materials at most hobby shops. RTV and West systems epoxy. Experiment before you make the big plung.
That will take car of the leery part. You'll be find.
Thanks again U812. I'll certainly take your advice. I don't want to jeopardize a very significant investment in both time and money by going off half cocked!
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