Three Methods to Determine Polymer Hardness: Shore, Rockwell, and Vickers
Introduction to Polymer Hardness Testing
Hardness is a measure of a material’s resistance to localised surface deformation, indentation, or scratching. For polymers — a material class spanning flexible elastomers, semi-rigid engineering plastics, and hard thermosets — no single hardness scale covers the full range of relevant materials. Instead, three principal methods are used depending on polymer type, stiffness, and application: Shore durometer testing, Rockwell hardness testing, and Vickers microhardness testing.
Selecting the correct hardness method is essential for generating meaningful, reproducible data. Using Shore A on a rigid engineering plastic or Vickers on a soft elastomer both yield invalid results.
Method 1: Shore Durometer Testing (ASTM D2240)
Shore durometer testing is the primary hardness method for elastomers, rubbers, flexible plastics, and semi-rigid polymers. A standardised indenter is pressed into the specimen surface under a defined spring force, and the depth of penetration is read directly as a hardness value on a 0–100 scale.
Shore A Scale
Shore A uses a truncated cone indentor and an 822 gf spring force. It is designed for soft to medium-hard rubbers, flexible elastomers, TPEs, silicones, and soft PVC. Shore A 20–30 is soft (gel-like); Shore A 70–90 is medium-hard (typical tire rubber); Shore A 95+ is very firm (approaching the limit of the scale).
Shore D Scale
Shore D uses a pointed cone indentor and a higher spring force (4550 gf). It is used for semi-rigid and rigid polymers,s including hard PVC, polyethylene, polypropylene, nylon, and ABS. Typical Shore D values range from 40 (flexible LDPE) to 80+ (hard thermoplastics).
Testing Procedure per ASTM D2240
Specimens must be at least 6.4 mm thick (or stacked to this thickness for thin specimens). Readings are taken after a defined contact time (instantaneous or 15 seconds — both must be specified). Instantaneous readings are higher than 15-second readings due to viscoelastic recovery. At least five readings are averaged with a minimum distance of 12 mm from specimen edges.
Method 2: Rockwell Hardness Testing (ASTM D785)
Rockwell hardness testing for plastics (ASTM D785) uses the same instrument and principle as metallic Rockwell testing (depth measurement under minor and major loads). Still, it employs lighter loads and ball indenters appropriate for polymer materials.
Rockwell Scales for Plastics
- Rockwell R scale: 60 kgf major load, 1/2-inch ball — for soft plastics (polyolefins, ABS, PS) with values typically R50–R130
- Rockwell L scale: 60 kgf major load, 1/4-inch ball — for hard plastics
- Rockwell M scale: 100 kgf, 1/4-inch ball — for harder thermoplastics and thermosets
- Rockwell E scale: 100 kgf, 1/8-inch ball — for very hard thermosets and composites
Advantages of Rockwell for Rigid Polymers
Rockwell testing provides more reproducible results than Shore D for rigid engineering plastics because it uses a defined minor load (which eliminates surface roughness effects) and measures the permanent indentation depth rather than the instantaneous contact pressure. It is widely used in quality control of injection-moulded parts from PA, PC, POM, and PEEK.
Method 3: Vickers Microhardness Testing (ASTM E384)
Vickers microhardness testing applies loads of 0.01–1000 gf through a diamond pyramid indenter and measures the diagonal of the residual indentation under optical microscopy. It applies to hard, brittle polymers and thermosets, composite-matrix characterisation, and polymer coatings, where the small indentation size enables spatial mapping.
Vickers hardness is less commonly specified for polymers than Shore or Rockwell hardness, but is used in research on PEEK, UHMWPE, and ceramic-filled polymer composites; polymer coating hardness mapping; and comparison of polymer hardness on the Vickers scale with metallic counterparts.
Choosing the Right Method
Polymer Type | Recommended Method |
Soft rubber, elastomers | Shore A (ASTM D2240) |
Semi-rigid plastic | Shore D (ASTM D2240) |
Rigid engineering plastic | Rockwell R/L/M (ASTM D785) |
Hard thermose, coating | Vickers (ASTM E384) |
Industrial Applications
Shore A durometer is the universal QC tool for rubber seals, gaskets, and flexible polymer components in the automotive, aerospace, and industrial equipment industries. Rockwell R/M is used for incoming inspection of engineering thermoplastic compounds for structural and electrical components. Vickers microhardness is used in polymer research and wear coating qualification.
Conclusion
Polymer hardness testing — utilizing methods such as Shore durometer (ASTM D2240), Rockwell hardness (ASTM D785), and Vickers microhardness (ASTM E384) — provides comprehensive evaluation of a material’s resistance to indentation across the full spectrum of polymer types. These techniques enable accurate characterization of soft elastomers, semi-rigid plastics, and hard engineering polymers for quality control, material selection, and product development. Selecting the appropriate hardness method and scale based on material stiffness, geometry, and application is essential to obtain meaningful and reproducible results, making method selection as important as the measurement itself.
Why Choose Infinita Lab for Polymer Hardness Testing?
Infinita Lab provides Shore A/D, Rockwell, and Vickers hardness testing for all polymer types through our nationwide accredited materials testing laboratory network. Our polymer testing specialists select the optimal method and scale for your material and application.
Looking for a trusted partner to achieve your research goals? Schedule a meeting with us, send us a request, or call us at (888) 878-3090 to learn more about our services and how we can support you.
Frequently Asked Questions (FAQs)
Can Shore A and Shore D results be directly compared? Approximately, using correlation charts — but not precisely. Shore A values above 90 are unreliable (the scale is near its upper limit), and Shore D values below 20 are unreliable. Materials in the transition zone (Shore A 90–100 / Shore D 20–40) may be tested on both scales for comparison.
Why must Shore durometer specimens have a minimum thickness? The Shore indenter must not be affected by the specimen support surface (anvil hardness). If the specimen is too thin, the measured hardness reflects a combination of specimen and anvil properties. ASTM D2240 requires a minimum specimen thickness of 6 mm.
What is the effect of testing temperature on Shore durometer values? Polymer hardness decreases with increasing temperature due to increased chain mobility. Shore hardness measurements must be performed at the standard laboratory temperature (23°C ± 2°C) and the test temperature must always be reported, as even ±5°C can produce significant Shore value differences for viscoelastic materials.
Is Rockwell hardness suitable for rubber and elastomers? No. Rockwell testing requires materials to exhibit significant permanent indentation — elastomers recover too completely from indentation, producing unreliable Rockwell values. Shore A or IRHD (International Rubber Hardness Degrees, ISO 48) are the appropriate methods for rubber and elastomers.
How does hardness testing detect changes in polymer formulation or degradation? Hardness is sensitive to polymer molecular weight, crystallinity, filler loading, plasticiser content, and crosslink density. Changes from ageing (oxidation, UV degradation, hydrolysis) or formulation variation produce measurable hardness shifts — making routine hardness testing an effective incoming material and in-process quality control tool.
