RTV silicone rubber is a commonly used material for making silicone molds. This raw material contains professional terms. In order to help you understand these terms, the meaning of each silicone term is explained in detail.
| Term | Definition |
|---|---|
| RTV | RTV is shorthand for Room Temperature Vulcanizing. It is a unique type of system that can be cured with a catalyst to form a tough silicone rubber. It cures without an exotherm (internal heat buildup) and very little shrinkage. This is why it can reproduce a surface feature as small as a finger print. There are two groups of silicone RTVs; addition or platinum cured and condensation or tin cured. |
| RTV-1 | A 1-part RTV system. No mixing, ready to use. |
| RTV-2 | A 2-part RTV system. The mixing of the two components is required prior to use. |
| Two-Part | Dual component A & B silicone material product, A silicone material which must be mixed with a catalyst or other additive to form a durable rubber. |
| Curing agent | A material which when added to a second material, brings about a chemical reaction, usually causing the hardening of the entire mass. |
| Viscosity | Describes the consistency of a liquid and is reported in centipoises (cps). Is a measure of how easily a material will pour. Thinner liquids will report a lower number; and thicker liquids will report higher numbers. For best flow material should at least be at room temperature. Heating the material may help in pouring some molds. If needed, heat the base before adding the catalyst. Be careful, heating will shorten the pot life of the mixed material. |
| Density and Specific Gravity | Density is weight of a material in a given volume, often compared to water. Typically reported in grams/cubic centimeter. Since RTV and most plastics are sold by the pound and are used by volume, specific gravity is an important factor in comparing the actual cost of materials. |
| Flash point | The temperature that an materials vapors will ignite when it is exposed to an ignition source. |
| Humidity | The amount of water vapor present in the air at a given time and place. |
| Durometer | Instrument used to measure hardness. The Shore A scale is used for elastomers and the Shore D scale for rigid plastics. This gauge has a needle which is pushed into the rubber. The further the needle is able to penetrate into the rubber, the lower the reading. The durometer requirement of your silicone is dependent upon your applications. |
| Pot Life | The time it takes a catalyzed material, once mixed, also be called gel time, working time, or pour time. It gives the user an idea of how long he has before the material becomes too thick to use. Most materials should be poured well ahead of time to ensure good flow and to permit air bubbles to rise and break. |
| Demold Time | The earliest time a cured product can safely be removed from a mold without distorting the part. Heating some materials will shorten their demold time. |
| Cure Time | The length of time before a material is finished and ready for use. Some materials require up to a week to fully cure. Most silicone RTVs will develop over 90% of their fully cured properties within 24 hours. Additional heating can be used to accelerate the cure of many urethanes, epoxies and addition cure RTVs. |
| Postcuring | To prevent deformation and tearing, part ejection from the mold typically occurs at 75%–95% completion of cure. Full conversion is often achieved through a postcure or postbake process. Although some materials and applications may not require a postcure, most material suppliers recommend heating parts after ejection for additional time. This heating ensures that the parts reach full conversion, thus achieving final properties and normalizing the final material properties. Typical postcure conditions are 4 h at 200°C in an oven. Experimental testing can be used to determine what the minimum postcure duration and/or temperature is required for a specific product. |
| Linear Shrinkage | The reduction in size, which a user may experience, during the life cycle of a silicone mold. The shrinkage is due to the release of a cure byproduct in condensation cure systems, as well as “oils” drawn out of the mold by aggressive casting resins. They may exhibit some progressive shrinkage when used with resins that tend to attack silicone polymers. |
| Tear Strength | The force needed to propagate a cut in an elastomer when it is stretched. |
| Elongation – (elongation at break) | The ability of the test piece to be stretched without breaking. Given in % of original dimension @ maximum elongation. Maximum elongation is determined by attaining the point of breakage. Typically measured via ASTM D-412. |