"Timothy O'Connor" wrote in message
...
This is a really stupid question (that someone asked me the other day, and
I
didnt have any reasonable answer), but why is it not possible to simply
mold
a mirror to shape, rather than grind it? I would have thought that it
would
be possible get it done with a great deal of acuracy these days.
(obviously
not, but Im not sure why)
It is just down to accuracy. The first problem, is that with most molding
processes, you need some form of 'release agent' to allow the mold and
molding to seperate. This degrades slightly the accuracy. Then you have the
problem that most (all?) molding materials change slightly in size as they
set (whether by chemical hardening, or by cooling). Again this distorts the
shape slightly (this is why designing molds to produce even reasonably
accurate surfaces is a complex process). Then you have the question of
whether the material itself is rigid enough to hold the shape when complete.
If you think in terms of other industrial processes, using casting or
molding, high accuracy surfaces (even though, in terms of a mirror, these
are relatively poor in accuracy), are usually still 'finished' by some form
of machining process after molding. The levels of accuracy required for a
good mirror, are several orders of magnitude beyond those achieved by all
but very complex molding processes. These processes are themselves
expensive, and then have tooling costs, that make normal mirror grinding
look very cheap indeed. You are talking an entire surface, which is accurate
to better than about 1/6000th mm...
There are some molding processes that approach this accuracy, but normally
only over quite small areas (these are the processes used for some lens
manufacture - these generally still only produce relatively 'budget'
lenses). The question at the end, is then still left, as to the suitability
of the final material for anything bigger.

Best Wishes