why i obtein this small grains?why the hollow of the model is black
?i clean the rubber very well
Normally, if your rubber is sold to you the way it comes to us, it’s
already clean, and the main thing you need do is not get it dirty.
ours comes with a yellow fabric cover on one side, and a thin blue
plastic peel off layer on the other side. We buy it in the pre-cut
pieces, since those happen to fit the mold frames we most often use,
and the time saved offsets the increased cost. That form of rubber
needs no cleaning if your hands are clean and you don’t touch the
surface of the rubber when packing the molds. I remember that back
when I bought the rubber in larger rolls, only one surface was
protected, and the open surface was powdered. I seem to recall this
benefited by cleaning, but i might be wrong. Perhaps Mr. Knight, of
Castaldo, who I think reads this list, might clarify that, as well as
commenting on other aspects of your post too.
I’m not familier with getting little grains in the rubber. However,
I wonder whether perhaps you’re cutting your molds with a sharp
enough blade? The tips of scalpel blades are easily dulled, and often
need to be replaced with new blades. There may be some readers of
this group who have success resharpeing their mold blades, but i’m
not one of them. I can put a decent edge on a blade, but for mold
cutting, the very tip of the blade must also be sharp, and I can’t
seem to get it as good as the commercial edge the blade comes with.
Again, the savings in time of using new blades when needed offsets
the cost of the blades. When cutting with a slightly dulled blade,
the rubber will be slightly torn, not totally cut, so it can seem not
quite as smooth. Is this perhaps what you’re seeing?
As to the dark color of the mold cavity, if you’re using models made
of silver or brass or the like, then this is normal. The rubber
vulcanizes because if it’s sulphur content. At vulcanization
temperatures, the sulphur in the rubber reacts with the copper and or
silver in the metal, to form sulphides, some of which get absorbed
into the rubber. If you wish to avoid this, some model makers go to
the trouble to rhodium plate their models. This means after fully
finishing your model, polishing as you need, etc, then you nickle
plate it (copper too, first, if it’s silver), and then rhodium plate
it. The rhodium is not affected by the rubber, and the mold cavity
then stays the rubber’s normal yellow color. However, the
discoloration of the rubber from unplated models doesn’t seem to have
a strong effect on the waxes the mold produces. I usually don’t
bother plating my models, and consider the discoloration normal. It
doesn’t seem to get in the way.
As to temperature, if the temperature is too low, the mold will
tend to be too soft, and perhaps even a bit gummy. If the
temperature is too high, then the molds can be stiffer, and tend more
to warping. Few vulcanizers are accurate in temperature control,
and a vulcanizer preheated to the right temperature gets quickly
chilled a bit when you put a new mold frame into it. So the
temperature the mold is cured at does vary a bit. Normal Castaldo
rubbers are fairly tolerant of this. Adjust your vulcanizer to a
temperature setting where the end result is acceptable, and then
don’t worry too much about the variences you see while the mold is
vulcanizing. The no-shrink pink rubber, though, is a LOT more
sensative. For that, you’ll need to pay closer attention. In
particular, measure the temperature of both platents of the press.
Many vulcanizers have considerable difference between one platen and
the other. The aluminum mold frames tend to conduct heat well enough
to somewhat even out the temperature differential, but some
vulcanizers are just too poorly matched between the platens to ever
give really consistant results.
Hope that helps.