As I stated before, organ pipes can be made of any material, providing
that the walls are sufficiently stiff to contain the air column.  I have
seen and heard organ flue pipes made of all kinds of natural woods,
plywood, organ metal, zinc, copper, brass, concrete basses (!), Bristol
board (a complete organ made by a Swiss), stoneware, Perspex, various
plastics, and papier mache.

I figure that they can be made of stainless steel or degraded uranium
as well!  I have never made flue pipes of MDF board, but I cannot see
why that would not make good sounding pipes.

Birch plywood was used by the late Carl Frei Jr. for making smaller
scale pipes like violins and stopped celestes.

Quite another question is: Why would you make pipes out of that
material?  Is it cheaper?  Maybe, but figure out what you will save
in time to construct MDF pipes which can be voiced properly.  I can
imagine that you make bass pipes from MDF, but in that case you will
still have the problem of shaping upper and lower lips and maybe the
looks of MDF pipes, which may be a problem for customers.  For voicing
purposes you will have to use normal wood for the lips.  Another
factor, already mentioned, may be the reaction to changes in relative
air humidity.

Planing of MDF is impossible, but thick walls do not harm the sound;
thin walls do, so the only precaution to take is not making the walls
too thin.

Some reactions point to the acoustic properties of natural wood.  I do
not agree with these, because the main purpose of pipe walls is NOT to
vibrate!  That is why most church organ pipes are made of organ metal,
a tin-lead alloy which has no structural resonance of its own!

Hans van Oost, Netherlands