Shore Durometer Hardness: Type A vs Type D Explained
Representative Infinita Engineering Visual explaining the four-step workflow for Shore Durometer Hardness Type A D.What Is Shore Durometer Hardness Testing?
ASTM D2240 measures the indentation hardness of rubbers, elastomers, and plastics using a durometer — a spring-loaded indenter that measures how far a standardised presser foot penetrates the material under a defined force. The depth of indentation is inversely related to hardness: softer materials allow deeper penetration and register a lower Shore value. ASTM D2240 defines twelve total scales (A, B, C, D, DO, E, M, O, OO, OOO, OOO-S, and R), but Type A and Type D are by far the most common in industrial use.
Type A vs. Type D: Key Differences
- Type A: uses a blunted, truncated cone indenter under a maximum force of 8.05 N (at a reading of 100); intended for softer, pliable materials — rubber seals, flexible silicones, TPEs, and soft plastics
- Type D: uses a sharp, pointed 30-degree conical indenter under a maximum force of 44.45 N (at a reading of 100); intended for harder materials — rigid plastics, safety helmets, and hard polymers like PTFE and PEEK
- At an identical reading of 50, Type A corresponds to roughly 4.3 N of applied force while Type D corresponds to roughly 22.2 N — the same numeric reading means something very different depending on the scale
Test Procedure
- Ensure the specimen is at least 6 mm (0.25 in) thick; thinner samples may require stacking multiple layers to avoid the anvil influencing the reading
- Confirm the durometer’s contact surface is parallel to the specimen table before testing
- Apply the durometer to the specimen surface in a consistent manner, without shock or sudden impact
- For initial hardness, record the reading immediately upon full contact; for timed hardness, hold the force for the specified duration (commonly 15 seconds) before reading
- Take multiple readings at different locations on the specimen, spaced far enough apart to avoid one indentation influencing the next, and report the median or average
Selecting the Right Scale
Choosing the wrong scale produces misleading results: testing a soft elastomer on the Type D scale (sharp indenter, high force) can over-penetrate and bottom out the reading, while testing a rigid plastic on Type A can barely register a dent at all. As a general rule, if a Type A reading exceeds approximately 90, or a Type D reading falls below approximately 20, switching to the adjacent scale gives a more accurate and repeatable result.
Industry Specifications Referencing Shore Hardness Testing
- ASTM D2240: twelve-scale US standard covering rubbers, elastomers, and plastics
- ISO 48-4: international equivalent focused specifically on vulcanised and thermoplastic rubber, using four scales
- ISO 868: separate method for Shore hardness of plastics using standardised specimen thickness
Conclusion
Shore A and Shore D aren’t interchangeable readings on the same scale — they use different indenters and force ranges entirely, so a hardness spec should always state which scale was used, and material selection or QC comparisons should never mix values across scales without a validated conversion.
What is the difference between Shore A and Shore D? Shore A is used for softer and more flexible materials, while Shore D is intended for harder rubbers and rigid plastics. The two scales use different indenters and spring forces.
Which materials are tested using Shore A? Shore A is commonly used for soft rubber, silicone, seals, gaskets, flexible PVC, footwear soles, and other elastomeric products.
Which materials are tested using Shore D? Shore D is typically used for hard rubber, rigid thermoplastics, nylon, acrylic, polyurethane, and other relatively stiff polymer materials.
How is a Shore hardness test performed? The durometer is pressed firmly against a flat specimen surface until the presser foot makes full contact. The indentation depth is converted into a hardness value from 0 to 100.
Can Shore A and Shore D values be directly converted? No. The scales use different test conditions, so there is no exact conversion between Shore A and Shore D. Conversion charts provide only approximate comparisons.
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