Despite only one letter difference, silicon and silicone are two different things. While silicon is the 14th chemical element on the periodic table, silicone is a synthetic material that, among other chemical elements, contains silicon. Although commonly observed as plastic, technically silicone is rubber. Among others, silicone processing equipment mainly involves several high-temperature machines, mixers and molds.
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Silicone processing equipment and techniques
Silicone is a synthetic material composed of silicon, oxygen, carbon, and hydrogen. Being one of the main ingredients in silicone, the chemical element silicon is most commonly found in quartz, chemically known as silicon dioxide, from which it is reduced in a silicon furnace at temperatures as high as 1800°C. The leftovers of metallurgical-grade silicon are ground into fine powder. For silicone synthesis, it is crucial to add methanol to silicon to create chloromethane. Subsequently, a Müller-Rochow process starts by using a fluidized bed reactor heated at 300°C to trigger a reaction among particles forming a crude silane mixture.
The mixture is separated into various chlorosilane versions during a distillation process, and each version can further be processed depending on the desired end-product. Converted to polysiloxane chains via hydrolysis and polycondensation, difunctional chlorosilanes can be used as silicone fluids in cosmetics, textiles, and silicone rubber. As water-repellent yet breathable, trifunctional chlorosilanes are used in the construction industry. The three-dimensional silicone-resin networks are produced by undergoing hydrolysis and catalytic condensation. Finally, tertrafunctional chlorosilanes is used to manufacture pyrogenic silica, made by burning a crude silane mixture under a stream of hydrogen and air.
Silicone rubber production methods: extrusion
Silicone rubber is a highly versatile material that can be produced by a number of methods such as extrusion, injection molding, liquid injection molding, compression molding, calendering, and transfer molding.
Extrusion is a preferred method for producing silicone rubber due to its practical and cost-effective nature. The essential silicone processing equipment in this method is an extruder through which all raw materials, once shaped into strips or slabs, are pressurized, heated and molded using a designed heated die. After exiting the die, the silicone is processed in ovens that circulate hot air vertically and horizontally, steam vulcanization, salt bath cure, and infrared radiation cure. Extrusion transforms fine silica particles into a translucent paste which is practical for its flexibility and ability to be aesthetically altered for the desired product specifications. This method produces silicone for end products such as gaskets, tubing, and seals.
Silicone rubber production methods: liquid injection molding
Liquid injection molding (LIM), also known as liquid silicone rubber (LSR), is used to produce high-volume durable parts and is suitable for creating complicated geometries.
The process starts in the pumping system where the base-forming silicone and the platinum catalyst parts of the liquid silicone rubber compounds are injected using supply drums, in addition to a separate container containing additives and pigments. The liquid silicone is pressurized into the machine’s pumping section with the injectors’ help. There, the metering pump will ensure that the liquid components mix and maintain a consistent ratio. Once it exits the metering pump, the mixture of base-forming silicone, catalyst, additives and pigments are combined by a static or dynamic mixer and forwarded into the nozzles. It travels from the cooled runner systems into the heated mold cavity, where the mixture is poured via the nozzles with automated shut-off valves to avoid leakage. Once the molding process is done and the product is cooled, the mold opens up.
Injection molding or liquid injection molding: where does the difference lie?
Among the production above methods, injection molding is the most common due to its ability to create products with a wide range of sizes, complexity, and application. The difference from liquid injection molding is the presence of mold halves and the raw material is forced into the hot cavity with a ram or screw-type plunger. The mold halves are clamped together, allowing for different shapes and sizes, from simple to complex and from small parts to large automobile body panels.
Solid and liquid silicone rubbers
Silicone rubber can be classified based on its viscosity – the difference between solid and liquid silicone rubber. Long chains of polymers with a high molecular weight result in a high consistency rubber, which is solid. Solid silicone is used for a variety of daily and industrial products. On the contrary, silicone rubber has a liquid texture when it contains polymers of lower molecular weight and shorter chains.
Due to it being safe as there were no added chemicals such as vulcanizing agents during molding, this type of silicone is used in the production of baby products as well as kitchen and medical supplies. Concerning the difference in production liquid silicone is injected into the mold through a vulcanization screw at a mold temperature of 170-200°C. Solid silicone, however, is cut into suitable size pieces and compression-molded.