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Choosing a Silicone
There are several aspects to consider for the novice mold maker when choosing a mold material. The foremost
aspect should be whether to use a silicone with a tin or platinum-based catalyst. A catalyst is a substance that
initiates or accelerates a reaction.
Tin catalysts work extremely well for the beginner because they generally have a low durometer, cure at room
temperature, and are virtually inhibition free
1 . Low durometer materials make it easy to create a parting line and
remove the master without damaging it. Inhibitors can be powder from gloves, rubbers, plastics, or chemicals
not cleaned from the surface of the master pattern.
Platinum catalysts have a cure cycle that can be accelerated with heat and have virtually no shrinkage, however
they have a higher durometer.
Patterns used to create molds with higher durometers may need draft and must have an excellent surface finish
to de-mold cast parts. It is also recommended that the novice mold maker use a translucent mold material so
the pattern is visible within the mold, making it easier to direct where the parting line is located.
Manufacturers of Molding Silicone
Hapco ( www.hapcoweb.com/ )
Dow Corning ( www.dowcorning.com )
General Electric ( www.gesilicones.com )
Innovative Polymers ( www.innovative-polymers.com )
US Composites ( www.shopmaninc.com/moldmaking.html )
Ebalta ( www.ebalta.de )
Building the Mold Box
The purpose of a mold box is to contain the silicone rubber (after it is poured
over and around a master pattern) until the silicone turns to a solid. A mold
box does not have to be a complex structure, depending on the size and
configuration of your part; often a can, small plywood box, pan or plastic
bucket will get the job done. For ease and convenience, plywood, Plexi-
glass, plastic or sheet metal work extremely well. Be sure not to use a
material that will inhibit the curing of the silicone. Consult your silicone
vendor for a list of these substrates.
The box must have four sides, all of which must stand higher than the master
pattern when mounted to the gate. The box can be screwed or nailed together, but clamps are used in most
cases for easy disassembly of the mold box. All seams of the mold box must be sealed to ensure the liquid
silicone does not leak. Hot melt glue, silicone caulk or clay are effective choices, again be sure the material
chosen will not inhibit the cure of the silicone.
Preparing and Pouring the Silicone Mold
Preparing the silicone is relatively easy but must be performed properly to ensure a good mold. Pay close
attention to the mix ratio of the two components of the silicones. Be sure to prepare a large enough volume to
completely encase the pattern. The open time
2 of RTV silicone is typically 1-2 hours (see your product’s Technical
Data Sheet for specific information) allowing enough time to carefully mix and prepare the liquid rubber.
Though mixing can be done by hand, it is difficult to determine if both components are thoroughly combined in
large volumes. The use of a Jiffy
®
Mixer is recommended. Mixing the components together will whip air into the
mixture. Air trapped in the mixture must be degassed, which separates the air out of the silicone mixture. Place
the mixture in a vacuum chamber and degass. Removing the air until a minimum of 27" Hg has been reached
(vacuum chambers and casting systems are commercially available for a range of budgets). Note that the
volume of the mixture will increase dramatically until all of the air has been removed. Choose a container that
holds approximately 3 times the volume of the mixed silicone.
1 Inhibition – The failure of silicone to cure within the recommended cure time. Surfaces of the mold will typically remain gummy, uncured or stick to the master pattern.
2
Open Time – The amount of time it takes to gel or double in viscosity.