A plastic hardness test report documents the measured indentation resistance of plastic materials using standardized methods such as Shore durometer, Rockwell, ball indentation, and Barcol testing. This article explains how plastic hardness tests are performed in accredited third-party laboratories, which international standards apply, how samples should be prepared and conditioned, and how engineers should interpret the reported values for quality control, material selection, and product compliance purposes.
Technical Overview
Hardness is one of the most frequently measured mechanical properties of polymeric materials. For plastics, hardness is defined as the resistance of the surface to indentation by a specified indenter under a defined load and time regime. It correlates loosely with other properties such as modulus, scratch resistance, wear behavior, and rigidity, which makes it a fast and economical indicator of material consistency in both production and qualification environments.
Unlike metals, plastics are viscoelastic: their indentation response depends on the magnitude of the applied force, the dwell time, the temperature, and the humidity of the test environment. A plastic hardness test report must therefore record not only the numerical hardness value but also the complete set of test conditions under which the value was obtained. Without this context, hardness data from different sources cannot be meaningfully compared.
Because indentation depth in a hardness test is localized to the surface layer, the method is classified as a quasi-non-destructive test in most practical situations, which allows it to be applied directly to finished components as well as to molded plaques and test coupons. Accredited third-party laboratories typically offer several hardness methods, and selecting the correct one depends on the stiffness of the material and the geometry of the part.
Testing Samples
Thermoplastic Materials
Standard engineering thermoplastics such as polyethylene (PE), polypropylene (PP), polystyrene (PS), ABS, rigid PVC, polycarbonate (PC), polymethyl methacrylate (PMMA), polyamide (PA), polyoxymethylene (POM), and polyethylene terephthalate (PET) are commonly tested using Shore D or Rockwell scales. Samples may be supplied as injection-molded plaques, extruded sheets, machined blocks, or representative sections cut from finished parts. The test surface must be flat, smooth, and free of sink marks, gate marks, flash, and texture, since surface irregularities directly distort the indentation measurement.
Elastomeric and Flexible Materials
Soft and flexible materials, including thermoplastic polyurethane (TPU), thermoplastic elastomers (TPE), silicone elastomers, flexible PVC compounds, rubber-plastic blends, seals, gaskets, tubing, and overmolded grips, are measured on the Shore A scale. Very soft gel-like materials may require the Shore 00 scale. For these materials, hardness is often the primary specification parameter, since it directly reflects formulation variables such as plasticizer content and hard-segment ratio.
Thermosets and Fiber-Reinforced Composites
Rigid thermosets, including epoxy and phenolic molding compounds, sheet molding compound (SMC), bulk molding compound (BMC), glass-fiber-reinforced laminates, and pultruded profiles, are typically evaluated using Barcol hardness or Shore D hardness. Barcol testing is particularly valuable for tracking the degree of cure of polyester and vinyl ester laminates, because under-cured surfaces register noticeably lower readings than fully cured surfaces.
Sample Preparation and Conditioning
Prior to testing, specimens are conditioned in a standard atmosphere of 23 ± 2 °C and 50 ± 5 % relative humidity in accordance with ISO 291 or ASTM D618, typically for a minimum of 24 to 40 hours depending on thickness. For Shore testing, a minimum specimen thickness of 6 mm is recommended; thinner sheets may be stacked, although not more than three layers should be used and stacked results should be flagged in the report. The combined thickness must be sufficient to prevent the indentation from being influenced by the supporting anvil.
Testing Items
A typical plastic hardness test report from an accredited third-party laboratory may include one or more of the following test items, each tied to a specific international standard:
- Shore A hardness (ISO 868, ASTM D2240) — soft elastomers and flexible plastics
- Shore D hardness (ISO 868, ASTM D2240) — hard and semi-rigid plastics
- Shore 00 hardness (ASTM D2240) — very soft gels and cellular materials
- Rockwell hardness, scales M, L, R, E, and K (ASTM D785, ISO 2039-2) — rigid engineering plastics
- Ball indentation hardness (ISO 2039-1) — results expressed in N/mm² for engineering design use
- Barcol hardness (ASTM D2583) — thermosets and fiber-reinforced composites
- Instrumented indentation and Martens hardness (ISO 14577) — thin specimens, coatings, and small parts
Testing Methods
Shore Durometer Hardness (ISO 868 / ASTM D2240)
A spring-loaded durometer indenter is pressed vertically onto the specimen surface. The Shore A indenter is a truncated 35° cone with a flat tip of 0.79 mm diameter, while the Shore D indenter is a 30° cone with a rounded tip of 0.1 mm radius. The hardness value, ranging from 0 to 100, is derived from the penetration depth: shallower penetration produces a higher reading. At least five readings are taken at points spaced at least 6 mm apart and at least 12 mm from any specimen edge, and the median value is reported. Readings are recorded after a defined dwell time, commonly 3 seconds per ISO 868, with optional extended readings at 15 seconds for materials that exhibit significant creep; the chosen dwell time is always stated in the report.
Ball Indentation Hardness (ISO 2039-1)
A hardened steel ball of 5 mm diameter is pressed into the specimen under a preliminary low load followed by a main test force selected from standardized levels of 49 N, 132 N, 358 N, or 961 N. The load level is chosen so that the resulting indentation depth falls within the range permitted by the standard, ensuring comparable data across materials. The indentation depth is measured after a fixed holding time, and the ball indentation hardness is calculated and expressed in N/mm². This method provides a hardness value with engineering units, which makes it useful for material comparison and design documentation.
Rockwell Hardness (ASTM D785 / ISO 2039-2)
A steel ball indenter of 3.175 mm (1/8 in), 6.35 mm (1/4 in), or 12.7 mm (1/2 in) diameter is applied first with a 10 kgf minor load to seat the indenter, then with a major load of 60, 100, or 150 kgf. The Rockwell number is derived from the difference in penetration depth between the minor and major load states. Common scales for plastics include the M, L, R, E, and K scales, with the M scale frequently applied to relatively hard engineering resins and the R scale to softer rigid plastics. Higher Rockwell numbers indicate harder materials, and the scale designation must accompany every reported value.
Barcol Hardness (ASTM D2583)
A Barcol impressor with a truncated conical steel indenter is pressed onto the specimen by spring force, and hardness is read directly from the 0 to 100 dial scale. The method is designed for rigid and semi-rigid thermosetting plastics and reinforced composites, and it is widely used for in-process verification of laminate cure as well as for laboratory qualification testing. Multiple readings are taken across the surface and the average is reported.
Instrumented Indentation and Microhardness (ISO 14577)
Instrumented indentation records the complete force-penetration curve during loading and unloading, from which parameters such as Martens hardness, indentation hardness, and indentation modulus are calculated. This approach is applied to thin coatings, small components, weld lines, and micro-scale features that cannot be evaluated with conventional bench methods.
Applications
Plastic hardness test reports support a broad range of industrial and engineering functions:
- Incoming material inspection — verifying that delivered resins, sheets, and molded components meet the hardness specified in purchase agreements and material datasheets
- Production quality control — monitoring batch-to-batch consistency of injection-molded parts, extruded profiles, films, tubing, rollers, seals, and overmolded handles
- Material selection and comparison — ranking candidate polymers or elastomer grades during product development
- Formulation development — quantifying the effect of plasticizer loading, filler content, copolymer ratio, or elastomer hardness grade in TPE and TPU compounds
- Cure verification — tracking the through-cure of thermoset laminates, SMC/BMC parts, and epoxy systems using Barcol or Shore D measurements
- Failure analysis — detecting hardening caused by plasticizer migration, oxidation, UV degradation, or post-curing in aged components
- Regulatory and customer compliance — supplying documented hardness evidence within certification and type-approval packages for automotive, medical, electrical, and consumer products
FAQ
What is the difference between Shore A and Shore D hardness?
Shore A uses a truncated cone indenter and is intended for soft, flexible materials such as elastomers and flexible PVC, typically reading below 90. Shore D uses a sharper, pointed indenter and is intended for hard plastics such as PP, PC, PMMA, and POM. The two scales overlap approximately in the region of 90 Shore A to 40 Shore D, which is the recommended transition zone where either scale may be applicable.
What sample thickness is required for hardness testing?
For Shore durometer testing, a minimum thickness of 6 mm is recommended so that the indentation is not affected by the test bench support. Thinner material may be stacked in up to three layers as a compromise, but stacked results should be identified in the report. For ball indentation and Rockwell testing, thickness requirements are generally greater, and the laboratory should confirm specimen dimensions before testing.
Why is dwell time important in a hardness test report?
Plastics creep under load, so the indenter continues to penetrate the specimen during the measurement. A reading taken at 3 seconds will therefore be higher than one taken at 15 seconds on the same material. Any valid comparison between laboratories or against a specification must confirm that the same dwell time was used.
Can hardness values be converted between Shore, Rockwell, and Barcol scales?
No exact conversion exists. Because each method uses a different indenter geometry, load level, and time regime, approximate cross-reference tables should only be used as rough guidance. When a specification cites a particular scale, testing must be performed on that scale rather than converted from another.
How many measurements are taken and how are results reported?
Shore methods require at least five valid readings, with the median reported in accordance with ISO 868 and ASTM D2240. The report lists the test standard, durometer type, dwell time, specimen thickness, conditioning history, laboratory temperature and humidity, individual readings, and the statistical result, together with sample identification, test date, and instrument calibration status.
How long does plastic hardness testing take?
The indentation measurement itself is rapid, and for specimens already conditioned to the standard atmosphere, results are typically available within one working day. When full conditioning of 24 to 40 hours is required before testing, the overall turnaround extends accordingly, usually to two or three working days.
