Page 114 - Casting of Angels- Dave Parvin
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Great one gram. But rather than without tearing. A durometer
of 8 to 15 is considered low
playing Archimedes and
and a low viscosity would be
placing the clay in water and
measuring its displacement or 15,000 to 20,000 centipoise.
filling the mold with water,
For more information on the
Mold there was an easier way. I used properties of silicone rubbers,
“Dave’s Law” that states, “The
(see “How to Select and Buy
amount of rubber required to Silicone Rubbers,” SJ, May
2005, by Michael J. Sisbarro.).
fill the space between a mother
Part III mold and a model is The rubber I selected was a tin
approximately equal to 70% of
cured silicone with a
the clay by weight.” In this durometer of eight and a
David E. Parvin, A.L.I. case, the clay weighed 1,680 viscosity of about 15,000
grams so about 1176 of rubber centipoise.
was estimated. This was only a I mixed the rubber
close guess because the model according to the directions
had been wrapped in plastic specified by the manufacturer.
preventing the clay from fitting I then de-aired the rubber with
snugly and filling every nook a vacuum chamber.
and cranny. In fact, I came out Photograph #26 shows me
a little short, the actual amount pouring the mixed components
of rubber needed was 1215 of the rubber into the mother
grams or two pounds and mold. Once I was sure that
eleven ounces. I just mixed there were no leaks, I placed
and added a little more. Since the mold into a pressure
rubber is not inexpensive, chamber, set the pressure at 50
having a pretty good idea of p.s.i., and let it cure overnight.
Photograph #26 the amount needed helps O.K., some readers are
prevent mixing too much and probably saying, “Oh great, I
wasting it. don’t have either a vacuum or
t the end of last month’s As to which rubber, there pressure chamber!” Don’t
article, I was ready to pour are two types most used by despair, it is possible to make
in the rubber and finish a mold makers, silicones and an acceptable mold without
really great mold. My next step urethanes. I much prefer either one. But using one or
was to determine how much silicones and have discussed both absolutely will result in
rubber would be required and why in detail in a previous longer lasting molds with
then choose which rubber to article (See “How to Make a fewer flaws. For a complete
use. Secondary Mold,” SJ, July discussion on both vacuums
Pouring in the Rubber 2004.). In this case, I preferred and pressure in casting, see the
a rubber with a low viscosity following Sculpture
What I needed to do was to so that it would pour in and Journal articles by yours truly:
fill the space with rubber that around the model filling the “Using Vacuums and
had been occupied by clay. In void completely. Since the Pressure in Casting,” August
other words, if I had known the model had significant hollows 2003
volume of the clay, all I would and undercuts, the rubber “Making a Vacuum
have needed was an equal needed to have a low Chamber,” September 2003
volume of rubber. It just so durometer meaning that it “Making a Pressure
happens that the specific would be soft enough to Chamber,” October 2003
gravity of most types of rubber separate first from the model “Putting Vacuum and
is very close to that of water or and then from the castings Pressure Chambers to Practical
one cubic centimeter weighs pulling out of tight areas Use,” November 2003.
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