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Foaming Experiences      Back ] Next ]


Radio Control Model World - Nov '95

by Stan Yeo



For a long time now the volume of work at PMP (Phoenix Model Products) has been such that we have had to sub-contract the manufacture of our foam wings. As a result, some of the experiences gained when we produced the wings ourselves had faded to the back of our minds so when we bought a computerised foam wing cutter recently of these experiences were rapidly retrieved! Not being one to keep useful information like this to ourselves I have put pen to paper once again. This article is not about how to do it but how to do it a little better and avoid some of the holes most of us fall into.


The first lesson we learnt was the selection of the raw material. Expanded polystyrene is produced by a steam foaming process. Steam as we all know is gaseous water that returns to liquid form when it cools below 100 deg. C. If you try to cut any foam that contains moisture then the moisture cools the cutting wire and it drags i.e. wire does nut cut cleanly. Unfortunately the moisture is never evenly displaced throughout the material so you end up with a ridges where the wet foam starts and the dry foam ends. The secret here is to buy your foam well in advance, several weeks if possible, and store it in a warm dry place. This is particularly applicable if you buy your foam from a builders merchant or direct from the manufacturer where the foam is so new it has not had time to dry out.

When selecting your foam keep an eye open for poor granule bonding i.e. easily detachable beads, (EPP and white foam). Only buy virgin foam. Some foam contains recycled material and this has the occasional hard lump in it which is disastrous if the cutting wire snags it during the cut.

The second lesson concerned stresses that are built up in the foam during the manufacturing process. Foam can react in a similar way to poorly seasoned wood when it is cut i.e. go in every way but which way. This again is another good reason for buying your foam well in advance and letting it mature. This will not relieve all the internal stresses but it does help. In some foams such as expanded polypropolene (EPP), the material we make our virtually crash-proof Com-bat 50 from, the material undergoes heat treatment during the manufacturing process to relieve these stresses. Unfortunately though, it doesn't remove all of the moisture so it still needs drying out before use. If possible, cut the foam into blocks slightly larger than that required before storing them. This will help relieve some of the internal stresses during storage and assist the drying out process.


For short runs and one offs 1.5mm ply templates are more than adequate. The secret to producing a good ply template is to make not one but 3 or 4 at the same time ! The method I use is to stick the 1.5mm ply together using double sided tape. Glue or double tape a photocopy or printout of the airfoil, complete with chord line, to the ply (Compufoil is an excellent program for generating airfoil templates). Drill a series of 0.8mm (1/32in) holes on the chord line where the leading and trailing edges start and finish. These holes act as reference points on the duplicate templates to draw in the chord line and leading / trailing edge positions. The next bit is easier if you have access to a bandsaw and a belt sander because it takes away all the hard work and allows you to concentrate on the accuracy of the template rather than the sweat you are producing! When working with thin materials it is very easy to get 'carried away' and remove too much material but by cutting the templates in a stack the cutting tool is loaded and easier to control. After sawing the templates roughly to shape finish off using, initially, 180grit wet and dry followed by 400 grit taking care to keep the edges square. Always use a reasonble size sanding block and double side tape the wet and dry to it. When satisfied, separate the templates and smooth the edges. All that is left to do now is draw in the chord lines and leading / trailing edge positions, mark off 10% increments along the templates (both sides of course starting from the LE), drill holes for the template retaining nails and candle wax the edges.

One of the problems you get using this type of template is template movement on the first cut, often due to too much downward pressure on the template. A way to overcome this is to use a template saddle and keep the downward pressure to a minimum. Make the saddle from 6mm birch ply, not forgetting to incorporate any washout in the tip. This is where the chord line comes in handy as it act as a reference for positioning the template on the saddle when marking out. Again a bandsaw is useful when cutting out as it removes the hard work and improves accuracy!


It is imperative that this is done on a flat surface. Check the surface with a spirit level, left and right, fore and aft and diagonally. I once rejected a considerable number of wings because a sub-contractor had allowed his working surface to develop a twist. Do not start blocking out until you are satisfied the working surface is completely flat.

The first cut is the most important as this becomes the reference for all other cuts so it is very important that it is a good one. To stop the foam moving during a cut it is necessary to weight it down. We use off-cuts of 6 - 10mm steel plate obtained from the scrap-yard. Only use sufficient weight to stop the foam moving. Be careful that the weight does not distort the foam as this could cause problems later. Having established a reference plane the other three sides can be cut to size. For blocking out either use specially prepared 1.5mm ply straight-edge templates or cheap aluminium rulers available from local DIY stores.

When blocking out position the foam block on the edge of the cutting table so that the cutting templates can extend beyond the edge of the block. This over-run is important because the cutting bow invariably drags during the cut and the over-run allows the bow wire to exit the foam cleanly. Alternatively if this is not possible put packing (offcuts of kitchen worksurface are ideal) underneath the foam checking of course that it does not foul the bow wire on exit.


Since cutting EPP foam the adage the tighter the bow the better the cut has become more significant. Cutting foam with a hot wire is principally achieved by the wire melting the foam but if the hot wire is allowed to do all the work then the finish is not as smooth as it could be if the wire was given a little help. This of course means applying a small amount of lateral pressure to the wire. Unfortunately unless the bow is very tight even the slightest pressure results in a curvature of the bow wire. A small amount can be tolerated but not a lot. Other people have obviously recognised this problem because when talking to the manufacturer of the computerised cutter it was mentioned that other customer had found it necessary to increase the bow tension and one even attached vibrators to the bow wire. Care has to be taken when increasing the tension however in case the wire snaps when it heats up. We use 26swg (0.45mm) nichrome wire. We have used 26swg nichrome locking wire as used in the aircraft industry to wire lock electrical connectors. This wire however is fully annealed and not as strong as the other. The current required is typically 2amps. Slightly more if blocking out or cutting blue / pink / EPP foam. The current is irrespective of bow length (it is the supply voltage that varies with bow length). Cutting speed is approximately 150mm / minute.


Before cutting the cores the templates must be pinned in position. This must of course be done with the utmost accuracy. If you have produced saddles for the templates it is a lot easier. I use 2 or 3 50mm thin wire nails to hold the templates in position. Take care when inserting the nails as many a wing has been ruined when the bow wire has snagged them. It is not normally a problem at the leading edge where the core is fairly deep but it is at the thinner trailing edge.

Unless you are experienced in cutting foam wings enlist the help of a friend, preferably not your partner!, and do it in a well ventilated room. The fumes can be hazardous to health (hot wire foam cutting has been banned in schools!). Always start at the leading edge. If the wing has a wrap around veneer leading edge push a pin into nose of each template to act as a wire guide when starting the cut. Set the bow at the correct temperature. If possible incorporate a foot operated micro switch in the bow power supply to switch the bow on and off. This way the bow can be switched on with the wire resting against the foam. This avoids over-cutting of the foam due to excess heat in the wire at the start of the cut. When cutting tapered wings a bit of trial and error may be necessary to find the optimum temperature as the cutting speed varies over the length of the wing. There will inevitably be some over-cutting at the tips. This must be allowed for when making your tip templates by reducing the allowance made for the thickness of veneer. The greater the taper the greater the allowance.

As mentioned previously the cutting action is a delicate balance between temperature and speed of feed. The bow must be kept moving smoothly throughout the cut if a smooth finish is to be achieved and damage to the template avoided. This can be difficult for the tip operator. A trick I learnt was to wax the templates and to move the bow back and forwards in a sawing action. This not only makes it easier to keep the bow moving laterally but gives a smoother finish and temporarily 'glues' the templates to the core helping keep them in position, particularly during the second unsupported cut. The herring bone affect is quite attractive as well! If fitting a spar to the wing prepare separate jigs to cut the spar slot and glue the spar in position using aliphatic resin.


The final pearls of wisdom, if that is the appropriate expression concern veneering but first fit the rear spar / trailing edge using aliphatic resin. These can then be locked in position by the veneer providing a neater and stronger joint. It is best to pre-shape these items first as any shaping after they are fitted runs the risk of damaging the core. Those of you who have built a PMP wing with the TE / rear spar fitted will agree it makes life easier and certainly looks neater.

Other veneering tips include ensuring the veneer grain is parallel top to bottom and parallel to the mean chord line. If not a warped wing could be the result. Obechi veneer is 'shaved' off a tree trunk, rather like sharpening a pencil with a pencil sharpener. This leaves the veneer with a natural curl in one direction. Let the veneer follow its natural curve when gluing it to the wing. Do not try to save weight by watering down the water based latex adhesive as this weakens the glue resulting in delamination. A good source for the latex glue (Copydex / Unibond etc.) is a carpet fitters. Apply the adhesive with a piece of scrap furniture foam. Allow the wing and veneer to dry overnight in a warmish room. It is very important that the moisture is allowed to evaporate from both the core and the veneer before joining them together. We have had wings where the individual sheets of veneer have had different moisture levels when attached to the core. As the moisture levels equalised the wings became distorted and sometimes the veneer split.


Well I hope you have found this article useful. It has been based on a number of years experience in making foam wings and I sure anyone who has made a few sets of foam wings will concur what has been written. Happy foam cutting.