Using Form

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Materials

There is an almost infinite number of types of foam. Most can not be safely cut with a hot wire under normal circumstances. The one type of foam that is readily available, has the desired mechanical properties, and can be hot wire cut with relative safety is polystyrene. Polystyrene foam comes in two major categories, expanded bead, and extruded. Expanded bead foam is made by superheating small pellets of styrene so that they rapidly expand (sort of like popcorn) to form a low density foam. Expanded bead foam is light (about 1.5 pounds per cubic foot typically) and inexpensive, and because of the way it is made, lends it self to all types of molded products. It is the type of foam found in beer coolers and most contour packaging. It is also used as a cheap insulator. Because expanded bead foam has air pockets between the beads, it is less suitable for exterior use, for flotation, or for any application where water or water vapor can invade the air pockets between beads. These same air pockets make it unsuitable for WACO style wing construction. The air pockets absorb epoxy, and are easily compressed by vacuums over (or is that less than?) 5 or 6 inches of mercury. For R/C model construction expanded bead foam is best relegated to use as core material for balsa and Obeche skinned wings. It is light, and as long as low (or is that high?) vacuums are used, and adhesives are applied sparingly, it works well in this conventional application.

Extruded polystyrene

foam is the same chemical product as the expanded bead stuff, but it is formed in a different fashion. As the name implies, it is extruded, and it is a much more homogenous material with no air pockets or visible voids. There is considerable confusion about extruded foam, mainly because it is made by many different companies, and appears in many different colors and densities. There is "artboard", green foam, pink foam, gray foam, blue foam and more. All of these are extruded polystyrene. The lowest density extruded styrene foams all range about 1.75 pounds per cubic foot. Curiously, extruded foam panels from the same manufacturer exhibit considerable variation in density. I have observed densities ranging from 1.8 to 2.1 pounds per cubic foot, all in the same batch. This variation is not as common as in balsa, so it probably won't do you any good to take your scales to the hardware store when you are "selecting" your foam. Different manufacturers also produce different qualities of extruded foam. Some foam seems to have more impurities and air bubbles than others. WACO has found that Dow foam is the most uniform. Dow foam also comes in a light gray almost dirty white color in its lowest density. Gray is an easy color to paint over. Dow Gray Board is the type of foam we use for all of our wings. For other applications where more stiffness or strength is required, Dow also makes blue foam in a range of densities from about 2.5 pounds per cubic foot all the way to over 12 pounds per cubic foot. Dow makes foam panels in thicknesses ranging from less than one inch to over six inches. The most common thickness for home insulation is 1.5 inch, and this is also a very handy thickness for cutting sailplane airfoils. You power pilots out there who need airfoils 2 and 3 inches thick can probably find the thickness if you call the right people. Gray foam is sometimes difficult to find because it is not specified in most building codes, therefore local construction material suppliers don't stock it. If you have a difficult time finding gray foam, or the thickness of foam you need, try calling Dow at their 800 number. Tell them you are doing a science project and maybe they'll send you a batch for free. At the least they will give you the name and number of a nearby supplier. As a last desperate measure, you can always call WACO, and we'll sell you a bundle at an exorbitant price which just barely covers the hassle involve in cutting, packing and shipping the stuff.

Final Thoughts on Foam

Styrene foam can be cut with all the methods outlined in this web, but do not neglect the thought that it can be machined in other ways. It can be routed, sawed, drilled, sanded, sliced, and worked in all sorts of fashions. It can probably be hot wire cut by methods which are not included here. When you are approaching a new and unique foam construction job, give consideration to all possible methods of attaining the desired results. Keep it simple.

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