Shore Hardness Testing of Plastics: Durometer Methods, Standards, and Applications

Shore hardness testing is the most widely used method for measuring the indentation hardness of plastics, rubbers, and thermoplastic elastomers, providing a fast and reproducible index of material softness or rigidity. This technical guide explains the operating principle of the durometer, specimen requirements, test procedures in accordance with ISO 868 and ASTM D2240, typical applications, and answers to frequently asked questions.

Technical Overview

Shore hardness, also known as durometer hardness, quantifies the resistance of a polymeric material to indentation by a spring-loaded indenter pressed against its surface under a defined force. The measured penetration depth is converted to a dimensionless scale from 0 to 100, where 0 corresponds to complete indenter penetration into a very soft material and 100 corresponds to no measurable penetration of a very hard material. Because the test is rapid, essentially non-destructive, and applicable to both test coupons and finished parts, it is one of the most frequently specified mechanical tests for plastics and elastomers in quality control, material qualification, and research.

The instrument used is called a durometer, and the two most common scales for polymeric materials are Shore A and Shore D:

• Shore A: intended for soft, flexible, and elastic materials such as vulcanized rubber, thermoplastic elastomers (TPE/TPU), silicone, and flexible PVC. The indenter is a truncated cone with a 35 degree included angle and a flat tip of 0.79 mm diameter, applied with a spring force of approximately 0.55 kgf to 8.1 kgf depending on the hardness value.
• Shore D: intended for hard and rigid plastics such as rigid PVC, HDPE, PP, PS, ABS, POM, and PA. The indenter is a sharp cone with a 30 degree included angle and a rounded tip of approximately 0.1 mm radius, applied with a substantially higher spring force of up to approximately 45 kgf.
• Auxiliary scales defined in ASTM D2240, such as Shore B, C, O, OO, DO, and M, cover intermediate hardness ranges, microporous materials, foams, and thin cross sections such as O-rings.

The principal governing standards are ISO 868 (Plastics and ebonite, determination of indentation hardness by means of a durometer, Shore hardness), ASTM D2240 (Standard Test Method for Rubber Property, Durometer Hardness), and the corresponding Chinese national standards GB/T 2411 for plastics and GB/T 531.1 for rubber and thermoplastic elastomers. DIN 53505, historically used in Europe, has been largely superseded by ISO 868.

Testing Samples

Soft Elastomers and Flexible Plastics (Shore A Scale)

Typical samples include natural rubber and synthetic rubber compounds, silicone parts, thermoplastic elastomers (TPE, TPV, TPU), flexible PVC compounds, seals, gaskets, hoses, tubing, and over-molded grips. These materials generally fall in the range of approximately 20 to 90 Shore A.

Rigid and Semi-Rigid Plastics (Shore D Scale)

Typical samples include rigid PVC profiles and sheets, polyethylene and polypropylene moldings, polystyrene, ABS, nylon, acetal, and glass-reinforced compounds. These materials typically exhibit values of roughly 60 to 90 Shore D.

Foams, Sponges, and Microporous Materials

Soft polyurethane foams, EVA foams, sponge rubber, and cellular materials that would be under-measured on the Shore A scale are tested with Shore OO or Shore C scale durometers, where the lower spring force and blunter indenter geometry are appropriate for cellular structures.

Specimen Preparation Requirements

• The test surface must be flat, smooth, and free of defects, voids, sink marks, and release agents.
• Minimum thickness: ASTM D2240 requires at least 6 mm; ISO 868 requires at least 4 mm for Shore A and 3 mm for Shore D. Thinner specimens may be stacked, preferably no more than three layers, provided they cannot slide against one another, although monolithic specimens are strongly preferred.
• The specimen must be large enough that all measurement points are at least 12 mm from any edge (9 mm per ISO 868) and at least 6 mm apart from one another.
• Specimens should be conditioned at 23 ± 2 °C and 50 ± 10 % relative humidity in accordance with ISO 291 or ASTM D618 before testing, since temperature strongly influences hardness readings.

Testing Items

Accredited third-party laboratories typically offer the following Shore hardness test items for plastics and elastomers:

• Shore A hardness (Shore A durometer, ISO 868, ASTM D2240, GB/T 531.1)
• Shore D hardness (Shore D durometer, ISO 868, ASTM D2240, GB/T 2411)
• Shore C, Shore B, Shore OO, Shore O, Shore DO, and Shore M hardness for special materials and geometries
• Median hardness value calculated from replicate readings, together with test temperature and dwell time
• Optional comparative hardness measurements after heat aging, UV weathering, chemical immersion, or compression set, used to evaluate material degradation

Testing Methods

The general test procedure, consistent with ISO 868 and ASTM D2240, is as follows:

• Verify the durometer before use: indenter geometry, protrusion height, presser foot condition, spring force calibration, and indication against certified reference blocks.
• Place the specimen on a rigid, flat, horizontal surface. For bench-mounted (stand-mounted) durometers, secure the specimen so that the indenter axis is perpendicular to the test surface.
• Apply the presser foot smoothly and without shock, ensuring full parallel contact with the specimen and a steady application of force.
• Take the reading at the specified dwell time: an instantaneous reading within approximately 1 second, or a timed reading after 3 seconds or 15 seconds, as required by the specification. Viscoelastic materials show a gradual decline in indicated hardness with longer dwell time, so the dwell time must always be reported.
• Perform at least five readings at points spaced at least 6 mm apart and at least 12 mm (or 9 mm per ISO 868) from the specimen edges.
• Report the median of the readings, the durometer type and scale, the test temperature, and the dwell time.

Scale selection follows a practical rule: when readings on the Shore A scale exceed approximately 90, the Shore D scale should be used; when Shore D readings fall below approximately 20, the Shore A scale is more appropriate. Similarly, materials softer than approximately 20 Shore A should be measured with Shore OO or an appropriate softer scale.

Applications

Shore hardness data are used across the full product life cycle of polymeric components, from material development to incoming inspection and failure analysis:

• Quality control of incoming resins, compounds, and molded or extruded parts against supplier datasheets.
• Material selection and comparison for seals, gaskets, diaphragms, bushings, rollers, wheels, and damping elements, where stiffness directly affects sealing performance and feel.
• Verification of cure state and crosslinking in vulcanized rubber and thermoset compounds.
• Evaluation of aging resistance by comparing hardness before and after heat, ozone, UV, or chemical exposure.
• Automotive components such as door seals, tires, hoses, engine mounts, and interior trim.
• Medical devices such as catheters, tubing, stoppers, and surgical grips, where hardness is often a regulated functional characteristic.
• Footwear midsoles and outsoles, consumer electronics keypads and over-molds, cable insulation and jackets, and construction profiles and waterproofing membranes.

FAQ

What is the difference between Shore A and Shore D hardness?

Shore A uses a blunt truncated-cone indenter with a lower spring force and is suited to soft rubbers and elastomers, while Shore D uses a sharper cone indenter with a much higher spring force and is suited to rigid plastics. The two scales overlap only loosely around the boundary between very hard elastomers and soft plastics.

Can Shore A values be converted to Shore D values?

There is no exact mathematical conversion between the scales because they use different indenter geometries and spring forces. Published comparison charts provide only rough approximate equivalences, and contractual or regulatory decisions should always be based on measurements made on the specified scale.

What specimen thickness is required?

ASTM D2240 requires a minimum thickness of 6 mm, while ISO 868 requires at least 4 mm for Shore A and 3 mm for Shore D. Insufficient thickness allows the indenter to sense the backing plate, producing falsely high readings.

Is Shore hardness testing destructive?

No. The test leaves only a shallow, localized indentation that does not impair the function of most parts, so it is suitable for finished-goods inspection and destructive testing of statistical samples is not required.

Why must the dwell time be reported?

Viscoelastic materials creep under the indenter, so the indicated hardness decreases with time. A reading taken within 1 second can differ significantly from one taken after 15 seconds on the same material; reporting the dwell time ensures comparability between laboratories and data sets.

What factors most commonly cause inaccurate results?

Frequent causes include specimens that are too thin, curved or textured surfaces, testing too close to an edge, testing at temperatures far from the standard 23 °C, inadequate conditioning, uncalibrated springs, and applying the durometer at an angle rather than perpendicular to the surface. Accredited third-party laboratories control these variables through calibrated instruments, verified procedures, and standardized conditioning environments.