Dear Trisha,
We have used (and re-used) polythene embedding molds and stubs from TAAB
Laboratories Equipment Ltd.(e-mail sales@taab.co.uk) fro methyl
methacrylate embedding for many years. They are available in the
following sizes; 16x2mm, 19x13mm, 16x8mm, 12x8mm. The stubs are designed
to act as a partial seal on top of  the embedding mold, and also to
provide an attachment to the microtome. You have to control the
embedding conditions to obtain a 3 hour polymerisation. If too rapid,
you will encounter problems with lack of section adhesion, prolonged
resin removal times, and poor stain uptake, particularly with
haematoxylin. With our supplies of methyl methacrylate, benzoyl
peroxide, DBTH, and dimethylaniline obtained from Aldrich, the following
protocol gives us the correct polymerisation conditions.
The embedding solution consists of:
MMA 80ml
DBTH 20ml
dried Benzol Peroxide 5g (60'C for 30mins)
Invert to dissolve the benzoyl peroxide and store at 4'C.

The accelerator is added at a ratio of 15ul per ml, mixed gently, then
added to the mold (which is surrounded by crushed ice slush which acts
as a heat sink). The stub is then applied and the rim sealed with molten
paraffin wax.
After 3 hours, the mold is removed from the crushed ice, and left at
room temperature for a further 30 minutes. The block is then prised from
the mold which can be washed and re-used.
This procedure is based largely on that of Neil Hand (Proc.Royal
Microscopical Society; Jan89), and has been adapted to meet our
laboratory conditions. You will probably have to make adjustments to
suit yours. A good indicator is provided by the time required to remove
the resin. If it takes longer than 5 mins in xylol to de-resin a 2/3u
section, there has been excess heat generated, and the polymerisation
too rapid.
Hope that this is of some help to you.
Alastair McKinnon