Simple Silicone Rubber Molds (no parting line)
Rhodia silicone rubbers are used extensively for
applications where a flexible mold material is required. A
truly high-quality line of silicone systems, these products
offer high accuracy, high heat resistance, and excellent
tear strength and elongation properties.
And since silicone rubber compounds, by nature, are
self-releasing, and therefore make an excellent choice for
a variety mold applications.
Preparing a mold
In preparing a silicone rubber mold,
we are first putting clay around the edges of our model to
make it adhere to the mold board.
Next, we aligned the model in the
center of our mold, removed the sides of the mold, and then
pressed the model onto the wood.
Next, we cut the excess clay from
the edges of the model.
Technically, you do not need a
release agent with silicone rubber, but in order for the
mold to slide off easier, we decided to apply a light coat
of Freeman Wax Release. Here we are using a
white-bristle throwaway brush to apply it, and then a
cloth to gently buff it.
Note that the wood has already been
sealed with the wood and plaster sealer so the surface
itself is not porous – otherwise, the silicone rubber will
reproduce the surface of the wood, which is not what we
want.
For more information, refer to our
other video on sealing wood.
Finally, we screw on the sides of
the mold
And now we’re ready to pour the
silicone rubber.
For this demonstration, we’re showing
Rhodia’s V-340 and the CA-55 catalyst –
the other option here is the CA-45 catalyst, which would
change the hardness.
Here is what the base looks like.
The catalyst, which can be shaken inside its bottle, will
be mixed using 10 parts base to 1 part catalyst by weight.
Here is what the mixture looks like.
For more information about weighing
and mixing, please see our other video on this topic.
Once the base and catalyst are fully
mixed, we poured the mixture into a larger bucket for the
degassing process. Since the mixture will rise while
degassing, you need a larger container to hold the silicone
rubber.
Once it is properly weighed and mixed, our
V-340 is ready to be degassed. While degassing not necessary
for many simpler or less demanding casting applications,
if you require an optimal, completely void-free mold, we
recommend degassing the material.
Pouring
When pouring a silicone rubber, you
want to pour it at a low point first. Notice that this is
thicker than a urethane like Repro.
When you pour the material, you do
enter a little more air into the mixture, but these bubbles
will break on their own.
This mold will take 16 to 18 hours
to cure. An addition Cure rubber can be accelerated only by
heating it. A condensation cure rubber can be accelerated
chemically.
Notice the larger bubbles that were
introduced while pouring are already breaking on their own.
Finishing
The next day, our mold is ready. We
remove the screws and then wedge the whole thing off with
very little effort.
Next, we pushed out our model from
behind. Notice how accurately the silicone reproduces the
surface of our model.
Next, we pushed out the mold itself
and pulled off the edges with our fingers.
Our mold is now good for 30-150
uses, depending on the material. Anything that gets really
hot will shorten the life of the mold. Here we have a mold
that is starting to show its age. You can see the
absorption of the urethane into the rubber – thus creating
a discoloration. While still perfectly usable, this is an
indication that the mold is reaching the end of its life.
As we pour our thoroughly mixed Repro into our mold, we typically like to
create a smaller stream in order to break the bubbles
better. Regardless, given its thin viscosity, bubbles
aren’t much of a problem with Repro and it is never
necessary to vacuum degas Repro.
After allowing our part to cure for
60 to 90 minutes, depending on the mass of the pour, we can
now demold it and immediately begin using the mold to pour
another casting. Even though the Repro is hard enough to
demold in 60 to 90 minutes, full hardness and strength are
not achieved until 24 hours. The part should not be placed
into service until then.
To demonstrate pouring and demolding epoxy mass cast
plastics, we’re showing Freeman 801 epoxy casting resin.
When pouring an epoxy mass cast material, it is important
to pour in a narrow stream to eliminate bubble.
The aluminum filler used in Freeman 801 will create the
separation of color that you will only see at the bottom
end of the pour
Pouring and Demolding a Polyurethane Elastomer
When pouring a polyurethane elastomer mass cast
material, it is important to begin in the lowest part of
the mold and pour in a narrow stream to avoid creating
new bubbles in the mixture. The thinner stream also
helps when directing the pour into intricate patterns.
Since we are using silicone rubber
mold, it is easy to flex the mold to remove the cured part.
Notice that the smaller mold isn’t
as shiny because the master model isn’t as shiny – thus
showing how well the Rhodia Silicone Rubber transfer the
surface features of your original part.
This little model also displays the
flex in our Freeman 1060 semi-rigid urethane elastomer, and
its superior impact resistance