Polyurethane is one of the most versatile products in the world, used from industrial machinery components to consumer products. Its versatility is largely accredited to its ability to be custom-made to certain hardness levels. These custom hardness levels are quantified by the Shore durometer scale, a scale that measures the hardness of materials, generally elastomers and polymers.
Choosing the correct Shore durometer value is a crucial part of selecting the right polyurethane product; if it’s too soft, the material may fail under load, while if it’s too hard, it may not provide the necessary flexibility or cushioning.
Today we look at choosing the correct polyurethane product based on Shore durometer values, in order to be certain that whatever the application, you have the correct material for the task. We start our discussion by examining the Shore Hardness Scale. We unpack the discussion of the scale by sharing practical use cases for polyurethane products within various Shore hardness scales. We cap it all off by looking at the factors to consider when choosing the correct Shore hardness for the application.
This won’t be too hard, so let’s get going.
Understanding the Shore Durometer Hardness Scale
Understanding how Shore hardness scales work is essential in choosing the right material for the intended application. As an example, the correct Shore durometer scale for a roller bearing will differ from the one for a gasket. To start, we first need to understand what Shore durometer means.
What is Shore Durometer?
Shore durometer is a standardized measure of the hardness of a material, mainly that of elastomers and polymers. The scale was developed by the American metallurgist Albert F. Shore and quantifies the resistance of a material to indentation. This numeric value is referred to as the Shore durometer value of a material.
Lower durometer values indicate softer materials, like gel shoe insoles, while higher values represent harder materials like hard hats. The scale is further divided into specific Shore scales, namely Shore OO, Shore A and Shore D.
Types of Shore Durometers:
Specific Shore scales are designed to measure different levels of hardness. The Shore OO scale measures products that are made from softer materials, like urethane sheeting for sound dampening. Shore A products are harder than Shore OO but generally softer than Shore D, think rubber bands or shopping cart wheels. Shore D products are the hardest of the lot, these products are used to make products like bushings, gaskets and automotive bumpers.
The next part may be a bit technical (so be sure to have a cup of coffee at hand!): the Shore durometer values of a product, within each specific Shore scale, are further numerically defined. These numeric values typically range between 1 and 100 and indicate the hardness of the material within that Shore scale. As an example, 25 Shore OO products are extremely soft, like the gel shoe insole mentioned previously, and 50 Shore OO products are harder, like a rubber band.
The different Shore scales also overlap, a perfect example of this is a rubber band. A rubber band can be measured on the higher end of the Shore OO scale, having a durometer value of approximately 50 Shore OO; but it can also be measured on the Shore A scale and there it has a value typically ranging between 10 Shore A and 35 Shore A, the softer end of this Shore hardness scale.
In summary, the Shore durometer value of a material will showcase the applicable Shore scale as well as its quantified relative hardness within the scale. Consulting with an expert is advisable to ensure you are fully informed regarding the material’s hardness.
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Testing and Measuring the Shore Hardness Scale
The Shore hardness value of materials is tested using a Durometer Gauge. This device resembles a pen with a gauge at its top and assesses a material resistance to indentation. It works by placing the tip of the durometer gauge on the surface to be tested and from there it is pressed into the surface. The durometer gauge then measures the resistance the material offers to indentation.
Durometer gauges are designed to measure a specific Shore scale, i.e. a specific durometer gauge would be calibrated to measure Shore OO products. To measure Shore A materials another durometer gauge would need to be used. Durometer gauges are calibrated according to ASTM D2240.
Watch our video on Polyurethane Durometers to better understand the various Shore scales and how they are tested.
Common Applications of Various Shore Durometers
Sure, measuring the Shore durometer allows the quantification of a material’s hardness, but what does this mean in practice?
To illustrate the use of different durometer values, we look at some common uses of each of the Shore hardness scales. Please note that some applications can also use Dual Durometer configurations like caster wheels and industrial rollers. This means the different materials used in a product differ in durometer value. As an example, a roller can be composed of a harder Shore D polyurethane at its core, while the outer layer can be made of a softer Shore A polyurethane. If you want to learn more about Dual Durometer parts click here.
With that understanding, let’s explore some specific applications:
Soft Polyurethane (Shore A):
Soft polyurethanes range typically between 10 Shore A and 60 Shore A. They are designed for applications where flexibility, cushioning and sealing are required. This would typically be a polyurethane gasket used in flow systems. Soft polyurethanes can also be used in applications that require impact absorption and vibration dampening, like protective padding and flexible couplings.
Medium Polyurethane (Shore A/D):
Printing urethane rollers are predominantly used in the printing and publishing industry, as the name suggests. These rollers work by providing a uniform surface that evenly distributes inks across printing substrates like paper and fabrics.
Medium Polyurethanes typically range between 60 Shore A and 90 Shore A, or 40 Shore D and 60 Shore D. They are used in applications where moderate load bearing and some degree of flexibility are required. Examples include wheels and rollers where a mix of resilience, toughness and vibration absorption is required.
Hard Polyurethane (Shore D):
Hard Polyurethane is at the upper end of Shore D durometer values, they typically start at 60 Shore D and are used in applications where toughness and resilience are paramount. Application of hard polyurethanes include hard hats, automotive bumpers, or boat dock bumpers. The high Shore durometer value of these products assures minimal deformation under stress, providing reliable performance and long service life.
Whether soft, medium or hard, PSI is there to assist you in getting the precise part required for the job.
Factors to Consider When Choosing a Shore Durometer
Now that a better understanding of the Shore durometer scales, values and testing has been obtained; the next step is to consider the factors that must be taken into account when selecting the correct hardness for your polyurethane product. Using a Shore D product for shoe insoles may turn out to be a bad decision for your toes, similarly, Shore OO products have absolutely no place in stopping an object on a collision course with your head.
Application Requirements:
Selecting the correct durometer value for your polyurethane product starts with understanding the specific demands of your application. Different applications require different hardness values. As an example, seals and gaskets often need to be soft to conform to surfaces and provide airtight and watertight seals. In such a case a lower Shore A Durometer is more suitable. On the other hand, the wheels of a shopping cart will benefit from higher durometer values, such as a high Shore A durometer.
Load and Stress Factors:
The load and stress that a product will endure are also an important factor to consider when choosing the correct Shore durometer value for the application. In environments where parts are subjected to heavy loads or constant mechanical stress, higher durometer values are likely best suited. In applications where grip is required and a product is not subject to constant mechanical stress, a lower durometer value may be better suited.
Environmental Conditions:
Environmental conditions also play a role when selecting the correct Shore durometer value for the application. High temperatures may lead to soft polyurethane products becoming more pliable with rising temperatures. Harder polyurethane, on the other hand, may become brittle at lower temperatures.
Other environmental factors like chemical resistance, such as in the case of choosing the correct polyurethane roller for a conveyor system, and UV exposure in the case of choosing bumpers at a dock, will also contribute to selecting the correct Shore durometer value.
If you are unsure what Shore hardness is needed for your polyurethane product, give PSI a call and let one of our experts assist you.
The different Shore durometer classifications and the numerous applications thereof truly show the versatility of Polyurethane. If you are curious about how PSI can assist you in implementing this versatile material in your operations, our experts are one click away from assisting you.