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Glossary Of Terms


If you are new to molding and casting, we recommend that you start with our product guide. Our product guide is full of information on our products and also includes tips on mold-making, casting, and finishing. You can order a copy here. If you are ordering by phone, you can request that a copy be added to your order for no extra charge.

Dry Tamp
Dry Tamp Architectural Precast Concrete (DTAPC) is fabricated with a mixture of cement, fine and course aggregates, water, pigments, and a variety of admixtures to produce an earth-moist zero slump mix. The mix is dumped or shoveled into the mold and then mechanically forced into place with hand held tamping equipment and is removed from the mold when tamping is complete. The same mold is then reset and continues to be used for the production of numerous pieces cast on the same day. The pieces cannot be handled after casting until they have been moist/steam cured overnight in a curing chamber. The product is then finished, usually by acid etching, to expose the fine aggregates and produce a fine "sugar cube" texture closely resembling natural cut stone.

Elongation is the percentage of stretch that a material will exhibit prior to breaking point. For example, Gel 10 elongation is ~960% at break. This means Gel 10 will stretch to almost 10 times its original size before breaking.

Library Life
Library Life represents the time a silicone mold can stay on the shelf before it looses its shape and integrity.

Once a mold had been made, its "library life" depends on the type of catalyst that was used. Tin based systems (e.g., RTV1000, RTV325) will probably last about 5 years if kept in a clean storage area. Beyond that, it will become brittle and crack upon releasing the casting. If you suspect that a mold is very old, you can always use your last casting to make a new mold.

Platinum-catalysed silicones have a much longer library life, up to 20 years or longer.

The lifetime of a mold (how many castings you can get before you lose details or the mold begins to tear) really depends on what you are casting, how many castings you have made and if you use a release agent or not.

The cavity or matrix into which the a composition is placed and from which it takes its form.

Platinum Cure Silicone
Platinum silicones, or addition cure silicones, cure with virtually no shrinkage to a durable, long-lasting, chemical resistant rubber. Platinum silicones are popular with the prototyping industry due to their incredible accuracy and long library life.

Most Platinum silicones such as 71-20 may be thickened with Plat-Thix for brushing onto vertical surfaces.

Molds made with Platinum silicones achieve the highest level of detail. Platinum silicones are recommended for resin casting, foam casting, low temp. metal casting, and archived molds for foundry patterns. Platinum silicones are easy to contaminate if proper care is not taken in surface preparation, mixing, or weighing. Platinum silicones yield excellent results but require a higher level of experience and patience than Tin cure or Urethane rubbers.

Polyfiber is a small, powder-like fiber that can be added to a mold rubber to thicken it during the mold making process.

Shore Hardness

Shore hardness gauges the softness or hardness of a given material. A durometer gauge (pictured) measures softness or hardness of a material by pressing a needle into a given surface until the base of the gauge is flush with the material surface.

There are two scales relevant to molding and casting, the "A" and the "D" scale.

The "A" scale measures soft, flexible materials and starts at 00 and goes to 100. The softest point on the "A" scale is 00, which is the equivalent of human skin. The highest point on the scale, 100, is similar to the feel of a car tire. The example shown of a silicone hand is approximately a 10.

The "D" scale measures rigid materials such as casting resins. The "D" scale overlaps the "A" scale. A 95 shore "A" is also a 45 shore "D". The low end of the "D" scale measures semi-rigid materials while the high end of the scale measures extremely rigid materials. Most of the common urethane casting resins measure from 60-85 on the "D" scale.


Tin Cure Silicone
Tin cure or condensation cure silicones are basic two-part systems that yield accurate, chemical resistant molds for resin, foam, plaster, and wax casting applications. Of our silicones, Tin cure silicones are the easiest to work with and do not inhibit (fail to cure) easily. Tin Cure silicones such as the 70 series RTV rubbers require an accurate gram scale for mixing.

Most Tin cure silicones such as 70-25 or 70-30 may be thickened with Tin-Thix for brushing onto vertical surfaces.

Tin cure silicones make great production molds but they do have a short storage life or "library life". Tin cure silicones have a life of 2-4 years depending on use and storage conditions. As Tin cure silicones age they lose elasticity and become brittle. Tin cure silicones such as 70-26 exhibit longer library life but archived molds for wax patterns (art bronze foundry work) may require a long lifespan of up to 20 years or more. We recommend urethanes (such as the 74 series) or Platinum silicones (Platsil® 71 or 73 series) for molds requiring an extended library life.

Measurement of a fluid's resistance to flow. Taffy and molasses are very viscous; water has low viscosity. It can be expressed as the ratio between applied shearing stress and resulting rate of strain in shear. The common metric unit of absolute viscosity is the poise (dyne-sec/cm2). In addition to kinematic viscosity, there are other methods for determining viscosity, including Saybolt Universal Viscosity (SUV), Saybolt Furol viscosity, Engier viscosity, and Redwood viscosity. Since viscosity varies in inversely with temperature, its value is meaningless until the temperature at which it is determined is reported.