Shore hardness testers
Durometer scale selection, specimen and timing controls, verification, and limits on interpreting Shore hardness.
Tags: inspection, hardness, polymers, durometers
On this page
What the tester measures
A Shore durometer measures indentation under a scale-specific indenter and force. Record the timing and contact arrangement. ZwickRoell: Shore hardness testing
PTC's Analog Pencil Style Durometers instructions, document 2192-7, provide a bounded example. They call for a clean, smooth specimen at least 6 mm thick, unless equivalent results with a thinner specimen are established, and an indenter position at least 12 mm from an edge, with a similar equivalence provision. The instructions permit stacking without glue but warn that stacked and solid specimens may differ. They specify 23 ±2 °C, with at least one hour at that temperature before testing. These are the cited product instructions, not a blanket recipe for all scales or standards. The PDF has a document identifier but no explicit revision date. PTC: instructions, page 2
ASTM D2240-15(2021) characterizes durometer hardness as an empirical control test influenced by elastic and viscoelastic behavior. Its public significance statement cautions that neither results from different durometer types nor hardness and fundamental material properties have a simple universal relationship. A Shore value therefore does not, by itself, establish tensile strength, modulus, wear life, or suitability for a seal application. ASTM: D2240 scope and significance
Select the scale and governing method
Specify the test standard and edition as well as the scale. ISO 48-4:2018, published August 2018 and confirmed in 2024, covers vulcanized or thermoplastic rubber. Its public scope identifies A for the normal-hardness range, D for high hardness, AO for low hardness and cellular rubber, and AM for thin rubber specimens in the normal-hardness range. ISO: ISO 48-4:2018
| Selection issue | Required distinction |
|---|---|
| Softer versus harder material | Choose the prescribed scale and useful range; do not compare equal numbers on A and D as equal hardness. |
| Thin specimen | Determine whether a thin-specimen method is required rather than assuming a conventional A or D test is valid. |
| Cellular or very soft material | Confirm the scale, indenter, support, and specimen requirements for that material. |
| Plastic versus rubber | Confirm the applicable material/test specification; ISO 868 and ISO 48-4 have different scopes. |
| Existing supplier result | Obtain scale, method, timing, and specimen conditions before comparing it with incoming inspection. |
ZwickRoell discusses ISO 868 for plastics and ebonite and distinguishes it from rubber testing under ISO 48-4. Its scale descriptions associate A with softer rubbers and elastomers and D with harder elastomers and plastics. Treat these as selection guidance rather than permission to substitute a method specified on a drawing. ZwickRoell: methods and scale applications
Specimen geometry, support, and conditioning
The presser foot needs appropriate contact with the specimen. Curvature, texture, a thin wall, an edge, or an underlying rigid support can change the tested response. Qualification should establish that the actual part geometry meets the selected method, or clearly identify the result as a separate product-specific check.
Record whether the specimen is a molded plaque, a cut section, stacked layers, or a finished component. Do not silently compare a curved production part with a flat reference plaque. Establish conditioning and material history appropriate to the product, including any time after molding or curing required by its specification.
Application and timing
Specify how the instrument approaches, how contact is established, and when the value is captured. A peak-hold indication and a reading at a defined elapsed time are different observations. PTC's instructions distinguish the maximum-reading pointer from a reading taken while the instrument remains motionless for a required interval. Its procedure also calls for multiple separated measurements. PTC: operating instructions
A stand can make approach and loading more repeatable, but the head and stand must suit the selected method. Bareiss's digi test II manual 00001103-B-EN, dated 8 April 2025, identifies different measuring devices for different scales and corresponding setup arrangements. The instrument's automation does not make those heads interchangeable or remove specimen preparation requirements. Bareiss: digi test II operating instructions, English section
Define the number and spacing of test locations and the required statistic from the governing procedure. Do not substitute a convenient mean for a specified median, or reuse an indentation because its location is easy to find. Preserve individual readings when investigating variation.
Bounded instrument example
PTC's undated product page for the 201A analog pencil-style Type A durometer lists a 0–100 point dial, manufacturer accuracy of ±1 point, and a maximum-hold pointer. It also offers compatible stands. The accuracy entry describes the instrument; it is not a guarantee that any operator, specimen, and timing combination produces a finished test result within one point. PTC: 201A product specifications
Illustrative reporting example: suppose an approved local procedure requires five readings at a specified dwell and their arithmetic mean. Invented readings of 68, 69, 69, 70, and 69 give a mean of 69. Report the scale, dwell, procedure, specimen, and individual values with that mean. The arithmetic does not establish method compliance or justify reporting the same value on another scale. Apply the procedure's rounding and acceptance rules separately.
Verification, calibration, and diagnosis
A reference check and full calibration have different purposes. PTC's product guidance separates a quick field check from complete checks of the mainspring and indenter and explicitly warns against using a test block as a calibration standard for adjusting the durometer. PTC: 201A calibration-check guidance
Bareiss's reference-block guide describes blocks as checks between instrument calibrations and states that their use does not replace calibration service. It identifies contamination, worn indenters, uneven surfaces, inclination, incorrect spacing, and environmental influences as possible causes of unexpected readings. The presser foot must sit flush under the prescribed conditions. Blocks themselves require controlled handling and condition assessment; a damaged reference is not a reliable basis for adjusting an instrument. Bareiss: reference-block operating guide
For repeatability studies, reproduce the actual loading, timing, operator, fixture, and specimen-location changes. Separate within-specimen variation from instrument variation. Assess uncertainty and the decision rule where results determine acceptance near a limit; display increments and a manufacturer accuracy rating are insufficient by themselves.
A useful report identifies the material and specimen geometry, scale and standard edition, instrument and stand, conditioning, timing, locations and readings, calculation, reference-check status, and acceptance criterion. This allows a later comparison without treating an isolated “Shore 70” as a complete material specification.