Plastic Hardness Testing Standards: Shore, Rockwell, Ball Indentation, and Barcol Methods

Plastic hardness testing standards define the instruments, procedures, and specimen requirements used to measure the indentation resistance of plastic and elastomeric materials. This technical article summarizes the major international and national standards—including ISO 868, ASTM D2240, ISO 2039, ASTM D785, and ASTM D2583—along with typical samples, testing items, methods, and practical applications. Use it as a professional guide to selecting the appropriate hardness method and interpreting results correctly.

Technical Overview

Hardness is one of the most frequently measured mechanical properties of plastics. It describes a material's resistance to localized deformation caused by a rigid indenter pressed into its surface under a defined force. Because polymers are viscoelastic, the measured hardness depends on the indenter geometry, applied load, dwell time, temperature, and specimen thickness, which is why standardized test methods are essential for obtaining comparable and reproducible data.

The principal families of plastic hardness testing standards are issued by ISO, ASTM, EN, GB, and JIS. The most widely applied methods include:

• Shore durometer hardness (ISO 868, ASTM D2240, GB/T 2411) — Shore A and Shore D scales
• Ball indentation hardness (ISO 2039-1, GB/T 3398.1) — for rigid and semi-rigid plastics
• Rockwell hardness (ISO 2039-2, ASTM D785, GB/T 3398.2, JIS K 7202) — R, L, and M scales
• Barcol impressor hardness (ASTM D2583) — for reinforced thermosetting plastics

Hardness values are dimensionless numbers specific to each scale and cannot be directly compared across different methods. Where conversion is required, only approximate empirical relationships should be used, and direct measurement according to the specified standard is always preferred.

Testing Samples

Molded Sheets and Plaques

Compression- or injection-molded flat plaques are the preferred specimen form. Surfaces must be flat, parallel, and free from sink marks, flash, or texture, since surface irregularities directly distort indentation readings. Typical minimum thickness is 4 mm to 6 mm depending on the standard, and thin films may be stacked in up to three unbonded layers for Shore testing.

Machined Test Blocks

For materials supplied as rods, plates, or semi-finished stock, test blocks can be machined to the required dimensions. Machined surfaces should be finished to a smooth condition, and any cutting-induced heat history or residual stress should be avoided because it can alter the near-surface properties of thermoplastics.

Finished Parts and Components

Hardness is often verified directly on molded or extruded products such as gaskets, tubing, rollers, trim panels, and housings. The test area must be supported so that the specimen does not deform internally during loading, and indentations must be placed sufficiently far from edges, ribs, and radiused regions.

Films and Thin Sheets

Flexible films require special attention because the indenter can sense the backing anvil. Sufficient thickness must be achieved by stacking identical layers, or a micro-hardness technique appropriate to thin specimens should be selected.

Unless otherwise specified, specimens should be conditioned in a standard laboratory atmosphere of 23 °C and 50 % relative humidity (ISO 291 or ASTM D618) before testing, since hardness is temperature-sensitive.

Testing Items

Typical hardness testing items requested by manufacturers, designers, and accredited third-party laboratories include:

• Shore A hardness (soft elastomers, flexible PVC, TPE, TPV, silicone)
• Shore D hardness (rigid and semi-rigid plastics such as PP, PE, PS, PMMA, nylon)
• Ball indentation hardness in N/mm² (rigid thermoplastics and thermosets)
• Rockwell hardness, scales HR-R, HR-L, and HR-M (engineering plastics such as PC, POM, PA, and phenolics)
• Barcol hardness (glass-fiber-reinforced polyester, SMC, BMC, and other thermosetting composites)
• Hardness retention after heat aging, UV exposure, or chemical immersion as part of durability studies
• Hardness profiling across cured or post-cured sections of thermoset parts

Testing Methods

Shore Durometer Method (ISO 868 / ASTM D2240 / GB/T 2411)

A spring-loaded indenter is pressed against the specimen by a presser foot. Shore A uses a truncated 35° cone with a 0.79 mm flat tip for softer materials, while Shore D uses a sharper 30° cone for harder plastics. The reading is taken after a defined dwell time—commonly 3 seconds in ISO 868, with 15 seconds used when creep is significant. A minimum of five readings is taken at points spaced well away from edges and from each other, and the median is reported. This method is fast, portable, and ideal for production quality control.

Ball Indentation Method (ISO 2039-1 / GB/T 3398.1)

A polished hardened steel ball of 5 mm diameter is pressed into the specimen under one of the standard loads (49 N, 132 N, 358 N, or 961 N). The load is selected so that the indentation depth falls within the specified range, and the depth is measured after a 30-second dwell. The ball indentation hardness is calculated with a correction term and expressed in N/mm², making it a physically defined value well suited to rigid thermoplastics and thermosets, as well as to datasheet comparisons.

Rockwell Method (ISO 2039-2 / ASTM D785 / GB/T 3398.2 / JIS K 7202)

A steel ball indenter is applied first with a minor load and then with a major load; the hardness value is derived from the difference in penetration depth. Common plastic scales are R (12.7 mm ball, 60 kgf), L (6.35 mm ball, 60 kgf), and M (6.35 mm ball, 100 kgf). Rockwell M is typically used for hard engineering plastics, while the R scale suits materials of medium hardness. Specimens must be thick and rigid enough to avoid anvil effects, and stacked layers should not be used.

Barcol Impressor Method (ASTM D2583)

A portable spring-loaded instrument with a sharp steel indentor is pressed onto the surface, and hardness is read directly on a 0–100 scale. Barcol testing is widely used for rigid reinforced thermosetting plastics and is a convenient field and in-process check for cure quality in composite manufacturing.

Applications

Plastic hardness testing supports a broad range of industrial and regulatory activities:

• Production quality control of extruded sheets, profiles, pipes, films, and molded components against specification limits
• Incoming material inspection to verify resin lots against supplier datasheets
• Research and development of polymer formulations, including plasticizer content in flexible PVC and filler loading effects
• Cure monitoring of thermosetting resins, coatings, and composite laminates
• Material selection and benchmarking for automotive interior trim, footwear, seals, gaskets, medical tubing, and consumer housings
• Durability assessment, such as hardness retention after thermal aging, weathering, or chemical exposure
• Compliance verification for product standards and customer technical specifications through accredited third-party laboratories

FAQ

Which hardness scale should be selected for my material?

As a general rule, Shore A applies to soft, rubber-like materials up to roughly 90 Shore A; Shore D applies to harder plastics and overlaps the upper Shore A range; ball indentation and Rockwell methods are suited to rigid engineering plastics; and Barcol testing is specific to reinforced thermosets. Materials near scale boundaries are often tested with both overlapping scales for confirmation.

Can Shore A results be converted to Shore D or Rockwell values?

No exact conversion exists because each method uses different indenter geometry and loading. Published conversion tables provide rough approximations only; for specification or certification purposes, the hardness must be measured directly in the required scale.

Are ISO 868 and ASTM D2240 equivalent?

The two standards are technically harmonized and results are generally comparable, but minor differences exist in default dwell times and minimum specimen thickness. The standard actually used should always be stated in the test report.

Why do hardness results vary between laboratories?

Common sources of variation include specimen thickness and support condition, test temperature, dwell time, indentation spacing from edges, surface texture, and instrument calibration. Adherence to conditioning requirements and regular durometer calibration minimize scatter.

How many measurements are required?

Most standards require at least five valid indentations per specimen, with the median value reported. For anisotropic or textured materials, additional measurements on both surfaces may be agreed upon.

Does a higher hardness number mean better scratch resistance?

Not necessarily. Indentation hardness, scratch resistance, and wear performance are distinct properties governed by different mechanisms, so hardness data should not be used alone to predict surface durability.