Rubber Hardness Measurements: The Micro IRHD vs the Micro Shore Scale Instruments

The hardness of rubber can be calculated using either the Shore Scale or the IRHD (International Rubber Hardness Degree). The instrumentation and test methods for each are different, for example, with regards to the time and method of indentor application and geometry.

This article examines the micro versions of these tests, which are usually used to measure small samples. For both methods, the instrumental parameters and sample limitations were investigated. This article demonstrates the practical differences between the two tests as well as their limitations and how these affect the results. This provides valuable information regarding the advantages and disadvantages of each test method.

Introduction

Hardness measurement is one of the most commonly used methods in quality control and product testing. Two scale tests are commonly used, providing results either on the Shore or the IRHD scale. Several instrument types exist for both types– the Shore-A and the IRHD dead load scales are most often used for rubber. Both of these methods are described in international standards.

The IRHD test, which takes 35 seconds, is the preferred method for final product inspection because it is usually non-destructive. Conversely, the Shore method only takes 1 (or 3) seconds but is often destructive and leaves a permanent indentation.

Although there is little relationship between the two tests, it is recognized that there can be a degree of correlation for some compounds over certain hardness ranges. Both methods have been used for decades, however, Briscoe and Sebastian1 (1993) state that the Shore-A test has not been analyzed in the same detail as the IRHD method.

This paper examines the Shore-A test, concluding that there is an approximate relationship between dead load IRHD hardness and Shore-A. In 1991, Brown and Soekarnein2 investigated the reproducibility of Rubber Hardness Tests on both the Shore-A and IRHD scales. Analog instruments were used, which lack the precision of modern electronic methods.

The IRHD scale has a micro counterpart for which a standard has been in place for more than 30 years (ISO 48 & ASTM 1415 ). There is currently no released standard for the proposed Shore Micro. The Micro IRHD instrument was designed in the 1950s as a smaller version of the dead load testing when testing smaller, thinner products and samples such as ‘O’ rings.

The Micro IRHD results are generally comparable to those produced by the standard dead load IRHD instrument. Conversely, there are a variety of ‘Shore-M’ scale hardness testers appearing on the market, but the results from these are not comparable to those produced by the Shore-A scale. Furthermore, little is known about the Shore-M scale, how it relates to the Micro IRHD scale and any instrumental differences that might be relevant.

This article discusses both micro instruments. There is often a degree of confusion between users of the two test methods. Moreover, it is not always possible for tests to conform precisely to a standard because of sample size and shape restraints. This article aims to compare the two methods, not to determine a relationship between the micro scales. It also aims to demonstrate the practical differences between the two tests as well as results obtained and instrumental parameters involved.

A Shore-M instrument was manufactured by Wallace to provide data for this article. This instrument is compared to the standard Micro IRHD hardness tester from Wallace.

Differences Between the Micro IRHD and Shore-M Instruments

The Shore-M instrument uses a sharp conical indentor, whereas the Micro IRHD instrument uses an indentor with a spherical tip.

The Micro IRHD uses dead loads as a basis. The test uses a foot with a force of 245 mN to hold the sample in place. To provide a datum position, a primary load of 8.3 mN is applied for five seconds. The second load of 145 mN is then applied for 30 seconds. The incremental displacement from the datum is then measured. This is converted to an IRHD value, which is a non-linear scale defined in the standard. The maximum depth of indentation is 0.3 mm.

Conversely, the Shore-M test uses a calibrated spring (applying forces that vary between 0.3 N and 0.8 N). This supplies an indentation force, which increases with the indentor displacement in a linear pattern. The presser foot applies a force that is greater than the spring force, allowing the presser foot to contact the sample. The indentation depth is recorded after a set time. The maximum depth of indentation is 1.25 mm. For the Shore-A scale, standard dwell times are 1 and 3 seconds. The Shore-M test also uses these values. On the Shore-M scale, each durometer point represents a displacement of 0.0125 mm.

Experimental

Before starting the test sequence, the Shore-M and Wallace Micro IRHD instruments were calibrated. They were then rechecked at the end of the sequence. Except where otherwise noted, a standard temperature of 23 ± 2 °C was used. Each test was carried out on the Micro IRHD and the Shore-M instruments in sequence. The Shore-M instrument used either a 1 or 3 second dwell time (timed to ± 0.1 s). Every sample was tested in five different places.

In order to provide comparative results for each instrument, four standard Wallace test blocks* covering a range between 40 and 80 IRHD were used. Various rubber samples were also tested on both instruments.

Micro tests are designed for use on small samples. By carrying out tests on thinner samples, the effects of sample thickness were investigated. Although ISO 483 allows the use of 1 mm samples, the standard recommends that two or more samples be plied together to conform to requirements and create the preferred thickness of 2 ± 0.5 mm. The Shore-M scale has similar requirements. Tests were carried out on samples that were 1, 2 and 3 mm thick. To determine the effect on the results, a latex surgical glove of 0.1 mm was used as a sample with up to 15 pieces plied together in incremental steps.

*The standard Wallace test blocks are varying compounds of natural rubber, supplied by MRPRA.

The Wallace test blocks are approximately 25 mm x 25 mm in size. The indentor needs to be at least 2 mm from the edge of a test piece in order to meet the standard3. By reducing the size of a standard sample, initially quartered and then reduced by regular amounts, this requirement was investigated. Five tests are performed on each sample and so the minimum sample size that is required in order to conform to the standard is approximately 6 mm x 6 mm. To measure the effects of decreasing the size below the standard specifications, the sample was further reduced.

A sample that is bent can give results that are apparently softer. This is because the load deflects the sample before indentation occurs. The foot force of the Shore-M scale is much larger and the effect of this was examined on both instruments.

By operating both instruments at 33 ± 2 °C, the effect of temperature on the test was compared. The samples and both instruments were kept in the same environment.

The effect of repeated measurements on the same spot was investigated for both instruments because tests might be conducted on a previously measured spot.

The effect of altering the applied foot force (presser foot weight) was investigated because it has been demonstrated that, in the case of the Shore-M instrument, the foot load simply needs to overcome the spring force. The Shore-M foot force was applied using a weight of 514 g and then this was reduced to 152.5 g. Weights were added in increments of 50 g, up to a maximum of 350 g. This meant a total weight of 502.5 g was applied. At each increment, tests were carried out on the standard Wallace blocks.

A microscope was used to view the indentations produced on different samples using both instruments. Samples were examined immediately after testing and had elapsed after a period of time.

Results

Standard Test Blocks

Repeatable results were achieved with both instruments using standard Wallace blocks. The 1 and 3-second Shore-M dwell times gave equivalent results. As hardness values increased, the Shore-M results had an increasing tendency to deviate from the IRHD results. See figure 1 below.

Increasing deviation between IRHD and Shore scales with increasing hardness.

Figure 1. Increasing deviation between IRHD and Shore scales with increasing hardness.

Variations Between Different Sample Types

On both instruments, different rubber samples generally gave repeatable results (table 1). The average results were also consistent between Shore dwell times and instrument types. However, when testing the IRHD instrument, some anomalies were noted. Due to the lack of flatness of the sample, there were imperfections in the sample that caused these anomalies. The same effects were not recorded with the Shore-M results.

Table 1. Samples tested with their standard deviations.

  Standard Deviation (to nearest d.p.)
Sample IRHD Shore, 1s Shore, 3s
Butyl 1.6 1.1 0.9
EPDM 0.9 2.9 1.6
Nitrile 0.7 0.3 0.2
Natural 0.6 1.0 0.6
Chloro compound 13.6H 0.5 1.1
Nitrile/PVC 1.0 0.4 0.8
EPM 0.7 0.4 0.2
NBR 1.4 1.3 1.3
Silicone 2.4 2.0 2.3
FKM 0.9 0.3 0.6

 

Effects of Thickness

As predicted, different results were obtained with the 1, 2 and 3 mm thick samples. Generally, there was a decrease in hardness value as thickness increased, however all results were repeatable. See figure 2.

Example of decreasing hardness with increasing thickness for an unspecified rubber type.

Figure 2. Example of decreasing hardness with increasing thickness for an unspecified rubber type.

HExample of an anomaly due to sample imperfections and the lack of flatness of the sample.

More than one piece was plied when testing samples of 0.1 mm thickness. Overall, the results of each plied thickness did not deviate much from the mean, although a few anomalies were recorded. These anomalies are likely due to the number of tests performed on the top piece of rubber and the flatness of the sample. With increasing thickness, the hardness value of the sample decreased. Initially, the Shore-M produced results that were about 10 IRHD units higher than those from the Micro IRHD. However, the results were comparable once the sample thickness increased to 1.54 mm (figure 3).

Increasing the thickness of a latex surgical glove of initial thickness of 0.1 mm.

Figure 3. Increasing the thickness of a latex surgical glove of initial thickness of 0.1 mm.

Effects of Lateral Sample Dimensions

Results on both the instruments were found to be repeatable despite reducing the sample size. Generally, the results from the Micro IRHD were consistent for the softer rubber (40 IRHD). In contrast, the Shore-M instrument produced rapidly increasing hardness values as sample size decreased (up to a maximum difference of approximately 15 units). Despite the 50 IRHD producing similar results, the Micro IRHD generally gave decreasing hardness values as dimensions decreased (to a maximum hardness difference of 3.5 units). Conversely, the Shore-M results showed the same upwards trend as previously, but with a reduced maximum hardness difference (approximately 7 units). When the 65 IRHD sample was tested with the Micro IRHD, results decreased at the same rate as previously, while the Shore-M instrument produced more stable results (despite a few fluctuations). The hardest sample tested (70 IRHD) produced a general downward trend with a maximum difference of approximately 4 IRHD. In contrast, the Shore-M instrument gave more stable results between the dimensions tested and exhibited a slight upward trend of approximately 1.5 units. Figures 4 and 5 show these extreme cases which test the hardest and softest rubbers.

Extreme differences in results between IRHD and Shore-M tests when reducing the dimensions of the sample of approximately 40 IRHD.

Figure 4. Extreme differences in results between IRHD and Shore-M tests when reducing the dimensions of the sample of approximately 40 IRHD.

Extreme differences in results between IRHD and Shore-M tests when reducing the dimensions of a sample of approximately 74 IRHD.

Figure 5. Extreme differences in results between IRHD and Shore-M tests when reducing the dimensions of a sample of approximately 74 IRHD.

Examples of Sample Flatness

To determine the effects of sample flatness on both instruments, tests were carried out on bent samples. For softer rubbers (40 and 50 IRHD), the samples quickly reverted to the ‘flat’ state and there was minimal difference between the IRHD and Shore readings. The Shore-M instrument gave consistent results for harder samples (60 and 70 IRHD), but the Micro IRHD instrument produced much lower values. For example, in an extreme case of a 70 IRHD sample, there was a difference of 30 IRHD between the ‘flat’ value and the ‘bent’ values. Figure 6 below shows each collection of three points for each of the four standard Wallace blocks. For each case, the first of the three points is a test carried out on an unbent sample, the second is a test on a sample which has been previously flexed and the third is a test on a sample which has been kept in a bent position for 10 minutes.

Harder samples being more affected by bending than softer samples.

Figure 6. Harder samples being more affected by bending than softer samples.

Effects of Temperature

Increasing the temperature to 33 ± 2 °C made minimal difference. The softest sample of 40 IRDH showed no significant change on either instrument. The samples of 60 and 70 IRHD showed a hardness decrease of 0.8 units on the Shore-M and 1.5 units on the Micro IRHD. The greatest effect was on the hardest sample (80 IRHD), which demonstrated a decrease of 1.5 units for the Shore-M and 2.4 units for the Micro IRHD instrument. Observations were carried out on various samples and no general trends were observed with increased temperature, although repeatable results were obtained.

Effects of Repeated Testing in One Location

When the same spot was measured repeatedly, there was a decrease in hardness as the number of tests increased. This was true for both instruments. The Micro IRHD produced more stable results, exhibiting a total decrease of 1.5 IRHD over 28 tests on a rubber that was approximately 65 IRHD.

On the other hand, the Shore-M instrument results decreased much more rapidly and then leveled out, exhibiting a total decrease of approximately 5 units over 28 tests on the same sample. When testing on the same sample, results produced from 1-second dwell time were also noted to be consistently lower than those produced with a dwell time of 3 seconds. Figure 7, below, graphically represents these results.

Repeated testing in one location.

Figure 7. Repeated testing in one location.

Effect of Altering the Shore-M Foot Force

Generally, increasing the foot force seemed to produce increasing hardness. On the samples tested, the effects were small.

Observations of the Sample Surface After Testing

All samples that were tested were examined for surface effects. Particular attention was paid to a small selection of samples, specifically silicone, the standard Wallace test blocks, EPDM, a chloro compound and a fluoro compound. The indentation left by the Micro IRHD instrument can be seen by the eye immediately after testing. The indentation disappears after a few minutes and this demonstrates the non-destructive nature of the IRHD test. In contrast, the samples that were tested on a Shore-M instrument showed a clear indentation both immediately after testing and after about 2 hours. These indentations were observed by eye and microscopically (see figure 8). A week later, the same indentation can be seen, illustrating a permanent deformation experienced by samples tested using a Shore-M instrument.

Photograph of an indentation left on the fluoro compound after being tested on the Shore-M. The magnification used here is 350x.

Figure 8. Photograph of an indentation left on the fluoro compound after being tested on the Shore-M. The magnification used here is 350x.

Discussion

As anticipated, a very thin sample of non-standard thickness influenced the results produced by both instruments. This is a result of the influence of the supporting tables and the effect becomes less apparent as the sample thickness increases.

The Micro IRHD results are generally constant for all hardness values and lateral dimensions tested. However, the Shore-M results demonstrated that, for decreasing sample dimensions on the softer rubber, the apparent hardness rapidly increases. This is less evident for harder samples. Here, the Micro IRHD exhibits a clear advantage.

The degree to which a sample is ‘bent’ is important and, between the two instruments, the different effects are very clear. Due to the small foot force involved, harder rubbers are more affected by lack of flatness on the Micro IRHD. Since the foot force of the Shore-M is large enough to flatten samples, none of the Shore-M results showed fluctuations. For bent samples, the Shore-M has a clear advantage. Increasing the foot force using the heavier foot weight, supplied as standard, can reduce the effects on the Micro IRHD instrument.

Although the effect is small, increasing the temperature by approximately 10 °C appears to have more of an influence on the IRHD results than the Shore-M results.

Repeated testing at the same location makes an appreciable difference to results and this affect is more apparent when the Shore-M is used. It is important to consider this when using standard test blocks to test calibration.

For a reasonable range of forces, the Shore-M result is relatively independent of foot force.

Interestingly, the results obtained from the Shore-M instrument with a dwell time of 1 second differ from those using 3 seconds. Consequently, although the increased time use for comparative Shore-M tests is unimportant, it is necessary for the timing to be repeatable and accurate.

While the Micro IRHD instrument is non-destructive and therefore suitable for testing end products, the Shore-M instrument creates sample penetration. The IRHD takes 35 seconds (specified by the standard) and this is a disadvantage. Previous work by Lackovic et al5 (1997) demonstrates that this time can be reduced using a predictive technique that takes it into direct competition with the Shore-M timing (i.e. 3 seconds).

Conclusion

This article has demonstrated many differences between the two instrument types. There are clear advantages to both instruments in different areas. For example, the Shore-M would be the preferred instrument for testing ‘bent’ samples while a Micro IRHD instrument would be ideal for testing samples with small dimensions. Other considerations that are important when selecting a test method have also been covered. These include having an awareness of the resulting sample penetration from the Shore-M instrument as this becomes important when repeatedly testing the same sample or calibration block and during final product testing. To provide consistent results with the Shore-M instrument, it is important to achieve accurate and repeatable timing. The user can choose and instrument that best meets the needs of the application once the Shore-M instrument has an associated standard.

Additional Work

Additional work is required to investigate the testing of curved samples such as ‘O’ rings, where instrumental differences might be expected to have an effect.

References

  1. Briscoe, B. J. & Sebastian, K. S., ‘An Analysis of the “Durometer” Indentation’, Rubber Chemistry and Technology, Vol. 66, pp. 827-836, 1993
  2. Brown, R. P. & Soekarnein, A., ‘An Investigation of the Reproducibility of Rubber Hardness Tests’, Polymer Testing, Vol. 10, pp117-137, 1991
  3. ISO 48: 1994, Physical Testing of Rubber, Methods for the Determination of Hardness
  4. ASTM 1415-88 (1994), Test Method for Rubber Property – International Hardness
  5. Lackovic, S., Morgans, R. & McGarry, B., ‘Reducing the Duration of IRHD Hardness Tests’, International Conference on Rubbers, Calcutta, Dec. 1997

R. Morgans1 BSc, S. Lackovic2 BSc, PhD, P. Cobbold2
1. School of Engineering, University of Greenwich, Medway Campus, Chatham Maritime, Kent, ME4 4TB
2. H W Wallace & Co. Ltd, 172 St James’s Road, Croydon, CR9 2HR

This information has been sourced, reviewed and adapted from materials provided by Wallace Instruments.

For more information on this source, please visit Wallace Instruments.

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