Rubber Hardness Testing Procedures: A Complete Step-by-Step Guide

Rubber hardness testing measures an elastomer's resistance to indentation and is one of the most widely performed quality-control measurements for products such as seals, O-rings, hoses, tires and vibration mounts. This article explains the complete rubber hardness test procedure, covering sample preparation, Shore A, Shore D and IRHD measurement steps, and the applicable standards such as ISO 48, ISO 7619-1, ASTM D2240 and GB/T 531.1. Whether testing in-house or through accredited third-party laboratories, following the correct procedure is essential for accurate and repeatable results.

Technical Overview

Hardness is a measure of how strongly a rubber surface resists localized indentation by a defined indenter under a specified force. Because rubber is a viscoelastic material, the measured value depends on the indenter geometry, the applied force, the dwell time and the test temperature. For this reason, every rubber hardness test must follow a strictly standardized procedure to ensure that results are comparable between laboratories and over time.

The two dominant families of test methods are:

• Durometer (Shore) methods, which use portable spring-driven instruments with a truncated-cone indenter (Shore A) or a sharp-cone indenter (Shore D), standardized in ISO 7619-1, ASTM D2240 and GB/T 531.1.
• Dead-load IRHD methods (International Rubber Hardness Degrees), which use bench-mounted instruments that press a 2.5 mm ball (normal test) or a 0.395 mm ball (micro test) into the specimen under calibrated weights, standardized in ISO 48 and GB/T 6031.

Both scales are dimensionless. Most engineering rubbers fall between 30 and 90 Shore A. Compounds reading above 90 Shore A are normally re-tested on the Shore D scale, while thin sections and small components such as O-rings are best measured with the micro-IRHD method.

Testing Samples

Standard Vulcanized Rubber Sheets

Preferred specimens are flat molded sheets with smooth, parallel surfaces and a thickness of at least 6 mm for Shore A and normal IRHD testing. If thinner material must be tested, up to three plies may be stacked, provided they are not bonded to each other and do not slip during measurement. The test area must be large enough for at least five measurement points spaced at least 12 mm apart and no closer than 6 mm to any edge.

Thermoplastic Elastomers

TPE, TPU and TPV materials are usually tested on injection-molded or compression-molded plaques. Because thermoplastics creep more than vulcanized rubber, a longer dwell time of 15 seconds is applied when reading the durometer.

Finished and Curved Parts

O-rings, seals, hoses and molded components should be measured on a genuinely flat region or supported in an appropriate fixture. Curved surfaces cause tilted contact and artificially low or scattered readings. Small cross-sections and cords are tested with the micro-IRHD method at a nominal thickness of about 2 mm.

Laboratory Conditioning

At least 16 hours should elapse between vulcanization and testing. Before measurement, specimens are conditioned in accordance with ISO 23529 at 23 °C ± 2 °C and 50 % ± 5 % relative humidity for a minimum of 3 hours, and the test itself is performed at 23 °C ± 2 °C unless otherwise specified.

Testing Items

Typical hardness test items requested for rubber compounds and products include:

• Shore A hardness (ISO 7619-1, ASTM D2240, GB/T 531.1) for soft to medium vulcanized rubbers in the range of approximately 10 to 90 Shore A.
• Shore D hardness (ISO 7619-1, ASTM D2240) for hard rubbers, ebonite-type materials and highly filled compounds exceeding 90 Shore A.
• Normal IRHD test (ISO 48, GB/T 6031) using a 2.5 mm ball indenter, often preferred for specification and arbitration purposes.
• Micro-IRHD test (ISO 48) using a 0.395 mm ball for thin or small specimens such as O-rings and thin-walled tubing.
• Hardness change after thermal ageing (delta hardness), evaluated in combination with heat ageing exposure according to ISO 188.

The reported result is the median of the individual readings, together with the test method, scale, dwell time, specimen thickness and test temperature.

Testing Methods

Step-by-Step Procedure: Shore A / Shore D Durometer Method

• Step 1 – Prepare the specimen: Verify that the surface is smooth, flat and free of pores, fabric layers, release agents or previous indentation marks. Confirm the thickness is at least 6 mm.
• Step 2 – Condition the specimen: Hold the sample at 23 °C ± 2 °C for at least 3 hours before testing, as required by ISO 23529.
• Step 3 – Verify the instrument: Check the zero reading on a flat glass plate, confirm the indenter protrusion and presser-foot condition, and validate the durometer against certified hardness reference blocks traceable to a national metrology institute. Mounting the durometer on an operating stand improves force consistency.
• Step 4 – Position the specimen: Place the sample on a rigid, flat, horizontal plate so that the indenter axis meets the surface exactly perpendicular. Tilting must be avoided.
• Step 5 – Apply the durometer: Press the instrument down smoothly and firmly, without shock, until the presser foot fully contacts the rubber surface.
• Step 6 – Read at the correct time: For vulcanized rubber, take the reading 3 seconds after the presser foot contacts the specimen; for thermoplastic elastomers, read after 15 seconds, in accordance with ISO 7619-1.
• Step 7 – Repeat the measurement: Take at least five readings at points spaced at least 12 mm apart and at least 6 mm from any edge, avoiding defects and previously indented spots.
• Step 8 – Report: Calculate and report the median of the readings, along with the scale (A or D), dwell time, thickness, conditioning and temperature.

Step-by-Step Procedure: IRHD Dead-Load Method (ISO 48)

• Step 1 – Mount the specimen: Place the conditioned sample, typically 8 mm to 10 mm thick for the normal test, on the flat anvil of the dead-load instrument.
• Step 2 – Apply the contact force: Lower the presser foot, which applies a defined force of 8.3 N, and hold for approximately 5 seconds to seat the specimen; this sets the zero reference.
• Step 3 – Apply the main force: Smoothly apply the additional indentation force so that the 2.5 mm ball carries a total force of 5.7 N.
• Step 4 – Read at the specified dwell: Record the indentation after 30 seconds of total force application; the instrument converts the penetration depth directly into IRHD units.
• Step 5 – Repeat and report: Take three to five readings at points spaced at least 10 mm apart (at least 0.8 mm apart for the micro test on 2 mm specimens) and report the median with the test conditions.

Quality-Control Best Practices

• Verify the instrument with certified reference blocks before each test session and perform full calibration at scheduled intervals as required by ISO 7619-1, ASTM D2240 or an ISO/IEC 17025 quality system.
• Never test over voids, inserts, air bubbles or the edge of a specimen.
• Retesting a previously indented point is not permitted, as rubber recovery is time-dependent.
• Record the laboratory temperature with every data set, since hardness drops measurably as temperature rises.
• For disputed or contractual results, use the dead-load IRHD method or engage accredited third-party laboratories to perform the verification testing.

Applications

Rubber hardness test procedures are applied across nearly every industry that uses elastomeric materials:

• Sealing products: O-rings, gaskets and oil seals, where a typical fluoroelastomer or nitrile compound is specified at 70 Shore A or 75 Shore A.
• Automotive engineering: engine mounts, bushings, suspension buffers, CV-joint boots, hoses and belts.
• Tire manufacturing: hardness checks on tread, sidewall and inner-liner compounds.
• Industrial rubber goods: sheets, linings, rolls, diaphragms, dampers and conveyor belting.
• Footwear and consumer products: outsoles, grips, elastic bands and sporting goods.
• Electrical and cable applications: insulation and sheathing compounds, where hardness complements electrical and ageing tests under IEC and UL standards.
• Research and development: compound optimization, cure characterization, plasticizer or filler studies, and hardness retention after heat, oil or ozone exposure.
• Incoming inspection, batch release and failure analysis, both in factory laboratories and in accredited third-party laboratories.

FAQ

What is the difference between Shore A and IRHD?

Shore A uses a spring-loaded handheld durometer with a truncated-cone indenter, giving a quick field measurement. IRHD uses a bench instrument with a spherical ball indenter and calibrated dead loads, providing better repeatability and reproducibility. For most compounds between 20 and 90 units, the two scales give broadly comparable values.

What is the minimum sample thickness for a Shore A test?

At least 6 mm. Thinner material may be built up from a maximum of three non-bonded plies; below roughly 2 mm, the micro-IRHD method specified in ISO 48 is the more appropriate technique.

How many readings should be taken, and which value is reported?

At least five readings for durometer tests and three to five for IRHD tests. The median, not the average, is the reported result because it suppresses outliers caused by local surface defects.

When should Shore D be used instead of Shore A?

When the material reads above 90 Shore A, the Shore A scale becomes compressed and insensitive. Above this level, testing continues on the Shore D scale, which uses a sharper indenter and a stiffer spring.

Why is temperature control so important?

Rubber softens as temperature increases, so hardness can drift by several units between a cold morning and a warm afternoon. All standardized procedures require testing at 23 °C ± 2 °C, and any non-ambient results must be clearly labeled with the test temperature.

How often should a durometer be calibrated?

The instrument should be functionally checked with certified reference blocks before each session and fully calibrated, including spring force and indenter geometry, on a documented schedule in accordance with ISO 7619-1, ASTM D2240 or the laboratory's ISO/IEC 17025 management system.