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General information

The hardness of plastics and hard rubber can be determined using a loaded ball indenter in accordance with standard ISO 2039-1 or ISO 2039-2.

Standard ISO 2039-1 describes determination of the hardness value by means of a ball indenter test. The measured indentation depth is used to calculate the surface area of the indent and subsequently the hardness.

Standard ISO 2039-2 describes determination of the hardness value with a Rockwell hardness testing machine, using Rockwell hardness scales E, L, M and R, similar to the  Rockwell method.

Categorisation of the plastic test

The plastic test is a static hardness testing method with the following characteristics:

• The procedure is standardised (ISO 2039).
• Hardness is tested in the macro range with this method, with a test force between 49 and 980,7 N.
• It is a differential-depth method. This means that the residual depth of indentation left by the indenter is measured to determine the hardness value of a test specimen.

Indenter shape and material: a steel ball, in the case of Rockwell hardness, with different ball diameter depending on the method.

Test procedure as per ISO 2039-1

In this plastic test method, the total test force is applied in two stages. This allows the impact of specimen surface roughness (e.g. grooves in the specimen) and measuring errors caused by backlash in indentation depth measurement to be eliminated.

The test sequence can be described as follows:

1. Test step:

First the indenter is pressed with a test pre-force of 9.8 N (also referred to as pre-force or preload) to a indentation depth of h0 in the specimen to be tested. h0 defines the reference level (basis) for subsequent measurement of the indentation depth (h).

2. Test step:

Next the test force is applied for a dwell time defined in accordance with the standard, whereby the indentation depth is measured. The test force must be selected from values of 49 N, 13 2N, 358 N and 961 N such that the indentation depth measured after a dwell time of 30 s has a value between 0.15 and 0.35 mm. If the indentation depth determined after the end of the test force dwell time has a value lower than 0.15 mm respectively higher than 0.35 mm, the test force needs to be increased respectively decreased.

The determined indentation depth (h) can then be used to calculate the surface area of the indent. This in turn is used to calculate the ball indentation hardness (HB) using a formula defined in the standard. The indents must be positioned such that there is sufficient clearance from the specimen edge and between the individual indents (no less than 10 mm in either case).

How is the hardness value according to ISO 2039-1 read and represented?

The plastics testing hardness value consists of two components:

1. A numeric hardness value;

2. The two letters "HR", standing for "Hardness according to Rockwell";

Example of how to represent and read a hardness value:

45 HR
45 …hardness value
HR …ball indentation hardness in newtons per square millimetre (N/mm²)

Test procedure as per ISO 2039-2

In the Rockwell method, the total test force is applied in two stages. This allows the impact of specimen surface roughness (e.g. grooves in the specimen) and measuring errors caused by backlash in indentation depth measurement to be eliminated.

The test sequence can be described as follows:

1. Test step:

First the indenter is pressed with a test pre-force of 98.07 N (also referred to as pre-force or preload) to a penetration depth of h0 in the specimen to be tested. h0 defines the reference level (basis) for subsequent measurement of the residual indentation depth (h).

2. Test step: 

Next the test force is applied for a dwell time defined in accordance with the standard (several seconds), whereby the indenter penetrates into the specimen to a maximum indentation depth of h1.

3. Test step:

On conclusion of the dwell time, the test force is removed, the indenter moves up by the elastic proportion of the penetration depth in the total test force and remains at the level of the residual indentation depth h. This is also referred to as the depth differential (difference in indentation depth before and after application of the test force). Now the Rockwell hardness (HR) can be calculated, using the residual indentation depth (h) and a formula defined in the standard, taking account of the applied Rockwell scale.

Methods as per ISO 2039-2

The individual methods are distinguished from each other by:

• Diameter of the indenter ball
• Magnitude of applied test force (or test load) 

The resulting test methods use two different ball diameters (steel balls with diameters of: 6.35 and 12.7 mm) and two different test loads (588.4 N and 980.7 N). The following table shows the test methods standardised according to ISO 2039-2.

How is the hardness value according to ISO 2039-2 read and represented?

The hardness value consists of two components:

1. A numeric hardness value;

2. The two letters "HR", standing for "Hardness according to Rockwell";

Example of how to represent and read a hardness value:

65 HR
65 …hardness value
HR …according to Rockwell