What Is the Hardness Range/Shore Hardness of LSR?
Liquid silicone rubber (LSR) is available in a wide range of hardness levels,
Liquid silicone rubber (LSR) is available in a wide range of hardness levels, allowing manufacturers to select the appropriate material for applications ranging from ultra-soft, gel-like components to semi-rigid structural parts. The hardness of LSR is typically measured on the Shore A scale, which is the standard method for evaluating the hardness of elastomeric materials. Understanding the available hardness range and how it relates to part performance is essential for product designers and engineers.
Standard Hardness Range
The most commonly cited hardness range for standard liquid silicone rubber grades is Shore A 10 to Shore A 80. Within this range, manufacturers can produce parts that range from very soft and conformable to relatively firm and dimensionally stable.
Some suppliers indicate a slightly narrower standard range of Shore A 20 to Shore A 80. The difference reflects variations in formulation and the specific product lines offered by different material suppliers. In practice, the usable range depends on the particular grade and the processing capabilities of the molder.
Ultra-Soft LSR Grades
At the low end of the hardness scale, LSR can be formulated to be extremely soft. Some specialty grades are designed for applications requiring a gel-like consistency or a very gentle tactile feel.
For reference, a Shore A hardness of 10 is comparable to the firmness of a gummy bear. Materials at this hardness level are often used for cushioning, anatomical models, prosthetic interfaces, and soft-touch surfaces. LSR grades in the Shore A 10 to 30 range provide excellent conformability and are frequently selected for medical devices and wearable products that contact human tissue.
Medium Hardness Range
The middle of the LSR hardness spectrum, roughly Shore A 40 to 60, represents the most widely used range for general-purpose sealing, gasketing, and molded components. For context, the tread surface of a car tire typically measures between 55 and 70 Shore A.
LSR grades in this range offer a balance of flexibility, compression set resistance, and mechanical strength. They are suitable for O-rings, diaphragms, valves, and seals in automotive, industrial, and consumer applications. This range also provides good demolding characteristics and dimensional stability in high-volume production.
Harder LSR Grades
At the upper end of the standard range, LSR can reach Shore A 70 to 80. These grades offer greater rigidity and dimensional stability while retaining the elastic recovery characteristic of silicone. They are used in applications where a firmer feel or greater resistance to deformation is required, such as structural seals, hard-soft overmolded components, and parts that must maintain shape under load.
Beyond the standard Shore A range, manufacturers have developed specialty LSR grades that extend into the Shore D scale. For example, a high-hardness LSR grade with a hardness of 90 Shore A (approximately equivalent to 33 Shore D) has been introduced for applications requiring exceptional dimensional stability. This level of hardness approaches that of rigid thermoplastics while retaining the temperature resistance and chemical stability of silicone. Applications include hard bonding partners in two-component molded parts, handles, and structural components.
Factors Affecting Hardness Selection
The choice of LSR hardness depends on several application requirements.
Sealing performance: Softer grades conform more readily to mating surfaces, while harder grades resist extrusion under pressure.
Tactile feel: Consumer products often require specific softness for ergonomic or aesthetic reasons.
Demolding and handling: Very soft grades can be more difficult to demold and handle without deformation, while harder grades maintain shape more easily.
Dimensional stability: Harder LSR grades provide better resistance to compression set and maintain tighter tolerances under load.
Overmolding compatibility: In hard-soft combinations, the hardness difference between the LSR and the substrate material affects bonding performance and mechanical function.
Measurement Considerations
Shore hardness is measured using a durometer according to standardized test methods such as DIN 53505 or ASTM D2240. For elastomeric materials like LSR, Shore A is the appropriate scale. When hardness exceeds approximately 90 Shore A, the Shore D scale is sometimes used to provide better resolution.
It is important to note that hardness values are typically specified with a tolerance range. For example, a grade designated as 70 Shore A may have a specification of 67 to 73 Shore A. This tolerance accounts for normal manufacturing variation in the material and the measurement itself.
LSR is available in a broad hardness range, typically from Shore A 10 to Shore A 80 for standard grades, with specialty formulations reaching 90 Shore A or higher in the Shore D range. Soft grades are used for cushioning and conformable applications, while harder grades provide dimensional stability and structural integrity. The appropriate hardness depends on the specific sealing, tactile, and mechanical requirements of the application. Designers should work closely with material suppliers and molders to select the optimal hardness grade for their product.