The Shore Durometer Scale is a standardized way of quantifying the hardness of a material, usually rubber or plastic. The higher the number (or farther to the right) on the durometer scale, the “harder” the material with greater indentation resistance. The lower the number (or farther to the left) on the durometer scale, the “softer” the material with less indentation resistance.
There are several variations of the Shore durometer scale, but the most commonly used for measuring rubber hardness are as follows:
- Shore 00 Hardness Scale which measures soft rubbers and gels.
- Shore A Hardness Scale which measures flexible mold rubbers that can be very soft and flexible to very hard with little to no flexibiltiy. Custom Rubber Corp. uses this scale.
- Shore D Hardness Scale which measures the hardest types of rubber.
As seen on the chart, These scales run parallel to each other on the same axis, meaning there is overlap between each one. For example, a Shore durometer of 65A is also a Shore 19D. Both labels can be true for the same material. It just depends on what Shore durometer scale you’re using.
“Durometer” vs. “Hardness”
Durometer is used interchangeably in the industry with the term “hardness.” Hardness is defined as a material’s resistance to being permanently indented or penetrated by an object.
Knowing the hardness, or durometer, of the material you’re working with matters for your final outcome. Softer rubbers should not be used for delicate or small parts that may break easily. Harder rubbers should not be used for parts like seals and gaskets that require a bit of flexibility to operate properly.
Another thing to remember is that hardness does not equal strength. A rubber can be hard as indicated on the durometer scale, but may tear easily and show poor resistance to wear and tear.
Is the Shore durometer scale different from rubber hardness?
Albert Ferdinand Shore developed both the Shore Hardness gauge and Shore durometer scale in the 1920s to measure the hardness of a material. However, his inventions were not the first or last of their kind.
The Shore durometer chart is sometimes more colloquially known as the “Rubber Hardness Chart,” or another, simpler term. But because there are several different rubber hardness scales and tests, it’s important to clarify you’re working within the Shore durometer chart for your measurements.
Other tests (like the Rockwell chart) use different terminology to describe the hardness of the same material. Tests can’t really be accurately compared as they tend to be so different. It would be akin to comparing apples and oranges. So while some people use the term “rubber hardness scale” or “rubber hardness test” to refer to the Shore durometer scale, it’s important to clarify and use specific terminology to make sure you’re on the same page.
How to measure durometer
Durometer is measured using a Shore Hardness gauge, which looks similar to a tire pressure gauge. Both are long tools with a needle on a spring attached to one end. That end is placed on the material being measured and the operator applies pressure.
Credit: Amazon.com
Once the needle has dug as far as it can go into the material without tearing or breaking, the Shore Hardness gauge will determine what number on the Shore durometer scale is indicated by the amount of pressure able to be applied.
With this in mind, know that there are two things represented by the term “durometer.” Within the industry, durometer can officially refer to:
- The needle-pressure gauge tool used to measure hardness
- The name of the scale itself
When used to measure durometer, the Shore Hardness gauge will display a number ranging from 0-100. Unlike a meter, which can be concretely defined as 100 centimeters or 1,000 millimeters, there is no dimension for durometer. The number on the scale functions only in comparison to the hardness of other materials measured on the same scale.
Why does the Shore durometer scale matter?
Simply put, hardness is subjective. You may think a pencil eraser is a hard rubber if you’re comparing it to something like a rubber band, while someone else may perceive it as soft and squishy, comparing it in their mind to a tire.
Rubber is a big industry with suppliers, distributors, molders and everyone in between. In order for all of these organizations to communicate effectively, everyone has to be speaking the same language. The Shore durometer scale gives a universal point of reference for rubber hardness that everyone can refer to.
With the durometer scale in mind, a customer will know how hard his rubber boots will be when they arrive, and if they will meet safety standards while maintaining flexibility. The boot manufacturer will be able to take those safety standards and repeatedly make identical boots of the same hardness over and over. Having a point of reference makes it easier for everyone on the supply chain to communicate effectively.
Common products and their corresponding durometer
Within the rubber molding industry, most products fit under the Shore A durometer scale, which ranges from flexible rubbers to rigid, immovable rubbers. Harder rubbers are measured with Shore D and super soft rubbers, or gels, are measured with Shore 00. At Custom Rubber Corp., we use the Shore A durometer scale to measure rubber hardness.
Here are some common products used to pinpoint certain levels of hardness on the Shore durometer scale. Presented here in order from softest to hardest.
- Rubber Band: Shore 20A
- Pencil Eraser: Shore 40A
- Car Tire: Shore 60A
- Tennis Shoe Sole: Shore 70A
- Belt: Shore 80A
- Shopping Cart Wheel: Shore100A
Remember, 100 is the “hardest” measurement on the Shore A scale. Rubbers harder than a shopping cart wheel should be measured using the Shore D durometer scale.
The Shore durometer scale is a self-referencing scale useful for uniting rubber producers, molders and consumers on common ground. Knowing the hardness of a material is non-negotiable in creating the best possible end product for your customers.
Contact Custom Rubber Corp. today to learn more about our experience in adjusting rubber hardness and choosing the best rubber material for your application.