Foam as it relates to model aircraft refers to a number of variations of polystyrene plastic foam, generically and commonly referred to by the Dow Chemical trade name of "Styrofoam." Occasionally viewed as the "quick and dirty" construction material because of its relative ease of use in comparison to traditional balsa construction, foam can be carved, sanded or molded to shape. Volume-wise, foam is also lighter than balsa.
Expanded polystyrene (EPS)Edit
This is the common "beaded" foam molded into products like portable coolers and drinking vessels. Blocks and sheets of beaded EPS as well as inexpensive Styrofoam products are made by filling a mold with individual beads of EPS and heating the material with steam. Other EPS products are extruded in a continuous process which results in a more uniform appearance.
EPS can take a lot of punishment when used properly but is easily marred and fairly low tension, so it is easy to snap or tear. The thousands of joints between the individual beads are the weak links in the finished material, so non-beaded varieties are preferred for modeling. Both varieties can be sanded very well, but care must be taken with beaded EPS or the beads will separate. EPS is vulnerable to attack by the solvents in many glues and paints; special "odorless" cyanoacrylate glue is readily available at any hobby shop as are vials of water-based acrylic paints and spray paints with foam-compatible propellant.
EPS sheets are generally available from home improvement centers in the guise of cheap insulation. Beaded EPS is usually white; non-beaded is pink or blue.
Fan-fold foam (FFF)Edit
This .25" EPS product is used for insulation and comes in blue (also referred to as Bluecor) and pink. Blue is often preferred because the pink material is a bit more brittle. Both variations have a thin plastic skin that can be peeled off. During construction of a model, leave the skin on for strength in stressed areas but remove it at joints so the glue can stick to the foam.
Some pink FFF is made of two layers of 1/8" foam and can be separated by hand.
FFF is most commonly used in models for building homemade pattern aircraft with little or no scale detail. This is due to the manufacturer's large black identifying marks, regular pinprick-sized holes and waves, making it difficult to create a nice model without the added weight of paint and decals.
The ink can be quickly removed from the skin using Golden West Super Solvent for cyanoacrylate glues. This debonder does not contain acetone and does not attack the foam.
Over the years, the thickness of Dow Bluecore has been decreasing so now standard 1/4" fanfold typically measures in at only 3/16" thick.
Similar to FFF but colored in white or dark grey. A larger variety of sizes (1,2,3 and 6mm) is available as well. No printing, holes, waves or outer skin, so it's often worth the extra price for the superior finish.
When bent, Depron is stiffer in one direction than the other.
Similar to Depron but covered with paper on each side. The paper adds quite a lot of weight and some minimal strength. Many people soak off the paper skin and use it as an alternative to Depron. Study board is also refered to as foamcore or foam board and is frequently used for architectural modeling. It is typically about 5 mm thick.
Expanded polypropylene (EPP)Edit
EPP is used in high quality packing foam, some newer ARF and RTF models and in automotive bumpers due to its extreme strength. EPP is not as stiff as EPS but it often recovers completely from impacts and is harder to tear, hence its use in more commercially-available models.
EPP is safe to use with most glues and paints. Cyanoacrylates and epoxies work well but are brittle and can crack when the foam bends. Hot melt glue works well but is heavy. Contact adhesives such as Shoe Goo or Zap-A-Dap-A-Goo work well. Polyurethane based adhesive is perhaps the best for and making large repairs; it is lightweight and expands to fill gaps, requiring extra care on the part of the modeler. 3M Scotch-Grip 4693H plastic adhesive was made for EPP and works well in constructing models. Many modelers find tapes do not stick as well to Epp as compared to other foams. Home insulation tape suck as TYVEK has great adhesive strength, but tears easily. Braided packaging tape such as extreme packing tape gives excellent strength and durability.
EPP can be sanded, but not to a nice finish. Sand in one direction to avoid tearing the material and melt any "hairs" that develop with a heat gun.
This variation is molded in large blocks and cut into sheets with a heated wire or saw into whatever part a modeler requires. Irregular shaped balls are visible in the cut surface. Closed-cell bends and is somewhat spongy, thereby preventing breakage upon crash.
EPP used to be difficult to obtain in Europe, but is available from epphobbies.com.
Open-cell is sold in precut sheets. Its molded surface or cut edge resembles a kitchen sponge and is just as flexible. This material is most often seen as packing material and extruded swimming pool toys. Open-cell sands better than beaded EPP.