Plastic hardness testing is a fundamental experimental procedure used to evaluate the resistance of polymeric materials to localized plastic deformation, scratching, and indentation. This technical guide provides a comprehensive overview of hardness determination methods for plastics, including Shore durometer, Rockwell, ball indentation, and microhardness techniques, along with applicable international standards such as ISO 868, ASTM D2240, and ISO 2039. Whether you are a materials engineer, quality control specialist, or researcher, this article will help you select the appropriate hardness test method and understand the factors that influence measurement accuracy.
Technical Overview
Hardness is one of the most frequently measured mechanical properties of plastic materials, providing a rapid and non-destructive indication of a material's resistance to surface deformation under an applied force. Unlike metals, plastics are viscoelastic materials whose hardness response depends strongly on time, temperature, humidity, and loading rate. Consequently, a plastic hardness determination experiment must be conducted under carefully controlled environmental conditions and in strict accordance with standardized test protocols to yield reproducible and comparable results.
Hardness values for plastics are not absolute physical constants; rather, they are relative indicators derived from the measured depth or recovered area of an indentation produced by a standardized indenter under a specified load and dwell time. The most widely recognized hardness scales for polymeric materials include the Shore A and Shore D durometer scales, Rockwell hardness scales (typically R, L, M, and E), ball indentation hardness, Barcol hardness, and microindentation (Vickers and Knoop) hardness. Each method occupies a different portion of the hardness spectrum and is suited to specific classes of materials, from extremely soft elastomers to rigid engineering thermoplastics and thermosets.
In an industrial context, hardness testing serves multiple purposes: incoming material verification, batch-to-batch quality control, comparison of competitive materials, assessment of curing or post-curing completion in thermosets, evaluation of aging and environmental degradation, and support for research and development of new polymer formulations. Accredited third-party laboratories routinely perform these determinations under internationally recognized standards to provide defensible, comparable data for manufacturers, buyers, and regulatory bodies.
Testing Samples
Proper sample preparation is critical to the validity of any plastic hardness determination experiment. The test specimens described below represent the principal categories of samples submitted for hardness evaluation.
Molded Thermoplastic Plaques and Plates
Injection-molded or compression-molded flat plaques are the most common specimen form. For Shore hardness testing, a minimum thickness of 6 mm (or a stacked total of at least 6 mm for thinner sheets, without bonding between layers) is generally required so that the indenter penetration is not influenced by the supporting surface. The test surface must be flat, smooth, and free of sink marks, flow lines, flash, and surface contamination, since surface irregularities directly distort the indentation measurement.
Soft Elastomeric and Rubber-Like Materials
Flexible thermoplastics, thermoplastic elastomers (TPE), thermoplastic polyurethanes (TPU), silicone rubbers, and vulcanized rubber compounds are typically evaluated on the Shore A scale. These materials require special attention to thickness, as insufficient specimen thickness causes artificially elevated readings. Specimens should be conditioned at standard laboratory atmosphere (typically 23 ± 2 °C and 50 ± 5% relative humidity) for a minimum of 40 hours prior to testing.
Rigid Engineering Plastics and Thermosets
Materials such as polycarbonate (PC), polyamide (PA), polyoxymethylene (POM), polyethylene terephthalate (PET), phenolic molding compounds, epoxy castings, and melamine-formaldehyde products are commonly tested using Shore D, Rockwell, or ball indentation methods. For ball indentation hardness per ISO 2039-1, the specimen must be at least 4 mm thick and possess two parallel, flat faces to ensure uniform load application.
Finished Parts and Components
When testing actual products—such as seals, gaskets, hoses, gears, housings, footwear components, or medical devices—the test area must be sufficiently flat and supported to prevent deflection or bending during force application. Curved or irregular surfaces should be avoided unless the geometry is accounted for, and clamping fixtures may be used to stabilize the part.
Films, Thin Sheets, and Coatings
Thin polymeric films, coatings, and paint layers are evaluated using microindentation techniques (Vickers or Knoop) under low loads, or by instrumented indentation testing per ISO 14577. Specimens must be securely mounted, typically on a rigid substrate, and cross-sections may be polished when through-thickness profiling of a coating is required.
Testing Items
A typical plastic hardness determination program includes the following measurable parameters and reported items:
- Shore hardness (Shore A, Shore D, and, where applicable, Shore AO, DO, AM, and Shore 00 scales) in accordance with ISO 868 and ASTM D2240.
- Rockwell hardness of plastics on scales R, L, M, and E in accordance with ISO 2039-2 and ASTM D785.
- Ball indentation hardness (H) in MPa (N/mm²) in accordance with ISO 2039-1 and GB/T 3398.1.
- Barcol hardness for reinforced thermosets and rigid plastics in accordance with ASTM D2583.
- Vickers hardness (HV) and Knoop hardness (HK) for micro-scale evaluation in accordance with ISO 6507, ASTM E92, and ASTM E384 as adapted to polymeric materials.
- Instrumented indentation modulus and hardness (ISO 14577) for advanced characterization of films and coatings.
- Hardness retention after thermal aging, UV exposure, chemical immersion, or sterilization, expressed as a percentage of the initial value.
- Statistical treatment of results, including mean value, standard deviation, and number of valid indentation points, with data typically reported to one decimal place or per the requirements of the governing standard.
Testing Methods
The choice of method depends on the expected hardness range of the material, the specimen geometry, and the requirements of the product specification. The following sections summarize the principal experimental procedures.
Shore Durometer Method (ISO 868 / ASTM D2240 / GB/T 2411)
A spring-loaded indenter is pressed against the specimen under a defined force. For Shore A, a truncated 35° conical indenter with a 0.79 mm diameter flat tip and an 822 g major load is used; for Shore D, a 30° conical indenter with a 0.1 mm radius tip and a 4536 g major load is applied. The reading is taken after a specified dwell time—commonly 15 seconds for standard readings or 1 second for "instantaneous" readings—with the durometer either handheld, mounted on a stand, or operated as a motorized bench instrument. At least five measurement points, spaced at least 6 mm from each other and at least 12 mm from the specimen edge, are averaged. Shore A is suitable for soft materials up to approximately 90 Shore A; above that range, Shore D should be used, which covers approximately 90 Shore D and above down to the transition from the A scale.
Rockwell Hardness Method (ISO 2039-2 / ASTM D785)
A hardened steel ball indenter (with diameters of 3.175 mm, 6.350 mm, or 12.700 mm depending on the scale) is first applied with a minor load of 10 kgf, followed by a major load (60, 100, or 150 kgf depending on the chosen scale). After a defined dwell period, the major load is removed while the minor load is retained, and the hardness value is derived from the difference in indentation depth before and after loading. The Rockwell R scale (12.7 mm ball, 60 kgf) is the most frequently used scale for plastics such as ABS, polystyrene, and polyolefins. Because plastics exhibit viscoelastic recovery, strict control of dwell time and load application rate is essential for reproducibility.
Ball Indentation Hardness Method (ISO 2039-1 / GB/T 3398.1)
A polished hardened steel ball of 5 mm diameter is pressed into the specimen surface under a preliminary load, after which the test load is applied for 30 seconds and the depth of indentation is measured. The ball indentation hardness H is calculated as the test force divided by the surface area of the conical (calculated) indentation, corrected by a factor that accounts for the real projected area of the spherical indentation. The result is expressed in MPa. This method provides a hardness value with a more direct physical interpretation than durometer scales and is particularly valuable for rigid plastics in European and international commerce.
Barcol Hardness Method (ASTM D2583)
A spring-loaded truncated conical indenter is pressed into the surface of rigid or semi-rigid plastics, including glass-fiber-reinforced thermoset laminates and panels. The instrument reads directly on a 0–100 scale. The method is widely used for quality control of cured composite panels, boat hulls, and glass-reinforced polyester sheets, and it also serves as an indirect indicator of cure completeness.
Microindentation Methods (Vickers and Knoop)
A diamond indenter of defined geometry is forced into the specimen under low loads, typically ranging from 10 gf to 1 kgf for polymeric materials. After unloading and a recovery interval, the diagonals of the indentation are measured optically, and the hardness number is calculated from the load divided by the indentation surface or projected area. Vickers uses a square-based pyramidal indenter with a 136° face angle, whereas Knoop uses an elongated rhombic pyramid suited to thin layers and anisotropic materials. These methods are appropriate for small specimens, individual phases in multiphase systems, coatings, and hardness profiling across cross sections.
General Experimental Precautions
- Condition all specimens at 23 ± 2 °C and 50 ± 5% relative humidity for at least 40 hours, or as specified, because hardness is temperature-sensitive.
- Verify and calibrate instruments regularly using certified reference blocks and calibrated dead-weight or force-measurement devices.
- Ensure specimen thickness exceeds the minimum requirement for the chosen method so that the supporting anvil does not influence the reading.
- Apply the indenter perpendicular to a flat, supported surface; angled contact produces invalid readings.
- Space indentations adequately to avoid interaction between stress fields of adjacent impressions.
- Report the exact method, scale, dwell time, temperature, humidity, specimen thickness, and number of readings together with the hardness values, since results from different scales or conditions are not directly interchangeable.
Applications
Plastic hardness determination experiments support a broad spectrum of industrial and scientific applications:
- Quality control of incoming resins, compounds, and molded parts against purchasing specifications and datasheet values.
- Material selection and comparative evaluation of candidate plastics for gears, bearings, rollers, wear strips, and sliding components where surface resilience matters.
- Verification of curing degree in thermosetting resins, epoxy castings, composite laminates, and potting compounds, where under-cure manifests as depressed hardness.
- Assessment of aging resistance after heat aging, weathering, UV exposure, hydrolysis, or chemical contact by tracking hardness retention over time.
- Footwear, seal, gasket, and hose industries, where Shore A and Shore D values are contractual quality parameters governing flexibility and fit-for-performance behavior.
- Medical device and pharmaceutical packaging sectors, where hardness consistency supports device function, biocompatibility screening programs, and regulatory submissions.
- Automotive and aerospace interior and exterior components, where hardness correlates with scratch resistance, tactile quality, and long-term durability.
- Failure analysis and research, where microhardness mapping reveals degradation gradients, phase segregation, weld-line weaknesses, and processing-induced property variations.
FAQ
Which hardness method should I choose for my plastic material? As a general guideline, use Shore A for soft, flexible materials below roughly 90 Shore A, Shore D for harder plastics that exceed the Shore A range, Rockwell or ball indentation for rigid engineering plastics and thermosets, Barcol for reinforced thermoset panels, and Vickers or Knoop microindentation for thin films, coatings, and small specimens. When a specification cites a particular standard, that standard governs the method selection.
Are Shore A, Shore D, Rockwell, and ball indentation values directly convertible? No. Hardness scales measure deformation under different indenter geometries, loads, and time regimes, so only approximate empirical correlations exist. For contractual or regulatory purposes, the hardness must be measured using the exact method and scale specified in the applicable document.
Why do my hardness readings vary between measurements? Common causes include insufficient specimen thickness, curved or textured surfaces, temperature deviation from the standard atmosphere, inadequate conditioning time, inconsistent dwell time, indentation too close to an edge or a previous indentation, and instrument calibration drift. Addressing these factors typically reduces scatter substantially.
How thick must a specimen be for a valid Shore hardness test? Under ISO 868 and ASTM D2240, the specimen should be at least 6 mm thick. Thinner sheets may be stacked to reach this thickness, provided the layers do not bond and the surface layer fully supports the indentation. For other methods, the specific standard's minimum thickness requirement—such as 4 mm for ball indentation hardness—must be observed.
Does hardness correlate with other mechanical properties of plastics? Hardness shows useful qualitative correlations with modulus, scratch resistance, and wear behavior within closely related material families, but it cannot substitute for tensile, flexural, or impact testing. It is best treated as a rapid comparative indicator rather than a predictor of bulk mechanical performance.
Can hardness testing be performed on finished products in place of prepared specimens? Yes, provided the test area is flat, adequately supported, thick enough for the chosen method, and representative of the material. Testing on curved, ribbed, or thin sections of finished parts may produce invalid results and should be validated against measurements on standard specimens.
