Building Foam Models
by Laddie Mikulasko
During the past 4 to 5 years I have built several, electric and glow powered models using foam as the primary building material. I've learned a great deal about successful foam techniques, both from reading about others designers projects, and from my own experiences. As is the case with most experimentation, I also discovered much about what does not work well!
My introduction to foam construction was born of necessity in 1963, while still living in Czechoslovakia. I couldn't obtain any balsa so I built the fuselages for my R/C models using foam. Later when balsa became readily available to me, I virtually forgot about foam. My recent interest in designing scale, electric powered aircraft has driven my desire to develop improved foam construction techniques.
Building with foam is not more difficult than with balsa but rather it is a different skill set that one must acquire. I would be pleased to share my foam construction techniques with you.
The models I mentioned earlier are often multi engine scale models utilizing complex shapes and compound curves. These shapes can be crafted easier and lighter from foam than from balsa. Should these models be subjected to a mishap, the damage is usually localized to the contact area. Ordinary white insulation foam is sufficiently strong for most construction. Blue or pink SM foam will provide a nicer finish after cutting or sanding, but it is twice as heavy as white foam. With proper technique, you will be able to achieve as good a finish using white foam as blue foam, but at a lower weight. As an experiment I built two identically sized scale models of the Canadian water bomber CL_415. One was built using very light balsa sheets and blocks while the other was built entirely from pink and white foam. The foam version came out lighter and looked much more scale like. The biggest obstacle to overcome is leaving your "comfort zone" to learn a new set of skills, but I assure you that the learning curve is not steep. Of course you will need to acquire foam_cutting equipment and you should also note that more care is required when handling foam models to prevent "hanger rash".
So, if you are ready, I will guide you through the art of using foam for model building. This is not intended to be the definitive guide, as is the case with any process, technique continuously evolves, as new products are introduced. Some may have found methods better suited to their needs, but the process presented here works very well.
Basic equipment requirements
A voltage regulated power source is must, 20 volts and 2 amps is sufficient for the longest bow I use. I constructed three bows. The lengths are: 5 feet, 3 feet and 18" long. The bow frame can be built using 2" X 1" pine board for the width piece and ¾" square for vertical pieces. These vertical pieces should be approximately 10" long and glued to end of the width or horizontal board. Drive a nail in the bottom of both vertical pieces to attach a length of wire between them. The Nichrome wire must be stretched tight so that heat will not allow the wire to slacken.
When cutting foam, the temperature of the wire is adjusted to be just high enough to melt the foam. If the foam starts to sizzle or smoke, the temperature is too high. There are two main advantages to cutting foam at a low temperature setting. First, you have more time to correct any error such as one end getting ahead of the other, and secondly, because the temperature is so low, even cardboard is sufficient to for templates. The cardboard I use is obtained from shoeboxes or similar material. The outside shape is cut out using scissors or a sharp utility or Xacto knife. The inside shape must be cut out using a knife because scissors tend to distort the template. Cardboard templates are used to cut out all parts except for the wing. I use plywood or metal templates for cutting wings. For box shaped fuselages, the foam must be cut into sheets. Use ordinary building grade foam and the following procedures to cut the foam into sheets of the thickness required.
First cut up 2' x 8' or 4' x 8' sheets to the largest size required for your project. To cut these sheets, you will need a smooth surfaced bench or table. Secure to the table, two stand off wood blocks of the required sheet thickness. Place the bow upside down so the wire rest on the blocks, and the frame hangs below the table. Clip the leads from the power supply to the Nichrome wire between stand off blocks. Place the sheet on the bench and slowly start pushing the foam into the wire. Do not pause or stop until the entire sheet passes through. These sheets can be used for fuselage sides, ribs or tail surfaces.
Often thicker sheets are required than the common 2" sheets that are readily available. You may join the sheets to create the desired thickness by using contact cement. The best contact cement I have found is 3M #77 spray adhesive. If the gluing surface is less than ½" wide, then full strength white or yellow glue can be used. Since these glues require air to set, wider surfaces will not cure properly. I do not use epoxy glues to make foam joints.
If the fuselage has compound curves, the best approach is to build right and left halves. Each half is made from several sections. For higher curvature areas, more sections are required. Each section consists of one foam block to which a cardboard template is pinned to either end. Each block should have a centre line drawn on it as reference for pinning the templates. Most of the templates have the inside cut out to produce a 3/8" to ½" thick fuselage wall to reduce the weight. Use the smallest bow when cutting the fuselage sections. Once all the segments are cut out, glue them together to obtain half of the fuselage. Similarity, after the other half is constructed, the halves may be joined together. Use water_based glue so that you have sufficient time to align the segments between the two halves. After the glue is dry, sand the outside of the fuselage to blend the contours of the various segments. On the inside of the fuselage epoxy the various plywood supports used for items such as the wing hold down blocks. The wing is cut out from foam in the usual manner, using templates at both the root and the tip. Depending on thickness of the wing, I often remove much of the foam from inside of the wing by using a template with the inside removed, such that it produces a wing with a 3/8" skin. On all my foam models the wing has one full depth spar, usually cut from 3/16" or ¼" balsa sheet. No balsa is used in the leading or trailing edges. The same applies to the tail surfaces.
Finishing the foam
Prepare the foam surface by sanding using 80 grit sandpaper. Then finish the raw foam by lightly sanding using 120 grit sandpaper. Next, brush a 50/50 mixture of white or yellow glue and water onto the foam. Let it dry and then sand lightly using 120 grit sandpaper. Recently I haveobtained very good results using FLECTO water base, clear urethane sealer instead of the glue mixture. There are several other brands on the market that will work, just be certain that they are water based. If the surface contains imperfections, fill them now with water based lightweight filler. You can use commercially available fillers or make your own, as I do. To white latex primer or flat latex paint, I add micro balloons until I achieve a thick consistency that can be applied by brush. For dents and gouges, I add more micro balloons to create a thicker paste that is applied with a spatula. After the filler is dry, sand it with 120_grit sandpaper. Foam surfaces can be covered using several materials. The lightest is Modelspan tissue paper. For small models this may be all that is necessary. A second layer of tissue paper will give the surface more durability. For models with a wingspan greater than 48" brown wrapping paper can be used. All paper must be made damp before is applied to the surface. I use a spray bottle for this task. Wipe the excess water from the paper. In the case of the wing, try to cover the top and bottom at the same time, to reduce the tendency to warp as it dries. Compound curves are covered using strips of paper. On no load bearing areas, newspaper can be used. The adhesive, to attach the paper to the foam, is a 50/50 glue and water mixture. In areas where there are sharp edges, use full strength glue. Another way to fasten paper to foam is to use the previously mentioned urethane sealer. Applying paper to the foam is the same procedure for either the glue mixture or urethane sealer as an adhesive. Brush a light coat of adhesive onto the area to be covered. Place damp paper on top of this area and brush a light coat on top of the paper. Use a soft squeegee and applying a light pressure, squeeze most of the glue out.
Wipe off the excess glue. After the entire area is covered, let it dry to the touch. Add the second layer (if required) and let it dry completely. Sand the entire surface using 120_grid sandpaper. If it is needed, brushon filler and sand once again. Once you are satisfied with the surface, brush on one more coat of sealer. Make certain that any filler material is well sealed before any paint is applied. I paint my foam models using Model Master spray paint, which isavailable in spray cans from most hobby shops that sell plastic models. This paint is lightweight and has great coverage. You now have sufficient information to get you started on the right track. As with any new technique, experimentation and practice makes for a much_improved end product. Please feel free to contact me personally, should you require additional information.
Tel. (905) 628-2749