The silicone rubber injection molding process

Liquid silicone rubber (LSR) Injection molding is a process to produce pliable, durable products in high volume.

Liquid silicone rubber is high purity platinum cured silicone with low compression set, great stability and ability to resist extreme temperatures of heat and cold ideally suitable for the production of parts, where high quality is a must. 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.

Typical applications for liquid silicone rubber are products that require high precision such as seals, sealing membranes, electric connectors, multi-pin connectors, infant products where smooth surfaces are desired, such as bottle nipples, medical applications as well as kitchen goods such as baking pans, spatulas, etc. Often, silicone rubber is over-molded onto other parts made of different plastics. For example, a silicone button face might be over-molded onto a Nylon 6,6 housing.

1.The Liquid Injection Molding (LIM) process starts with a two part liquid silicone compound formula ( A & B parts) that are delivered at a fixed ratio, typically 1:1, from material drums or pails by a hydraulic or pneumatic metered pumping system to a static mixer.  Liquid Silicone Rubbers are normally supplied in barrels. Because of their low viscosity, the material can be pumped through pipelines and tubes to vulcanization equipment.

The mold is usually heated by water flowing inside the mold or by electrical heaters. Additionally, a cold runner system can be used to prevent the premature curing of the flowing silicone in the runners or its vicinity.The static mixer blends the two components into a homogenous suspension; this mixing process activates the compounds platinum cure system.

2.The liquid silicone material from the static mixer flows to the injection unit. The LSR material is then injected into the mold cavity through a runner and gate system where it is held in the mold under high pressure and elevated temperature until material is cured (rubber is vulcanized). The cycle time is established to reach an optimal level of cure. At the end of the cycle, the parts are removed or ejected from the cavities and the next cycle begins. In the second step, the mold is closed by a clamping machine, and the injection process starts.

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.

With the injection molding machine with that equipment, The injection can be made either under atmospheric pressure (an efficient ventilation system is required) or in vacuum conditions. The injection time depends on the injection pressure determined by the product to be manufactured and the silicone properties, such as viscosity. The latter depends on both the shear rate, increasing during the flow, and the temperature.

Once the mold is filled with silicone, the mold temperature is increased to accelerate the crosslinking process of the silicone rubber, resulting in the material transformation from a liquid to solid state; this step is called the curing stage. The time of the curing stage is specific for a given silicone grade and, of course, product. This is another critical step in the processing, because the material overheating and significant pressure increase inside the mold can be observed as a result of the exothermic effect of a curing reaction and high thermal expansion of silicone.

The final step of the injection molding process of silicone rubber is the cooling of the product inside the mold, followed by a product demolding and final cooling in ambient conditions.

Contact Z.S.R Group for Custom Liquid Silicone Rubber injection modling:

In Z.S.R Group, we produce custom-designed and manufactured, high-quality LSR products and LSR 2-Shot products. We produce products for the Consumer and industrial markets, as well as medical and consumer applications. Contact us today for a free quote.