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Liquid injection molding is a process which uses premixed liquid silicone rubber to mold products rather than the thermoplastic resins used in other methods. In this process, inert liquid silicone rubber is mixed with an activation catalyst outside the mold prior to injection into the mold cavity. The activated silicone compound then hardens in the heated mold at which point the finished product is ejected. The liquid injection molding process produces highly accurate and predictable results in a medium that possesses specialist physical properties.
The process of liquid injection molding is similar to other injection molding methods in its basic principles. A fluid material is forced or injected into a closed mold under pressure where it cures or hardens and then is dropped or ejected from the opened mold. The main differences between this particular technique and others include the materials used to manufacture the finished product and the external processes used to prepare the material. Silicone rubber used for liquid injection molding is an inert, low-viscosity fluid which remains in this state until mixed with an activation agent or catalyst then heated or vulcanized. Once activated and heated, the compound undergoes a chemical reaction which causes it to solidify to form the final production material.
The equipment used to facilitate liquid injection molding consists of two or more tanks or plungers, a metering unit, a mixer unit, an injection nozzle, and the mold itself. One tank contains the inert base material and the other the catalyst agent. Additional additives or coloring agents will then be contained in additional tanks. The outlets from all raw material tanks lead to a metering unit which maintains a constant relationship between the materials. The mixing station combines all the various materials thoroughly and then feeds the mixed compound to the injection nozzle under pressure for injection into the mold.
Molds are generally preheated to ensure uniform filling and to complete the curing or vulcanization process. The mixed compounds in the feed section of the system are kept at a lower temperature to prevent complete curing and resultant blockages. The injection nozzle is typically fitted with an automated shut off system to prevent leakage and overfilling of the mold. Nozzle assemblies may also be of a spring loaded pin design to maintain higher injection pressures which further aids in keeping the extruder channel clear of clogging. Given the low viscosity of the materials used in liquid injection molding, all parts of the system need to be well sealed to prevent leaks.
Silicone rubber compounds exhibit excellent tensile strength, flexibility, and heat resistant characteristics. In addition, unique forming capabilities are possible with these materials which allow for extensive coloring and transparency options in the finished product. These characteristics are ideal for the production of seals, smooth surface flexible infant products such as pacifiers and feeding bottle nipples, medical equipment, and kitchenware.