This provides enough space between the border
and the model to add some plastic hemispherical
keys to help the mold halves align (“register”) properly; we’ll discuss this in Step 9.
The clay bump you see in the middle is built
around the pre-filled silicone plug I pointed out before. This clay plug will leave a void when I pour the
first silicone blanket. That way, when I remove the
clay plug and pour the second silicone blanket, the
void will lead my silicone right to that pre-filled plug.
Also note the clay plug in the gun barrel. Unlike
the other plug, I didn’t pre-fill this shallow barrel
with silicone. I didn’t need to because it’s large, easy
to get bubbles out of, and has no threading. But the
completed first blanket will have a trough that leads
the silicone right into the barrel.
compressed air over the part (don’t get too close) to
eliminate any bubbles (Figure G).
When the first layer is just past the tacky stage,
brush on another layer of silicone, making sure it’s of
uniform thickness all over the model (Figure H). For
this application, you can add thixotropic agents to
increase the silicone’s viscosity (but not on the first
layer, as these agents make it difficult to get the silicone into all the nooks and crannies for high detail).
6. POUR THE FIRST BLANKET.
The first layer of silicone is the most important
one, because it’s what grabs all the detail from your
model. To avoid bubbles, pour slowly from one location and from high up, letting the silicone drift slowly
into the detail on the model (Figure F). You can use
accelerator in the silicone (or use more kicker) to
make it kick faster, but that will make for a weaker
mold. If you need only 1 or 2 castings, it’s OK to use
an accelerator (they can speed up the setting time
from 10 hours to 3), but if you want to make dozens
of castings, be patient.
With the first thin layer covering the model, blow
7. APPLY SILICONE KEYS.
Once the second layer is applied for the first blanket
of silicone, and while that layer is still wet and tacky,
start applying the silicone keys. These will help
the silicone stay adhered to the inside of the hard
plaster mold. While the advantage of this method
is that it uses less silicone, the disadvantage is that
the thin silicone layer lacks structure and must be
married to the plaster mold so it doesn’t collapse.
I made these keys from an old silicone mold by
cutting small wedges about 1½" long by about a
pinky width (make sure that the silicone for the keys
and the mold are the same brand — it helps them
stick better). For this mold, I set keys in the wet
silicone about every 3". As every mold is different,
you have to imagine your mold upright and think
through the weak spots, where it will buckle, and
place the keys accordingly.
Figure I shows the mold with the keys in place —
all the silicone is poured (for the first half), and it’s
112 Make: Volume