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Foaming
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Radio
Control Model World - Nov '95
by
Stan Yeo
INTRODUCTION
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.
RAW
MATERIAL
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.
TEMPLATES
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!
BLOCKING
OUT
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.
THE
BOW
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.
CUTTING
THE CORES
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.
VENEERING
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.
SUMMARY
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.
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