Everything You Need to Know About Liquid Silicone Rubber

Liquid Silicone Rubber (LSR) is a two-part platinum-cured elastomer that can be injected into a mold to produce various parts. LSR is renowned for its adaptability within the elastomer sector and is utilized in an extensive array of applications, from consumer goods to medical devices, encompassing a wide spectrum of products.

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LSR operates efficiently in temperatures ranging from -65°C to 150°C, with peaks reaching up to 260°C. Post-curing enhances its low compression set, achieving low outgassing values of TML < 1% and CVCM < 0.1%. These polymers typically exhibit translucence, allowing for pigmentation to meet any color specifications. Additionally, LSR facilitates fast cycle times and reduces raw material handling, translating to expedited delivery and increased production rates.

To empower manufacturers to harness the full potential of this innovative material, we've created a comprehensive guide to everything you need to know about liquid silicone rubber.

Creating the Polymer

Several methods exist for producing the base polymer of LSR. As with all elastomeric materials, a polymerization reaction is necessary to develop molecular chains. The polymerization of vinyl and methyl siloxane groups generates polydimethylsiloxane chains, which provide both strength and flexibility. This stage transforms the initial liquid, nearly aqueous components into the thicker, more viscous form of LSR. Here, various other characteristics can also be engineered into the material.

Once the primary polymer is established, the material gets reinforced, commonly with fumed silica. Unlike organic materials that typically use carbon black or clays as fillers, LSR employs different silicates to enhance the tensile strength of the polymer chains.

The material is then divided into two parts, with the catalyst (platinum) and crosslinker introduced into each half. This approach significantly extends the materials' shelf life. After blending the components, LSR exhibits a short working period averaging around eight hours. By separating the components, the material can potentially be stored for years without concern for premature curing.

Different Types of LSR

There are numerous varieties of LSR materials, including:

• Medical Grades: These LSRs undergo extraction to remove any elements unsuitable for medical applications. This stripping enhances the strength of the material while also catering to the strict biocompatibility requirements necessary for interacting with human tissue. Medical grades are categorized as either restrictive (implants lasting less than 30 days) or unrestricted (implants exceeding 30 days).

• Self-Lubricating: Incompatible fluids can be blended into the components, and once the silicone cures, it will bloom to the surface, creating a slippery finish. The addition of 1.5 to 3 percent phenyl fluid into the silicone controls the rate of blooming.

• Conductive: Conductive-grade carbon black is incorporated into LSR components, leading to superior mechanical properties when compared to metal particle-filled silicones. These grades can achieve a volume resistivity of as low as 9 ohm-cm.

• Self-Bonding: Adhesion promoters enhance the material's ability to adhere without primers to various substrates, thus enabling better sealing in complex components.

• Radio-Opaque: Barium sulfate is included to allow silicone parts to be visible under X-ray imaging, which is particularly useful in medical scenarios such as wound drains and catheters.

• FLSR: Fluorine is chemically integrated into the polymer chain, typically in a 60/40 or 100 percent fluorosilicone ratio. This process results in silicone highly resistant to oils and fuels.

• Flame Retardant: Most LSRs typically meet a UL 94 HB rating without additional aids, while achieving higher ratings (V1 or VO) necessitates additives that release water upon decomposition.

• Fast Cure: Like ethylene propylene, adjusting the functional groups allows manufacturers to control the extent of cross-linking in LSR, thereby enabling rapid production cycles.

The LSR Manufacturing Process

Upon entering our facility, high-consistency silicone generally requires labor to modify it to a mold-ready size and shape. Often this just involves converting the material for the mold cavity, referred to as performing the material.

Some materials necessitate adding cures during processing (which could have been omitted to enhance shelf life). In contrast, LSR streamlines the process. Here's an overview of the LSR manufacturing steps:

1. LSR components A and B are usually packaged in 5-gallon pails or 55-gallon drums. A controlled pumping system is employed to move the material into a static mixer and subsequently into the injection unit.

2. Pigments can be added and managed during injection. The material is then mixed via a screw mechanism and pushed forward to create a 'shot' of material, sufficient to fill the mold.

3. Once injected into the mold, it remains clamped while the LSR cures. Subsequently, an operator or robot extracts the final product, and the process repeats, averaging a quick 30 seconds per cycle.

4. In contrast, creating parts from organic materials or high-consistency silicone typically takes around six minutes, with peroxide-cured materials taking even longer. Therefore, LSR can significantly reduce labor costs when compared with high-consistency silicone materials.

5. Additional steps like post-baking can reduce volatilities and enhance compression set, while a deflashing process eliminates excess materials, followed by cleaning and packing for shipment.

For further information on liquid silicone rubber, engage with us on Twitter or LinkedIn, or reach out to our engineering team for assistance.

If you're interested in gaining further details, please check out Liquid Casting Rubber.

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