Two shot (2K) injection thermoplastic molding dates back several decades, from the multicolor molding of typewriter keys (to produce permanent characters on the keys), to the integration of the multiple parts that make up a car’s headlights. This process has since advanced to allow for the consistent and cost-effective manufacture of multicolor/multifunction thermoplastic/thermoset products for a variety of innovative applications. SIMTEC Silicone Parts prides itself in being at the forefront of two shot LSR/Thermoplastic technology.
Injection molding of Liquid Silicone Rubber (LSR) has existed and evolved over the past 20 years. Due to the thermosetting nature of the material, injection molding of liquid silicone rubber requires special treatment, such as uniform distributive mixing; in addition, the material must be maintained at a constant temperature up until it is pushed into the heated cavity and vulcanized.
Liquid Silicone Rubber (LSR) is part of the family of thermoset elastomers that have a backbone of alternating silicone and oxygen atoms with methyl (-CH3) or vinyl side groups (-CH=CH2). They offer outstanding properties for making silicone parts, not obtainable with today’s TPEs. The solidification process of all heat-activated thermosets, including silicone rubber, is dominated by an exothermic and irreversible chemical reaction called cure, vulcanization or network polymerization. The curing process when making silicone parts, forms a three dimensional network in which each chain is connected to all others by a sequence of junction points. The curing process of making silicone parts improves the general properties of the final product when making silicone parts and provides resistance to heat and hot air (180˚C, stabilized up to 250˚C, short-term up to 300˚C), due to the fact that these network polymers do not melt upon heating. The curing of Liquid Silicone Rubber (LSR) is almost exclusively carried out with a platinum-catalyzed hydrosilylation reaction, which does not generate by-products.
Plastics can be divided into uncross-linked thermoplastics and cross-linked thermosets. The first have intermolecular bonds between the molecule chains, but can move freely relative to their neighbors. They can be melted and solidified multiple times. In contrast, thermosets have “bridges,” or covalent, bonds interconnecting the polymer molecules. Once the material is solidified by chemical curing, it cannot be melted again.
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