Balsa Molding

I'm trying out doing molded fuselage blocks. I've read a bit about it, but thought I'd ask what other's techniques were. I've got the blocks shaped and sanded, pretty much how I would have them before hollowing them out were I doing a normal construction.

What's the best way to undersize them so I can use them as molds? Is it better to shave the bottoms or try to take the material off the already shaped surface?

Any great tricks?

Although balsa can be used for the mold, he generally uses foam, and has used both white and blue foam. It should be noted that foam does not carve well. However, you can band saw it to its basic dimensions, and then sand it to its final shape. Use coarse sand paper to start with, then move to finer grits as the carving progresses. You will need, at some point, to put a plywood stip under the block that is to be used as the mold. 1/4 inch plywood is fine, although size is not crucial. You just want to make sure that it is stiff and will not twist, because we are going to slice the mold vertically to fit the formers.

Yes, we are going to need formers for the molded piece, and the easiest way to fit them is to slice the mold at the point where the formers go (1/8 inch slice if you use 1/8 inch formers) and then slide a piece of balse into the slot and mark it so you can cut it to the exact shape needed for fit. If you do this before you finish carving, you can slide a piece of wood or paper into the slot, mark it, then turn it around and check to see if both sides of the mold are even. If not, sand away on the high point until it becomes symmetrical. It is good to place the mold on the plane periodically, also, as you progress, to ensure that the shape is correct and that it will be properly undersize all the way around.

Now, on to molding. Matthew normally uses three 1/32 in thick layers to form the actual part for the airplane. This ends up with a piece of 3/32 thickness, that is both stronger and easier to mold. Cut the pieces oversize to mold them. You can cut them to actual size later. Take the first piece, soak it in water, lay it on a towel to take off the excess water, and then form it around the mold while wrapping it in gauze bandages or old rags. You want something that will hold it tight to the mold, but will, also, allow it to dry. Elastic bandages work well, also. After it is wrapped, let it dry for 24 hours, then mold a second piece over the first. Let this dry, then mold the third over the first two. After all three pieces have been molded and dried, remove them from the stack and glue them together with a thin layer of yellow glue. Put this sandwitch back in the mold and let it dry again. Cut the formers to shape using the slots in the mold to mark them. Then glue these into place, trim the wood, and you are done. (Or, alternately, you may wish to trim it first, then add the formers.)

Just remember, compound curves get more difficult and may require slicing pieces out or molding sections at a time. If anyone else has additional helps, pass them on. I know I have only hit the highlights and there are certainly other techniques that can be added as well.

I've molded before. The leading edges of the stab and wing are molded components in the current project. I used gray structural foam (very heavy, thick stuff) for the forms. Very easy as you can cut out the form and use the female and male pieces as a clamp for the balsa. With the fuse blocks, I could do the same thing for the turtle deck and bottom block (though the bottom block would be tricky). I was most concerned with the nose block since it is a double compound curve.

I went ahead and took the original block and just reshaped it to undersize 3/32". I was planning to use 3 pieces of 1/32" sheet for the nose block, putting carbon fiber veil in between.

>>These pieces get quite strong all by themselves.

You clearly don't understand my level of paranoia.

The wing sturncture I'm using is my own design and I take full credit for it (or full blame depending on how it works out). There is very limited internal structure, at least not in the conventional sense. The molded leading edge/sheeting combination is pretty much all the structure there is apart from the ribs. I used a very modified millenium (lincoln log) wing design that used a vestigal leading edge to plug into the jig and strip ribs ala I-beam. It has a spar that is contructed within the wing after the ribs are in place. It's a long story, but the upshot was; a 700sq inch, relatively high aspect ratio wing that with tips and hardware installed came out at about 9.5oz. It's very stiff and I have to say, I was pretty happy with it. The molded leading edge takes away the problem of meeting the conventional leading edge with sheeting and causing. Since this is continuous, it cures a problem. It's also stronger.

I wrapped up the nose block sheeting awhile ago. The balsa was like putty (water and alcohol soaked), but what a pain getting it to conform to the mold. Kept wanting to wrinkle up at the ends. We'll see how it comes out tomorrow. This was just my first try.

I've just never tried doing just top and bottom blocks on a fuse before. It's a different experience. I did manage to get Windy's exceptional article on doing this and we'll see what we get. Hey, worse case is I have to do a conventional block.

I would imagine that using just 3 layers of 1/32" is quite strong. Not sure I would try cross graining, but hey, worth a try. The nice thing about the molds is you can try stuff.