Conventional Hardness Testing of Plastics: Methods, Standards, and Applications

Plastic hardness testing is a fundamental quality control measurement that evaluates a material's resistance to localized surface deformation and indentation. This technical guide covers the conventional hardness test methods for plastics—including Shore durometer, Rockwell, ball indentation, and Barcol hardness—along with the relevant ISO, ASTM, GB, and JIS standards, sample requirements, and typical industrial applications. Whether you are qualifying rigid engineering thermoplastics or flexible elastomeric compounds, understanding these standardized procedures helps ensure accurate, repeatable, and comparable hardness data.

Technical Overview

Hardness is one of the most frequently measured mechanical properties of plastic materials. It describes the ability of a polymer surface to resist indentation, scratching, or penetration by a harder indenter under a defined load. Although hardness is not a fundamental material constant—it depends on the test method, indenter geometry, load, dwell time, and temperature—it remains a fast, economical, and highly informative indicator of material consistency, cure or molding state, and suitability for a given application.

Because plastics span an enormous stiffness range—from ultra-soft thermoplastic elastomers to glass-fiber-reinforced engineering resins—no single hardness scale can cover all materials. For this reason, international standards organizations such as ISO, ASTM, and the Chinese national standardization bodies have developed a family of complementary test methods. The most widely applied conventional methods are Shore hardness (Shore A and Shore D), Rockwell hardness, ball indentation hardness, and Barcol hardness. Selecting the correct method is essential: a scale suited to soft elastomers will bottom out on rigid plastics, while a scale designed for rigid materials will lack sensitivity on flexible compounds.

Hardness data are typically used for incoming material inspection, batch-to-batch quality control, comparison of competing grades, verification of post-processing effects (annealing, conditioning, plasticizer migration), and specification compliance in industries such as automotive, construction, electronics, medical devices, and consumer products.

Testing Samples

Molded or Extruded Flat Specimens

Flat plaques, sheets, or compression-molded panels are the preferred sample form for hardness testing. For Shore durometer testing, ISO 868 and ASTM D2240 require a minimum thickness of approximately 4 mm to 6 mm depending on the standard, so that the indenter cannot sense the support surface beneath the specimen. Thinner specimens may be stacked to reach the required thickness, provided the layers are free of gaps and cannot slip against each other. The test surface must be flat, smooth, and free of sink marks, gates, weld lines, or texture that could bias the reading.

Finished Parts and Components

Hardness may also be measured directly on manufactured articles such as seals, gaskets, hoses, handles, trims, tubing, and housings. When testing finished parts, the measurement location must offer adequate material thickness and a sufficiently flat, supported area. Curved surfaces should be avoided or tested with appropriate fixtures, since curvature changes the contact geometry between the presser foot and the specimen. Readings should be taken at least 9 mm to 12 mm away from edges, ribs, or other discontinuities.

Films, Sheets, and Coatings

Thin films and coatings generally exceed the capability of macro-scale hardness methods. When the layer thickness is insufficient for Shore or Rockwell testing, laboratories either stack multiple film layers, test the coating on its substrate, or apply microhardness techniques. Sample preparation for such cases must be documented, because measured values can shift significantly with backing and layer count.

Soft Elastomers and Rubber-Plastic Blends

Thermoplastic elastomers (TPE), thermoplastic polyurethanes (TPU), flexible PVC, and rubber-modified compounds are conventionally tested on the Shore A scale. These materials are viscoelastic, so their readings depend strongly on dwell time; specimens must therefore be conditioned and tested under a strictly controlled procedure to obtain comparable results.

Testing Items

Conventional plastic hardness testing services typically cover the following measurement items, each matched to a specific stiffness range and governed by recognized international or national standards:

• Shore A hardness (soft elastomers, flexible PVC, TPE, TPV) — ISO 868, ASTM D2240, GB/T 2411
• Shore D hardness (hard plastics: HDPE, PP, PS, PA, PC, POM, rigid PVC) — ISO 868, ASTM D2240, GB/T 2411
• Rockwell hardness, scales M, R, L, K, E (rigid and semi-rigid thermoplastics, thermosets) — ASTM D785, ISO 2039-2, GB/T 3398.2
• Ball indentation hardness H (engineering plastics, thermosetting materials) — ISO 2039-1, GB/T 3398.1
• Barcol hardness (glass-reinforced plastics, composites, cured resins) — ASTM D2583
• Additional durometer scales (Shore C, DO, O, E, and others) for intermediate or specialized materials per ASTM D2240

Testing Methods

Specimen Conditioning and Test Environment

Prior to testing, specimens are conditioned in a standard laboratory atmosphere—commonly 23 °C ± 2 °C and 50% ± 10% relative humidity in accordance with ISO 291 or ASTM D618—for a period of at least 40 hours unless otherwise specified. Because polymer hardness is temperature-dependent, all measurements must be performed in the same controlled environment in which the specimens were conditioned, and the test temperature must be reported with the results.

Shore Hardness (Durometer) Testing

Shore hardness is the most common routine method for plastics and elastomers. A spring-loaded indenter is pressed against the specimen by a presser foot: the Shore A indenter is a truncated 35° cone with a flat tip, intended for soft materials, while the Shore D indenter is a sharp 30° cone for hard materials. The penetration depth of the indenter relative to the presser foot is converted to a scale from 0 (no resistance) to 100 (no measurable penetration).

Measurements are taken at multiple points spaced far enough apart to avoid interference from previous indentations—typically at least 6 mm apart and at least 9 mm to 12 mm from any edge. ISO 868 specifies the reading after 15 seconds of load application unless otherwise agreed, whereas ASTM D2240 permits instantaneous (approximately 1 second) readings or other specified times; for viscoelastic materials, longer dwell times yield lower values. The median of at least five readings is reported together with the scale (for example, "Shore D 78, 15 s") and the test temperature.

Rockwell Hardness Testing

Rockwell hardness, performed per ASTM D785 and ISO 2039-2, is used for rigid plastics where Shore D approaches saturation. A steel ball indenter (diameters of 12.70 mm, 6.35 mm, or 3.175 mm depending on the scale) is first applied with a minor load, then with a major load, and the hardness value is derived from the difference in indentation depth. Common scales for plastics include the R scale (60 kgf major load, 12.70 mm ball), the M scale (100 kgf, 6.35 mm ball), and the L, E, and K scales for other stiffness ranges. ASTM D785 defines two procedures: one in which the depth is read after a 15-second dwell under the major load, and one in which the indenter is returned to the minor load before reading to account for elastic recovery. Specimens must be flat, parallel, and thick enough that the anvil does not influence the result, and results are only comparable within the same scale.

Ball Indentation Hardness Testing

Ball indentation hardness, standardized in ISO 2039-1 and GB/T 3398.1, is widely used in Europe and Asia for engineering plastics and thermosets. A polished hardened steel ball of 5 mm diameter is pressed into the specimen surface under a specified load—selected from 49 N, 132 N, 358 N, or 961 N—so that the corrected indentation depth falls within the standard range. The hardness value H is expressed in N/mm² and is calculated from the applied force and the corrected indentation depth, giving a result in physical units rather than an arbitrary scale. This provides a hardness value that can be correlated with other mechanical data and is particularly valuable for material datasheets and international comparisons.

Barcol Hardness Testing

Barcol hardness per ASTM D2583 is applied to rigid reinforced plastics, fiber-reinforced composite panels, and cured thermosetting resins. A spring-loaded sharp steel indenter is pressed onto the surface, and the penetration depth is read directly on a 0–100 dial scale. The method is popular in composite fabrication because it provides a rapid, portable indication of cure completion: under-cured resin systems register lower Barcol values, so the test is routinely used for in-process quality control of laminates and gel coats.

Method Selection Guidance

• Use Shore A for soft, flexible, and elastic materials typically below approximately 90 Shore A.
• Use Shore D for hard plastics; materials above approximately 90 Shore D are better characterized by Rockwell or ball indentation.
• Use Rockwell (M, R, L, K, E scales) for rigid thermoplastics and thermosets requiring finer discrimination at the high end of hardness.
• Use ball indentation hardness when a hardness value in engineering units (N/mm²) is required for design or datasheet purposes.
• Use Barcol hardness for glass-reinforced laminates, composites, and cure-state monitoring.

Applications

Conventional hardness testing supports quality assurance, material selection, and failure analysis across a broad range of industries. Typical applications include:

• Automotive: interior trim, seals, gaskets, instrument panel skins, tires, bushings, and under-hood components
• Construction: PVC pipes and fittings, window profiles, flooring, waterproofing membranes, and cable jackets
• Consumer goods: handles, footwear components, toys, sporting goods, and appliance housings
• Electrical and electronics: connector housings, insulators, cable insulation, and switch components
• Medical devices: tubing, catheters, seals, syringe components, and equipment housings
• Packaging: rigid containers, closures, and film materials assessed for consistency
• Composites manufacturing: cure verification of laminates, gel coats, and resin systems via Barcol hardness

Accredited third-party laboratories routinely perform these tests for incoming material verification, supplier qualification, regulatory submission support, and comparison against manufacturer datasheet values.

FAQ

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

Answer: Shore A uses a blunter, truncated-cone indenter and is intended for soft, flexible materials such as elastomers and flexible PVC. Shore D uses a sharper cone indenter and is intended for hard plastics. The two scales overlap in the middle range—roughly 90 Shore A corresponds to approximately 40 Shore D—but they are not directly convertible, and results must always be reported with the scale used.

Question: Can Shore hardness values be converted to Rockwell or ball indentation values?

Answer: No exact conversion exists. Different methods use different indenters, loads, and dwell conditions, so hardness values are only comparable within the same scale and test conditions. Published conversion tables provide rough approximations for specific material families and should never be used for specification or acceptance purposes.

Question: Why does the specimen thickness matter so much?

Answer: If the specimen is too thin, the indenter penetration is artificially restricted by the supporting anvil beneath it, producing falsely high readings. Standards therefore specify minimum thickness (approximately 4 mm to 6 mm for Shore testing) and permit stacking of thin, clean layers that do not slip, although stacked results should be noted in the report.

Question: How does temperature affect the results?

Answer: Polymer hardness decreases as temperature increases, particularly for materials near their glass transition. All specimens must be conditioned and tested at the standard laboratory atmosphere of 23 °C and 50% relative humidity per ISO 291 or ASTM D618, and the test temperature must be recorded.

Question: How many measurements are taken, and how is the result reported?

Answer: For Shore testing, a minimum of five readings at different, well-separated points is standard, and the median value is reported along with the scale, dwell time, and temperature. For Rockwell and ball indentation methods, the number of indentations and the averaging convention of the applicable standard apply.

Question: Which method should be chosen for a glass-filled engineering resin?

Answer: Rigid, filled, or reinforced plastics are normally evaluated using Rockwell hardness (commonly the M or R scale), ball indentation hardness per ISO 2039-1, or Barcol hardness for laminated composites. Shore D can be used for screening, but it loses resolution at the upper end of the scale.