How to Compare Shore A Hardness Samples for Custom Rubber Parts

A sample marked 60 Shore A gives a buyer a useful starting point for discussing firmness. It leaves several purchasing questions open: which compound was tested, how the reading was taken, and whether the finished part will meet its sealing or assembly requirements. A structured sample comparison helps turn a request for “the same softness” into a specification that suppliers can quote and inspectors can check.

Understand what the hardness value describes

ASTM D2240, Standard Test Method for Rubber Property—Durometer Hardness, measures resistance to indentation under defined conditions. Its significance-and-use section explains that results depend on the material’s elastic and viscoelastic response. The method is primarily an empirical control test; a hardness number has no simple relationship to every other material property.

Consequently, two compounds with similar Shore A readings still need separate evidence for tensile behavior, compression set and fluid exposure. The number also cannot, by itself, predict the installation force or sealing performance of a particular geometry.

Agree on the comparison method before requesting samples

Specify the test method and scale together. ISO 48-4:2018, Rubber, vulcanized or thermoplastic—Determination of hardness—Part 4: Indentation hardness by durometer method (Shore hardness), includes the A scale and separate scales for other hardness ranges or specimen types. Avoid treating readings from different scales as directly interchangeable.

Ask both parties to record:

• The compound identifier, sample batch and sample preparation history.
• The test-piece shape, thickness, measurement location and supporting surface.
• The conditioning and test temperatures, and the time between applying the indenter and recording the result.
• The instrument identification, calibration status and whether a test stand was used.
• The individual readings and the reporting method required by the agreed procedure.

A thin sealing lip or curved ring may be unsuitable for the same measurement arrangement used on a flat laboratory plaque. Agree on suitable specimens or an appropriate alternative method. If plaques are supplied alongside parts, document how they relate to the production compound and cure conditions.

Define the purpose of each sample set

For an initial firmness trial, request a small selection of agreed hardness grades within the same material family. Keep the part geometry and evaluation procedure consistent, and ask the supplier to identify formulation differences. This makes the trial easier to interpret, although changing hardness may also change other properties.

For a proposed material substitution, compare the full application specification. An EPDM sample, a silicone sample and a flexible PVC sample with similar hardness labels should each have their own material identification and performance evidence. Record why the alternative is being considered, such as assembly behavior or exposure conditions.

Connect hardness to the tests that matter in service

For a part held under compression, ASTM D395, Standard Test Methods for Rubber Property—Compression Set, provides relevant methods for evaluating recovery after specified compressive exposure. Agree on the method, temperature, duration and acceptance limit.

For tensile requirements, ASTM D412, Standard Test Methods for Vulcanized Rubber and Thermoplastic Elastomers—Tension, is relevant within its material scope. For liquid exposure, ASTM D471, Standard Test Method for Rubber Property—Effect of Liquids, evaluates changes associated with immersion. Select applicable tests with the supplier rather than applying every rubber test automatically to every soft polymer.

Also evaluate representative parts in the assembly: insertion and removal, compression, interference, visible damage and the required functional test. Define the fixture and pass criteria before comparing results.

Build a reviewable RFQ and approval record

Include the drawing revision, material family, target hardness and tolerance, test method, operating environment, critical dimensions, sample quantity and intended annual demand. Identify which values are requirements and which are provisional targets awaiting trials.

After evaluation, keep the approved sample identifier, compound specification and test report together. Describe the reference sample’s role, such as appearance, fit or functional performance, so later orders have a clear acceptance basis.

To discuss a sample request, use the IKSONIC contact page and provide the application requirements with the proposed comparison plan.