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Mold grains with castaldo rubber


#1

Sir Peter Rowe, I’m Ivan from spain. i have problems with the
castaldo gold rubber,all the rubber molds that i do have a lot of
small graind like sand for all the rubber.i vulcanize the rubber at
310�F FOR 7:30minutes per layer.i put the rubber in the vulcanizer at
310�f,and tighten three times in the first eight minutes.when i cut
the rubber all the times i have small grains like sand in all the
rubber,why?what is the problem?,please reply me sir Peter.


#2
Sir Peter Rowe, I'm Ivan from spain. i have problems with the
castaldo gold rubber,all the rubber molds that i do have a lot of
small graind like sand for all the rubber.i vulcanize the rubber
at 310B7F FOR 7:30minutes per layer.i put the rubber in the
vulcanizer at 310B7f,and tighten three times in the first eight
minutes.when i cut the rubber all the times i have small grains
like sand in all the rubber,why?what is the problem?,please reply
me sir Peter. 

Ivan, Please just call me Peter, or Peter Rowe, or if you wish to
be very formal, Mr. Rowe. The word “sir” in front of a name is too
formal, and usually means someone who’s of military or royal rank, a
knight, or something like that, and I have no such rank or title.
(“sir” can be used by itself, in the place of a name, but it’s still
then rather formal). I’m certainly not offended, and in fact am a
little flattered by you calling me “sir Peter”, but it’s really not
needed.

Anyway, I think you asked this same question before. As I said
then, I’ve never seen quite this problem before, and can only guess
what you’re seeing. It occurs to me that what you consider a problem
might just be what I see as normal rubber, but without seeing one of
your molds, there is no way I can know for sure. The main thing
that first jumps to mind would be wondering if your mold knife is
sharp, since a blade that isn’t quite sharp enough can cut the
rubber with sort of grainy feel to it, but this is because the blade
isn’t slicing the rubber cleaning, but only partly cutting, and
partly tearing, the rubber. it’s a subtle difference, perhaps not
obvious. Try a brand new scalpel blade, and cut the molds while they
are still slightly warm to the touch. The rubber should cut
smoothly. If this isn’t the answer, then I’d have to wonder about
the rubber itself. Perhaps you’re buying rubber that’s been stored
too long, or stored in too warm a place before being used, or perhaps
it’s been allowed to get dirty, and you have actual dirt grains in
the mold. All these are possible. Also, if you are using models that
are brass, or silver, then you can expect the mold cavities to be
dark, maybe with a stained or blotchy look, rather than smooth light
tan/yellow. this is due to the rubber reacting with the metal. But
the rubber that has been colored by contact with the model should
still be smooth, not grainy. It only looks uneven. I don’t think
I’ve ever seen gold label (or other brands and types either, for that
matter) have an actual grainy texture within the rubber

However, please understand that while I’ve cut plenty of molds,
and seem to be known for a willingness to answer questions with a
longer posting than needed, there are a number of other people who
subscribe to Orchid who have MUCH more experience with rubbers than I
do. Several of the full time casting experts, such as Dan Grandi or
John Heinkel and a number of others are here, and unlike the two or
three molds I cut per week, they probably cut dozens or hundreds,
and have done so for a lot longer than I have. And also reading this
list is Michael Knight. He is the manufacturer of the gold label
rubber, and has MUCH more available to him as to what can
go wrong. What I would suggest doing is sending an example of one of
your problematic molds to Michael Knight, and asking for his expert
opinion.

Peter Rowe


#3

I tend to agree with you as to the scalpel blade. There is a
difference in sharpness from one company to the next. More people
have been injured with dull blades than sharp. Change blades often,
blades are an expendable expense and in the relative scope of things
cheap. Larry

Larry Paul Casting Co. Inc.
740 Sansom St
Philadelphia, PA , 19106
215-928-1644


#4

Ivan, Castaldo Gold is very picky about time and temp. I set my
vulcanizer at 307F I tighten my handle hand tight in the beginning
and 5 min later I tighten hand tight again. all my mold frames have
a small 2-3 mm hole to allow the mold material to flow around the
piece better. all ring size molds (1" thick and 2" X 3" or less I
run for 30 min total) if the mold is thicker and or larger I run it
about 35 min total. Good luck Rich