ASTM D785-23

Standard Test Method for Rockwell Hardness of Plastics and Electrical Insulating Materials

SIGNIFICANCE AND USE
4.1 A Rockwell hardness number is a number derived from the net increase in depth impression as the load on an indenter is increased from a fixed minor load to a major load and then returned to a minor load (Procedure A). A Rockwell alpha (α) hardness number represents the maximum possible remaining travel of a short-stroke machine from the net depth of impression, as the load on the indenter is increased from a fixed minor load to a major load (Procedure B). Indenters are round steel balls of specific diameters. Rockwell hardness numbers are always quoted with a scale symbol representing the indenter size, load, and dial scale used. This test method is based on Test Methods E18. Procedure A (Section 11) yields the indentation of the specimen remaining 15 s after a given major load is released to a standard 10-kg minor load. Procedure B (Section 12) yields the indentation of the indenter into the specimen after a 15-s application of the major load while the load is still applied. Each Rockwell scale division represents 0.002-mm (0.00008-in.) vertical movement of the indenter. In practice, the Rockwell hardness number is derived from the following relationship:  
     where:
  HR  =  the Rockwell hardness number, and   e  =  the depth of impression after removal of the major load, in units of 0.002 mm. This relation only holds for the E, M, L, R, and K scales.    
4.2 A Rockwell hardness number is directly related to the indentation hardness of a plastic material, with the higher the reading the harder the material. An α hardness number is equal to 150 minus the instrument reading. Due to a short overlap of Rockwell hardness scales by Procedure A, two different dial readings on different scales may be obtained on the same material, both of which may be technically correct.  
4.3 For certain types of materials having creep and recovery, the time factors involved in applications of major and minor loads have a considerable effect on the results of the measureme...
SCOPE
1.1 This test method covers two procedures for testing the indention hardness of plastics and related plastic electrical insulating materials by means of the Rockwell hardness tester.  
1.2 The values stated in SI units are to be regarded as standard. The values given in parentheses are for information only.  
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.
Note 1: This test method and ISO 2039-2 are equivalent. Procedure A of this test method is equivalent to the test method in the main body of ISO 2039-2. Procedure B of this test method is equivalent to the test method in the integral annex part of ISO 2039-2.  
1.4 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

General Information

• Status: Published
• Publication Date: 14-Mar-2023
• ICS: 29.035.20 - Plastics and rubber insulating materials
• Technical Committee: D20 - Plastics
• Drafting Committee: D20.10 - Mechanical Properties

Relations

• Refers: ASTM D883-24 - Standard Terminology Relating to Plastics
• Effective Date: 01-Feb-2024
• Refers: ASTM D883-23 - Standard Terminology Relating to Plastics
• Effective Date: 01-Nov-2023
• Refers: ASTM E456-13a(2022)e1 - Standard Terminology Relating to Quality and Statistics
• Effective Date: 01-Apr-2022
• Refers: ASTM D883-20 - Standard Terminology Relating to Plastics
• Effective Date: 01-Jan-2020
• Refers: ASTM D883-19c - Standard Terminology Relating to Plastics
• Effective Date: 01-Aug-2019
• Refers: ASTM D883-19a - Standard Terminology Relating to Plastics
• Effective Date: 15-Apr-2019
• Refers: ASTM D883-19 - Standard Terminology Relating to Plastics
• Effective Date: 01-Feb-2019
• Refers: ASTM D883-18a - Standard Terminology Relating to Plastics
• Effective Date: 01-Dec-2018
• Refers: ASTM D883-18 - Standard Terminology Relating to Plastics
• Effective Date: 01-Nov-2018
• Refers: ASTM E18-18 - Standard Test Methods for Rockwell Hardness of Metallic Materials
• Effective Date: 01-Jul-2018
• Refers: ASTM E2935-17 - Standard Practice for Conducting Equivalence Testing in Laboratory Applications
• Effective Date: 01-Oct-2017
• Refers: ASTM E456-13A(2017)e1 - Standard Terminology Relating to Quality and Statistics
• Effective Date: 01-Oct-2017
• Refers: ASTM E456-13A(2017)e3 - Standard Terminology Relating to Quality and Statistics
• Effective Date: 01-Oct-2017
• Refers: ASTM D883-17 - Standard Terminology Relating to Plastics
• Effective Date: 15-Aug-2017
• Refers: ASTM E18-17 - Standard Test Methods for Rockwell Hardness of Metallic Materials
• Effective Date: 01-Jul-2017

Overview

ASTM D785-23: Standard Test Method for Rockwell Hardness of Plastics and Electrical Insulating Materials outlines two validated procedures for determining the indentation hardness of plastics and related electrical insulating materials using a Rockwell hardness tester. Developed by ASTM International, this standard helps manufacturers, laboratories, and quality control professionals assess material hardness, which is crucial for ensuring product reliability and performance. The test method supports comparability and traceability in hardness measurement, providing standardized Rockwell hardness numbers across the plastics industry.

Key Topics

• Rockwell Hardness Measurement: The standard details two procedures (A and B) that involve increasing and decreasing major and minor loads on an indenter to generate a Rockwell hardness number or Rockwell alpha (α) number.
• Indentation Hardness: Rockwell hardness is directly related to a plastic’s resistance to surface penetration or deformation, with higher values indicating harder materials.
• Applicable Scales: Several Rockwell scales (including E, M, L, R, and K) are used, depending on the indenter size and test load, ensuring appropriate measurement across material types and hardness ranges.
• Test Conditions and Equipment: Provides requirements for Rockwell hardness testers, specimen preparation, conditioning, and test conditions, such as temperature and humidity.
• Precision and Reproducibility: Specifies guidelines for obtaining repeatable and reliable results, including the number of tests, calibration, and reporting requirements.
• Material Suitability: Addresses the impact of material properties such as creep, recovery, and anisotropy on test reproducibility and interpretation.

Applications

ASTM D785-23 is widely applied in quality assurance, research, and specification of plastics and electrical insulating materials. Specific applications include:

• Quality Control: Manufacturers use Rockwell hardness testing to verify that polymer and insulation materials meet hardness requirements before product release.
• Material Development: R&D labs employ the standard to compare hardness among new formulations, monitor curing processes, and identify optimal compositions.
• Product Specification and Compliance: Provides a reliable method for reporting material hardness in datasheets and satisfying procurement or regulatory requirements.
• Comparison Across Suppliers: Standardized Rockwell hardness values allow users to compare hardness across different sources and batches of materials.
• Process Monitoring: Indentation hardness is used as an indirect indicator of curing in thermosetting plastics and to assess consistency in mass production.

The standard covers a wide range of plastic forms including sheets, molded parts, and electrical insulating material, making it highly versatile across industries such as automotive, electronics, construction, and consumer goods.

Related Standards

For enhanced applicability and compatibility, ASTM D785-23 references and aligns with several key international and ASTM standards, including:

• ISO 2039-2: Plastics-Determination of Hardness-Part 2: Rockwell Hardness (equivalent test methods for global interchangeability)
• ASTM D618: Practice for Conditioning Plastics for Testing
• ASTM D2240: Test Method for Rubber Property-Durometer Hardness (for softer plastics)
• ASTM D883: Terminology Relating to Plastics
• ASTM E18: Test Methods for Rockwell Hardness of Metallic Materials (methodological basis)
• ASTM D4000: Classification System for Specifying Plastic Materials

By adhering to ASTM D785-23, organizations ensure consistent, internationally-recognized hardness testing methods for plastics and electrical insulating materials, supporting product quality, safety, and regulatory compliance.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: D785 − 23
Standard Test Method for
Rockwell Hardness of Plastics and Electrical Insulating
Materials
This standard is issued under the fixed designation D785; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the U.S. Department of Defense.
1. Scope* E18 Test Methods for Rockwell Hardness of Metallic Ma-
terials
1.1 This test method covers two procedures for testing the
E456 Terminology Relating to Quality and Statistics
indention hardness of plastics and related plastic electrical
E691 Practice for Conducting an Interlaboratory Study to
insulating materials by means of the Rockwell hardness tester.
Determine the Precision of a Test Method
1.2 The values stated in SI units are to be regarded as
E2935 Practice for Evaluating Equivalence of Two Testing
standard. The values given in parentheses are for information
Processes
only. 3
2.2 ISO Standards
1.3 This standard does not purport to address all of the
ISO 2039-2 Plastics—Determination of Hardness—Part 2:
safety concerns, if any, associated with its use. It is the
Rockwell Hardness
responsibility of the user of this standard to establish appro-
3. Terminology
priate safety, health, and environmental practices and deter-
mine the applicability of regulatory limitations prior to use.
3.1 Definitions used in this test method are in accordance
with Terminology D883.
NOTE 1—This test method and ISO 2039-2 are equivalent. Procedure A
of this test method is equivalent to the test method in the main body of ISO
3.2 For definitions of terms that appear in this practice
2039-2. Procedure B of this test method is equivalent to the test method
relating to quality and statistics (such as precision and bias),
in the integral annex part of ISO 2039-2.
refer to Terminology E456.
1.4 This international standard was developed in accor-
3.3 Definitions of Terms Specific to This Standard:
dance with internationally recognized principles on standard-
ization established in the Decision on Principles for the 3.3.1 indentation hardness—the resistance of a material to
surface penetration or plastic deformation.
Development of International Standards, Guides and Recom-
mendations issued by the World Trade Organization Technical
4. Significance and Use
Barriers to Trade (TBT) Committee.
4.1 A Rockwell hardness number is a number derived from
2. Referenced Documents
the net increase in depth impression as the load on an indenter
is increased from a fixed minor load to a major load and then
2.1 ASTM Standards:
returned to a minor load (Procedure A). A Rockwell alpha (α)
D618 Practice for Conditioning Plastics for Testing
hardness number represents the maximum possible remaining
D883 Terminology Relating to Plastics
travel of a short-stroke machine from the net depth of
D2240 Test Method for Rubber Property—Durometer Hard-
impression, as the load on the indenter is increased from a fixed
ness
minor load to a major load (Procedure B). Indenters are round
D4000 Classification System for Specifying Plastic Materi-
steel balls of specific diameters. Rockwell hardness numbers
als
are always quoted with a scale symbol representing the
indenter size, load, and dial scale used. This test method is
1 based on Test Methods E18. Procedure A (Section 11) yields
This method is under the jurisdiction of ASTM Committee D20 on Plastics and
is the direct responsibility of Subcommittee D20.10 on Mechanical Properties. the indentation of the specimen remaining 15 s after a given
Current edition approved March 15, 2023. Published April 2023. Originally
major load is released to a standard 10-kg minor load.
approved in 1944. Last previous edition approved in 2015 as D785 - 08(2015). DOI:
Procedure B (Section 12) yields the indentation of the indenter
10.1520/D0785-23.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on Available from American National Standards Institute (ANSI), 25 W. 43rd St.,
the ASTM website. 4th Floor, New York, NY 10036, http://www.ansi.org.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D785 − 23
into the specimen after a 15-s application of the major load and/or testing parameters covered in the materials specification
while the load is still applied. Each Rockwell scale division shall take precedence over those mentioned in this test method.
represents 0.002-mm (0.00008-in.) vertical movement of the If there are no material specifications, then the default condi-
indenter. In practice, the Rockwell hardness number is derived tions apply.
from the following relationship:
5. Factors Affecting Reproducibility and Accuracy
HR 5 130 2 e (1)
5.1 Rockwell hardness readings have been found reproduc-
where:
ible to 62 divisions for certain homogeneous materials with a
HR = the Rockwell hardness number, and
Young’s modulus in compression over 3400 MPa (5 × 10 psi).
e = the depth of impression after removal of the major
Softer plastics and coarse-filled materials will have a wider
load, in units of 0.002 mm. This relation only holds for
range of variation. A large ball indenter will distribute the load
the E, M, L, R, and K scales.
more evenly and decrease the range of test results (Note 2). The
4.2 A Rockwell hardness number is directly related to the
sensitivity of the instrument decreases with an increase in the
indentation hardness of a plastic material, with the higher the
dial reading and becomes very poor for readings of 100 and
reading the harder the material. An α hardness number is equal
over due to the shallow indentation of the steel ball. It is
to 150 minus the instrument reading. Due to a short overlap of
desirable to use the smallest ball and highest load that is
Rockwell hardness scales by Procedure A, two different dial
practical because of this loss of sensitivity. Rockwell hardness
readings on different scales may be obtained on the same
readings over 115 are not satisfactory and shall not be reported.
material, both of which may be technically correct.
Readings between zero and 100 are recommended, but read-
ings to 115 are permissible. For comparison purposes, it may
4.3 For certain types of materials having creep and recovery,
be desirable to take readings higher than 115 or lower than zero
the time factors involved in applications of major and minor
on any single scale. In such cases, Rockwell hardness readings
loads have a considerable effect on the results of the measure-
may be reported, but the corresponding correct readings shall
ments.
follow in parentheses, if possible. Such alternate readings are
4.4 The results obtained by this test method are not gener-
not always feasible when the specimen is subjected to con-
ally considered a measure of the abrasion or wear resistance of
stantly changing conditions or irreversible reactions.
the plastic materials in question.
NOTE 2—Molded specimens containing coarse fiber fillers, such as
4.5 Indentation hardness is used as an indication of cure of
woven glass fabric, will influence the penetration obtained. These varia-
some thermosetting materials at room temperature. Generally,
tions in hardness may be reduced by testing with the largest ball indenter
an uncured specimen has a hardness reading below normal. consistent with the overall hardness of the material.
4.6 Each Rockwell hardness scale in Table 1 is an extension 5.2 If the bench or table on which a Rockwell hardness
of the proceeding less severe scale, and while there is some tester is mounted is subject to vibration, such as is experienced
overlap between adjacent scales, a correlation table is not in the vicinity of other machines, the tester should be mounted
desirable. Readings on one material may be satisfactory for on a metal plate with sponge rubber at least 25 mm (1 in.)
such a table, but there is no guarantee that other plastic thick, or on any other type of mounting that will effectually
materials will give corresponding readings because of differ- eliminate vibration from the machine. Otherwise the indenter
ences in elasticity, creep, and shear characteristics. will indent further into the material than when such vibrations
are absent.
4.7 Before proceeding with this test method, reference
should be made to the specification of the material being tested.
5.3 Dust, dirt, grease, and scale or rust should not be
Any test specimen preparation, conditioning, dimensions, allowed to accumulate on the indenter, as this will affect the
results. Steel ball indenters that have nicks, burrs, or are out of
round shall not be used.
TABLE 1 Rockwell Hardness Scales
5.4 The condition of the test equipment is an important
Rockwell Indenter Diameter
factor in the accuracy of the test results. Dust, dirt or heavy oil
Hardness Minor Major
act as a cushion to the load supporting members of the test
Scale Load, Load,
A
(Red Dial kg kg
equipment and cause erroneous readings of the instrument dial.
in. mm
Numbers)
The shoulders of the instrument housing, indenter chuck, ball
R 10 60 0.5000 ± 12.700 ±
seat in the instrument housing, capstan, capstan screw, and
0.0001 0.0025
L 10 60 0.2500 ± 6.350 ±
anvil shoulder seat should be kept clean and true. The capstan
0.0001 0.0025
and screw should be lightly oiled. Pitted anvil surfaces may be
M 10 100 0.2500 ± 6.350 ±
refinished with 600 grit paper.
0.0001 0.0025
E 10 100 0.1250 ± 3.175 ±
5.5 Surface conditions of the specimen have a marked effect
0.0001 0.0025
K 10 150 0.1250 ± 3.175 ±
on the readings obtained in a test. Generally, a molded finish
0.0001 0.0025
will give a higher Rockwell reading than a machined face due
A
This major load is not the sum of the actual weights at the back of the frame but
to the high resin content or filled materials or better orientation
is a ratio of this load, depending on the leverage arm of machine. One make and
and lower plasticizer content of unfilled plastic materials.
model has a 25 to 1 leverage arm.
Injection mold specimens in such a way that sink marks and
D785 − 23
a pile-up of several pieces of the same thickness, provided that precaution
warpage are minimized. Tubular or unsupported curved speci-
is taken that the surfaces of the pieces are in total contact and not held
mens are not recommended for plastic hardness testing. Such
apart by sink marks, burrs from saw cuts, or other protrusions and
curved surfaces have a tendency to yield with the load and
provided the hardness values are not affected by the stacking of thin
produce an unsymmetrical indentation pattern.
specimens.
5.6 Many plastic materials have anisotropic characteristics
8. Calibration
which cause indentation hardness to vary with the direction of
testing. In such cases, the hardest face is generally that one
8.1 Check the Rockwell hardness tester periodically with a
perpendicular to the molding pressure. Specimens with flash-
small machinist’s level along both horizontal axes from a flat
ing on the side supported by the anvil also may give erroneous
anvil for correct positioning. Minor errors in leveling are not
results.
critical, but correct positioning is desirable.
5.7 Ambient temperature variations can significantly affect
8.2 The adjustment of speed-of-load application is of great
hardness for many materials.
importance. Adjust the dashpot on the Rockwell tester so that
5.8 Rockwell hardness tests of the highest accuracy are
the operating handle completes its travel in 4 to 5 s with no
made on pieces of sufficient thickness so that the Rockwell
specimen on the machine or load applied by the indenter to the
reading is not affected by the supporting anvil. A bulge, change
anvil. The major load shall be 100 kg for this calibration. When
in color, or other marking on the under surface of the test
so adjusted, the period taken for the mechanism to come to a
specimen closest to the anvil is an indication that the specimen
stop with the specimen in place will vary from 5 to 15 s,
is not sufficiently thick for precision testing. Stacking of thin
depending upon the particular specimen, the indenter, and the
specimen is permitted provided they are flat, parallel, and free
load used. The operator should check the instrument manual
from dust or burrs. The precision of the test is reduced for
for this adjustment.
stacked specimens, and results should not be compared to a test
specimen of standard thickness.
8.3 Select a standardized test block as near as possible to the
hardness of the material being tested. If more than one
6. Apparatus
hardness scale is used in testing, choose a standardized test
6.1 Rockwell Hardness Tester, in accordance with the re- block for each scale used (Note 4). Make five impressions on
quirements of Section 7. A flat anvil at least 50 mm (2 in.) in
the test surface of the block. Compare the average of these five
diameter shall be used as a base plate for flat specimens.
tests against the hardness calibration of the blocks. If the error
is more than 62 hardness numbers, bring the machine into
6.2 For Rockwell hardness testing, it is necessary that the
adjustment as described in 8.4 or in 5.3 and 5.4. If adjustment
major load, when fully applied, be completely supported by the
can not be achieved, more extensive servicing of the instru-
specimen and not held by other limiting elements of the
ment may be needed.
machine. To determine whether this condition is satisfied, the
major load should be applied to the test specimen. If an
NOTE 4—Standard test blocks for the R, L, E, K, and M scales are
additional load is then applied, by means of hand pressure on
available from Wilson Instruments, 100 Royal Street, Canton, Ma 02021
the weights, the needle should indicate an additional indenta-
(A Division of Instron Corporation).
tion. If this is not indicated, the major load is not being applied
8.4 Check the index lever adjustment periodically and make
to the specimen, and a long-stroke (PL) machine or less severe
adjustments if necessary. To adjust the index lever, place a
scale should be used. For the harder materials with a modulus
5 specimen (plastic with low creep or soft metal) on the anvil and
around 5500 MPa (8 × 10 psi) or over, a stroke equivalent to
turn the knurled elevating ring to bring the specimen in contact
150 scale divisions, under major load application, may be
with the indenter. Keep turning the ring to elevate the specimen
adequate; but for softer materials the long-stroke (250 scale
until positive resistance to further turning is felt, which will be
divisions under major load) machine is requir
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: D785 − 08 (Reapproved 2015) D785 − 23
Standard Test Method for
Rockwell Hardness of Plastics and Electrical Insulating
Materials
This standard is issued under the fixed designation D785; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the U.S. Department of Defense.
1. Scope Scope*
1.1 This test method covers two procedures for testing the indention hardness of plastics and related plastic electrical insulating
materials by means of the Rockwell hardness tester.
1.2 The values stated in SI units are to be regarded as standard. The values given in parentheses are for information only.
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety and healthsafety, health, and environmental practices and determine
the applicability of regulatory limitations prior to use.
NOTE 1—This test method and ISO 2039-2 are equivalent. Procedure A of this test method is equivalent to the test method in the main body of ISO 2039-2.
Procedure B of this test method is equivalent to the test method in the integral annex part of ISO 2039-2.
NOTE 1—This test method and ISO 2039-2 are equivalent. Procedure A of this test method is equivalent to the test method in the main body of ISO 2039-2.
Procedure B of this test method is equivalent to the test method in the integral annex part of ISO 2039-2.
1.4 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
D618 Practice for Conditioning Plastics for Testing
D883 Terminology Relating to Plastics
D2240 Test Method for Rubber Property—Durometer Hardness
D4000 Classification System for Specifying Plastic Materials
E18 Test Methods for Rockwell Hardness of Metallic Materials
E456 Terminology Relating to Quality and Statistics
E691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
E2935 Practice for Evaluating Equivalence of Two Testing Processes
This method is under the jurisdiction of ASTM Committee D20 on Plastics and is the direct responsibility of Subcommittee D20.10 on Mechanical Properties.
Current edition approved Oct. 1, 2015March 15, 2023. Published October 2015April 2023. Originally approved in 1944. Last previous edition approved in 2008 as
D785 - 08. DOI: 10.1520/D0785-08R15.2015 as D785 - 08(2015). DOI: 10.1520/D0785-23.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D785 − 23
2.2 ISO Standards
ISO 2039-2 Plastics—Determination of Hardness—Part 2: Rockwell Hardness
3. Terminology
3.1 Definitions used in this test method are in accordance with Terminology D883.
3.2 For definitions of terms that appear in this practice relating to quality and statistics (such as precision and bias), refer to
Terminology E456.
3.3 Definitions of Terms Specific to This Standard:
3.3.1 indentation hardness—the resistance of a material to surface penetration or plastic deformation.
4. Significance and Use
4.1 A Rockwell hardness number is a number derived from the net increase in depth impression as the load on an indenter is
increased from a fixed minor load to a major load and then returned to a minor load (Procedure A). A Rockwell alpha (α) hardness
number represents the maximum possible remaining travel of a short-stroke machine from the net depth of impression, as the load
on the indenter is increased from a fixed minor load to a major load (Procedure B). Indenters are round steel balls of specific
diameters. Rockwell hardness numbers are always quoted with a scale symbol representing the indenter size, load, and dial scale
used. This test method is based on Test Methods E18. Procedure A (Section 11) yields the indentation of the specimen remaining
15 s after a given major load is released to a standard 10-kg minor load. Procedure B (Section 12) yields the indentation of the
indenter into the specimen after a 15-s application of the major load while the load is still applied. Each Rockwell scale division
represents 0.002-mm (0.00008-in.) vertical movement of the indenter. In practice, the Rockwell hardness number is derived from
the following relationship:
HR 5 130 2 e (1)
where:
HR = the Rockwell hardness number, and
e = the depth of impression after removal of the major load, in units of 0.002 mm. This relation only holds for the E, M, L,
R, and K scales.
4.2 A Rockwell hardness number is directly related to the indentation hardness of a plastic material, with the higher the reading
the harder the material. An α hardness number is equal to 150 minus the instrument reading. Due to a short overlap of Rockwell
hardness scales by Procedure A, two different dial readings on different scales may be obtained on the same material, both of which
may be technically correct.
4.3 For certain types of materials having creep and recovery, the time factors involved in applications of major and minor loads
have a considerable effect on the results of the measurements.
4.4 The results obtained by this test method are not generally considered a measure of the abrasion or wear resistance of the plastic
materials in question.
4.5 Indentation hardness is used as an indication of cure of some thermosetting materials at room temperature. Generally, an
uncured specimen has a hardness reading below normal.
4.6 Each Rockwell hardness scale in Table 1 is an extension of the proceeding less severe scale, and while there is some overlap
between adjacent scales, a correlation table is not desirable. Readings on one material may be satisfactory for such a table, but there
is no guarantee that other plastic materials will give corresponding readings because of differences in elasticity, creep, and shear
characteristics.
4.7 Before proceeding with this test method, reference should be made to the specification of the material being tested. Any test
Available from American National Standards Institute (ANSI), 25 W. 43rd St., 4th Floor, New York, NY 10036, http://www.ansi.org.
D785 − 23
TABLE 1 Rockwell Hardness Scales
Rockwell Indenter Diameter
Hardness Minor Major
Scale Load, Load,
A
(Red Dial kg kg
in. mm
Numbers)
R 10 60 0.5000 ± 12.700 ±
0.0001 0.0025
L 10 60 0.2500 ± 6.350 ±
0.0001 0.0025
M 10 100 0.2500 ± 6.350 ±
0.0001 0.0025
E 10 100 0.1250 ± 3.175 ±
0.0001 0.0025
K 10 150 0.1250 ± 3.175 ±
0.0001 0.0025
A
This major load is not the sum of the actual weights at the back of the frame but
is a ratio of this load, depending on the leverage arm of machine. One make and
model has a 25 to 1 leverage arm.
specimen preparation, conditioning, dimensions, and/or testing parameters covered in the materials specification shall take
precedence over those mentioned in this test method. If there are no material specifications, then the default conditions apply.
5. Factors Affecting Reproducibility and Accuracy
5.1 Rockwell hardness readings have been found reproducible to 62 divisions for certain homogeneous materials with a Young’s
modulus in compression over 3400 MPa (5 × 10 psi). Softer plastics and coarse-filled materials will have a wider range of
variation. A large ball indenter will distribute the load more evenly and decrease the range of test results (Note 2). The sensitivity
of the instrument decreases with an increase in the dial reading and becomes very poor for readings of 100 and over due to the
shallow indentation of the steel ball. It is desirable to use the smallest ball and highest load that is practical because of this loss
of sensitivity. Rockwell hardness readings over 115 are not satisfactory and shall not be reported. Readings between zero and 100
are recommended, but readings to 115 are permissible. For comparison purposes, it may be desirable to take readings higher than
115 or lower than zero on any single scale. In such cases, Rockwell hardness readings may be reported, but the corresponding
correct readings shall follow in parentheses, if possible. Such alternate readings are not always feasible when the specimen is
subjected to constantly changing conditions or irreversible reactions.
NOTE 2—Molded specimens containing coarse fiber fillers, such as woven glass fabric, will influence the penetration obtained. These variations in
hardness may be reduced by testing with the largest ball indenter consistent with the overall hardness of the material.
5.2 If the bench or table on which a Rockwell hardness tester is mounted is subject to vibration, such as is experienced in the
vicinity of other machines, the tester should be mounted on a metal plate with sponge rubber at least 25 mm (1 in.) thick, or on
any other type of mounting that will effectually eliminate vibration from the machine. Otherwise the indenter will indent further
into the material than when such vibrations are absent.
5.3 Dust, dirt, grease, and scale or rust should not be allowed to accumulate on the indenter, as this will affect the results. Steel
ball indenters that have nicks, burrs, or are out of round shall not be used.
5.4 The condition of the test equipment is an important factor in the accuracy of the test results. Dust, dirt or heavy oil act as a
cushion to the load supporting members of the test equipment and cause erroneous readings of the instrument dial. The shoulders
of the instrument housing, indenter chuck, ball seat in the instrument housing, capstan, capstan screw, and anvil shoulder seat
should be kept clean and true. The capstan and screw should be lightly oiled. Pitted anvil surfaces may be refinished with 600 grit
paper.
5.5 Surface conditions of the specimen have a marked effect on the readings obtained in a test. Generally, a molded finish will
give a higher Rockwell reading than a machined face due to the high resin content or filled materials or better orientation and lower
plasticizer content of unfilled plastic materials. Injection mold specimens in such a way that sink marks and warpage are
minimized. Tubular or unsupported curved specimens are not recommended for plastic hardness testing. Such curved surfaces have
a tendency to yield with the load and produce an unsymmetrical indentation pattern.
5.6 Many plastic materials have anisotropic characteristics which cause indentation hardness to vary with the direction of testing.
D785 − 23
In such cases, the hardest face is generally that one perpendicular to the molding pressure. Specimens with flashing on the side
supported by the anvil also may give erroneous results.
5.7 Ambient temperature variations can significantly affect hardness for many materials.
5.8 Rockwell hardness tests of the highest accuracy are made on pieces of sufficient thickness so that the Rockwell reading is not
affected by the supporting anvil. A bulge, change in color, or other marking on the under surface of the test specimen closest to
the anvil is an indication that the specimen is not sufficiently thick for precision testing. Stacking of thin specimen is permitted
provided they are flat, parallel, and free from dust or burrs. The precision of the test is reduced for stacked specimens, and results
should not be compared to a test specimen of standard thickness.
6. Apparatus
6.1 Rockwell Hardness Tester, in accordance with the requirements of Section 7. A flat anvil at least 50 mm (2 in.) in diameter
shall be used as a base plate for flat specimens.
6.2 For Rockwell hardness testing, it is necessary that the major load, when fully applied, be completely supported by the
specimen and not held by other limiting elements of the machine. To determine whether this condition is satisfied, the major load
should be applied to the test specimen. If an additional load is then applied, by means of hand pressure on the weights, the needle
should indicate an additional indentation. If this is not indicated, the major load is not being applied to the specimen, and a
long-stroke (PL) machine or less severe scale should be used. For the harder materials with a modulus around 5500 MPa (8 × 10
psi) or over, a stroke equivalent to 150 scale divisions, under major load application, may be adequate; but for softer materials the
long-stroke (250 scale divisions under major load) machine is required.
7. Test Specimen
7.1 The standard test specimen shall have a minimum thickness of 6 mm ( ⁄4 in.). The specimen may be a piece cut from a molding
or sheet. Care should be taken that the test specimen has parallel flat surfaces to ensure good seating on the anvil and thus avoid
the deflection that may be caused by poor contact. The specimen shall be at least 25 mm (1 in.) square if cut from sheet stock,
2 2
or at least 6 cm (1 in. ) in area if cut from other shapes. The minimum width shall be 13 mm ( ⁄2 in.) plus the width of the
indentation resulting from the conduct of a test using the chosen indenter.
NOTE 3—Specimen with a thickness other than 6 mm may be used if it has been verified that, for that thickness, the hardness values are not affected by
the supporting surface and that no imprint shows under the surface of the specimen after testing. The specimen may be composed of a pile-up of several
pieces of the same thickness, provided that precaution is taken that the surfaces of the pieces are in total contact and not held apart by sink marks, burrs
from saw cuts, or other protrusions and provided the hardness values are not affected by the stacking of thin specimens.
8. Calibration
8.1 Check the Rockwell hardness tester periodically with a small machinist’s level along both horizontal axes from a flat anvil
for correct positioning. Minor errors in leveling are not critical, but correct positioning is desirable.
8.2 The adjustment of speed-of-load application is of great importance. Adjust the dashpot on the Rockwell tester so that the
operating handle completes its travel in 4 to 5 s with no specimen on the machine or load applied by the indenter to the anvil. The
major load shall be 100 kg for this calibration. When so adjusted, the period taken for the mechanism to come to a stop with the
specimen in place will vary from 5 to 15 s, depending upon the particular specimen, the indenter, and the load used. The operator
should check the instrument manual for this adjustment.
8.3 Select a standardized test block as near as possible to the hardness of the material being tested. If more than one hardness scale
is used in testing, choose a standardized test block for each scale used (Note 4). Make five impressions on the test surface of the
block. Compare the average of these five tests against the hardness calibration of the blocks. If the error is more than 62 hardness
numbers, bring the machine into adjustment as described in 8.4 or in 5.3 and 5.4. If adjustment can not be achieved, more extensive
servicing of the instrument may be needed.
NOTE 4—Standard test blocks for the R, L, E, K, and M scales are available from Wilson Instruments, 100 Royal Street, Canton, Ma 02021 (A Division
of Instron Corporation).
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