What is Liquid Silicone Rubber Injection Molding?
Liquid Silicone Rubber Injection Molding (LSR) is a process used to produce pliable, durable parts in high volumes. The process involves the utilization of various components, including an injector, metering unit, supply drum, mixer, nozzle, and mold clamp, among others.
This process is used to create flexible silicone parts. LSR moldings are highly durable and well-suited for challenging applications due to their low compression set and ability to withstand extreme temperatures. LSR elastomer provides outstanding optical clarity, durability, and design flexibility while maintaining excellent mechanical properties over a wide temperature range (-50°C to 250°C).
LSR injection molding is suitable for manufacturing complicated geometries and can combine multiple functional elements into a single part due to its design and tooling flexibility\
Equipment for LSR Injection Molding
To ensure the successful implementation of the liquid injection molding process, various mechanical components need to be in place. An integrated system can aid in precision and process efficiency. Typically, a molding machine consists of the following critical components:
Injectors: The injector device is responsible for pressurizing the liquid silicone, enabling its injection into the pumping section of the machine. The operator can adjust the pressure and injection rate as needed.
Metering Units: Metering units are responsible for pumping the two primary liquid materials, the catalyst and the base forming silicone, to ensure a consistent ratio between the two materials during their simultaneous release.
Supply Drums: Supply drums, also known as plungers, serve as the main containers for the mixing materials. They are connected to the main pumping system along with a pigment container.
Mixers: A static or dynamic mixer combines the materials once they exit the metering units. The mixed materials are then pressurized and driven into a designated mold.
Nozzle: A nozzle is used to facilitate the deposition of the mixed material into the mold. It often incorporates an automatic shut-off valve to prevent leakage and overfilling of the mold.
Mold Clamp: A mold clamp is employed to secure the mold during the injection molding process. It ensures that the mold remains in place and opens once the process is complete.
Characteristics of LSR for Injection Molding
Biocompatibility: Liquid silicone rubber (LSR) has undergone extensive testing and has shown excellent compatibility with human tissue and body fluids. It surpasses other elastomers in terms of resistance to bacterial growth, and it does not cause staining or corrosion of other materials. LSR is tasteless, odorless, and can be formulated to meet strict FDA requirements. It can be sterilized using various methods such as steam autoclaving, ethylene oxide (ETO), gamma radiation, e-beam, and other approved techniques, complying with standards like BfR XV, FDA 21 CFR 177.2600, and USP Class VI.
Durability: LSR parts are highly durable and can withstand extreme temperatures, making them suitable for automotive components located near engines. These parts are also fire retardant and do not melt.
Chemical Resistance: Liquid silicone rubber is resistant to water, oxidation, and certain chemical solutions like acids and alkalis.
Temperature Resistance: Compared to other elastomers, silicone exhibits exceptional resistance to a wide range of high and low temperatures.
Mechanical Properties: LSR has excellent elongation, high tear, and tensile strength, remarkable flexibility, and a hardness range between 5 and 80 Shore A.
Electrical Properties: LSR demonstrates outstanding insulating properties, making it a desirable choice for various electrical applications.
Transparency and Pigmentation: LSR possesses inherent transparency, enabling the production of colorful custom-molded products.
The Steps of Liquid Silicone Rubber Injection Molding
The first step involves preparing the mixture for liquid silicone rubber (LSR). Typically, LSR consists of two components: pigment and additives (such as fillers) chosen based on the desired properties of the final product. During this step, the mixture’s ingredients are thoroughly blended and can be combined with a temperature stabilization system to achieve better control over the silicone’s temperature, either by maintaining ambient temperature or preheating the silicone.
Inserts, which are internal parts of the product embedded in the silicone coating after the injection process, are placed into the mold. These inserts can be kept in ambient conditions prior to injection or preheating.
LSR is commonly supplied in barrels since it has low viscosity, allowing it to be pumped through pipelines and tubes to the vulcanization equipment.
The mold is typically heated using either water flowing inside it or electrical heaters. Additionally, a cold runner system can be employed to prevent premature curing of the flowing silicone in the runners or nearby areas.
In the second step, the mold is closed by a clamping machine, initiating the injection process.
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. Viscosity is influenced by both the shear rate, which increases during flow, and the temperature.
Once the mold is filled with silicone, the mold temperature is raised to expedite the crosslinking process of the silicone rubber. This causes the material to transform from a liquid to a solid state, known as the curing stage. The duration of the curing stage is specific to the particular silicone grade and the product being manufactured. This step is critical in the process, as the exothermic effect of the curing reaction and the high thermal expansion of the silicone can result in material overheating and a significant increase in pressure inside the mold.
The final step in the injection molding process of silicone rubber is cooling the product within the mold, followed by demolding the product and allowing for final cooling under ambient conditions.
All the aforementioned parameters greatly influence the course of silicone molding. Therefore, optimizing these aspects becomes crucial to achieve the shortest production cycle time while maintaining the highest quality of the final product.
Advantages of liquid silicone injection molding
Batches stability: Liquid silicone rubber (LSR) is a ready-to-use material, ensuring consistency and stability in each batch.
Process repeatability: LSR injection molding offers excellent process repeatability, allowing for consistent and reliable production of parts.
Direct injection (no waste): The direct injection process of LSR eliminates waste, as the material is precisely injected into the mold without excess material.
Short cycle time: LSR injection molding has relatively short cycle times, enabling efficient production and high throughput.
‘Flashless’ technology (no burrs): The use of LSR and the injection molding process often results in ‘flashless’ parts, meaning there are no unwanted burrs or excess material on the finished product.
Automated process: LSR injection molding can be automated, resulting in increased productivity and reduced labor costs.
Automated demolding systems: The demolding process, where the finished parts are removed from the mold, can also be automated, further enhancing efficiency and reducing manual labor requirements.
Applications
Liquid silicone rubber injection molding is used in thousands of applications across a variety of industries, including medical, automotive, healthcare, consumer food and beverage, appliance and electronic products, infant and geriatric care, and industrial markets.
LSR injection molding is an excellent manufacturing method for a wide range of medical products. It finds applications in various areas including drug delivery, fluid management, diagnostics, and biotechnology.
In the automotive sector, LSR mold includes wire harness seals and connectors, bellows, gaskets, vibration dampeners, housings, and sensors.
Consumer products: LSR molds include watertight barriers and bacteria-resistant showerheads and faucets, vibration-resistant sealed housings for personal hygiene products like shavers and electric toothbrushes, soft pliable surfaces for headphone and hearing aid ear tips, and is safe and durable for baby bottle tips and soothers
Contact A1 Silicone to learn more about liquid silicone rubber molding or to request a quote.
See more: RTV- 2 Liquid Silicone Rubber Molding