MAKING A SILICONE
OK, so you want to make molds? You want to make SILICONE
molds...well, here you go. It's not as easy as it looks,
but here's the basic idea!
This is just my method of doing things, for more information
you can buy a video on making case molds from various suppliers
of special effects materials and books. Cybergraphic
Designs sells it.
Find something that you want to mold... I've selected the "grenade"
on the right since it looks nice and will be simple to use for this
walkthrough. It's about 4 inches tall.
So, due to my own oversight, this IS NOT the right picture, but
I didn't take one of the right method. This picture would
be correct if you were making a 2 piece skin mold. A skin
mold would have you painting a thin layer of quick setting silicone
on this, back it with a thicker layer of thixotropic silicone,
and then back that with a support shell.
The reason I have included this picture on here is that they
are similar techniques. You still need to build up your
clay bed and seat the item you're molding into it 1/2 way. If
you want to make a case mold, just disregard the keys and make
a flat wall of clay.
This is the correct method for the case mold. I've fixed
it from the above picture and added 1/4 inch of clay over the
grenade, along with registration, pour, and escape holes. I've
also coated the clay with a clear acrylic spray to prevent it
from mixing with the ultracal while I make the support shell,
or case. I'm using pre-made silicone keys. (See below for
information) I am now
ready to build the first part of the case, which I'll be using
the standard ultracal 30 and burlap.
A thin brush coat of ultracal 30 is painted on with a 1"
chip brush. Since it's so thin, you will need to watch it
and brush it back onto the model. Your desired thickness is
1/4 inch over the entire surface. While doing the brushing,
it helps to slightly wiggle the brush from side to side to remove
air bubbles that may be trapped on the surface. It's not entirely
necessary, but you spent all this time trying to get everything
as smooth as glass, why cut corners now?
After that layer has set, but before it fully sets have your
next batch of ultracal ready to go, along with your burlap. I
wet and thoroughly wring out my burlap to ensure that it bonds.
For this mold I've cut the burlap into 1x3 inch rectangles,
and I apply 3 full layers. Also, it is best to smooth the
ultracal into the burlap with your fingers, rub ultracal into the
burlap, then run it between 2 fingers to remove any excess.
The final step on this side of the case, as far as ultracal
is concerned. I mix this layer slightly thicker and apply,
then as is begins to thicken more, I smooth it out. While
it is about the consistency of a thick peanut butter you can use
a cake decorating tool or large toothed comb to add some ridges,
these will aid in preventing you from dropping the smooth surface
later. When it is hard to the touch you can smooth it with
your hands, this takes and sharp edges off of the ridges. It
is also a good idea to create a flat surface on at least one side
of your mold so it will sit flat on a table.
In this picture, I have removed the clay wall, smoothed out the
clay and am ready to begin the second half of the case. It
is done in the same manner as above.
The 1/4 inch of clay has been built up and smoothed out to
as glass smooth as possible. On this half of the mold I
have decided to only put one pour hole, and will drill the vent
holes to allow any trapped air to escape. This was done for two
reasons, one is ease of knowing which hunk of silicone goes into
which hunk of ultracal, and I wanted to test which method worked
(Note: the single pour hole with drilled escape holes seemed
to work better then two large holes)
Clear acrylic spray, such as "Crystal clear" has been
sprayed onto the entire surface, and petroleum jelly (Vaseline)
has been thinly applied onto all of the ultracal that may come
in contact with this side of the case. This aids in separation,
and cleanup. Also pry points have been added . We're ready
to finish the case up. This is done in the same manner as steps
2-6, so I'll skip the boring blabbing. One thing to note
is that you will want to make sure not to allow the ultracal to
flow off the sides and down the mold. It's best to be neat
here and save yourself time later.
Well, after cleaning up the mold, opening it and removing the
clay from ONE side, while leaving the other side intact this is
what you should have. Save the clay in a container off to
the side for using to determine how much silicone you need. At
this point things get easier for you. The many small steps
begin now, first, you will need to once again smooth your clay out
as much as possible, remember GLASS smooth. After you do that,
you need to cut a gutter key, a small half circle shaped trench
about 1/4 inch away from the item you're molding. I use a
round sculpting tool to do this, it's visible in the picture in
the lower right corner. After that's been done, smooth your
clay if necessary. You can spray clear acrylic on again if you want
to seal it, but I don't find it necessary. Now get some petroleum
jelly (Vaseline) and apply a small "bead" around the edge
of the clay, onto the ultracal. This will act as a gasket
and help to seal your mold and keep that expensive silicone from
I've drilled the vent holes in the highest section of the case
(the lowest in the negative mold) these will allow the air to escape
as the silicone flows over the surface and fills the cavity.
The mold has been sealed shut, with two layers of defense, the
Vaseline, and now with hot melt glue from a glue gun on the outside.
You can see the large pout hole flanked by two smaller vent
Directly behind the mold I'm working on
is a silicone key mold. This is simply a mold of "keys"
the things that are used to aid a mold in closing the right way
and stopping it from slopping around. I sculpted some in
clay on a sheet of plastic, then made a small ultracal mold of
them. If I'm pouring silicone I always have it handy to
pour any leftovers in. The keys you make can be used to
save time on future molds.
|Another view of the sealed mold, make sure to cover those pry
points, just in case. I apply a large squirt of glue above
them and let it drip down over the hole rather then filling it,
this way I can still use it by pulling the glue off, sure beats
digging glue out of a tiny hole. I have also had to glue a
piece of styrene onto the rear of the mold to seal the large hole
Time to mix up the silicone. I have already determined the
amount I need, as well as weighing it out on an Ohaus triple beam
balance. No big secrets here, you just want to avoid mixing
in any air bubbles. Easier said than done, but we can still
get good results without an expensive degassing chamber.
A very boring step indeed, but essential without a degassing chamber.
If you pour the silicone into the mold in a very thin stream
any trapped air bubbles will be broken as they fall into the mold.
This doesn't get ALL of them out, but anything big will certainly
be broken. If you're making a larger mold and can't simply
put the container on a shelf or something like that get comfortable
and prepare to stay like that for a while.
Waiting isn't the hardest part. With your mold filled with
silicone, it's time to check the seal to make sure it isn't leaking.
If it is, you can use a piece of clay to seal the hole, or
more hot glue. If it's not leaking, go away and do something
else for 16 hours.
OK, now that you're back and rested, you peel off the glue and
open the mold, try to keep the object in the silicone side. This
is what you should see after peeling off the clay from the other
side and cleaning any clay off of it.
What you want to do now is Vaseline the silicone so that the other
side of the mold will not stick to this half. After you've
done that repeat the process of sealing the mold, weighing out and
mixing your silicone. Pour it like before, then go do something
else for another 16 hours.
The silicone has been poured just like the last time, you need to
wait 16 hours. Work on another project if you have one, or
go to sleep.
YOU'RE DONE! All the time spent hasn't been wasted, the mold
is wonderful and you can begin casting copies. OK, well, you
aren't really done, you still need to trim off your pour holes and
vent holes, easily done with a pair of scissors. You can also
trim any silicone that managed to overlap onto the ultracal, but
you made a great mold and don't have to do this. Now you can
begin casting copies in whatever material you want.
|Pictures of the mold disassembled above and below.
|The first casting, done in a urethane resin and painted quickly.
|Another picture of the first casting. This casting required
very little seam trimming or sanding. The only real areas
that had flashing were inside the fuse area and the bottom of the
grenade where the pour hole filled with resin. An excellent
job if I do say so myself, and I do.
Silicone Keys made while making this mold. There were more,
but I just took a picture of a few. These are small keys
suitable for a mold of this size.
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