Thermoplastics are divided into two material classes (amorphous and semi-crystalline). Their classification is determined by both their transition temperatures and their structure in different states, which works great for molded silicone parts.
The solidification of most materials is defined at a discrete temperature; however, amorphous polymers do not exhibit a sharp transition between the liquid and the solid states. Instead, an amorphous thermoplastic polymer vitrifies as the material temperature drops below the glass transition temperature (Tg) – a benefit for molded silicone parts.
Amorphous thermoplastics can be used below their glass transition temperature. They can also be processed at approximately 100˚C above Tg. They do not change their coiled chain structure upon solidification. Due to this random structure, the characteristic size of the largest ordered region is on the order of a carbon-carbon bond. This dimension is much smaller than the wavelength of visible light; therefore amorphous thermoplastics are generally transparent and good for molded silicone parts.
The shear modulus versus temperature diagram (below) provides a very useful description of the mechanical behavior of these materials. It shows two general regions: where the modulus appears fairly constant and where the modulus drops significantly with increasing temperature. With decreasing temperatures, the material enters the glassy region where the slope of the modulus approaches zero. At high temperatures, the modulus is negligible and the material is soft enough to flow. Although there is not a clear transition between “solid” and “liquid“, the temperature at which the slope is highest is Tg.
In a solid state, the amorphous thermoplastic is brittle. Its stiffness decreases with increasing temperature, causing it to become leathery or rubbery. Above Tg, its strength decreases significantly as the maximum strain continues to increase. Once viscous, the mechanical strength becomes negligible. This makes molded silicone parts manufacturing more effective and efficient.
In conclusion, the main characteristics of amorphous thermoplastics are their hardness, their transparency, and their high gloss. They exhibit minimal thermal shrinkage as compared to other materials, which makes them a good choice for high precision molded silicone parts applications, such as ski goggles.
Although their impact resistance is already high, it can be further improved with rubber additives.
Learn more about the Materials and Properties of Liquid Silicone Rubber.
 Osswald, T. A.; Baur, E.; Brinkmann, S.; Oberbach, K.; Schmachtenberg, E.: International Plastics Handbook; 4th edition, Hanser Gardner Publications, Cincinnati, 2006
 Ehrenstein, G. W.: Polymeric Materials: Structure, Properties, Applications; Hanser Gardner Publications, Cincinnati, 2001
 Campo, E. A.: Industrial Polymers; Hanser Gardner Publications, Cincinnati, 2007
At SIMTEC Silicone Parts, a leading company in manufacturing high precision parts and components, we are exclusively focused and specialized in the production of LSR and LSR/Thermoplastic (Two-Shot) components.
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