ASTM D785 / ISO 2039 Rockwell Hardness Testing for Plastics
ASTM D785 / ISO 2039-2 Rockwell Hardness Testing measures the indentation hardness of rigid plastics, reinforced thermosets, and electrical insulating materials on the R, L, M, E, and K scales — used for material selection, incoming lot QC, processing verification, and compliance reporting across automotive, electrical, medical device, aerospace, and additive manufacturing applications.
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- Overview
- Scope, Applications, and Benefits
- Test Process
- Specifications
- Instrumentation
- Results and Deliverables
ASTM D785 / ISO 2039 Rockwell Hardness Testing Overview
ASTM D785 and ISO 2039-2 measure the Rockwell indentation hardness of plastics and plastic-based electrical insulating materials. A steel ball indenter is pressed into the specimen under a minor load (10 kgf) and a major load (60, 100, or 150 kgf), and the net increase in indentation depth is reported as a Rockwell hardness number. It is a static indentation test — not an impact test — used for material selection, QC, and ASTM/ISO compliance. ASTM D785 is the US standard; ISO 2039-2 is its direct international counterpart, while ISO 2039-1 is a separate ball-indentation test and is not interchangeable with Rockwell values.
Results are reported on five scales—R, L, M, E, and K—, and the correct scale must be chosen based on the plastic’s hardness. Scale R suits softer plastics (PP, PE, ABS, nylon), Scale L covers medium plastics (acrylic, PC), Scale M is used for harder thermosets, epoxy, and PEEK, and Scales E and K are used for hard, rigid, and filled/reinforced plastics. Higher values indicate greater resistance to indentation; if a reading falls outside the 50–115 range, the next harder or softer scale should be used.
Scope, Applications, and Benefits
Scope
ASTM D785 and ISO 2039-2 apply to rigid plastics, reinforced thermosets, and plastic-based electrical insulating materials evaluated for indentation hardness on the R, L, M, E, or K scale. Typical Rockwell hardness ranges for common plastics help engineers validate QC results, compare supplier grades, and shortlist materials during design:
- ABS — 85 to 110 HR R
- Polycarbonate (PC) — 70 to 90 HR R, or 80 to 95 HR L
- Nylon 6/6 — 65 to 85 HR R
- HDPE — 40 to 70 HR R
- Polypropylene (PP) — 80 to 110 HR R
- Acrylic (PMMA) — 85 to 105 HR M
- PEEK — 85 to 99 HR M
- Epoxy (cured) — 80 to 110 HR M
- Glass-filled thermosets — 100 to 125 HR E or HR K
Applications
- Electrical insulation material qualification — phenolics, epoxies, and laminated insulators per ASTM D785 scope
- Automotive interior plastic compliance — dashboards, trim, and HVAC components
- Additive manufacturing part qualification — 3D-printed PEEK, PEI, and reinforced photopolymers
- Recycled vs virgin polymer QC — quantifying hardness drop from recyclate addition
- Mould condition monitoring — hardness mapping to detect cold spots or incomplete cure
- Industrial wear components — gears, bushings, and bearing cages
- Aerospace and defense composite tooling and reinforced thermoset hardware
- R&D screening of new polymer formulations, fillers, and additives
- Failure analysis — hardness comparison of field-returned parts against original specification
Benefits
- Provides standardized hardness values for plastics
- Supports comparison between plastic materials
- Assists in evaluating durability and wear resistance
- Helps in selecting materials for specific applications
- Supports manufacturing and product evaluation
ASTM D785 / ISO 2039 Rockwell Hardness Test Process
Sample Preparation
The plastic specimen is prepared with a smooth surface and placed on the Rockwell hardness tester.
1Minor Load Application
A preliminary load is applied, and the dial indicator is set to zero.
2Major Load Application
The main load is applied for a specified time to produce an indentation in the specimen.
3Data Recording & Evaluation
After removing the major load, the hardness value is read directly from the tester dial.
4ASTM D785 / ISO 2039 Technical Specifications
| Parameter | Details |
|---|---|
| Applicable Materials | Rigid plastics, reinforced thermosets, electrical insulating materials |
| Hardness Scales | R, L, M, E, K |
| Indenter Ball Diameter | Typically about 6.4 mm (0.25 in) |
| Minor Load | 10 kgf (all scales) |
| Major Load | 60 kgf (R, L) / 100 kgf (M, E) / 150 kgf (K) |
| Dwell Time Under Major Load | 15 seconds |
| Recovery Time After Major Load Removal | 15 seconds |
| Minimum Specimen Thickness | 6.4 mm (0.25 in) |
| Minimum Spacing Between Indents | 6 mm |
| Minimum Edge Distance | 6 mm |
| Conditioning | 23 °C / 50 % RH, 40 h minimum (ASTM D618 / ISO 291) |
| Measurement Principle | Net increase in indentation depth under static load |
Instrumentation Used for Testing
- Rockwell hardness tester compliant with ASTM E18 calibration requirements
- Steel ball indenters in 12.7 mm, 6.35 mm, and 3.175 mm diameters
- Specimen support anvil with adjustable height
- Calibrated weight stack or closed-loop load cell for 10 / 60 / 100 / 150 kgf loads
- Digital dial indicator or LVDT depth sensor (0.001 mm resolution)
- Conditioning chamber maintaining 23 °C / 50 % RH
- Rockwell test blocks for daily verification on R, L, M, E, and K scales
Results and Deliverables
- Rockwell hardness values
- Hardness scale identification (R, L, M, E, or K)
- Indentation depth measurement results
- Test condition summary
- Compliance report
Frequently Asked Questions
Plastic Rockwell hardness is measured by ASTM D785 and ISO 2039, which measure the resistance to indentation, providing comparative hardness values that are useful in evaluating the rigidity and performance of plastics.
In testing using ASTM D785 or ISO 2039, a small load and then a major load are applied. The hardness value is computed from the indentation depth difference, giving a Rockwell hardness reading depending on the plastic scale.
Use R for softer plastics (PP, PE, ABS, nylon), L for medium (acrylic, PC), M for harder thermosets and PEEK, and E or K for the hardest filled or reinforced plastics. Move to a different scale if readings fall outside the 50 – 115 range.
No. Rockwell hardness is a static indentation test measuring resistance to permanent plastic deformation under a slowly applied load. Impact strength (ASTM E23, Charpy, Izod) measures energy absorbed during a dynamic impact and is a separate test family.
ASTM D785 and ISO 2039 standards test for surface indentation hardness only and do not test for tensile strength or impact resistance. Material thickness and surface finish can affect test scale selection for hardness testing.
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