Shore Hardness Testing for Plastics: Principles, Standards, and Applications

Shore hardness testing, also known as durometer testing, is one of the most widely used methods for evaluating the indentation hardness of plastics, elastomers, and rubber-like materials. By measuring the resistance of a specimen to penetration by a spring-loaded indenter, this quick and non-destructive test supports quality control, material verification, and product development across industries. This article explains the principles, sample requirements, test procedures, and applications of Shore hardness testing in accordance with ISO 868, ASTM D2240, and GB/T 2411.

Technical Overview

Shore hardness is a dimensionless indentation hardness value determined with an instrument called a durometer, which presses a spring-loaded indenter against the surface of a test specimen under defined conditions. The depth of indentation is inversely related to hardness: the shallower the indenter penetrates, the higher the hardness value. Results are expressed on a scale from 0 to 100, for example "85 Shore A" or "70 Shore D".

For plastic materials, two durometer scales are predominantly used: Shore A for soft, flexible materials and Shore D for rigid or hard materials. Because the scales overlap, the general convention is to switch from Shore A to Shore D when readings consistently exceed approximately 90 Shore A, and to use Shore A instead of Shore D when readings fall below roughly 20 Shore D. Additional scales such as Shore B, C, O, OO, and DO exist in ASTM D2240 for very soft or specialty materials, but they are rarely applied to plastics.

Principal Standards

• 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
• GB/T 2411 – Chinese national standard for determining Shore hardness of plastics
• ISO 291 – Standard atmospheres for conditioning and testing of specimens

Indenter and Force Characteristics

• Shore A: truncated cone indenter with a 35° included angle and a flat tip of approximately 0.79 mm diameter; applied force increases from about 0.55 N to 8.05 N across the scale (F = 550 + 75H, in mN)
• Shore D: sharp cone indenter with a 30° included angle and a rounded tip of 0.1 mm radius; applied force increases from zero to about 44.5 N across the scale (F = 445H, in mN)

Both durometer types feature a presser foot that contacts the specimen surface surrounding the indenter, ensuring that the measurement reflects the local resistance of the material and not deflection of the instrument.

Testing Samples

Soft and Flexible Plastic Materials (Shore A)

Shore A testing is appropriate for flexible plastics and thermoplastic elastomers, including flexible PVC, TPE, TPU, soft polyolefin compounds, silicone-based plastics, rubber-plastic blends, foam sheets, and elastomeric seals and gaskets. These materials typically exhibit hardness values ranging from around 20 to 95 Shore A.

Rigid and Hard Plastic Materials (Shore D)

Shore D testing is applied to rigid engineering plastics such as hard PVC (uPVC), HDPE, PP, PS, ABS, PMMA (acrylic), POM, PA (nylon), PC, and reinforced thermosetting compounds. Values for these materials commonly fall between 50 and 90 Shore D.

Acceptable Sample Forms

• Molded or machined test plaques and panels
• Extruded sheets, films, pipes, and profiles
• Injection-molded components and finished parts with suitable flat surfaces
• Calendared or calendered-laminated products

Sample Requirements

To obtain valid results, the test surface must be flat, smooth, and free of defects, contamination, and release agents. The specimen must be sufficiently thick so that the indenter does not sense the supporting surface. Where necessary, thinner layers may be stacked to reach the minimum thickness, although results on layered specimens may differ slightly from those on solid specimens and this must be noted in the report.

Testing Items

Accredited third-party laboratories typically offer the following Shore hardness test items for plastic materials:

• Shore A hardness (soft plastics and elastomeric materials)
• Shore D hardness (rigid plastics and hard thermoplastics)
• Hardness verification against material datasheets or product specifications
• Hardness measurement after thermal, humidity, or aging conditioning
• Comparative hardness evaluation for incoming material inspection and supplier qualification
• Hardness profiling across finished parts to assess cure, cure uniformity, or processing consistency

Testing Methods

Specimen Preparation and Conditioning

Specimens are conditioned in a standard laboratory atmosphere, typically 23 ± 2 °C and 50 ± 10% relative humidity in accordance with ISO 291, unless otherwise specified. Minimum thickness requirements differ between standards: ASTM D2240 specifies at least 6 mm for Shore A and at least 3 mm for Shore D, while ISO 868 recommends a thickness of at least 4 mm for both scales. The test area must be large enough to allow multiple indentations with adequate spacing.

Test Procedure

• Place the specimen on a rigid, flat, horizontal surface
• Apply the durometer perpendicular to the test surface with steady pressure and without shock
• Maintain firm contact between the presser foot and the specimen throughout the measurement
• Take the reading after the specified dwell time; ISO 868 specifies 15 seconds unless otherwise agreed, while ASTM D2240 permits an instantaneous reading or a specified time interval
• Perform at least five measurements at points spaced at least 6 mm apart and no closer than 12 mm from any specimen edge
• Report the median of the individual readings as the Shore hardness value

Expression of Results

The test report should identify the durometer scale used, the median hardness value, the specimen thickness, the dwell time, the conditioning atmosphere, and any deviations such as layered specimens or curved surfaces. Because viscoelastic plastics exhibit creep under the indenter, instantaneous readings are typically higher than delayed readings, so the dwell time must always be documented.

Instrument Verification

Durometers must be periodically verified for indenter geometry, spring force characteristics, and presser foot flatness, and checked against certified reference hardness blocks as required by ISO 868 and ASTM D2240. Regular verification ensures traceability and consistency of results between operators and laboratories.

Applications

Shore hardness testing serves a broad range of practical purposes in the plastics industry:

• Quality control: routine monitoring of extrusion, injection molding, and calendering processes to detect batch-to-batch variations
• Incoming inspection: verification that received resins, compounds, or semi-finished goods conform to purchase specifications
• Material selection and comparison: ranking candidate materials for stiffness and tactile properties during design
• Compliance demonstration: confirmation of hardness values declared in technical datasheets or contractual requirements
• Research and development: evaluation of formulation changes, plasticizer content, filler loading, and polymer blends
• Aging and durability studies: assessment of hardness changes after heat aging, UV exposure, chemical contact, or weathering
• Failure analysis: identification of degradation, over-plasticization, or incomplete curing in failed components

Typical product categories benefiting from Shore hardness testing include pipes and fittings, cable insulation and sheathing, seals and gaskets, footwear components, flexible tubing, rollers and wheels, instrument panels, packaging films, and medical device components made from elastomeric plastics.

FAQ

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

Shore A uses a blunter, truncated-cone indenter with lower spring force and is intended for soft, flexible materials, while Shore D uses a sharper conical indenter with higher spring force and is intended for rigid plastics. The two scales overlap in the middle range, and the appropriate scale is chosen based on the expected hardness of the material.

Which standard should be specified: ISO 868, ASTM D2240, or GB/T 2411?

All three standards describe fundamentally similar durometer procedures, but they differ in specimen thickness requirements, dwell time conventions, and reporting details. The choice usually depends on the market, customer specification, or regulatory framework involved; ISO 868 and GB/T 2411 are closely aligned, while ASTM D2240 is common in North American supply chains.

Can thin plastic films be tested for Shore hardness?

Very thin films are difficult to test accurately with a standard durometer because the indenter may sense the backing surface. Where possible, layers should be stacked to reach the minimum thickness, and the result should be reported as obtained on a layered specimen; alternative methods may be more suitable for very thin films.

Why do Shore hardness readings change over time under the indenter?

Plastics and elastomers are viscoelastic, so the indenter continues to penetrate the material under sustained load. Readings taken immediately (instantaneous hardness) are therefore higher than readings taken after a dwell period. Consistent timing, such as the 15-second interval specified in ISO 868, is essential for comparable results.

How is Shore hardness related to other hardness scales?

Shore hardness is not directly convertible to other scales such as Rockwell hardness (ASTM D785) or ball indentation hardness (ISO 2039-1), although approximate comparison tables exist for overlapping ranges. For very hard plastics, Rockwell or ball indentation methods typically provide better resolution, while Shore durometers remain the preferred method for soft and medium-hardness polymeric materials.

How reproducible are Shore hardness results?

With properly conditioned specimens, calibrated instruments, and trained operators, repeatability within a single laboratory is typically within one to two hardness units. Interlaboratory variation can be larger, particularly near the extremes of a scale, which is why standardized procedures and clear reporting of test conditions are critical.