Elastomers have many physical properties that impact a parts ability to provide an adequate seal in varying environmental conditions. These characteristics include tensile strength, elasticity, compression set resistance, low temp performance, high temp performance, and swell in certain fluids and chemicals, just to name a few.
One property often discussed when considering rubber for sealing applications is the hardness of the material, also called the durometer. The higher the durometer, the harder the compound. For example, rubber bands are typically around 20A to 40A durometer, pencil erasers approximately 40A durometer and shopping cart wheels around 95A durometer.
Why is durometer important to your application?
Durometer plays a crucial role in the manufacturing process of many silicone, rubber, and plastic materials and can help determine the item’s resistance to penetration. Being able to establish the hardness of the materials used in products is important regarding quality control and even design approval.
Softer compounds stretch easier, making the installation of a seal easier. They also provide a better seal on rough surfaces, by filling the microvoids and imperfections in the sealing surface. When sealing delicate hardware that cannot withstand a lot of pressure or force without deforming or breaking, a softer compound is often a better option. Harder compounds typically perform better in high-pressure conditions because they offer superior abrasion resistance and extrusion resistance.
In dynamic applications with moving parts, determining the appropriate hardness of the material becomes even more critical and complicated. Although a harder compound generally has a lower coefficient of friction than a softer material, its break-away friction values can be higher due to the compression required to achieve a sufficient seal with the harder material.
While the hardness of the rubber compound is critical to its performance in a sealing application, this seemingly simple concept, durometer, is one of the most contested and least understood physical traits in rubber compounds, primarily because it can be so difficult to measure.
Durometer Testing of Rubber
Durometer is measured using a Shore Scale named after Albert Shore, the founder of the Shore Instrument Company. Shore was an American metallurgist who invented the first quadrant durometer in 1915 to measure the hardness of polymers and other elastomers.
The durometer scales are known as types of Shores, with the difference being the shape of the indenter and the tension of the calibrated spring. Shore durometers most often used for manufacturing plastics, rubber, and silicones are rated on the A, D, and OO Shore scales. The Shore OO Scale measures softer materials such as gels, the A Scale ranges from soft to flexible and hard materials that offer minimal flexibility, and the Shore D Scale is better suited to measure the hardness of very hard plastic and rubber materials.
In terms of molding rubber, a durometer is important to help determine how easily the original model can be extracted from the rubber mold after the curing process has been completed. The Shore Hardness Scale ranges from extra soft up to extra hard. Extra soft materials, for example, could be gummy type candies or gel shoe insoles. When the scale reaches the higher points of hardness, it is typically referring to objects such as shoe heels, shopping wheel carts, and hard hats. Throughout the seal industry, the Shore A durometer scale is the standard used to measure hardness for most rubber compounds. Manufacturers only test and certify hardness based on the Shore A Scale.
Shore Durometer Testing
The Shore durometer tester consists of a reference presser foot, an indenter, an indicating device, and a calibrated spring that applies force to the indenter. When exposed to a softer material, there will be very little resistance present on the indentor pin. However, a harder material will cause the indentor pin to push back further against the indentor pin on the durometer.
During the test, force is applied as rapidly as possible, without shock, and the hardness reading determined after a duration of 15 sec +/- 1 sec. Hardness is measured based on the depth of indentation by a standard size and shape of impacting gauge. The reading will be between 0 and 100 when determining the amount of pin movement that was present while testing the material.
The shape of the indenter, the force applied, and the test duration all influence test results.
Components of Shore Durometer Tester
- Presser Foot: this can usually be found toward the bottom of the durometer and is flat and perpendicular to the indentor.
- Indentor: the indentor sticks out from the presser foot, and any movement in its placement during testing helps determine the hardness of the material
- Calibrated Spring: attached to the indentor the calibrated spring is what offers the resistance to the indentor’s movement. With recent technology improvements, the testing has become more accurate and precise than before in older durometers
- Indicator: indicates the hardness of the material after testing has been completed. The durometer can include either a digital or analog display to reveal the results
Limitations of Durometer Testing for your Rubber Parts
- Rubber manufacturers only test and certify hardness based on the Shore A Scale.
- The Shore A tester is only valid for flat surfaces with a minimum thickness. [ASTM D-2240 – the minimum thickness is .24 inches; ISO-868 – the minimum thickness is .157 inches]
- Many rubber components are not flat, and do not have the required minimum thickness for Shore A testing accuracy
- Hardness measurement accuracy is +/- 5 points due to mechanical limitations of instrumentation
- Rubber batches contain inherent minor variations from batch to batch based on slight differences in raw materials and processing techniques causing variations in durometer readings
- If specimen thickness is not sufficient the gauge reading is impacted by the base material (In the case of ASTM requirements, this is a flat steel surface)
International Rubber Hardness Degrees – IRHD Scale
Many organizations are now using the IRHD Scale which is a tester used for curved surfaces. The size and shape of the indenter is much smaller, therefore allowing for a more accurate reading on curved surfaces such as an O-ring. Unfortunately, there is not a direct correlation between the IRHD readings and the readings of a Shore A Scale. Manufacturers only test and certify hardness based on the Shore A Scale. So until the manufacturers of the industry adopt this standard it is not a valid test for ensuring the durometer of your parts.
Important Considerations for Durometer and your Rubber Parts
Durometer is primarily a measurement of the elastomer’s response to a very small surface stress (surface indentation), which is only one component in determining its effectiveness for performance in your application. Candidly this measurement of surface indentation to determine hardness usually does not bear significant relation to the ability of the elastomeric part to perform properly. Stiffness and compression modules measure the response to large stresses of the entire part and are a much more accurate indicator as to how the seal will perform.
That being said, hardness can be a factor for a seal in certain instances. If durometer is important in your application and it is a required measurement for approval through incoming inspection, then it is critical to request buttons with each new order shipment to increase the possibility for accurate durometer readings.