What is the "Lost Foam" Technique?
Everyone has its own preferred way of building; some like to carve balsa, some like to bend sticks, some like to get deep in epoxy and polyester or foam and paper. There are certainly a lot of different ways. The Lost Foam technique is just one of them. I will present a bit of a description for this method. Maybe some day you will be tempted to try it.
A related molding technique, called the Lost Wax Molding, has been in use for several centuries. In the case of the Lost Wax, the wax is often used to build the second part of the mold but during the process the wax is being replaced by metal in fusion. The wax in the mold melts or evaporates upon contact of the hot liquid metal leaving space for the metal to fill the mold. When the metal turn to solid, the mold is open and the metal piece retrieved.
In the Lost Foam technique, the foam is the mold and you dissolve the foam (or just part of the foam to keep more rigidity) when the piece is done to leave you with the resulting part. In short, you carve the shape you want in foam, cover the foam shape with layers of fiberglass and epoxy, and then get rid of the foam. It is used mostly for fuselages and nacelles, but there is nothing preventing you from doing a hollow composite wing this way.
Here are some advantages and disadvantages to consider:
Some may use polyester resins instead of epoxy, but because of the smell and appetite for foam, I never used that. So I will only describe and talk about epoxy instead.
You will probably need only 1 layer over all with another layer of lighter glass in high stress areas, if you use a heavy fiberglass cloth. You may also want to add carbon fiber and/or kevlar in high stress areas.
Wear protective gloves when handling epoxy. Work in a well-ventilated area.
I use this technique when building scale curved fuselages. I decided to explore with the Lost Foam technique after reading about it being used by several UK modelers on PSS planes. I prefer to use this technique instead of wasting a lot of good balsa wood on a design showing plenty of compound curves.
The Hurricane in the pictures is my second attempt at such a technique and gave decent results. Much lighter than my first attempt. But still lots of space for improvement. The Hurricane is 1/15 standoff scale and powered by an AP29 on 8 cells. The dark area on the bare fuselage is where the microballoons stayed in a thicker layer.
The finished fuselage for my Hurricane, created using the technique described in this article.
The finished Hurricane.
The technique is fairly simple and fast. I'll give a short description with additional tips and improvements.
Mold-making and Fiberglass Lay-up
1) Take a big chunk of dense foam - pink, gray or blue extruded foam are much easier to carve and sand - to match the fuselage. I take junk pieces of foam and glue the pieces to cover the fuselage outline. Then cut the side outline using a band saw, a scroll saw, a carpet knife or a kitchen knife with small teeth. Complete the shaping and rounding by rough cutting with the knife then sand to final shape - I use a dry wall sanding pad, this is a mesh of abrasive material with plenty of holes where the foam crumbs can lodge themselves instead of rolling and making dents in your foam. All this can be done in half an hour or an hour.
2) Once satisfied with the accuracy and level of details, wrap the plug with one layer of packing tape to prevent the epoxy from getting into the foam. Apply some wax to help remove the tape later on - I will use Johnson floor wax next time.
3) If you want, you can mount it on a fixture with a nail at each end to hold the foam core off the working table. You want to be able to rotate the core like a chicken on a rotisserie so that you can work on all sides of it.
4) Apply one layer of thick fiberglass cloth - 2oz or 3oz - using 3M#77 spray to stick before the epoxy is applied. Apply the cloth in 2" or 3" overlapping strips to ease the placement. Make the strips as wide as will conform to the shape of the core. In some areas, they may be quite narrow, and others quite wide.
5) Mix your epoxy per the manufacturer's recommendations. Then brush epoxy into the glass, thoroughly saturating it.
6) Use toilet paper or paper towels to absorb as much of the epoxy as possible. Lay the paper on a portion of the fuse. Press the paper onto the glass so that it absorbs epoxy. Pull off the paper, being careful not to pull up glass cloth with it. Continue this process until the paper can absorb no more epoxy.
7) Apply the number of layers needed to obtain the rigidity you want. I used a couple of layers of medium grade cloth with additional layers in the nose and tail.
8) Let the epoxy harden for at least 24 hours if no more glass is to be applied. If more glass is needed, apply it before 24 hours. You may want to rough up the surface with sandpaper first.
1) After the epoxy has hardened, sand off the high spots and rough up the surface all over.
2) Mix up a blend of epoxy and microballoons that is just liquid enough to be brushed on, but no so fluid as to be runny. You want plenty of microballoons so that it will be easy to sand. Other fillers that will stick to epoxy can be used. Just be sure they are light in weight.
3) Brush the filler all over the fuse. Let it harden until it can be sanded. Don't let it harden any longer than necessary, because it just makes it more difficult to sand.
4) Using sandpaper of decreasing roughness, sand all the mixture off, except what is left in the hollows. One application may be enough to get a smooth surface, but you may have to touch up a few areas.
1) It is better to get rid the foam after all the sanding was done as the foam provides a solid base to apply pressure. Now get the solvent out - I use acetone. Use a large bucket to receive the soon to be dripping solvent (or your neighbor's lawn if you hate him) in an area with excellent aeration. Open up an area of the fuselage where you will pour the solvent - I use the wing opening in the fuselage. Pour a bit of solvent at a time and let it dissolve the foam. You will be left with a bit of thick paste at the end; don't worry.
2) Next, remove the packing tape; if you waxed it, pulling a corner from inside the wing bay will have most of the tape extracted and leave a nice clean interior, bringing with it the foam paste that was left.
1) Spray with gray, white or light gray (depending on the finish color you want) Krylon primer or something similar. Wet sand almost all of it off. Repeat this until you are happy with the surface smoothness.
2) If you make a sport model you can spray with one or two coats of Krylon fast dry enamel. Silver Krylon makes for a very nice aluminum.
- Remember that there is wax inside and that you will need to sand the places you want to glue formers or reinforcements.
- To be more accurate; make cardboard templates.
- For boxy type fuselage a stick method would be faster and more rigid.
- Apply your reinforcement (carbon fibers, kevlar cloth, etc) in the first layers, as you don't want to have to sand those if they protrude.
- I make my firewalls out of discarded PCB from work (RRR; Reduce, Reuse, Recycle) that I have predrilled and glued to the plug's tape using 3M77 contact cement; the firewall is an integral part of the completed fuselage.
- I used 2" thick pink foam that was a bit narrower that the scale fuselage and didn't bother to do the wing fairing on this first attempt at a scale airplane. Therefore there is a fairly large flat area on the sides and these are not very stiff. Next time, I will use balsa planking (1/32 or maybe 1/16) then add fiberglass to provide more rigidity to the side.
- You may find that the sanding has weakened a couple of spots on your final fuselage. You can reinforce those up from the inside by applying patches of cloth soaked with epoxy.
- If you make it flexible enough is will absorb energy much more and there will be less destruction. A thicker fuselage will propagate the energy of a crash to the components instead of absorbing it. On the other hand, a soft fuselage may bend or distort when moving control surfaces at high speed.
- Fiberglass is weaker on sharp corners. Round the foam plug a bit and add more microballoons/epoxy paste to be able to carve/sand the sharp corner in the end.
One of my friends, Waid Reynolds, while attending the June Slope Jamboree at Richland, camped at the Columbia River Park right next to a real nice guy named Jerold Converse. It turned out that Jerry is an RC glider pilot living in Grand Coulee. He likes to scratch build gliders, including using the lost foam technique to make his own fuselages. In searching for better ways to build fuselages, he found that using sleeves of modern woven materials over the foam fuse cores saved time and produced a stronger and prettier end product compared to using strips of material to do the lay-up. He knew that other scratch builders would like to do the same. But, sleeve material was expensive and difficult to acquire from normal modeling sources. He searched out manufacturers who would sell sleeve stock in relatively small quantities at reasonable prices. To make this material available to the rest of us Jerry has started his own Web based business.
His Web based business, R/C COMPOSITES in Grand Coulee, Washington sells Biaxial Woven Sleeves made from Fiberglass, Kevlar, Carbon fiber and Kevlar/Carbon fiber blend. The material will expand 25 to 30% and contract 65 to 70%. It comes in various sizes and weights (1/4" to 11" dia. and 5.9 oz./sq.yd. to 30.7 oz./sq.yd.).
Check out his home page at http://homepage.usr.com/c/composites.
I like the technique and will use and improve on it. A B-25 on two SP400 with retracts would be an interesting next candidate...
If you discover new tricks or just want to share ideas and projects, I can be reached through email at louis_dionne(at)truevision.com. I will be glad to hear from you, in French or English.
Indianapolis, IN, USA
Why not make several formers (as we used to do when making stick fuselages) and glue in place along an upper and lower backbone. Then glue spreader styrofoam blocks separating the formers by 3-4" thick blocks between formers. This would allow a guide for the final fuse shape as well as a support after removing the styrofoam. Should allow much more accuracy in fuse shape as well as a ver rigid structure as the formers would remain epoxied in place....... email@example.com
Some questions for you.
My name is Christopher from Indianapolis as well. I am a newbie to the R.C. World and have decided to use the technique described in this thread. My question to you is this...Does one layer of packing tape really keep the epoxy resin from getting to or even melting the foam? Have you you tried any other types of tape?
Thank you for your time and energy,
i like it-seems better for one off models-no molds plugs all that did my head in. Is the fuselage done in two halves or as a whole piece(or carve out of the faom as on piece then cut in half for access to fit out with gear)???? I look forward to trying it- i think it could be easily adapted to model boats
Last edited by lots of power; Aug 27, 2011 at 06:44 AM.
If you use common expanded styrofoam - you don't need to use Acetone - common gasoline will melt it. It will also if you swill some around the inside of the finished article after tape removed - wash out the wax.
Acetone is a softener agent for Polyester resin and I suggest is best only when using epoxy.
I used to cut the foam out with a soldering gun with a shaped piece of copper wire in place of the soldering tip. The reason for this was the gooey mess that the dissolved foam makes using gasoline. I used styrofoam dock billets. Somewhere around 4 ft long 8 in.thick and 2 ft wide. This was quite a few yrs ago, so this is an educated guess.
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