Plastic hardness testing measures a material's resistance to localized indentation and is one of the most widely performed quality control tests in the plastics industry. This step-by-step guide explains the complete plastic hardness test procedures for Shore durometer, Rockwell, ball indentation, and Barcol methods in accordance with ISO 868, ISO 2039, ASTM D2240, ASTM D785, and ASTM D2583 standards. From specimen preparation and conditioning to instrument operation and result reporting, these procedures help manufacturers and accredited third-party laboratories obtain accurate, repeatable hardness data.
Technical Overview
Hardness is a mechanical property that describes a plastic material's resistance to localized deformation, indentation, or penetration under an applied force. Unlike metals, plastics are viscoelastic materials, which means their hardness response depends on the magnitude of the load, the duration of loading, the ambient temperature, and the loading rate. For this reason, standardized plastic hardness test procedures strictly define the indenter geometry, the applied force, the loading sequence, and the reading time so that results remain comparable between laboratories.
The most commonly applied hardness test standards for plastics include:
- Shore durometer hardness — ISO 868, ASTM D2240, and GB/T 2411
- Rockwell hardness — ISO 2039-2 and ASTM D785
- Ball indentation hardness — ISO 2039-1
- Barcol impressor hardness — ASTM D2583
- Instrumented indentation (microhardness) — ISO 14577 for coatings and thin specimens
Hardness correlates loosely with elastic modulus and wear behavior, but it is not a direct measure of scratch resistance, tensile strength, or durability. Its main advantages are speed, low cost, minimal specimen preparation, and suitability for incoming material inspection, production quality control, and batch-to-batch consistency checks. Because results are time-dependent, any reported value is only meaningful when the method, scale, dwell time, specimen thickness, and conditioning state are all clearly documented.
Testing Samples
Specimen Geometry and Thickness
- The test surface must be flat and smooth, with parallel opposite faces, and free from sink marks, flash, voids, scratches, and flow defects.
- For Shore durometer tests, the specimen should be at least 6 mm thick; thin sheets may be stacked in up to three layers of the same material, without bonding agents, to reach the required thickness.
- For ball indentation hardness per ISO 2039-1, the specimen must be at least 4 mm thick.
- For Rockwell tests, the specimen must be thick enough that no visible bulge or marking appears on the supporting face after testing; otherwise the reading is invalid.
- For Barcol tests on reinforced thermosets, specimens are typically at least 1.5 mm thick and must be rigidly supported.
- Finished parts with curved surfaces should be tested on a flat machined or gated region, or a flat plaque molded under equivalent conditions should be used.
Surface Preparation
Molded surfaces should be tested as-produced whenever possible, since machining, sanding, or polishing can alter the surface layer and change the local stress state. If surface preparation is unavoidable, it must be applied consistently and noted in the test report. All specimens must be clean and free of release agents, oil, dust, and moisture.
Conditioning Before Testing
Specimens shall be conditioned in the standard laboratory atmosphere of 23 °C ± 2 °C and 50 % ± 10 % relative humidity for at least 40 hours, in accordance with ISO 291 or ASTM D618, unless the material specification states otherwise. Hygroscopic resins such as polyamide (nylon) show significantly different hardness values in the dry-as-molded state versus the moisture-conditioned state, so the conditioning history must be recorded along with the results.
Testing Items
Depending on material stiffness and the governing product specification, the following hardness parameters are routinely determined:
- Shore A hardness — soft thermoplastics, thermoplastic elastomers (TPE, TPU), flexible PVC, and rubber-like compounds
- Shore D hardness — rigid plastics such as HDPE, PP, PS, ABS, PMMA, PC, and POM
- Rockwell hardness (scales R, L, M, E, and K) — rigid and semi-rigid plastics where a defined preliminary-plus-total load sequence improves discrimination
- Ball indentation hardness H in N/mm² — engineering plastics, widely referenced in European automotive specifications
- Barcol hardness — thermosetting resins, glass-fiber-reinforced polyester laminates, and gel coats for cure monitoring
- Instrumented microhardness — thin coatings, films, and small specimens where macro-scale indenters cannot be applied
Testing Methods
Shore Durometer Procedure (ISO 868 / ASTM D2240)
The Shore method uses either a type A durometer, with a truncated 35° conical indenter and a 0.79 mm flat tip for soft materials, or a type D durometer, with a sharper 30° conical indenter for hard materials. The operating steps are as follows:
- Step 1 — Verify that the durometer is within calibration; check indenter geometry, spring force, and presser-foot condition, and confirm performance against certified reference blocks.
- Step 2 — Condition the specimen at 23 °C ± 2 °C and 50 % ± 10 % relative humidity for at least 40 hours, and perform the test in the same atmosphere.
- Step 3 — Select the correct durometer type: type A for soft, flexible materials; type D for readings above approximately 90 Shore A or for rigid plastics.
- Step 4 — Place the specimen on a rigid, flat, horizontal support surface so that the test point cannot deflect under the indenter.
- Step 5 — Apply the durometer vertically and perpendicular to the specimen surface without shock, keeping the presser foot parallel to and in full contact with the surface, using only enough hand pressure to ensure firm seating.
- Step 6 — Read the indicated value within the specified time interval, typically an instantaneous reading or a reading after a 15-second dwell, and always record which interval was used.
- Step 7 — Repeat the measurement at additional points, spaced apart and away from edges as required by the standard (at least 5–6 mm in ISO 868; at least 12 mm in ASTM D2240).
- Step 8 — Take at least five valid readings at different positions and report the median value, rounded to a whole number, together with the scale letter and dwell time.
Rockwell Hardness Procedure (ISO 2039-2 / ASTM D785)
The Rockwell method applies a 10 kgf preliminary load followed by a total load using a hardened steel ball indenter. The common plastic scales are R (60 kgf with a 12.7 mm ball), L (60 kgf with a 6.35 mm ball), M (100 kgf with a 6.35 mm ball), E (100 kgf with a 3.175 mm ball), and K (150 kgf with a 3.175 mm ball). The operating steps are:
- Step 1 — Select the Rockwell scale appropriate to the material stiffness and the product specification, choosing the combination that gives valid, non-saturating readings.
- Step 2 — Confirm the specimen thickness is sufficient; if any deformation is visible on the underside after testing, the result is invalid and a smaller ball, lower load, or thicker specimen must be used.
- Step 3 — Place the specimen securely on the anvil and apply the 10 kgf preliminary load without shock to seat the indenter and establish the zero datum.
- Step 4 — Apply the major load smoothly, within the time window defined by the standard, to reach the total load for the selected scale.
- Step 5 — Hold the total load for the specified dwell; for example, ASTM D785 Procedure A releases the major load after 15 seconds and reads the value after a further recovery period with the preliminary load retained, compensating for the creep behavior of plastics.
- Step 6 — Remove the major load while retaining the preliminary load, then read the hardness value from the dial or digital display at the prescribed moment.
- Step 7 — Position successive indentations far enough apart, and far enough from edges, that the deformation zones do not interact, as specified by the standard.
- Step 8 — Take at least five valid readings, report the median and range, and always identify the scale letter (for example, 85 Shore D equivalent procedures differ, so a Rockwell M value must be reported as HRM).
Ball Indentation Hardness Procedure (ISO 2039-1)
The ball indentation method presses a polished, hardened steel ball of 5 mm diameter into the specimen under an electronically or mechanically controlled load sequence and calculates hardness from the load and penetration depth. The steps are:
- Step 1 — Prepare a specimen at least 4 mm thick with a flat, smooth test surface, properly conditioned in the standard atmosphere.
- Step 2 — Mount the specimen rigidly on a flat support in the testing machine so that the load is applied normal to the surface.
- Step 3 — Bring the ball into contact and apply the preliminary force of 9.8 N, then set the zero reference for depth measurement.
- Step 4 — Increase the force to the selected test force (49 N, 132 N, 358 N, or 961 N) within 2 to 8 seconds, without shock.
- Step 5 — Maintain the test force for 30 seconds and record the penetration depth h; the depth must fall within the valid range of approximately 0.15 mm to 0.35 mm, so the test force may need to be adjusted to suit the material.
- Step 6 — Apply the machine-deflection correction defined in ISO 2039-1 (h′ = h − 0.04 mm) and calculate the ball indentation hardness as H = F / (π × D × h′), expressed in N/mm², where F is the test force, D is the ball diameter, and h′ is the corrected depth.
- Step 7 — Repeat the test at several positions spaced well apart and away from edges, then report the mean value, the test force used, and the scatter of results.
Barcol Hardness Procedure (ASTM D2583)
The Barcol impressor uses a spring-loaded, truncated steel cone and is particularly useful for thermosets and fiber-reinforced plastics, including in-service cure monitoring of gel coats. The steps are:
- Step 1 — Verify the instrument against certified high- and low-value reference blocks before each test session.
- Step 2 — Support the specimen on a rigid, flat backing plate thick and stiff enough to prevent deflection.
- Step 3 — Hold the instrument perpendicular to the surface and press down steadily until the presser foot is in full contact, using hand pressure only.
- Step 4 — Read the dial value immediately after full contact is achieved.
- Step 5 — Take at least ten readings at points distributed over the surface and spaced according to the standard, discarding any readings influenced by voids, fibers, or surface defects.
- Step 6 — Report the average to the nearest whole unit, noting that the method is most reliable in the range of roughly 25 to 100; softer materials should be tested with a Shore durometer instead.
Result Reporting and Documentation
A complete and traceable hardness test report should include the following elements, regardless of the method used:
- Material identification, grade, color, and lot or batch number
- Test method, standard edition, and scale (for example, Shore D with a 3-second or 15-second reading, Rockwell scale M, or ball indentation at 358 N)
- Specimen form, thickness, stacking details, and surface condition
- Conditioning atmosphere, duration, and moisture state where relevant
- Individual readings, the reported median or mean, and the scatter of the data
- Test temperature and relative humidity
- Instrument model, calibration due date, operator, and test date
Applications
Plastic hardness testing supports quality assurance, material qualification, and research across virtually every sector of the polymer industry. Typical applications include:
- Injection molding quality control for automotive interior trim, appliance housings, and consumer electronics enclosures
- Footwear manufacturing, where Shore A values of outsoles, midsoles, and TPU components define cushioning performance
- Pipes, fittings, and valves in PP, PE, and PVC-U, where hardness verifies compound consistency against EN and ISO product standards
- Wire and cable production, controlling the hardness of PVC and polyethylene insulation and jacketing compounds
- Medical device manufacturing, including catheter tubing, syringe components, and rigid device housings
- Seals, gaskets, and O-rings, where elastomer hardness directly affects compression and sealing behavior
- Composite fabrication, using Barcol readings to monitor the degree of cure of gel coats and glass-reinforced laminates
- Research and failure analysis, comparing materials before and after heat, UV, or chemical aging exposure
- Supply-chain verification, where buyers and accredited third-party laboratories confirm that incoming resins and finished parts meet contractual hardness specifications
FAQ
What is the difference between Shore A and Shore D hardness?
Shore A uses a blunter, truncated conical indenter with a lower spring force and is intended for soft, flexible plastics and elastomers. Shore D uses a sharper 30° conical indenter with a stiffer spring and is intended for rigid plastics. When a material reads above about 90 Shore A, the Shore D scale provides much better discrimination, which is why specifications switch scales in this region.
Can hardness values be converted between different scales?
Approximate comparison tables between Shore A, Shore D, Rockwell, and Barcol exist, but because each method applies different indenter geometries, loads, and dwell times, such conversions are indicative only. Acceptance decisions should always be based on testing performed with the exact method and scale stated in the specification.
What should I do if my specimen is thinner than 6 mm for a Shore test?
Thin sheets may be stacked in up to three layers of the same material, placed loosely on top of one another without adhesive, to reach the minimum thickness. Because trapped air and support effects can still bias readings slightly upward, stacked-specimen results should be clearly identified as such in the report.
Why is the reading time so important in plastic hardness testing?
Plastics creep under load, so the indenter continues to sink after initial contact. A reading taken after 15 seconds can be several points lower than an instantaneous reading on the same specimen. This is precisely why the standards define the measurement interval and why the dwell time must accompany every reported value.
How do temperature and humidity affect hardness results?
Raising the temperature generally lowers hardness because the polymer softens, and absorbed moisture can plasticize hygroscopic resins such as nylon, shifting readings significantly between the dry-as-molded and conditioned states. Testing is therefore performed at 23 °C and 50 % relative humidity after standardized conditioning, and the environment is recorded with the data.
How often should hardness instruments be calibrated?
Durometers, Rockwell machines, and Barcol impressors should be verified regularly against certified reference blocks, with indenter geometry and applied-force accuracy checked as specified in the applicable standard. The verification frequency is defined in the laboratory's quality management system, and full calibration by an accredited calibration provider is typically performed at least annually, or whenever the instrument is dropped or serviced.