Injection Molding Process
Liquid silicone rubbers are supplied in barrels or hobbocks. Because of their low viscosity, these rubbers can be pumped through pipelines and tubes to the vulcanization equipment. The two components are pumped through a static mixer by a metering pump. One of the components contains the catalyst, typically platinum based. A coloring paste as well as other additives can also be added before the material enters the static mixer section. In the static mixer the components are well mixed and are transferred to the cooled metering section of the injection molding machine. The static mixer renders a very homogeneous material that results in products that are not only very consistent throughout the part, but also from part to part. This is in contrast to solid silicone rubber materials that are purchased pre-mixed and partially vulcanized. In contrast, hard silicone rubbers are processed by transfer molding and result in less material consistency and control, leading to higher part variability. Additionally, solid silicone rubber materials are processed at higher temperatures and require longer vulcanization times.
From the metering section of the injection molding machine, the compound is pushed through cooled sprue and runner systems into a heated cavity where the vulcanization takes place. The cold runner and general cooling results in no loss of material in the feed lines. The cooling allows production of LSR parts with nearly zero material waste, eliminating trimming operations and yielding significant savings in material cost.
Liquid silicone rubbers are supplied in a variety of containers, from tubes to 55 gallon drums. Because of their viscous nature, these liquids are pumped at high pressures (500 - 5000 psi) based on the durometer of the material. The raw materials are shipped in two separate containers (known in the industry as a kit) identified as "A" and B" compounds, with the "B" side usually containing the catalyst, but may vary based on the brand of silicone used. The two (A and B) compounds must be mixed in a 1 to 1 ratio, usually by way of a static mixer. Once the two components come together the curing process begins immediately. A chiller supplying cold water to jacketed fittings is typically used to retard the curing process prior to the materials introduction to the mold. A color pigment can be added via a color injector used in conjunction with the material pump (closed loop metering system) before the material enters the static mixer section.
In a cold deck scenario, the 1 to 1 mixed compound is pumped through cooled sprue and runner systems into a heated cavity where the vulcanization takes place. The cold runner and general cooling results in minimal loss of material as the injection occurs directly into the part or cavity. The cooling allows production of LSR parts with nearly zero material valve gate waste, however this does not guarantee a "flash free" finished part. Molds and tooling are varying in design, execution and cost. A good cold runner is expensive as compared to conventional hot runner tooling, and has the potential to provide a high level of performance.
Read more about this topic: Injection Molding Of Liquid Silicone Rubber
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