Following on from my short description of the fabrics available to modellers we now switch our attention to foam. Foam is used in two ways; either for the cores material in wood veneered or glass vacuum bagged wings or as the sandwich material for the skins of hollow moulded wings. We've all come across white and blue foam but there are some other foams we could look at...
1) Styrofoam SP, 2) Rohacell 51, 3) Conticell, 4) Styrofoam IB, 5) De-Q-cell, 6) Termanto, 7) Polyfoam Plus and 8) Nomex Aramid Honeycomb.
Polystyrene foams come in two types: expanded foam and extruded foam. Common bead-type white polystyrene is expanded foam (in a mould) and it's the cheapest available. It comes in a variety of densities from about 17 kg per cu. m. upwards for the large bead foam. Expanded foams are only really suitable for covering with wood veneer since the fine trailing edge needed for vacuum bagged wings is hardly obtainable with this foam. There are, however, better white foams other than the standard builder's yard material in DIY stores. Left to right the samples here are:
One such PS foam is De-Q-cell found in Germany which has a sort of crystalline appearance when you look at it. I haven't seen it for sale here in the UK but it maybe available from Windsurf board manufacturers, I don't know. It comes in 3 weights: 17, 30 and 40 kg/cu.m.
Extruded polystyrene (forced through a die) foams are eminently more suitable for wing cores - their closed cell structure (hit the DOW logo to see it) giving a much finer trailing edge. In the UK we call it Blue foam, in France they call it Roofmate. It doesn't have to be blue, of course for I have seen Gray board from the States, an orange one from Germany and I'm told you can even buy a green variety from the Bricolage at the Calais Hypermarket. An old DOW catalogue I have actually lists 17 types of Roofmate, Wallmate, Floormate boards with densities from 28 to 45 kg/cu.m. The one I generally use is called Styrofoam IB with a density of 28 kg/cu.m. but I have also used Styrofoam SP (34kg/cu.m.) which I found less easy to use.
Polyfoam Plus, the pink extruded variant, available in the UK from LinPac, is once again an insulation material for building which boasts the use of steam instead of CFC's (Freon?) as the blowing agent. Its cavity wall board has a density of 20 kg/cu.m. while the floorboard comes in at 27 kg/cu.m. Lighter but a little more care needs to be taken because it's easier to damage its surface through heavy handedness and workshop dings. Cuts OK though with a standard hot wire set up.
Otherwise known as Rohacell (PMI) foam, this is a comparitively expensive white coloured foam. It apparently does cut with a hot wire like polystyrene foam although it is often cut with a knife or saw. It is available in ready cut sheets at 1 mm, 2, 3, 4.... up to 65 mm thick and more. Modelling applications require 1 or 2 mm thick sheets. Its superior compressive strength over PS foams means that it's ideally suited for sandwich construction. Modern moulded models (especially tailplanes) may use Rohacell as the sandwich between lightweight (40 -70gsm glass or 100gsm carbon) plies of reinforcement cloth. It can be cut and sanded easily and is not subject to attack by most solvents (styrene, acetone, MEK, etc). Rohacell is made by Röhm of Germany as is classified as Rohacell 31, 51, 71, 110, 170, 190, the numbers denoting it's density in kg/cu.m. We would use the 51 type for modelling. UK suppliers are EMK plastics I believe.
PVC rigid foams are rarely used in modelling but full size gliders sometimes use it. Trade names are Termanto, Conticell, Divinycell. These cannot be hot wire cut easily but must be sawn to size, they are also used as sandwich materials. They are yellowish brown in colour. Densities range from 30 to 400 kg/cu.m. Divinycell Webpage
We're more likely to see polyurethane foam from an aerosol can but it's also available in sheets as a sandwich material. It has few uses in modelling but I have heard of the aerosol foam used for repairing broken fuselage booms.
POLYPROPYLENE FOAM (EPP)
This is the latest foam on the modelling scene. In the US it comes under the brand name of Eperan 2, the bead precursor for the foam (Neopolen P) is made by BASF and is expanded in a low pressure steam chamber without using blowing agents. In the jargon it's called a closed-cell thermoplastic olefinic foam material. Nowadays it's used for wings and fuselages for Combat flying although was originally developed for use by the car (automotive) industry for impact absorption (i.e. bumpers, dashes, etc). EPP is a beaded foam which has a memory; that is to say that it returns to its shape after being distorted. It is a white coloured foam with a waxy feel and appearance and really needs to be sprayed with 3M adhesive before applying fibreglass reinforced tape and/or plastic film. It can be cut with a hot wire, though not as well as PS foams. I hear tell some cut it with a flexible wire saw too. As far as I know it comes in two standard grades, 1.3 lb and 1.9 lb per cubic foot. The lighter version is generally used for models.
EPP FOAM LINK: http://epp540.com/
POLYETHYLENE FOAM (Ethafoam)
Polyethylene foam is similar to polypropylene foam and is made by the DOW Chemical company. As far as I know two grades are available: Ethafoam Nova and Ethafoam Select. They are low-density polyethylene foam products. For more info: http://www.dow.com/ethafoam
Nomex is not a foam but since it is used as a sandwich material in full size (and can be used for models) I include it here. It's made by DuPont and is really a sort of expanded Kevlar paper. Layers of this paper material are laid one on top of the other with lines of adhesive. Each layer has the adhesive lines in a different place. When the adhesive has set and the layers are expanded the honeycomb is formed. It is then sawn into sheets. Available in 2 mm sheets upwards for modelling. Once again a very expensive material and, although difficult to use, my brother did make a set of moulded wings using it as the sandwich material. I sometimes use it between two plies of carbon cloth as a lightweight wood substitute for ply servo trays and the like. Even though Nomex is mostly air, it still comes in at 29 kg/cu.m.
If you're looking for foam suppliers a good place to start is the Yellow Pages under Insulation. In the UK, Pink Foam came to me via LinPac and Blue Foam through Sheffield Insulations. Rohacell made by Röhm in Germany (www.roehm.com) used to be available here but the company stocking it in the UK ceased trading. De-Q-cell comes from Bacuplast in Germany and Nomex is available form R&G Flüssigkunststoff, also in Germany.