Could you elaborate a bit for a total novice on the "powder
separated" mold method? I'm in the process of learning how to make
molds & cast silver, etc., and this is a method I have not seen
discussed in anything I've read thus far. Thanks!
Powder seperated molds is a method used to seperate molds instead of
hand cutting. It can be done in any type of rubber, but is best used
in heat cured silicon e based rubbers . It comes in 1/4 inch layers so
if you were to make a 1/2 in thick mold, it will only require 2 layers
of material. The frame must be a new frame or a clean frame that has
been sandblasted clean of residue from organic yellow rubber Have you
seen how your models turn grey/ black after making a mold in yellow
organic rubber… thats the residue that I’m talking about…
It’s not the easiest stuff to remove from a model or a mold frame.A
new frame is guaranteed to work. If the frame and model has
contamination from this other rubber on it, the rubber will not cure
around the sides of the mold where it touches the frame and it will
not cure properly where the residue is left on the model. To make
such a mold, insert 1 layer into the aluminium frame… ( more layers
if the mold is thicker than 1/2 in.). To start, use a simple model of
a charm … Press the charm into the rubber… the rubber is almost as
soft as siliputty .Press until the edge of the charm is level with
the rubber.
Next, you should have brass locators to locate the mold. there will
be 4 fold overs on each locator. fold them flat facing inwards by
press them against a hard surface… when you have seven done, insert
them into the mold with three in a pyramid shape above the model and 2
on each side of the sprue . The locators should be about 1/8 to 1/4
in. away from t he model or the edge of the frame. These are what makes
your mold “locate” after it has been seperated , so don’t forget to
put them in. Next, Powder the layer of rubber with the model in it
with a mica based powderremove the excess powder by taping the frame
lightly on a trash can or vacuum it off or blow it off… just get the
excess off 
Press your next layer on top of the first layer… then use a heavy
duty paint scraper to shave the excess rubber of the top of the the
frame as there will be some excess. This rubber has a high expansion
rate and you do not want any excess sticking over the frame … having
more rubber than that in the frame will cause the seperation /parting
line to move from where you put it ( this is called shifting) The
rubber ( from Contenti supply Co) normally comes with 2 layers of
specially waxed paper as a protector. when your mold is ready to put
in the vulcanizer,cut 2 pieces of this paper the size of your mold
frame and put th e shiny side against the rubber … the rough side of
the paper will stick to the rubber and is somewhat difficult to remove.
Place the mold frame in the vulcanizer and tighten the vulcanizer
just past
finger tight… or rather, a 1/4 turn of the handle. It takes 15
minutes per 1/4 in to cure at 325 oF. so leave it in the vulcanizer
for 30 minutes, then remove the frame, remove the paper on top of the
frame and dunk it in a bucket of water to cool down or let it cool on
the floor if you are not in a rush. Floor cooling gives you about 1%
less shrinkage than dunking in water, but takes longer.
To remove the rubber from the frame, slide a thin steel rule between
the rubber and the frame walls to break the seal all the way
around.press it out of the frame with your fingers.
Now, look in the middle of the flat edge around the mold and you will
see a white line caused by your seperating powder. use you finger
nails to pull the mold apart where this line is.
Remove your model, vent the mold as you see fit and shoot some waxes.
Another good point about this rubber is that it is very easy to cut
with a mold blade and handle set … you can cut 10 times more molds
with a single blade than you can with any other rubber.
Contenti supply sells a kit with enough rubber, locators and mica
powder as well as an instruction sheet with pictures showing how to do
it.The mold rubber is called " moldex" Their Tel: 800-343-3364
More complex molds can be made using diffrent techniques. Hope this
helps.
Daniel Grandi Racecarjewelry.com
We do casting ,finishing in gold,silver,brass/bronze and pewter for
people in the trade.