Foam Techniques: Part 1
DUE TO THE amount of material which will be presented; this article is divided into three parts. This month's installment deals with foam as a material, types of foam, chemical characteristics, compatible solvents and glues, etc. Part two will deal with methods and techniques of cutting foam, cutting bows, power supply, safety, etc. Part three deals with covering or skinning, cements, construction, and finishing. If this first installment seems excessively detailed, it is only because of the need to supply a comprehensive reference which will be found useful at a later date.
Foam Pros and Cons
Construction: Speed of work is an important advantage. Once you master the techniques of cutting foam wing cores, you can be ready to cover your wing in minutes after you get two rib templates made, and it takes only a little longer to make an extra wing while you're at it. If your foam does break, you can often epoxy it together and be back in the air without leaving the flying field. In one instance a Silent Squire was flown into a post at 40 mph, hitting the wing about 15 in. from the root, and putting a 1-1/2-in. deep dent in the wing. A built-up wing would have been sheared in two. But the foam wing was back on the tow line after merely covering the damaged portion with a few strips of masking tape.
What Is Foam
Many plastics can be produced as a "foam" by adding chemicals (called foaming agents) as gases, or that generate gases on heating which cause the plastic to assume a cellular structure. Those with open cells absorb liquids and those with closed cells generally absorb very little liquid. Foam plastics are produced in rigid, semi-rigid and flexible form. Several closed-cell rigid plastic foams have been used successfully for wings and fuselages of model aircraft.
Polystyrene Expanded Bead Foam: Polystyrene expanded bead foam (often called bead board) is the most popular. It is produced by expanding polystyrene beads which usually contain pentane gas as the expanding or foaming agent.
How Foam is Made: Polystyrene beads of the appropriate grade and bead size are fed into a steam or vacuum pre-expander and expanded to the cell size required for the desired product density. The expanded beads are dried and aged for one to six hours. In the most common molding systems they are then fed into preheated molds and steam is applied to heat the beads to a point where they will complete their expansion and fuse together. The mold is then cooled and the molded foam is removed.
There are a number of variables that affect the final product. Some are design variables, such as bead size, high or low volatility of beads, resin compounds, and resin quality.
Some are processing equipment variables; there are several alternative processes for pre-expanding and for molding the beads. There are also a number of operator controlled variables. All of the above affect the characteristics of the finished product, either by design or circumstance. Therefore, some variation will be encountered in quality, even in foam of approximately the same density. The density of the foam affects the strength of properly made foam as shown in the comparative strength tables.
Styrofoam—Extruded Polystyrene Foam
Styrofoam is the Dow trademark name of polystyrene foam made from the same basic raw material (polystyrene) by a different process. While the exact process is secret, it is generally as follows: Polystyrene in liquid form is pumped into processing equipment, where it is heated and foaming agents are introduced, and it is then extruded in continuous sheets. The material has a continuous matrix with relatively uniform bubbles of gas trapped in it. Freon is one of the gases used and, being a large molecule, it does not gradually escape as the pentane gas does from the expanded bead type foam. Due to the solid matrix between the bubbles, Styrofoam generally has a minimum density of 1.7 pcf (lbs./cu. ft.) as opposed to .08 pcf for expanded bead. The term Styrofoam is widely used to describe all the polystyrene foams. Even where the distinction is made between the expanded-bead type and the extruded Styrofoam brand foam, many modelers think of all Styrofoam as the relatively light, brittle DB (decorator board) type widely used by florists.
Styrofoam of the type used by aircraft modelers is a light solid blue color and has a fine texture like sandpaper. It is called SM (sheathing material) and is usually about 1.8 pcf (about 80% heavier than the 1.0 pcf expanded-type foam generally used). Styrofoam is advantageous for unskinned wings due to its greater strength and finer grain. Expanded bead foam is preferred where weight is a factor and the wing is skinned (which provides ample strength and rigidity). For example, a typical 60-in. Styrofoam wing core could be 4 to 5 oz heavier than one cut from expanded bead foam.
Ease and accuracy of construction are two more big advantages. You get a true airfoil, it is easy to get graduated washout from root to tip and the covering material follows the exact plotted airfoil rather than flattening out between ribs. Only the root and tip ribs need be plotted for a tapered wing, eliminating the plotting or stack cutting of the remaining ribs in a built-up wing. The inherent accuracy also makes foam ideal for modern scale models.
There are a few disadvantages. Polystyrene expanded bead foam is attacked by a wide variety of solvents and a core can easily be ruined if you don't know what you are doing. A major purpose of this article is to help you avoid such pitfalls and to take maximum advantage of the good properties of foam.
Flight Characteristics: The accuracy of the airfoil, the ability to build in graduated washout, and the high strength with no weight penalty, offer significant advantages well supported by the domination of foam wings in Formula 1 racers and pattern ships. Its use in all types of gliders is well established and spreading. Moreover, foam wings provide shock absorbing characteristics that have saved many a ship. When foam does break, you can often epoxy it together and be back in the air without leaving the flying field.
Chemical Properties
Solvents: Polystyrene expanded bead foam is especially sensitive to attack due to the thin cell walls. The same solvents also attack Styrofoam.
Fast Acting Solvents: The following type solvents dissolve foam quickly: aromatic hydrocarbons like benzene; chlorinated hydrocarbons like carbon tetrachloride; ketones like acetone and MEK (Methyl Ethyl Ketone); esters like polyester resins; and essential oils of high terpene content like oil of lemon or orange and turpentine.
Slower Acting Solvents: The aromatic hydrocarbon portion of gasoline, solvent naptha (common paint solvent thinner), fuel oil, and mineral oil exert solvent action on polystyrene. Higher alcohols, ether alcohols, glacial alcohol, acetic acid, and some unsaturated hydrocarbons, coal-tar pitch, cut-back asphalt, and essential oils of low terpene content, cause relatively slower softening or swelling.
Polystyrene has relatively good resistance to most weak acids, caustics, salt water, bleach, alcohol-based paints, water-based paints, cements, asphalt, paraffin wax, and most lubricating oils. Whenever there is a possibility that a chemical to be used with polystyrene may dissolve it, test it on a piece of scrap.
Adhesives for Polystyrene Foam
As a general rule, avoid coatings and adhesives for direct application to foam which contain solvents such as:
- Acetone
- MEK
- Styrene
- Butanone
- Esters
- Ketones
- Distillates
- Naptha
- Nitrocellulose
- Petroleum
- Toluol
- Turpentine
- Xylol
- Silicone
- Urea
Adhesives which often contain these solvents include: Model and jewelry cements, contact cements, butyrals, and many general purpose cements. Some specific examples are: Ambroid modeling glue, Hot Stuff, and Superepoxy.
Coatings in this category include: Airplane dope, lacquer, oil-base paints, polyester resin, and most spray paints of the acrylic, epoxy, synthetic base, and urethane types. Even some adhesives and coatings which list polystyrene foam as one of the materials on which they can be used may prove disastrous on foam wings. There are cases where materials containing some of these solvents may work if applied in very thin coats and the solvents are allowed to largely evaporate before assembly.
As a general rule, the following adhesives and coatings can safely be applied directly on polystyrene foam: White glues (usually must dry tacky before assembly), water-base contact cements, epoxies which are mixed by user (watch out for spray cans), silicone adhesives, and the natural glues (casein, animal, etc.).
Some specific products known by the authors to be satisfactory in contact cements are Light Dex, Carl Goldberg's Blue Goo, Core Grip, Southern Sorghum, and 3M 77 Scotch Grip; and in the slow drying adhesives: Kwik Tak, and Wilhold Glu Saver; and in the paints: Foam Luster, Perfect Paint and Styromate. There are no doubt other good products. We have only mentioned those we have had an opportunity to try and test. Test all adhesives on scrap first to be sure.
Emulsion Solvent Water Asphalt Asphalt Natural (Casein animal glue, etc.)
Inorganic Inorganic Inorganic
Resin* Resin* - Rubber** Rubber** Rubber** Silicone Silicone
*Typical resins are: Polyvinyl Acetate, Polyvinyl Chloride, and Polystyrene. **Natural and Polymer.
The principal components of the reactive-type adhesives used are: Cement • Epoxy • Inorganic • Mortar • Resourcinate
Reactive—which thickens and hardens due to a chemical reaction between two components which both remain in the joint.
Hot Melt—which are composed of solids which become fluid when heated and return to a solid state in the joint as they cool.
The principle solid components of the drying-type adhesives used are listed under the three basic types of liquid carriers as follows:
A pressure sensitive silicone is also used.
While a wide variety of adhesives have been used successfully, there is only one sure way to test the suitability of a specific adhesive. Test it on scraps of the foam you are using. Because of the many formulations, testing is the only way to be sure, unless the adhesive is specifically labeled for use on polystyrene foam and has instructions for application.
The drying type generally work best when the solvent escapes thoroughly in the open assembly time (before the two pieces are joined).
Coatings
Polystyrene foam is damaged by exposure to the ultraviolet rays in sunlight. If you do not plan to cover it with balsa, cardboard, plywood, etc., it should be painted with a suitable material. Latex paints work well and some lacquers, enamels and varnishes can be used if one or two prime coats of shellac are applied first.
Epoxy paint is probably the best protective coating if any material is going to be used which is a solvent for polystyrene foam. Even with epoxy at least two coats should be applied due to the tendency of all coatings to form pinholes when applied to foam. Other commercial resins can also be used depending on the solvent. Again, as
COMPARATIVE STRENGTH TABLE
Density (pcf)
- Balsa: 8.0
- Expanded Bead Polystyrene: 1.0, 2.0, 3.0
- Styrofoam: 2.0, 3.0
Compressive Strength (psi)
- Balsa: 150
- Expanded Bead Polystyrene: 15, 35, 60
- Styrofoam: 45, 140
Tensile Strength (psi)
- Balsa: 100
- Expanded Bead Polystyrene: 21, 55, 90
- Styrofoam: 70, 200
With the grain (psi)
- Balsa: 2000
Shear Strength (psi)
- Balsa: -
- Expanded Bead Polystyrene: 19, 35, 60
- Styrofoam: 45, 75
Flexural Strength (psi)
- Balsa: 200
- Expanded Bead Polystyrene: 22, 90, 145
- Styrofoam: 95, 150
With the grain (psi)
- Balsa: 3000
Data from Modern Plastics magazine, technical publications from ARCO/Polymers, Inc., and DOW Chemical USA. Differences noted between sources are shown as a range in some cases. While the range is the most valid due to variations in foam, this table provides a useful comparison of what appeared to be average strengths. with adhesives, first test any coating on foam scraps.
Sources of Foam
Expanded Bead Polystyrene Foam—General Sources: There are many independent manufacturers and some of them will sell directly if as a club you purchase in sufficient quantity. Otherwise they can direct you to dealers who sell foam of the density you are interested in, typically 1–1½ lbs./cu. ft. Both manufacturers and retailers may be found in the yellow pages under "Plastics—Foam."
Beadboard insulation is one of the more common forms usable by the modeler. It is typically available in 1 lb./cu. ft. density in thicknesses of 3/4, 1, 1-1/2, 2 and sometimes 3 in. Many building materials suppliers carry this material. It is also widely used for many aquatic applications, typically molded at 1.5 pcf density. The name of your nearest supplier can be obtained by writing to: Arco Polymers Inc., Philadelphia, PA 19101.
Custom cutting of cores is done by some hobbyists. Your local RC club is a good place to start looking.
Styrofoam Extruded Polystyrene Foam: All "styrofoam" is manufactured by Dow Chemical Company or its licensees. Styrofoam "SM" (Sheathing Material) is the most widely available type suitable for aircraft modelers. It ranges from 1.8 to 2.0 pcf and is available through many building material suppliers in 4×8-ft. sheets in thicknesses of 1, 1½, 2, and sometimes 3 in. Look for Dow Chemical in your yellow pages under Plastics or write to: Dow Chemical USA, Midland, MI 48640.
(To be continued)
Transcribed from original scans by AI. Minor OCR errors may remain.
