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Talcum powder and rubber molds


#1

Hi Orchid, I decided this year that I wanted to increase my
production by outsourcing the mold making and wax injection for my
line of cast filigree pieces. I took great care in perfecting my
original models, makingsure they had a super high polish and sent
them off to a well known company. They were very helpful and
friendly. A couple of weeks later I got my waxes and I was
disappointed to find that they had used quite a lot of talcum powder
or some other type of powder in the molds, leaving a very rough and
pitted surface on the wax. When I make my own waxes I use rtv
silicone molds and just a tiny bit of cornstarch in the vents, the
first wax that comes out of the mold that has the rough surface from
the powder gets thrown out and then I keep the waxes that have the
nice smooth polished surface. The waxes I received look like a
coarser powder like talc was used every time and all the waxes are
very rough. So I guess my question is, are my standards too high? Is
it common to powder the mold every time? Apart from that I do not
have time to heat-polish every wax before casting and I don’t have
time to wait until I get better waxes, so I casted a couple flasks
with about 80 little pieces yesterday. They look rather rough. So
I’m going to try running them through some blue pyramids in the vibe
tumbler, then green, and then steel shot. I would appreciate any
advice on this issue… Thanks. Douglas


#2

From you bit of description of the situation, I would think that the
company has made molds using some sort of heat vulcanizing mold
rubber, in that every wax has release powder. Silicon RTV molds are
GREAT in that they make a perfect replication of the master and
usually no release is ever needed, save “making” an air release.
Heat vulcanized molds, especially rubber verses silicon, HAVE TO BE
release nearly every injection, at lease in my limited experience.
Silicon molds, have a shorter life and are a more expensive
material, than “real vulcanized rubber” molds. I bet the company you
went to is using rubber, not silicon, thus the need to use release
on every “shoot” and the poor texture of the waxes/castings. I doubt
tumbling will result in giving you an acceptable piece.

Talk to the company you are having make the molds and waxes and see
what they are actually doing. It they are using “real rubber” ask if
they could make the molds with silicon. If they will, ask them to
keep the release to an absolute minimum if at all. Silicon heat
vulcanizing molds are ok, if it were my “thing” I would use silicon
RTV (room temperature vulcanizing) mold material. You then have a
decision to use either Tin catalyzed mold materials, cheaper but
shorter mold life (about 10 years) or Platinum cured silicon, more
pricy but a VERY LONG mold life. Either of these materials would not
require much if any release and if needed/wanted, I would recommend
using a spray-on Teflon (if you want the source, drop me an e mail
as I have to go look it up in my studio/workshop). This release
works fantastically well, create NO texture in the waxes. I get it
in aerosol cans, by the case, but I use it for a separator release
when making multiple part molds for sculptures my wife Cynthia
creates. She is a retired goldsmith/master mode maker now making
bronze sculptures, mixed media pieces and pastels. If interested you
can see both of our works at MLCE. net. Just click on the
picture/description of the site you want to go to (there are 3) on
the MLCE. net splash page.

If you or anyone else has any questions about what I have posted,
please feel free to contact me via this site’s e mail (for list
perusal) or directly (john at MLCE. net) and I will reply ASAP.

john dach
john@mlce. net


#3

You’ve mentioned it in passing, but only slightly. The choice is not
limited to natural rubber vs. liquid rubber RTV compounds.

There are two main categories of vulcanized (that is, heat-cured)
rubber molds. There are natural rubber compounds and silicone rubber
compounds.

Vulcanized silicone rubber molds do not need a release spray as they
are loaded with silicone oil. Plus you just put them in the
vulcanizer-- no measuring, no mixing, no vacuuming, no bubbles, no
mess.

Advantages and disadvantages both ways. Of course, it’s a personal
choice.

Michael Knight