Molded Balsa

I tried a couple of experiments over the weekend molding balsa: one was a "hot" thecnique the other two were "cold".

Luthiers and Shiprights have been using "steam bending" and heat for wood bending for hundreds of years. I haven't seen any comments on its application to balsa but I thought I'd give it a try. I soaked a piece of 1/16th inch balso in water and while it was soaking I heated a 3/8" diameter aluminum rod with a propane torch. Then I gently wraped the wet balsa around the hot rod forming a fairly small diameter (3/8") bend with out cracking the balsa. It's quick and simple and works well. It looks like this technique would be good for making molded leading edges. The "trick" is to get the rod hot enough to make the wet wood "sizzle" (steam...) without getting it so hot it chars the wood. It also works bending across the grain as well as with the grain of the wood. You'd be suprised at the small radius you can get with a little patience.

The other experiments were with "cold" molding: The first was using the now famous "Polycrylic" (sp?). I took two pieces of 1/32" sheet and soaked them in water. The after removing the excess water, I gave each piece a coat of Polycriyic, laminated them together with a layer of "OO" silkspan in between, pulled them over the mold with a damp piece of old cotton sheet, and let them dry over night. The result was a light, stiff molded piece. I used Polycrylic becasue it is water based and shouldn't be bothered by the wet balsa. Others on the forum have discussed this technique... The third experiment was with a glue called "Gorilla Glue" It is a "water cured" glue and foams slightly during cure. I used the same procedure as with the Polycrylic, sans the "OO" silkspan. The Gorilla Glue is about the consistancy of honey, and I scraped off as much a spossible before laminating the balsa sheets. Since this stuff cures with water, air is not needed. I used "Saran" wrap to pull the laminate around the mold. Since this stuff foams as it cures, it might be just the ticket for gluing balsa sheet to a foam wing core. I have't tried that yet... It remains to be seen how well these test pieces will hold up.

Has anyone else tried any of these techniques? and how well did they work for you?

You can use epoxy or "yellow" glue between layers, except that you wrap the wet balsa with the rags and let it dry over the form first, then remove it after it is dry, glue it, and place it back on the form for the glue to dry. It is a two part process, but your chances of getting a good glue joint are better.

Although the gorilla glue fills gaps, it is not as strong and I would be concerned with gaps anyway. If you bond the pieces tightly together there should be no gaps and then a thinner glue (and less glue) would be in order.

Do you have results for the different procedures in terms of weight and strength?

Which one would you use on a model?

I'm about to laminate a leading edge for a nobler out of two sheets of 1/32 and the only question is what to put in between.

I did some experiments where the weight of the glue overcame any value from increased stiffness.

I'm perilously close to just using one thickness of light balsa, but everyone says lamination is the way to go.

Molded 3/32 works real well with no need to laminate anything unless you are doing a top nose block. A few support formers and go....takes little effort and plenty strong.

John S.

Hey Leonard....I am locked out again...cant log in and my password has changed from the one you gave and the one i retrieved wont allow me to log in either. Tell ma what i need to do!!! Sorry, not molding related.

I tried a curling iron as to maintain constant heat source and found it worked great! Also, you can buy a curling iron in 3 or 4 different diameters for only $10.00.

>Molded 3/32 works real well with no need to laminate
>anything unless you are doing a top nose block. A few
>support formers and go....takes little effort and plenty
>strong.
>
>John S.
>
>Hey Leonard....I am locked out again...cant log in and my
>password has changed from the one you gave and the one i
>retrieved wont allow me to log in either. Tell ma what i
>need to do!!! Sorry, not molding related.

On the molded balsa issue, the balsa is less likely to split, so becomes stronger when laminated. It is sort of a plywood type of thing even though it is not truly cross grained. The glue helps, also, but make sure you don't use much. It is also a little easier to mold when using thinner stock. Matt had started with three pieces of 1/32 but found two 1/32 pieces laminated together (as above) worked fine.

On John's password thing, I don't konw how you do it John, but once again I show you with two identical accounts. The forum is set up to not take a second username that is the same or e-mail address, but you got lucky.

I don't know if this was your problem on the pasword, but this is for everybody: The password is case sensitive. If you use capitals when you put it in, then you must always use capitals. If you capitalize the first letter, then it must always be capitalized. This is another way of adding security, although I don't think we need it quite that high. Anyway, I have e-mailed you, John, and you are good to go.

Frequently "stiffness" and light weight are far more important than the yield strength of a laminated piece. Does the "Polycrylic" method produce a stiffer, more rigid piece that the "white glue" method? On my small test pieces I couldn't measure a weight difference (less than .1oz for my small sample) but the Polycrylic test "seemed" stiffer. I have no quantitative data on this... What is your experience?

"the" current method for applying balsa to a foam core is to use polyurethane. Sort the sheeting onto a dead flat table(formica), push them together and block sand with an orbital sander. Vacuum and tape together. Cut to size. Turn the skin over and apply as thin a layer of polyurethand as you can- put on thin beads of glue and spread them with a card or piece of soft balsa, scrape as much glue off as you can. Lay the skin on the foam, cover with a sheet of polyethylene plastic, put on the wing shuck and weight down with magazines, bricks, etc. There is no need to glue the sheets together, the glue will do that when if foams and cures.

>"the" current method for applying balsa to a foam core is to
>use polyurethane. Sort the sheeting onto a dead flat
>table(formica), push them together and block sand with an
>orbital sander. Vacuum and tape together. Cut to size.
>Turn the skin over and apply as thin a layer of polyurethand
>as you can- put on thin beads of glue and spread them with a
>card or piece of soft balsa, scrape as much glue off as you
>can. Lay the skin on the foam, cover with a sheet of
>polyethylene plastic, put on the wing shuck and weight down
>with magazines, bricks, etc. There is no need to glue the
>sheets together, the glue will do that when if foams and
>cures.

...sorry for the dumb question, but...

polyurethane as in "Gorilla Glue" or as in "Polycrylic"?