Durometer and Shore hardness scales are international standards for measuring the hardness of materials.
When selecting a material for a new application, one of the most important factors to consider is the material’s hardness. It can severely impact its success in an application. For example, compare the hardness of a shoe insole to that of a shoe heel. While both items are often made of rubber, one is far more durable.
Measuring material hardness can also be beneficial to performing maintenance on your system’s materials. A great example is NASA monitoring the hardness of ship tiles in space to ensure they are ready for re-entry into Earth’s atmosphere.
Generally, material hardness is determined using the Shore hardness scale and the durometer hardness test.
What Is a Durometer?
A durometer is a tool designed to measure the hardness of many non-metallic materials, including rubber, vinyl, polyester, leather, nitrile, and neoprene. This tool resembles a tire pressure gauge, as it is composed of a needle on a spring and a number gauge ranging from 0 to 100.
To estimate a given material’s Shore hardness scale, the durometer’s needle will attempt to penetrate the material. The material’s hardness is determined by measuring its resistance to penetration and any resulting permanent indentation. The pressure of this penetration then activates the gauge on the device, giving a number reading between 0 and 100.
Depending on the gauge’s number reading and the size of the durometer needle used, this measurement will fall under certain Shore hardness scales.
What Are Shore Hardness Scales?
To understand Shore hardness scales, it’s important to note that a durometer is not a unit of measurement. Rather, a durometer is a dimensionless measurement based on the relative hardness of various materials. Thus, the Shore hardness scale establishes a common reference point for comparing these materials.
Durometer is bucketed into three Shore categories that rank on a hardness scale from 0 to 100:
- 00 – Measures light foams, sponge rubber, and soft gels
- A – Measures soft rubber, plastics, and rubber-like elastomers
- D – Measures hard rubber, plastics, and thermoplastics
Each of the three Shore scales uses a durometer with a different-sized needle. Therefore, certain Shore categories are best for measuring certain kinds of material.
Regardless of which Shore scale the measurement falls under, higher numbers mean higher resistance to the durometer needle, while lower numbers show lower resistance. Materials that score lower are softer, while materials that score higher are harder.
The image below depicts various materials and what Shore hardness scales they fall into.
Note: Materials can fall into more than one Shore category. If the material does fall into more than one category, the measurement numbers will likely differ from one Shore type to another.
The illustration below shows the difference between a Shore A and a Shore D material.
Various industries and applications will require different materials, so it’s important to perform durometer hardness tests before implementation. For example, softer compounds that offer less resistance are fine for low-pressure seals where the material must conform to rough or uneven surfaces. In contrast, harder materials are more appropriate for high-pressure seals.
How is the hardness of plastics determined precisely?
To measure the plastics’ hardness, a piece of plastic is put into the Durometer’s indenter foot.
Before applying pressure to the material, a needle is put against the plastic.
A Shore hardness test, carried out in accordance with ASTM D2240, ISO 868, and ISO 7619-1, can be extremely important to engineers.
The test material’s surface will be level and set up in an area with enough room for the Durometer’s foot to readily come into contact with it.
This will be within a minimum 6mm radius of the indenter point. The smooth surface is crucial because surfaces that are rounded, rough, or uneven prevent a durometer from providing an accurate hardness value.
- In cases where it is practicable, test items shall be conditioned for a minimum of one hour before testing.
- The test piece will be placed on a hard, flat, or solid surface, and pressure will be exerted as rapidly but smoothly as feasible.
- Durometer measurements will be collected at specific intervals of three seconds for vulcanized rubber and 15 seconds for thermoplastic rubber.
- Five measurements will be made in total, at various locations on the test item. The mean value will be computed, recorded, and each will be noted.
- The needle used to measure the plastic will display a corresponding measurement pertaining to its hardness if the needle has entered the plastic as far as it can and the gauge has been firmly placed against the material.
It all boils down to how flexible the rubber or plastic is, and since this reading might fluctuate, it’s crucial to record both the time the measurement was made and the number that corresponds to its hardness.
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