Durometer Shore Hardness Scale Explained

July 30, 2020

Whether you’re making molds or are in the midst of a new construction project, having the right materials is essential to get the job done. When working with non-metallic objects like rubber and plastic, it’s important they’re either flexible or hard enough for the project you’re working on.

To determine if your materials are the right hardness to complete what you’re building, you can use something called the durometer scale. The durometer scale measures the Shore hardness of rubber, plastics, silicone, and other polymers and elastomers.

To help you get started on your task, we’ve created this guide on understanding the durometer scale, how to measure Shore hardness, and other useful tips.

What is Durometer Shore Hardness?

Durometer Shore hardness is the measure of how resistant materials, typically rubber, plastics, and other non-metallic materials, are to indentation. This is just one of the many terms you’ll find in our mold casting glossary that will prove useful. A durometer scale, also called a Shore hardness scale and rubber hardness scale, was created so people can have a point of reference to compare different materials, such as soft rubbers, rigid plastics, gels, and other non-metallic materials. The scale looks at how resistant these materials are to indentation, which can also be thought of as how flexible the materials are.

There are three main Shore hardness scales that measure the hardness of various materials. There’s the Shore 00 Scale, Shore A Scale, and Shore D scale. Here’s what each scale measures:

• Shore 00 Scale: The Shore 00 scale measures the softest rubbers, gels, and other similar materials. Examples of items that are measured using the Shore 00 scale include objects like gel insoles for shoes and gummy bears. The Shore 00 scale indenter has a spring force of 113 g.
• Shore A Scale: The Shore A scale measures the hardness and flexibility of a wide range of materials, from very soft mold rubbers to semi-rigid plastics. Examples of items that are measured using the Shore A scale include soft items like rubber bands to more rigid objects like shoe heels on dress shoes. The Shore A scale indenter has a spring force of 822 g.
• Shore D Scale: The Shore D scale measures the hardest of rubbers that have little to no flexibility. The Shore D scale is used to measure hard rubber, semi-rigid plastics, and hard plastics such as hard hats worn by construction workers and PVC pipes. The Shore D scale indenter has a spring force of 10 lbs.

Overall, there are about 15 different Shore hardness scales, which depends on their intended use. Each scale ranges from 0 to 100, with 0 representing the softest materials and 100 representing the hardest materials. However, Shore scales 00, A, and D are the most common scales used to measure the hardness of rubber, plastic, and other non-metallic materials.

Why Does Durometer Shore Hardness Matter?

Durometer shore hardness matters for a variety of reasons. Whether you’re casting small, fragile objects in a mold or creating hard and rigid objects for a construction project, it’s important you use the right durometer scale to ensure the rubber, plastic, or non-metallic material you’re using is soft or firm enough for the intended purpose of the object. Engineers are often faced with the dilemma of determining which rubber is best for a certain application. The Shore hardness scales serve as a tool to identify which rubbers are best for certain types of applications.

For example, you wouldn’t want to use rubber with a Shore hardness of 75A to mold a small figurine like a doll or action figure, as this rubber is thick and usually used for items like car tires. If you used rubber with a Shore hardness of 75A to cast an object like a doll, fragile pieces, like the doll’s arms and legs, would break because the rubber is too hard. Instead, you’d want to use rubber or silicone with a Shore hardness around 28-32A, such as RTV silicone rubber. Rubber or silicone with a Shore hardness around 28-32A will be flexible enough to release from the mold without breaking.

Rubber with a Shore hardness around 75A or more is most common for flat objects, such as paving stones. In a case like this, you’d use concrete stamps made from urethane rubber to cast flat objects that can easily be released from the mold without breaking.

What is the Durometer Scale?

Durometer measures the Shore hardness of a material, and this hardness is placed on a durometer scale. The durometer scale is used by engineers and others working with rubber, plastics, and other polymers to compare the hardness of a wide range of objects.

To give you an idea of the Shore hardness of different materials, we’ve created two durometer scales. The first durometer scale includes the Shore hardness of common objects, such as pencil erasers and hard hats, while the second durometer scale consists of the Shore hardness of a variety of AeroMarine’s products.

The first rubber hardness scale contains everyday household items. To give you an idea, here are some everyday objects and their shore hardness:

Download the Durometer Shore Hardness Scale here. 

Here at AeroMarine, we offer a wide range of products, such as epoxy and silicone, that can be used for various projects, ranging from mold making to potting and encapsulation. Take a look at the Shore hardness of some of our most popular products:

Download AeroMarine’s Durometer Shore Hardness Scale here. 

How Do You Measure Shore Hardness

The Shore Scale was created by Albert Shore back in the 1920s. He was a metallurgist and is credited as the founder of the durometer, which is the instrument used to measure Shore hardness. A durometer gauge is used to measure Shore hardness and is a relatively simple device. It contains a spring-loaded steel rod that compresses the surface of the rubber, plastic, or other non-metallic samples, to produce a reading.

When Shore first invented the durometer, he created the durometer scale. It first began with scale “A,” which measured softer materials. However, as materials became stronger and harder, new scales needed to be made, which resulted in scales B, C, D, and so on. As time went on, some materials were even becoming softer, which then led to scale “00.”

Each scale uses different spring forces and different indenters to measure the indentation levels of certain materials. This indenter, or needle, penetrates the sample and measures how far the indenter travels, up to .100”.

The Shore hardness scales also have different hardness ranges. The Shore 00 scale measures ultra-soft materials, the Shore A scale measures the hardness of soft to semi-rigid materials, and the Shore D scale measures extremely rigid materials. Today, scales Shore 00, Shore A, and Shore D are the most commonly used scales for measuring the hardness of rubber, plastics, silicone, and other polymers.

When measuring durometer hardness, follow these steps:

• Step 1: Choose the appropriate Shore hardness scale to measure the hardness of your test sample.
• Step 2: Place your test sample on a flat, horizontal, and hard surface that is at least ¼-inch thick.
• Step 3: Hold your durometer with both hands and place it on your test sample. Ensure the indenter is touching and firmly-pressed on the sample.
• Step 4: Take a reading. If possible, take multiple readings on different parts of the sample and calculate the average for a more accurate result.

When you read the results from your durometer, it’s important to know what the numbers mean. Higher numbers closer to 100 mean there’s a greater resistance to indentation, which means the object is harder and more rigid. Numbers closer to 0 are more prone to indentation and are less rigid. It’s also worth remembering that each scale has its own range. So, a hardness of 90 on the Shore 00 scale is only a 60 on the Shore A scale, and a 0 on the Shore D scale.

During some projects, you may realize the materials you’re working with have a higher Shore hardness than you need. If you’re working with silicone, such as our silicone encapsulation rubber, for example, you can reduce shore hardness with a silicone thinner that can be added to the silicone base to reduce both viscosity and Shore hardness.

Understanding Durometer Shore Hardness

The durometer scale is used to measure the Shore hardness of non-metallic materials such as rubber, plastic, silicone, and other polymers and elastomers. Whether you’re an engineer or an everyday hobbyist, understanding durometer hardness is key to ensuring your molds come out perfect.

There are numerous Shore hardness scales. Each dependent on the materials you are using and their hardness. The three most common rubber hardness scales are the Shore 00 scale, Shore A scale, and Shore D scale. With this guide on hand, you’ll be able to compare the hardness of different materials, whether it be soft rubbers or rigid plastics.

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