Liquid Silicone Rubber—Versatility and Variety of Applications

Liquid Silicone parts and O-rings

Liquid Silicone Rubber (LSR) has become very popular for various applications in different industries because it is a versatile elastomer that performs well under demanding operating conditions. LSR technology has grown dramatically over the last decade thanks to its lower-cost customization options for manufacturing.

Components made of SLR are injection molded in a Class 1000 cleanroom and can be formed into specific end products. They lend themselves well to producing parts when following Production Part Approval Process (PPAP).

In this post, we’ll detail some of the advantages of using LSR in manufacturing automotive parts, components for medical devices, sealing products for the Oil & Gas industry, and a wide range of consumer or general application products.

LSR and Shore hardness

In case you need to refresh your memory–Silicone Liquid Rubber is a two-part elastomer composed of silicone (a polymer made up of siloxane) that is cured using platinum to form a clear, strong, and flexible rubber. Other curing agents can also be used.

LSR is available in different shore hardness levels, from soft to hard. The Shore durometer is used to measure a material’s hardness, typically of polymers.

The hardness of an elastomer is measured based on the depth of indentation by a standard size and shape impacting gauge. The hardness is obtained by comparing the difference between a small initial force and a much larger final force.

The International Rubber Hardness Degrees (IRHD) scale has a range of 0 to 100, corresponding to the elastic modulus of 0 (0) and infinite (100), respectively. The measurement is made by indenting a rigid ball into the rubber specimen.

The Shore A scale is the most prevalent in the United States. The readings range from 30 to 95 points. Harder elastomers use a pointed conical indenter with the Shore D scale.

  • 60 Shore A is softer than 70
  • 70 Shore A is the standard
  • 90 Shore A is very stiff
Durometer Hardness test for LSR

(Image source: Public Domain)

Advantages of using Liquid Silicone Rubber

LSR products have many key properties that make them an excellent choice for use in a variety of applications. They are:

  • Withstanding temperatures as low as –155ºF (68.33ºC) and as high as 392ºF (200ºF)
  • Customizable hardness from 10 to 80 Shore A durometer
  • Resistance to environmental conditions such as ozone, water, steam, and UV light while not showing significant aging
  • Biocompatibility with the end product being FDA-Grade
  • It is odorless and tasteless, it doesn’t support bacteria growth, and will not stain or corrode other materials
  • Since liquid silicone rubbers are highly flexible, they do not suffer permanent deformation even when compressed for a long time

Chemical structure of Liquid Silicone Rubber

The chemical structure of LSR is similar to that of other silicones, such as polydimethylsiloxane (PDMS). However, LSR is formed when long polysiloxane chains gain stiffness and tensile strength from specially treated silica. Polysiloxanes differ from other polymers in that their backbones consist of Si–O–Si units, unlike many other polymers that contain carbon backbones.

LSR’s chemical structure is based on a cross-linked polymer, with the addition of a platinum catalyst. The methyl-hydrogen siloxane in the rubber acts as a cross-linking agent. At the same time, the platinum catalyst is essential to the curing process. It helps to ensure that the final product is solid and durable.

Chemical structure of PDMS

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Variety of applications using LSR

LSR materials are used in many products, including electrical insulation, potting and encapsulation, and components for medical devices. Its versatility lends itself to various applications for different industries such as Aerospace & Military, Automotive, Food & Beverage, and Medical & Pharmaceutical.

Liquid Silicone Rubber molding is preferred over compression molding because lower costs are associated with prototyping in many manufacturing situations.

The rapid cure cycle of LSR products allows the combination of existing components and then injection molding around them. This allows for reliable and repeatable processing in high-volume batches. In addition, LSR can be molded into complex shapes, making it perfect for precise molding applications.

  • Medical grades: LSR is often used in medical devices and implants because it is highly biocompatible, and the components can be sterilized.
  • Electrical parts:  LSR is a popular material for making electrical parts. It’s suitable for high-voltage applications due to its resistance to high electrical stress.
  • Self-lubricating: LSR is suitable for manufacturing parts that don’t require lubrication or where lubrication would be difficult or impossible to apply.
  • Automotive parts: Used in engine seals and gaskets.
  • Self-bonding: LSRs contain adhesion promoters that allow them to bond to themselves and other materials such as aluminum, stainless steel, and glass-filled nylon.
  • Radio opaque: This type of LSR contains barium sulfate that makes them visible on X-rays which is useful in medical imaging applications.
  • Flame retardant: LSR can resist burning and meets a UL 94 HB rating, so it is used in environments where fire safety is a concern.

Liquid Silicone Rubber vs. Thermoplastic Elastomers

In the past, Thermoplastic Elastomers (TPEs) were the go-to choice for injection molding applications. However, LSR has quickly overtaken TPEs in many cases. So, why is LSR a better option than TPE for injection molding? There are several reasons:

  • Durability: LSR  has a much higher tolerance for temperature extremes, which ensures long-term stability. Also, the material has a low compression set and is more resistant to wear and tear.
  • Non-Toxic: LSR is non-toxic and doesn’t degrade over time. It is also resistant to harsh chemicals and cleaning agents, making it a safe choice for medical and food-grade applications.
  • Viscosity: LSR has a much lower viscosity, making it easier to work with and producing a smoother finish. Its exceptional flexibility allows it to be molded into intricate shapes that are less likely to crack or break during injection molding.
  • UV Resistance: It is resistant to UV light, making it best suited for manufacturing products that must meet rigorous standards.

One thing to be aware of is that LSR is more expensive than TPE, but the increased durability and flexibility make it a worthwhile investment.

To summarize, liquid silicone rubber (LSR) offers design engineers and manufacturing professionals a versatile material with multiple uses in different industries. With so many potential applications, there’s no doubt that LSR will continue to grow in popularity.

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