What is Liquid Silicone Rubber Moulding?
Liquid silicone rubber is a high purity platinum cured silicone with low compression set, good stability and ability to resist extreme temperatures of heat and cold ideally suitable for production of parts, where high quality is required. Due to the thermosetting nature of the material, liquid silicone injection molding requires special treatment, such as intensive distributive mixing, while maintaining the material at a low temperature before it is pushed into the heated cavity and vulcanized.
Chemically, silicone rubber is a family of thermoset elastomers that have a backbone of alternating silicon and oxygen atoms and methyl or vinyl side groups. Silicone rubbers constitute about 30% of the silicone family, making them the largest group of that family. Silicone rubbers maintain their mechanical properties over a wide range of temperatures and the presence of methyl-groups in silicone rubbers makes these materials extremely hydrophobic, making them suitable for electrical surface insulations.
Equipment
In order for the liquid injection molding process to fully occur, several mechanical components must be in place. Typically, a molding machine requires a metered pumping device in conjunction with an injection unit—a dynamic or static mixer is attached. An integrated system can aid in precision and process efficiency. The critical components of a liquid injection molding machine include:
Injectors. An injecting device is responsible for pressurizing the liquid silicone to aid in the injection of the material into the pumping section of the machine. Pressure and injection rate can be adjusted at the operator's discretion.
Metering Units. Metering units pump the two primary liquid materials, the catalyst and the base forming silicone, ensuring that the two materials maintain a constant ratio while being simultaneously released.
Supply Drums. Supply drums, also called plungers, serve as the primary containers for mixing materials. Both the supply drums and a container of pigment connect to the main pumping system.
Mixers. A static or dynamic mixer combines materials after they exit the metering units. Once combined, pressure is used to drive the mixture into a designated mold.
Nozzle. To facilitate the deposition of the mixture into the mold, a nozzle is used. Often, the nozzle features an automatic shut-off valve to help prevent leaking and overfilling the mold.
Mold Clamp. A mold clamp secures the mold during the injection molding process, and opens the mold upon completion.
Characteristic of LSR
Biocompatibility: Under extensive testing, liquid silicone rubber has demonstrated superior compatibility with human tissue and body fluids. In comparison to other elastomers, LSR is resistant to bacteria growth and will not stain or corrode other materials.
Durable: LSR parts can withstand extreme temperatures, which makes them an ideal choice for components under the hood of cars and in close proximity to engines. Parts fabricated via liquid silicone rubber injection molding are fire retardant and will not melt.
Chemical resistance: Liquid silicone rubber resists water, oxidation and some chemical solutions such as acids and alkali.
Temperature resistance: Compared to other elastomers, silicone can withstand a wide range of high/low temperature extremes.
Mechanical properties: LSR has good elongation, high tear and tensile strength, excellent flexibility and a hardness range of 5 to 80 Shore A.
Electrical properties: LSR has excellent insulating properties, which offer an appealing option for a host of electrical applications. Compared to conventional insulating material, silicone can perform in far higher and lower temperatures.
Transparency and pigmentation: LSR possesses a natural transparency. This attribute makes it possible to produce colorful, custom, molded products.