In this post, we’ll show you how to check the hardness of soft silicone and TPE parts and items with the Shore hardness test. Much of the time the hardness (or softness as it may well be seen) of these items is critical, as they need to have a certain level to reach their expected level of functionality or their applications, quality standard, performance in terms of durability, and ability to comply with industry regulatory standards.

There is a certain scale used, the Shore hardness scale, so let’s get acquainted with this and learn how to check the Shore hardness of silicone and TPE items here.

Introduction to Shore Durometer Scales

The Shore durometer measures the hardness of soft materials such as rubber and elastomers. It operates by measuring the depth of the indentation made by a spring-loaded indenter placed on the surface of the material being examined. The gadget provides a reading on a 0 to 100 scale, with higher numbers representing harder materials.

Here’s an example Shore durometer (courtesy of Schmidt control instruments):

Shore A and Shore D are the two primary scales of the Shore durometer, however, some materials can not be measured with these scales and one of the other durometer scales is required.

Shore A scale

The Shore A scale (seen above) is the most used and is intended for testing softer materials, including natural rubber, synthetic rubber, and polyurethane. The Shore A scale extends from 0 to 100, where 0 represents no resistance to indentation and 100 represents the highest resistance. The Shore A scale may use a cone-shaped indentor with a flat top.

Shore D scale

The Shore D scale is intended for tougher materials, such as thermoplastic elastomers, harder polymers, and rigid thermosets. The Shore D scale similarly spans from 0 to 100, although its values tend to be greater than those of the Shore A scale. The Shore D scale may use a more pointed tapered pin-shaped indentor.

Other scales

In addition to the Shore A and Shore D scales, there are also the Shore B, C, O, and OO scales, which are less frequently employed. These scales are utilized for particular materials and have distinct indentation geometry compared to the A and D scales.

The OO Durometer Scale

The Shore OO scale is intended primarily for the Shore hardness test for extremely soft materials, like gels and soft silicone. Using a Shore OO durometer, the Shore OO scale assesses the indentation hardness of various materials between 0 and 100.

The Shore OO durometer features a larger indenter diameter and shorter spring than the Shore A and Shore D durometers. It is specifically intended to deliver less force to the material being tested, which is required for softer materials to avoid severe deformation.

When testing materials with a Shore OO durometer, it is essential to adhere to the same general testing protocols employed with other Shore durometers. This involves ensuring that the surface being tested is flat and homogeneous and collecting repeated readings from different regions to guarantee uniformity.

In addition, Shore OO values can be altered by temperature and humidity, so it is essential to maintain constant testing circumstances while using this scale.

You can learn more about the Shore hardness test using a durometer in this video:

Shore hardness test probe (indentor) shapes and sizes

There are different probe shapes for each of the Shore durometer scales to accommodate different types of materials. Here are the typical probe shapes for each of the Shore durometer scales:

Shore Scale	Probe Shape	Typical Material Applications
Shore A	Conical, 35° angle	Soft to medium-hard materials, such as natural and synthetic rubber and elastomers
Shore B	Conical, 30° angle	Soft to medium-hard materials, such as foam, sponge rubber, and cellular materials
Shore C	Conical, 35° angle	Materials too hard for Shore A but too soft for Shore D, such as resin and thermoplastic elastomers
Shore D	Conical, 30° angle	Harder materials, such as thermoplastic elastomers, hard plastics, and rigid thermoset materials
Shore O	Circular, 2.40mm diameter	Soft and flexible materials, such as foam and sponge rubber
Shore OO	Blunt circular, 2.40mm diameter	Extremely soft materials, such as gels and soft silicone

 

Correct Checking Procedure

The procedure for the Shore hardness test of a material with a handheld durometer tester:

1. Ensure that the durometer tester is calibrated and in good working condition.
2. Clean the surface of the material to be tested to remove any debris or contaminants that could affect the accuracy of the test.
3. Place the durometer tester on the material, making sure that it is perpendicular to the surface being tested.
4. Apply a firm and steady pressure to the durometer tester, holding it in place for a few seconds to allow the indentor to fully penetrate the material.
5. Release the pressure on the durometer tester and read the hardness value from the tester display.
6. Repeat the test in multiple locations on the material to ensure consistent results.
7. Record the hardness values obtained from the durometer tester and calculate the average value if necessary.

We also created this video about how silicone’s hardness is checked:

 

No reading unless you use the correct durometer device

When hardness testing different materials, it is crucial to use the correct scale because the hardness of a material can vary greatly based on its composition and structure. Using the improper scale can lead to erroneous assumptions regarding the material’s characteristics and performance potential.

Each Shore durometer scale is intended to measure a distinct range of material hardness levels. For instance, the Shore A scale is utilized for measuring the hardness of soft, elastomeric materials such as rubber, whereas the Shore D scale is utilized for evaluating the hardness of stronger, more rigid materials such as plastics.

If the incorrect scale is employed, the durometer may not be sensitive enough to detect minute variations in hardness, or it may be overly sensitive, resulting in inaccurate or inconsistent readings. The results may not be directly comparable when comparing hardness measures across different materials or testing settings.

Using the incorrect scale may also result in problems in material selection and specification. For instance, if a material is defined to have a particular hardness value based on an inaccurate durometer scale, it may not perform as anticipated in the intended application. This can lead to product failures, safety concerns, and increased expenses as a result of rework or product recalls.

 

Graphical representation of the Shore Hardness Scales

The graph below shows a graphical representation of the hardness scales used for each Shore hardness test if they were compared to each other. This graph shows an estimate of the value positions for each scale if they are compared to each other.

Upon examination of the Shore durometer scales, it becomes apparent that certain numerical values are more closely spaced in some areas than others. A notable example is the Shore OO scale, where the values 0 to 50 all fall within the super soft zone. This clustering of values indicates that the scale is more finely tuned to provide greater accuracy in this specific range, particularly when measuring the hardness of super soft materials like silicone implants. Therefore, it is critical to select the appropriate Shore durometer scale to achieve precise and accurate measurements of the hardness of various materials, and in the case of super soft materials, the Shore OO scale may be the most suitable option.

Similarly, the Shore D scale features closely spaced numerical values in the upper range of the scale, specifically within the range of 50 to 100. These values correspond to harder materials that require greater precision in hardness measurements.

A thorough understanding of the characteristics of different Shore durometer scales is essential for selecting the appropriate scale for a given material and obtaining reliable and accurate hardness measurements.

 

Physical examples of different Shore hardnesses

These examples will clarify the typical hardness for some common applications:

SHORE SCALE	PRODUCT EXAMPLE
Shore OO scale Breast implants 10 Shore OO
Shore OO scale Gummy bears 10 Shore OO
Shore OO scale Gel shoe inserts 30 Shore OO
Shore A scale Chewing gum 20 Shore A
Shore A scale Eraser 55 Shore A
Shore A scale Car tire 60 Shore A
Shore A scale Leather belt 80 Shore A
Shore D scale Book covers 10 Shore D
Shore D scale Shopping cart wheels 60 Shore D
Shore D scale Hard hat 80 Shore D

 

Step-by-Step Guide on Measuring TPE Materials

Here are the detailed work instructions for measuring TPE materials using a Shore durometer:

1. Select the appropriate durometer device: Choose a handheld Shore durometer device that is designed to measure the hardness of elastomeric materials, such as TPE. Ensure that the device has the correct probe shape and size for the material being tested.
2. Determine the appropriate Shore durometer scale: Review the properties of the TPE material and determine the appropriate Shore durometer scale to use for the specific range of hardness values expected. For TPE materials, the Shore A or Shore O scales are commonly used.
3. Prepare the sample: The sample should be at room temperature before testing, so allow it to acclimate for at least 30 minutes before testing. The sample should also be clean and free from any dirt or debris that may affect the reading.
4. Position the device: Hold the durometer device perpendicular to the surface of the sample, with the probe in contact with the surface.
5. Apply pressure: Apply enough pressure to fully compress the spring in the device, ensuring that the probe is fully engaged with the surface of the sample.
6. Take the reading: Read the hardness value from the scale on the device where the needle has come to rest. Record the reading for future reference.
7. Repeat the test: Repeat the test at least three times in different locations on the sample to ensure consistency and accuracy of the results. If the results vary significantly, adjust the pressure or positioning of the device and retest until consistent readings are obtained.
8. Interpret the results: The hardness value obtained from the Shore durometer test represents the material’s resistance to indentation or deformation. The higher the number, the harder the material. Compare the results to the expected range for the material to determine if the material is within specification.

Using the correct durometer device, selecting the appropriate scale, preparing the sample properly, and taking multiple readings to ensure accuracy are all critical steps for obtaining reliable and accurate measurements of the hardness of TPE materials.

 

P.S.

For more information about the Shore hardness test and other tests you should be doing on your product, listen to this podcast episode:

• 7 Tests You SHOULD Be Doing On Your Hard Goods! [Podcast]

And you can keep learning about silicone and its properties here:

• Silicone Production Processes, Properties, And Testing [Videos]