Molding Silicone Parts: Thermoplastics and Liquid Silicone Rubber (LSR)

Molding Silicone Parts

Thermoplastics and Liquid Silicone Rubber (LSR)

Plastics are either organic or semi-organic materials that have (as their main attribute) a very large molecular weight. These very large molecules, or macromolecules, give them their distinct properties and material behavior, when compared to other materials used in manufacturing or found in nature.

Polymers can be placed into either a thermoset, thermoplastic or elastomer category. In contrast to the already known thermosets like Liquid Silicone Rubber (LSR), which are cross-linked and cannot be melted after cure, thermoplastics are polymers that solidify as they are cooled, and once cooled, the long molecules are no longer allowed to move freely. When heated, these materials regain the ability to “flow“, as the molecules are able to slide past each other with ease.

Furthermore, thermoplastic polymers are divided into two classes: amorphous and semi-crystalline polymers. Amorphous thermoplastics, such as polystyrene (PS) or polycarbonate (PC), are those with molecules that remain in disorder as they cool, leading to a material with a fairly random molecular structure. An amorphous polymer solidifies, or vitrifies, as it is cooled below its glass transition temperature.

Semi-crystalline thermoplastics, on the other hand, solidify with a certain order in their molecular structure. As they are cooled below what is usually referred to as the melting temperature, the molecules begin to arrange in a regular order as they harden. The molecules in semi-crystalline polymers that are not transformed into ordered regions remain as small amorphous regions. These amorphous regions within the semi-crystalline domains lose their “flowability” below their glass transition temperature. Most semi-crystalline polymers have a glass transition temperature at sub zero temperatures, hence, behaving at room temperature as rubbery or leathery materials. Typical representatives of this class are polyolefins, such as polyethylene (PE) and propylene (PP), polyamides (PA) or polybutylentherephtalate (PBT).

In the plastics industry, it is common to define a polymer by the chemical family it belongs to, and to assign an abbreviation based on its chemistry. Instead of using the standardized descriptive symbol, engineers often use the trade name given by the resin supplier; polycarbonate for example, would be referred to as Makrolon (Bayer), Lexan (was GE, now Sabic), or Calibre (Dow).

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In order to benefit from the advantages of different materials in one product when molding silicone parts, two materials can be combined using 2K (two shot) injection molding silicone parts. With this technology, the best features of different materials for molding silicone parts can be integrated; for example, the soft touch of Liquid Silicone Rubber (LSR) can be married with thermoplastics like polyamides (PA) to increase the stiffness of the part in the molding silicone parts process. The end result is a high quality product that can be manufactured at a competitive price.

Amorphous and semi-crystalline thermoplastics

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[1] Osswald, T. A.; Baur, E.; Brinkmann, S.; Oberbach, K.; Schmachtenberg, E.: International Plastics Handbook; 4th edition, Hanser Gardner Pubilcations, Cincinatti, 2006

[2] Ehrenstein, G. W.: Polymeric Materials: Structure, Properties, Applications; Hanser Gardner Publications, Cincinatti, 2001

At SIMTEC Silicone Parts, a leading company in manufacturing high precision parts, components, and molding silicone parts, we are exclusively focused and specialized in the production of LSR and LSR/Thermoplastic (Two-Shot) components.

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