Sloping Off - March 2002

Working in Pink Foam
Mike Roach reports on a new technique

Cartoon Courtesy of Phil Lockwood

No Trevor, not the pink foam from that disco in Corfu (the usual fee, and I won't tell Sloping Off about that episode). This is PolyFoam Plus from Travis Perkins the builders' merchant in Christchurch. It comes in sheets 18" x 48" x 2" (one cu ft exactly) which weigh 22oz (630 grams), which is about 1/5th of the weight of a similar block of medium balsa. The material is of a consistent structure, completely different from white foam. It's a little like the finest honeycomb you could imagine, but without a magnifying glass I can't tell whether it has a regular structure at the thou level. As bought, it has rebated edges and a hard external finish on the main faces. The edges should be squared up, and what I think of as the "crusts" discarded, as they are much more brittle and probably heavier than sheets cut from the main part of the bread. (I'm a toast fan, you see).

With conventional (but sharp) tools it cuts, sands, carves and forms like balsa. With the Club's Hot Wire Cutter (HWC) it can be cut into sheets as thin as you like, down to 1 mm and even thinner, and after a little practice a section can be cut to give a feather edge, so it is perfect for flying surfaces. Comparing a sheet with balsa of the same thickness, you can see that:

• Pink Foam has no grain and cuts equally easily in any direction, but keep the knife blade at an acute angle to the foam to avoid tearing.
• It bends more easily in one direction than another, but this seems to be the result of HWC'ing rather than anything in the foam.
• It can be "moulded" to form an airfoil section, rolling it round a worktop edge or something similar. As far as I can tell, it then keeps the shape indefinitely.
• Thin sheets can be hand moulded to form amazingly tight curves, such as the headrest on a fuselage, without cracking. Once moulded, it holds the shape. With care, a 1 mm sheet can be bent round a 1 mm curve.
• It glues securely with wood glue or epoxy, and best of all with B&Q's solvent-free All Purpose Glue. (£1.65, in a green tube). This and the wood glue take longer to set than when working in balsa: say 60 minutes compared with 5 minutes.
• HWC'ing produces a rough surface, which can then be sanded to give a very good surface by using a standard orbital sander, with fine paper of course. Hand sanding can also give good results but is very tedious! This surface can then be improved by using a lightweight filler (Light & Easy by Pollyfilla works just fine), spread as thinly as possible, using a scrap of 0.4mm ply. This process also whitens the surface, which is useful.
• The foam can be painted in rough, sanded or plastered form using acrylics, either brushed or sprayed. I've had great success with Tamiya paints. The difference between the finishes can be used to simulate metal, ply or fabric on the original. Weathering can be simulated using dry brushing or finger painting and even my very first efforts with a spray gun were encouragingly good.

In all these respects, pink foam is at least the equal of balsa, if not its superior. Like all foams, its disadvantages are based on its lack of compressive strength. Bend it and it will snap, press it and it will bruise, drop a pin on a wing and it may puncture. Building with it is a constant battle against "hanger rash". And yet without any special efforts, I have a Sopwith Tabloid of 36"span that weighs 4oz bare, exactly half the weight of a similar balsa model made just a few weeks earlier to the same scale. For a second model I expect a target bare weight of 3.5oz, which will give a flying weight with the Pico Stick gear of 7oz. The quality of surface and detail that can be achieved is amazing and makes my earlier efforts with white foam look raw and childish.

My only concern is with its toxicity when being HWC and the dust it produces when being sanded. Until it gets a clean bill of health, wear a mask at all times and keep the room ventilated. But give it a try - at £6.50 a sheet it's a bargain!