Molding balsa parts

Molding balsa has always been a problem with other people wanting to build my airplanes. My airplanes always use molded parts to give rather exotic shapes and keep weight at a minimum. Much of the problem has always been obtaining molds like the originals. I've molded parts for some builders, reluctantly. I've shipped molds in specially made wooden boxes which cost an arm and a leg to ship and risks damage.

I've had an idea that might be of interest to designers who would want builders to use molded parts. Include enough of the molds that completion would be fast, simple and easy. One would just assemble the mold forms with CA, fill them with balsa, plaster or foam, and water proof them with paint or resin (with or without a layer of glass). The mold forms would add about $6 of $7 to the cost of the kit made from lite-ply which is strong enough.

I used to mold fuselages for the classic Bearcat in two halves. It is too difficult for most people to mold. When designing the BBQB Bearcat, I used eight molded parts dividing the fuselage into fore and aft, left and right, top and bottom shells. this also contributed to the accuracy of the airplane as these shells were applied in sequence to the fuselage sub-structure while it was still in the jig. The problem with the BBQB Bearcat molding is the fuselage molds are still two full size halves even though only sections of the mold are used are used at one time to make smaller parts.

There is some justification for very large half fuselage molds for the Bearcat but not for an airplane having simpler lines like the Mustang which is essentially slab sided. My original molds for the Mustang utilized just one shell for nearly the entire fuselage. It still had cowl, scoop and aft lower fuselage molds. It was a bitch to make shells for that airplane. Now, I'm smarter, or at least smart enough to realize the awkwardness and unnecessary effort I used to put into those shells. The Millennium had mostly 3/32" sheet fuselage sides with small molds for the forward fuselage top, left and right, and the aft fuselage top, left and right. I eliminated the scoop mold and still make cowl and aft lower fuselage shells. I guess it took me twenty wears to realize that two small shells glued to the same longeron is no heavier or weaker than one big shell covering the same longeron.

Anyway, for what its worth, here are a few opinions on balsa molding. First, a 3/32" molded shell is strong enough. There is little justification for laminating shells with or without layers of cloth or mat between. Laminated shells are much too strong and very heavy, to say nothing of being hard to make. Second, balsa molding is useful for much more than making fuselage parts. I also mold wing leading edges from lower spar to upper spar in one piece. Third, I've never found it necessary to use ammonia. Bath water works just fine for big pieces. I use 6-7 lb wood very carefully selected for "A" grain. Softer wood molds easily but is subject to damage until you get some finish on it to strengthen the wood surface. Heavier wood is too hard to mold. Finally, I hear nothing but good about Poly-Crylic for finishing wood, but it is water based and scares me. I'm afraid that water based finishes would swell molded parts with the possibility they might harden while swollen

Anyway, back to my original thought. Would laser cut mold forms encourage molding enough to be useful?

As for the photos, they are the new and old way of molding necessary parts for the same exact airplane, "Snaggletooth" and "Snagletooth 2", representing the easy new way and the old way of using one monstrous shell for most of the fuselage. Both photos show all of the molds (frames) for the fuselage. The blue airplane is "Easy Two Sugar", the first of the "Snaggletooth" series.

"Sanggletooth 2" is one of the original molded Mustangs. It was the basis for the Millennium Mustang which was basically a 10% bigger "Snaggletooth". Both use the same engine, a Ro-Jett or PA65. Why resurrect "Snaggletooth"? I think the Millennium would score better in light to moderate conditions as it is a much more impressive airplane. The smaller airplane with the same engine should compete better in extreme conditions but is very much more weight critical in construction. "Snaggletooth 2" is updated with improved construction and optimized for the Ro-Jett or PA 65 for a few competition minded friends.

Ah, my favorite technology...

>Anyway, for what its worth, here are a few opinions on balsa
>molding. First, a 3/32" molded shell is strong enough.
>There is little justification for laminating shells with or
>without layers of cloth or mat between. Laminated shells
>are much too strong and very heavy, to say nothing of being
>hard to make.

I'm a ragged newbie trying to figure this all out. Len keeps saying that a laminated set of 1 x 1/32 will be stronger, stiffer, lighter and thus better than one sheet of 3/32, but I'm finding it really hard to laminate the layers without adding a lot of weight.

What I finally settled on is to mold the layers in place, then take them apart put the bottom one on the mold, lay a sheet of silkspan that has been run through a pan of water that's been spiked with some titebond, so that the silkspan has a light film of thin titebone glue in it, lay the next sheet down and wrap it back up around the mold.

I end up with parts sticking to the mold, parts sticking to the ace bandage, etc.

I got one turtledeck (a fairly simple structure) for my 'Zoot Suit' Nobler to come out well, but it seems too much trouble. The belly pan for the Nobler is a single sheet of 3/32 and the front top of the fuselage is three layers of 1/32 that are drying on my mold and have been for months because I'm hesitant to go through the lamination step.

The point is (for me) that the lamination process moves molding past the point of diminishing returns. I'd have been better off to just carve a block for all the effort I've put into learning molding.

I think I'll take your word for it until I get any better at building and (aside from the part I'm about to describe) use 3/32 solid sheets, mold once and be done with it.

>Second, balsa molding is useful for much more
>than making fuselage parts. I also mold wing leading edges
>from lower spar to upper spar in one piece.

I've got a plan for my second Nobler wing. The idea is to jig the wing up nice and firm and then mold the leading edge skins IN PLACE on the wing itself, using the ribs as the mold for the leading edge.

I'm hoping to have a perfectly formed 1/16 in leading edge build out of two sheets of 1/32 with the layer of silkspan in between (I've already got the balsa from Lone Star.) that will be stiffer than the 1/16 sheet.

Is there anything wrong in your experience with using the actual wing structure on a jig as a mold?

Or do I have to go through a drill to align a pair of tip and root ribs, wire cut a foam wing, use that as a mold and then hope everything matches.

Cy,

As stated, I think that laminated parts are both too heavy, too strong and too hard to make, and I've made a lot of molded parts. My first parts were 3/32" a grain 6lb balsa. This is what I still use,(with a fling to epoxy glass cowls before returning to balsa cowls). Improvements have come in the use of simpler molds and smaller parts installed on jigged frames. For what its worth, I don't think you will be able to mold on wing leading edge structure. Wing ribs are too weak. 1/32" balsa is too weak to try to install over unsupported structure and wet silkspan will probably cap the effort with enough frustration to make you a believer in ARF's. Make a foam buck or buy one. There are many people who cut foam for a price, Bob Hunt being one. Richard Oliver cut the foam buck for the new "Snaggletooth 2" wing leading edges and one for himself. Richard dose not cut foam for hire. If there was an easier way to get good results......... Improvements in technology come slowly with many side tracks down fruitless paths. Stick to the straight and narrow known path. What did you think of the new mold forms?

What did you think of the
>new mold forms?
>
>Al

I too had Kyle laser cut my mold forms for the Mr. Hyde.

I used 1/4 plywood bases and 1/8" lite ply formers, filled between with balsa and planed everything down. This airplane was the first time that the finished plane looked just exactly like the print. Very fast and easy. I will never do it voluntarily another way.

The laser just allows so much stuff, and makes everything so much easier and accurate.

From your pictures it seems the laser cutting tricks are all out of the bag. Like putting together a big jigsaw puzzle.

Brad,

Yea, sometimes you just have to mold to get just that correct shape, particularly, if you need repeatable parts.

Building semi-scale airplanes like the Mustang also raises questions about correct shapes which define the airplane in subtle ways. For example, many Mustang lovers don't know the cowling isn't round behind the spinner. It is quite squared off, in the real airplane, to cover the front of the cylinder banks of the V-12 Packard Merlin engine. This makes a noticeable difference in the appearance of a semi-scale Mustang with a "too pointy" nose. The color photo of E2-S shows the more realistic cowling bulge which doesn't show at all in the next side view photo.

>Cy,
>
>Make a foam buck or buy one.

OK.

Thanks.

>What did you think of the new mold forms?

I'm assuming your suggestion to provide laser cut mold forms as part of the kit.

I think it's a good idea, although there's a lot of work left to go from the formers to the male plug part of the mold. It's certainly one step less to take by hand and certainly would make it easier to make an accurate molded part.

One thing I notice about your technique is that it's in general more labor intensive than -- well, what I used to do in the old days. I've seen shots where you build custom jigs for everything fuselage, wing, the whole plane in alignment, etc.

I'm coming from pinning the wing ribs on to a flat piece of building board over some saran wrap over the plans, lining each rib up with a 90 degree piece of balsa and hoping for the best.

And using big sticks sanded down for the Leading Edge and blocks on the fuselage.

Clearly your more professional method of airframe fabrication and assembly is better than the old days and results in a lighter monocoque fuselage etc.

I need to figure out how to utilize some of these methods without getting lost in Fab Hell.

For one thing, I have yet to even attempt getting my own parts laser cut.

If nothing else, your work is an inspiration.

L.

Cy,

All of the jigging is not only necessary for a very accurate competition model but it is quite likely faster and easier than what you may be used to. The fuselage jig, for example, is 3/4" particle board or MDF from Home Depot, about 4" wide and 40 " long. The bulkhead jig pockets come on the same sheets of balsa and ply as the bulkheads themselves. The pockets have a tic mark on center so all that you have to do is to line them up on a jig base centerline, space them and glue the pockets in place. These hold the bulkheads in their final relationship as the airplane is assembled. takes about 30 minutes to build a jig base complete with pockets.

The wing jig base is another chunk of 4" wide MDF about 6' long with (3) vertical standoffs to hold the jig tubes. the jig tube standoffs are 4" wide and 8" tall (to allow installation of the gear in the jigged wing). Home Depot will not only sell you the material for the bases but will cut it for you. The wing jig uses (2) pieces of 3/8" aluminum tubbing on which the ribs are spaced in their final relationship for assembly of the spars, bellcrank mount, gear blocks and etc. The four jig base pieces can be cut in about 15 minutes. I can get two complete wing bases from a 6' x 1' x 3/4" MDF shelf. It is easy to position the pieces in their final spacial position and glue and assemble parts. In all, it is a very civilized way to build an airplane and gives a bit more satisfaction than building on wax paper and hoping for the best. It makes building better. When you are into the techniques, you will find that even without laser cutting, the airplane builds well as all of my molded Mustangs were until a couple of years ago when I had my first airplane (the Millennium) cut. Now it would be hard to go back. but there is no need to go back. Already, there are laser cut kits which use all of these techniques. I know Ultra-Hobby kits would be very familiar to me. I'm sure there must be others. There is a somewhat greater cost up front but a great deal easier building with a better likelihood that you will have a good airplane for the effort.

Welcome to the 21st century. Put away the Ambroid and join up.

>Welcome to the 21st century. Put away the Ambroid and join
>up.
>
>Al

Interesting quip. Last time I looked Ambroid is still in production, but the average "21st century" young man of today has never even heard of a P-51. If I wanted to be similarly glib I might suggest that a legitimate "21st century" modeler would be flying an all-composite model, perhaps of a current production full-size acrobatic airplane, instead of one fashioned from an oversized Costa Rican weed.

Ambroid is no more old-fashioned than models of old airplanes, or the wood they are made from, or for that matter, the desire to fly models of antique airplanes, with lines and a handle.

I tried to put the Ambroid away but I can't....its glued to my hand!

>
>I've got a plan for my second Nobler wing. The idea is to
>jig the wing up nice and firm and then mold the leading edge
>skins IN PLACE on the wing itself, using the ribs as the
>mold for the leading edge.
>
>I'm hoping to have a perfectly formed 1/16 in leading edge
>build out of two sheets of 1/32 with the layer of silkspan
>in between (I've already got the balsa from Lone Star.) that
>will be stiffer than the 1/16 sheet.
>
>Is there anything wrong in your experience with using the
>actual wing structure on a jig as a mold?
>
>Or do I have to go through a drill to align a pair of tip
>and root ribs, wire cut a foam wing, use that as a mold and
>then hope everything matches.
>
>
>Cy.
I have been experimenting with balsa molding. It has been my experience so far that simple conical sections, such as the turtle deck on my Sig Chipmunk, can be molded easily in situ. I soaked the balsa, covered one side with silkspan and Polycrylic while the wood is damp but not dripping wet, and simply wrapped into place. Maybe that doesn't really qualify as "molding"? For the wing leading edges, where the radius gets smaller, I used the same procedure. But before putting the leading edge sheet in place I "pre bent" it over a hot 1/2" steel rod. Both of these procedures worked without issue.

In response to the original question: YES, the laser cut frames would significantly simplify the mold making process!

I agree with Al here. I've done laminated parts. I just take 2 sheets of wood (1/32" or 1/16"), soak them and mold them together. Then pop the mold apart, put a very thin layer of glue (I've used Titebond and epoxy) and put them back in the mold. Sometimes with carbon veil, sometimes not. Wrapped it bakc up and waited for it to cure. Make a very tough piece, but I don't think it's any improvement over a good piece of 3/32" and it's going to be heavier.

But, to each his own. I do sometimes do the laminated method for a top nose block to give it a bit more stiffness, but I do't imagine it really matters that much.

I agree, a mold with the kit would sure make things simpler. The Top Flite Gieske Nobler is a case in point. Virtually every kit of it I have seen or owned had the molded parts split. Making new ones would have been easier that trying to glue the splint one back together. My only experince with molding is in free flight rubber power such as Mulvihill and Coupe. We used fluoresant (sp) tubes mostly. Once I used a tube that had skin graftings in it. Empty of course. But it was a heavy duty bottle. My 59 Ares has molded top and bottom parts. I made the molds from heavy balsa (baseball bat weight stuff good for not much else). Nice thing about molds is that you can make two or three planes almost as fast as just one. Doing the same thing three times in a row does not take all that long.

1) Including the ability to make a mold seems to make sense to me, for what it's worth, good idea.

2) Here's a little wood bending stuff from the woodworkers, who routinely bend 3/4" oak. Always steam instead of soak, should work just was well for balsa. Much less messy, but slightly dangerous. Knowing that when you bend wood successfully, you have put all the wood into compression, helps. Wood can't stretch, but it will compress.

3) I really don't think the ammonia helps either. Won't bet the farm, but I don't use it when I bend hardwood.

4) Ty, sent you an email then went to FL for a week. Just found that my email didn't go through, I'll try to remember to give you a call tomorrow.

As I learned more about molding, I found I could get by with smaller, easier to mold parts. For example, pictured below are two Sea Furies. One is a semi-scale version of the standard airplane, the other is a version of a racing Sea Fury. They both have the same wing and similar moments and areas.

The second photo is of the molds used to mold each of the airplanes. In the rear are the two fuselage molds which were used to mold the original Sea Fury. In the foreground are the two molds I used to mold the molded sections of the racing Sea Fury, Critical Mass. The larger mold is for the top front of the fuselage, just behind the cowl ring. That mold is also used to mold the engine cowling, which is the bottom front of the fuselage. The smaller mold also has two uses. It is used to mold the turtle deck and the aft lower fuselage bottom. The rest of the airplane uses flat 3/32" sheet to form the rest of the fuselage. It was simpler, but not necessarily easier as it takes a bit of doing to build round fuselages out of flat sheet balsa. Still.....

Al, you obviously have the right idea. What I think may be a wrong perception by many modelers is that it is hard to make a mold or mold balsa. It is very easy. If I can do it, any average joe out there can do it. You inspired me as to molding parts for my Ares and once I did it, other than basic round, I found it was not unlike making my first ever model an F6F Comet kit, .10, and we cut out the former shapes with scissors and carved the tiny fuselage to these shapes. If I could do this in 1947 at age 6 and with a Boy Scout knife and a double edge razor blade, then modern guys my age or younger can do it. It just has to be attempted the first time and it does get easier. ps I painted it with blue water colors!
Putting the mold formers in a kit that had compound curves would make it simple and the kit price would not be that much higher than many out there now.

I just got around to completing my molds using the laser cut mold forms. In the past I have used balsa, foam and 20 minute joint compound (plaster) to fill molds.

Foam is the easiest to shape once the form is filled. filling the form, however, takes just as much time with a table saw and chop saw as cutting balsa scraps to fill the mold.

Balsa filling of the forms makes a lovely, durable and attractive mold. Once again it takes hours to cut and fit all the plugs necessary to fill the mold form, and that is with the use of a selection of power saws. I decided to go with the balsa fill as I had a bunch of cheap scrap blocks from Lone Star. The filling balsa would cost only about $2. No problem, until the molds were filled. It is one thing to carve balsa wing tips, cowlings and canopy molds which are all fairly small. It is another thing altogether to carve a 30" long fuselage form. After more than an hour with bucher knife, electric palm sander, disc and belt sander, wood rasp and sanding block it finally turned into a mold form that I can certainly be proud of. Now I have to paint it, at least. The original Mustang molds were carved essentially the same and finished with a layer of glass cloth and polyester resin and polished to a high gloss. These are beautiful molds which have turned out many molded Mustangs and are, in fact, still usable as Sanggletooth 2s have the same exterior size and shape. But I wanted to try the new mold forms..... I have come to the conclusion, that I am too old to build molds this way again, to say nothing of my garage floor being covered ankle deep with balsa shavings and saw cutoffs and a thick layer of balsa dust on everything.

Third way, filling molds with plaster is, in my opinion, the fastest, cheapest and easiest method. Instead of spending hours cutting and fitting balsa or foam into a mold, powdered plaster is mixed with water and slapped into the mold with bare hands. As the mold begins to overfill the excess is carved off after about 20 monutes with a tool called a Stanley Sureform Shaper from Home Depot. It cuts new plaster like slicing a firm cheese. When the mold is completely full, the Sureform tool makes short work of carving the new plaster to the desired shape. There are no boundaries between the layers of plaster and the shaper can be used with delicacy to artfully carve with a very high degree of accuracy, better in fact, than can be achieved with balsa or foam. After drying over night the plaster filled mold can be sanded easily with an electric palm sander and/or a block hand sander to remove shaper tool marks. I'm sure I could have filled and carved the mold forms with plaster in about a third the time it took to fill and carve balsa. Also, the plaster carves with greater ease and accuracy than the balsa or foam. When the shaping is complete, the forms, filled by any method, still have to be painted to waterproof them. Any paint will do. Glassing the molds is overkill but nice. Until you have tried cutting new plaster with a Sureform Shaper, you can't appreciate just how valuable a tool for making parts you may be missing.

Photo below shows both the old molds and the new unpainted replacements. Now to wet some balsa!

Hi Al.....thanks for doing this thread, its very neat stuff and highly informative. I was wondering, have you ever tried the artists molding plaster? It sets up very quick (its a chemical reaction not water evaporation, so no shrinking), no air pockets, and carves like a dream. You can work on it after an hour and don't have to worry about it being wet still inside. It actually cures from the inside out. You can find it in any arts and crafts store and is quite inexpensive.

Glen,

No, I haven't tried artist plaster but I can't imagine any plaster product working better or costing less than 20 minute joint compound from Home Depot. I can fill and carve two fuselage half molds in about two or three hours. Each filling is ready to carve by the time I finish filling the alternate form. The material is carvable as fast as I can work on two fairly large molds.

If isn't bad enough that filling and carving my large Snaggletooth mold, I noticed in the photo above I had accidentally sanded a bit of "sway back" into the middle of my mold, left side (near). I fixed the mold with a coat of Bondo to the affected area but it added another layer of effort on what I hoped to be a fast, easy process. It would have been much harder to sand/carve a shallow depression using the relatively long Sureform shaping tool. With plaster, this would should have taken no more than an hour or two plus a couple coats of waterproofing paint.