Liquid silicone rubber (LSR) hardness testing measures the indentation resistance of cured silicone elastomers using Shore A, Shore 00, and IRHD methods in accordance with standards such as ASTM D2240, ISO 48, ISO 7619-1, and GB/T 531.1. Accurate hardness data are essential for material qualification, incoming inspection, and quality control of medical, automotive, baby care, and consumer silicone components. This guide explains the applicable standards, sample preparation requirements, test procedures, and the key factors that influence test results.
Technical Overview
Liquid silicone rubber (LSR) is a two-part, platinum-catalyzed (addition-cure) silicone elastomer that is processed primarily by liquid injection molding. Hardness, defined as the resistance of the cured material to indentation by a specified indenter under a specified force, is one of the most frequently specified and verified properties of LSR. It correlates directly with the elastic modulus of the elastomer and strongly influences sealing performance, compression behavior, tactile feel, and assembly compatibility.
Commercial LSR grades typically range from approximately 5 to 80 on the Shore A scale, while ultra-soft gel grades fall below the practical range of the Shore A scale and are characterized on the Shore 00 scale. Because measured hardness is sensitive to cure completeness, test temperature, specimen thickness, and dwell time, hardness testing must be performed under standardized conditions to produce comparable and repeatable results.
- Formulation factors: silica filler loading, crosslink density, and silicone oil extension determine the nominal hardness of the cured elastomer.
- Processing factors: mix ratio accuracy, mold temperature, cure time, and post-cure baking affect the final hardness achieved.
- Measurement factors: durometer scale selection, specimen geometry, indentation dwell time, and ambient temperature directly affect the reported value.
Testing Samples
Cured Test Plaques
The preferred specimen is a flat cured plaque with smooth, parallel surfaces and a minimum thickness of 6 mm for Shore A testing. Thinner sheets may be stacked in no more than three layers, provided the layers are in intimate contact and free of trapped air. Plaques should be fully cured, post-cured where specified, and conditioned at 23 ± 2 °C and 50 ± 5 % relative humidity for at least 16 hours prior to testing.
Finished Molded Parts
Production components such as seals, valves, nipples, gaskets, and wearable pads can be tested directly, provided a sufficiently flat area exists for the presser foot to seat evenly. Curved or complex surfaces introduce measurement errors; in such cases, testing is performed on a flat section of the part or on companion test plaques molded from the same material batch.
Gel-Grade and Ultra-Soft Materials
Materials softer than approximately 10 Shore A, including silicone gels used for wound care, cushioning, and vibration isolation, are tested on the Shore 00 scale with adequate specimen thickness and rigid backing support.
Thin-Walled Sections
For thin-walled or intricately curved parts that cannot meet the dimensional requirements of durometer testing, micro-IRHD hardness testing (ISO 48, method M) using a small-diameter ball indenter is the appropriate alternative.
Testing Items
Hardness testing of liquid silicone rubber typically covers the following items, depending on the stage of product development and the applicable specification:
- Shore A hardness of cured LSR per ASTM D2240, ISO 7619-1, GB/T 531.1, and JIS K 6253.
- Shore 00 hardness of gel-grade and ultra-soft silicones.
- IRHD normal and micro hardness per ISO 48 and GB/T 6031.
- Hardness verification after post-cure baking.
- Hardness retention after heat aging, for example after 72 h or 168 h at an elevated temperature.
- Hardness uniformity across a part, between mold cavities, or between production lots.
Testing Methods
Durometer Method (ASTM D2240 / ISO 7619-1 / GB/T 531.1)
The Shore A durometer presses a truncated 35° cone indenter with a 0.79 mm flat tip against the specimen under a spring force that increases from approximately 0.55 N to 8.05 N over the 0–100 scale range. The instrument should be mounted on a stand that applies the presser foot perpendicular to the specimen at a controlled rate, which greatly reduces operator-induced variability compared with hand-held operation.
At least five readings are taken at points no closer than 12 mm to any edge and spaced at least 6 mm apart, and the median value is reported together with the dwell time (instantaneous or 3 s, as specified by the governing standard or product specification). For Shore 00 testing, the spherical-indenter instrument and its lighter spring loading are applied to the ultra-soft material in the same general manner.
IRHD Method (ISO 48 / GB/T 6031)
The International Rubber Hardness Degree (IRHD) method uses a dead-load instrument in which a 2.5 mm diameter ball indenter is pressed into the specimen, and the difference between the total and contact indentation depths is converted to a hardness value on a scale from 0 to 100. Because loading is by dead weight rather than a spring, IRHD instruments provide excellent reproducibility and are well suited to referee testing. The micro method (method M) uses a 0.395 mm ball indenter and is intended for thin or curved specimens such as catheter walls and small seals. IRHD and Shore A values correlate approximately but are not interchangeable; the test method must always be stated with the result.
Instrument Verification and Calibration
Durometers should be verified at regular intervals using calibrated hardness reference blocks and by checking indenter geometry and spring force in accordance with the relevant annexes of ASTM D2240 and ISO 7619-1. Accredited third-party laboratories maintain traceable calibration systems and controlled environments to ensure the validity of reported hardness values.
Conditioning and Ambient Control
All hardness tests should be conducted at 23 ± 2 °C and 50 ± 5 % relative humidity. The hardness of silicone elastomers decreases measurably as temperature rises, so specimens must be allowed to reach thermal equilibrium in the test laboratory before measurement, and the test temperature should be recorded in the report.
Applications
Hardness testing of liquid silicone rubber supports material selection, supplier qualification, first-article inspection, and ongoing production control across a broad range of industries:
- Medical devices: catheters, septa, check-valve seals, implant-grade components, and wearable sensor pads, where hardness affects patient comfort and device function.
- Baby care products: nipples, pacifiers, and teethers, where hardness is both a safety and a functional attribute.
- Automotive engineering: sealing systems, connector seals, grommets, diaphragms, and vibration dampers exposed to wide temperature ranges.
- Consumer electronics: keypads, waterproof seals, earbud tips, and wrist straps, where hardness defines tactile feel and long-term resilience.
- Industrial applications: O-rings, gaskets, thermal interface gels, and potting compounds.
Incoming material verification against datasheet hardness, cavity-to-cavity comparison during mold qualification, and hardness retention after heat aging are among the most common quality-control programs executed by accredited third-party laboratories on behalf of manufacturers.
FAQ
What is the typical hardness range of liquid silicone rubber?
Most commercial LSR grades fall between approximately 10 and 80 Shore A. Ultra-soft gel grades below about 10 Shore A are characterized on the Shore 00 scale, while the Shore D scale is generally not applicable to silicone rubber.
What is the minimum specimen thickness for Shore A testing?
A minimum thickness of 6 mm is required. Thinner sheets may be stacked in up to three layers, provided the layers make intimate contact without trapped air or gaps, and the result is noted as measured on a stacked specimen.
Why do measured hardness values sometimes differ from the datasheet value?
Common causes include insufficient specimen thickness, reading the durometer at a different dwell time, testing at a different temperature, testing before post-cure is complete, measuring on curved or textured surfaces, and normal lot-to-lot variation. Standardized specimen preparation and conditioning eliminate most discrepancies.
Are Shore A and IRHD results interchangeable?
No. The two methods use different indenter geometries and loading principles, and results are only approximately correlated. For specification and certification purposes, the test method and standard must be explicitly stated together with the hardness value.
How is hardness tested on small or curved finished parts?
When a flat area large enough for the durometer presser foot is not available, micro-IRHD testing per ISO 48 method M, testing on a flat section of the part, or testing companion plaques molded from the same batch are the accepted alternatives.
How soon after molding can hardness be measured?
Hardness should be measured only after the material has fully cured and cooled to room temperature. Where a post-cure bake is specified, for example 2 to 4 hours at 150 °C to 200 °C, hardness verification is normally performed after post-curing, since volatile removal and crosslink equilibration can shift the hardness value slightly.