What is Shore Hardness & Why Does It Matter?

Shore hardness is a property that determines how hard a material is, or rather, its resistance to indentation when force is applied. When talked about in the polymer manufacturing industry, you’ll often hear people say, “what durometer are you using?” Durometer is the tool used to test hardness and is done on an A or D scale, as the hardness of one material is compared to that of other materials using the same scale.

Shore hardness is an essential property to know because it helps users determine which material will work best for their application. Volatile Free, Inc.’s on-site lab staff tests the durometer of all our products using Shore hardness scales, so you know a material’s capabilities by looking at its properties.

Types of Shore Hardness Scales

In 1920, Alfred Shore invented a device similar to a tire pressure gauge to determine material hardness. With this tool, different Shore hardness scales were developed to group and test materials with similar characteristics. Determining which scale to use depends on whether you’re looking at a flexible and soft or stiff and hard product.

Though there are many types of durometer scales, the ones commonly used for polymers are Shore A and Shore D. They use a standard test method called the ASTM D2240. Each scale ranges between 0 and 100, but materials are tested using a different combination of force and indenter shapes. Regardless of the scale, lower numbers mean the material is softer and has less resistance to indentation. Higher numbers mean the material is more rigid and has higher resistance to indentation.

The American Society for Testing and Materials (ASTM) does not recommend using data outside the 20-90 range for each scale because that data may not be accurate. If a hardness is determined to be outside this range, you might be using the wrong scale. Even though the data from one scale may overlap with data from another, they should not be compared.

The numbers on these scales are typically categorized as extra soft, soft, medium soft, medium hard, hard, or extra hard. Shore A is best utilized for softer materials, while Shore D is best utilized for hard materials. Also, be aware that the scales do not predict other properties, such as tensile strength, elongation, and resistance to scratching, abrasion, or wear.

What is the Shore A Scale?

Shore A is one of the scales used to test the durometer of softer materials. It is widely used in the polymer industry for rubbers, elastomers, rubber-like coatings, and other flexible materials. If the material is extra soft and gel-like, it may be measured on the Shore OO scale rather than the A scale to get the most accurate reading.

This scale uses a hardened steel rod with a blunt, truncated 35° indenter cone. The tip diameter is 0.79 mm, and the applied spring force is 8.05 N (822 g). When the force is applied, softer materials will have deeper indents from the presser foot than harder materials.

What is the Shore D Scale?

The Shore D scale compares the hardness of semi-rigid to very hard materials such as plastics, rigid rubbers, or plastic-like hard coatings. Though the Shore A scale can be used for some of these materials, it’s recommended to use the Shore D Scale when the tested material passes 90 A for accuracy. Shore D hardness is good to know for part making and hard coating, as it factors into whether the material will have the durability and longevity needed for a specific application.

This scale uses a hardened steel rod with a sharp 30° indenter cone. The tip diameter is 0.1 mm, and the applied spring force is 44.45 N (4536 g).

How is Durometer Tested?

To test for hardness, a set of conditions must be met to get a proper reading. The test material has to be at least ¼ of an inch thick with a smooth surface. Ensure your sample surface and the indenter tip are clean. The test material must be in a climate-controlled environment of about 73.4°F (23°C), as temperature and humidity can affect the reading. Some materials may provide lower readings at higher temperatures and high readings at lower temperatures.

The process of testing is as follows, regardless of which scale you are using:

  • Place the test material on a hard, flat, consistent surface.
  • Place the presser foot against the test material so it is flat and in full contact with the surface. The calibrated spring within the device will apply the necessary pressure for an accurate reading.
    • Optionally, there are stands that will be perfectly perpendicular and will press at a controlled weight for the best repeatable results.
  • The final reading depends on the indenter depth after pressure has been applied for 15 seconds. Record the value found by looking at the gauge on the tool.
  • Repeat this process a minimum of 5 times in different places on the test material to minimize errors caused by external factors. Each spot tested must be a minimum of ½ an inch from each edge on the sample. Calculate the average to determine the Shore hardness.

Note: Always note where in the cure cycle you are when testing the hardness of the material, because the hardness will continue to change until full hardness is reached.

Importance of Durometer for Rubbers, Plastics, and Coatings

Comparing products using Shore hardness scales will allow you to identify if a material is suitable for your application. Manufacturers like VFI can also customize formulas to accommodate a specific hardness based on the needs of the user.

If you are working with molding rubber, you should ask yourself how flexible the mold needs to be to easily demold from the model. If you have a very delicate model, you will want to use a softer rubber (20-30 A). A softer material will release more easily from delicate pieces, undercuts, and extreme details without breaking the model. These lower hardness rubbers will be great for making cast stone and manufactured stone molds. On the other hand, you probably want a harder material if undercuts and flexibility are no longer a concern. For example, rubber formliners are typically between 50-90 A. Higher durometer rubbers also have better abrasion resistance, which is necessary when dealing with an abrasive material like concrete.

If you are in the part making industry, you will want to look at materials on the Shore D scale or the higher end of the Shore A scale. These products typically provide the necessary rigidity to make long-lasting parts or, in specific applications, provide rigidity with enough flexibility to not break, crack, or tear. Harder plastics will have more strength and can be machinable, while softer plastics or rubbers will provide more impact resistance.

Coatings are also tested under these hardness scales. They are desirable when they are hard but flexible enough to resist cracking or tearing, which could expose the underlying substrate. Many thick film coatings will fall into the Shore D hardness range. Hard coatings with Shore hardnesses of 65 D or higher will feel very plastic-like after curing and are best for hardening foam and other fragile surfaces. On the other hand, coatings around 50 D and under will have more flex for protecting firmer surfaces like metal, wood, and concrete. However, some coatings are rated on the Shore A scale because they feel rubber-like after they cure.

Contact VFI if you need help figuring out whether a material has the hardness you need for your project.