1. What Hardness Testing Measures
Run ASTM D2240 for rubber, thermoplastic elastomers, and many softer polymer materials that suit a durometer scale such as Shore A or Shore D. Run ASTM D785 for rigid plastics and electrical insulating materials that suit Rockwell indentation testing. Choose the scale and method from the material stiffness and specification.
2. Full Comparison
Hardness numbers from different scales do not represent the same physical measurement. Use the scale named in the product specification and avoid informal conversions for acceptance decisions.
| PARAMETER | ASTM D2240 | ASTM D785 |
|---|---|---|
| Primary materials | Rubber, elastomers, cellular materials, some plastics | Rigid plastics and electrical insulating materials |
| Instrument | Durometer | Rockwell hardness tester |
| Indenter and load | Scale-specific durometer indenter and spring force | Steel ball indenter with defined minor and major loads |
| Result | Durometer hardness, such as Shore A or Shore D | Rockwell hardness on the selected scale |
| Specimen sensitivity | Thickness, support, dwell time, temperature | Thickness, flatness, scale selection, indentation spacing |
| Best fit | Flexible to semi-rigid polymer hardness | Rigid plastic indentation hardness |
3. Materials Covered
D2240 spans elastomeric and some plastic materials through multiple durometer scales. D785 focuses on rigid plastics that support a Rockwell indentation measurement.
Standard Test Method for Rubber Property: Durometer Hardness
- Vulcanized rubber and thermoplastic elastomers
- Soft to semi-rigid plastics where a suitable durometer scale applies
- Cellular materials within the applicable durometer type
Standard Test Method for Rockwell Hardness of Plastics and Electrical Insulating Materials
- Rigid thermoplastics and thermosets
- Electrical insulating plastics
- Rigid molded or machined plastic specimens with sufficient thickness and flatness
4. How Each Test Is Run
Both methods use indentation, but the instruments, load systems, time response, and scales differ.
Standard Test Method for Rubber Property: Durometer Hardness
- Select the durometer type that matches the material and specification.
- Place the specimen on a firm support and apply the presser foot and indenter according to the method.
- Record the hardness reading at the required time and report the durometer scale.
Standard Test Method for Rockwell Hardness of Plastics and Electrical Insulating Materials
- Select the Rockwell scale and ball indenter that fit the material.
- Apply the minor and major loads through the Rockwell cycle.
- Determine the hardness number from the indentation response and report the Rockwell scale with the result.
5. Testing Equipment
| EQUIPMENT | ASTM D2240 | ASTM D785 |
|---|---|---|
| Hardness instrument | Durometer | Rockwell hardness tester |
| Indenter | Scale-specific durometer geometry | Round steel ball indenter |
| Load application | Spring force built into durometer | Defined minor and major loads |
| Specimen support | Firm, flat support and adequate thickness | Flat anvil and adequate specimen thickness |
6. Applications
Standard Test Method for Rubber Property: Durometer Hardness
- Rubber compound QC
- Seal, gasket, roller, and elastomer product inspection
- Shore A or Shore D comparison of polymer formulations
Standard Test Method for Rockwell Hardness of Plastics and Electrical Insulating Materials
- Rigid plastic resin and molded part QC
- Electrical insulating material inspection
- Rockwell hardness comparison of engineering plastics
7. Strengths and Limitations
| STANDARD | STRENGTH |
|---|---|
| ASTM D2240 | Provides fast hardness measurements across a wide range of elastomeric and polymer materials through multiple durometer scales. |
| ASTM D785 | Provides standardized indentation hardness for rigid plastics using defined Rockwell loads and scales. |
8. Which Standard Should You Run?
| IF THIS DESCRIBES YOU | RUN THIS |
|---|---|
| Your sample is rubber, a TPE, or a soft to semi-rigid polymer | ASTM D2240 |
| Your sample is a rigid plastic and the specification calls for Rockwell hardness | ASTM D785 |
| You need to compare Shore and Rockwell values | Do not convert for acceptance without a validated correlation |
Still unsure? Infinita Lab can review your material, specimen form, governing specification, and reporting goal before you submit samples for ASTM D2240 or ASTM D785 testing. Talk to our team.
9. Frequently Asked Questions
Is Shore D the same as Rockwell hardness?
No. Shore D and Rockwell use different indenters, loads, and measurement systems.
Can ASTM D2240 test plastics?
Yes. The method covers some plastics in addition to rubber, elastomers, and cellular materials. Select the durometer type that fits the material and specification.
Why does specimen thickness matter in durometer testing?
A thin specimen can allow the support surface to influence indentation. Follow the method requirements for thickness, stacking, and support.
Which method works better for a rigid engineering plastic?
Use the method named by the governing specification. ASTM D785 fits Rockwell hardness requirements, while ASTM D2240 can fit products specified on a Shore scale.
