Products used in this How To:
Two Piece Mold
A two or multiple piece mold is typically required when the
object you need to mold has detail on all sides of the piece.
As you can see with this bear, the back is not flat and has
detail in which the mold and casting will need to have in order
to reproduce the piece exactly. In order to mold both
halves, you will need to start by making a mold of the front
and then making a mold of the back. You will need to
start by determining a parting line which is where the two
halves of the mold will meet. This is typically an edge
or perhaps the half way point from the front and back.
You begin by building a clay base and pressing your piece into
Press the bear down until it is close to where
you want the parting line.
Using clay tools, smooth and flatten the clay
to establish a perfectly smooth and clean parting line where
the clay touches the part. The cleaner the edge, the smaller
your parting line will be and the less clean up will be required
on the seam line of your cast resin piece.
Once the part is completely clayed up to the parting line
you've established, you are ready to build your mold box
to contain the liquid rubber.
Using corrugated plastic, angle iron, wood, a recycled piece
of plastic, Legos, or any other non porous material, construct
a mold box to contain the liquid silicone rubber. Build your
box so that the walls are ¼”-3/8” around the
outside of your original. Building your walls too big around
the outside of your piece require much more silicone which wastes
Make sure to seal the box well to prevent leaks. Hot
melt, super glue, clay, or even caulk works well.
Properly measure your silicone RTV and mix thoroughly to
ensure a proper cure and physical properties of the rubber mold. If
you need help calculating the volume needed for your piece, visit
the Calculating Volume page on our website.
Once the silicone is thoroughly mixed, pour the silicone
in the sealed mold box. Start from one corner and allow
the liquid rubber to flow naturally over the original rather
than pouring the rubber directly onto the piece. This technique
will reduce the possibility of trapping air on the surface of
Continue to pour the silicone until the entire piece is covered
by at least ¼” of liquid silicone.
Allow the silicone rubber to fully cure. In this case,
the Alumilite Quick Set silicone RTV was used and the cure time
is 4-8 hours. With most silicones, it is always a good
idea to allow them to cure overnight.
Once the rubber has cured, remove the mold and box from the
You can also remove the mold from the box but if possible,
leave the box in the mold to prevent breaking the seal between
the box and the mold.
Remove the clay from the back side of the piece but DO NOT
REMOVE THE PIECE FROM THE POURED RUBBER MOLD. This will
break the seal and damage the clean parting line you clayed up.
Remove the bulk of the clay and then go back and clean
off all of the left over clay.
Using your Alumilite clay tools, remove every tiny bit of
clay from the mold and original.
Once you have cleaned the mold and master off completely
of all clay residue, you are ready to cut your locators so the
second half of the mold aligns with the first.
Using an Excel knife or better yet a U-Channel knife, cut “v” or “u” channels
in the rubber mold. This will allow the rubber you pour
during the second half to flow into these channels and provide
great locators to align the two halves of the mold.
Cut locators on at least two or three sides of the mold.
Place your mold back into the mold box and reseal it if necessary.
You are now ready to mold release the first half of your
mold. The only thing silicone rubber sticks to is another
silicone. Failing to use mold release will allow the first
and second pour of your silicone rubber to bond perfectly together
and not allow you to remove your original without cutting blind
to the piece. This would destroy the perfectly placed parting
line you established with your clay line. So MOLD RELEASE
IS REQUIRED to separate the two halves. Use Alumilite's
UMR spray all over the cured rubber from the first pour. It
is not required on your original but some overspray is expected
and will not hurt.
The other option is to use Alumilite's Rubber to Rubber
Mold Release. Shake the bottle well before using and brush
on multiple coats waiting 5-10 minutes between coats before applying
Before pouring your second half of your silicone rubber mold,
be sure to check that your mold box is sealed and seal any gaps
you may find to eliminate the chance of your entire silicone
batch leaking out a hole in your mold box. Once the silicone
starts to leak, it is nearly impossible to stop and creates a
huge mess along with wasted time and material. It is always
best to take a minute to double check to make it is sealed well.
Measure and mix the proper amount of silicone required to
fill the second half of the mold. Once again, if you need
help calculating the volume needed for your piece, visit the
Calculating Volume page on our website.
As you did with the first have of the mold, pour the rubber
from one corner of the mold box and allow the material to flow
naturally over your original and cover it by at least ¼”.
Allow the rubber to fully cure and remove it from the mold
Remove the mold from the mold box.
Using your hands, find the seam line and begin to separate
the two halves of the mold.
As you can see, the mold will separate exactly where the
two halves were poured and mold released. You will also
see how the second half of the mold filled in the locators from
the first half and how it will give you perfect alignment between
the two halves.
Remove the unharmed original from your two piece mold.
You are now ready to cut your pour hole and any vents if
they are needed. Using an Excel knife, find the most non-cosmetic
area to use as your pour area. Ideally this will allow
for natural flow of the resin and area in which air bubbles will
float to the top and escape without trapping air on flat surfaces
of your mold. In this example, we chose to use the bottom
of the bear's feet. So using an Excel knife, cut
a “u” shaped channel from the bottom of the bear's
feet to the outside of the mold on both halves of the mold.
Once you have both channels cut out and place the mold together,
you will find a perfect pour hole in which you are ready to pour
an exact replica of your original. You can also use 1/8” copper
or brass tubing to twist and cut holes through the silicone to
place vents and/or injection points throughout the mold.