Shore Durometer

A Shore Durometer is one of the most useful tools that one can use to determine the material used for the mold cast. Depending on how intricate the design of the object to be casted, the flexibility of the mold rubber will be determined. Objects that are more complicated features or many protruding and extending arms would require a mold with more flex that will allow for easy removal after it has been casted.

As such, a Shore 30A durometer rubber would be more suitable than a Shore 70A durometer rubber that is firmer and is less flexible. Instead, a Shore 70 durometer rubber is more suitable for making molds of flat objects with an even surface like tiles and paving stones. These objects require less flexibility in their molds as they can be easily removed.

Durometers are also critical when it comes to choosing the type of silicone rubber for use in heat sealing packaging applications. High temperature heat sealing silicone rubber that has a Shore 70A to Shore 90A score is recommended for use on the side that comes into contact with the heating tools. As compared to silicone rubbers that have a lower durometer score, a Shore 70A durometer rubber is able to provide better heat transfer which translates to more efficient heat sealing.

To understand Shore hardness scales, it's important to note that a durometer is not a unit of measurement. Rather, a durometer is a dimensionless measurement based on the relative hardness of various materials. Thus, the Shore hardness scale establishes a common reference point for comparing these materials.

Durometer is bucketed into three Shore scales. Each Shore scale ranks in hardness from 0 to 100:

00 – Measures light foams, sponge rubber, and soft gels
A – Measures soft rubber, plastics, and rubber-like elastomers
D – Measures hard rubber, plastics, and thermoplastics

Each of the three Shore scales uses a durometer with a different-sized needle. Due to this variety in needle size, certain Shore scales are best for measuring certain kinds of material.

Regardless of which Shore scale the measurement falls under, higher numbers mean higher resistance to the durometer needle, while lower numbers show lower resistance. Materials that score lower are softer, while materials that score higher are harder.

To measure the hardness of rubbers and plastics, a needle attached to the durometer will be placed against the material. Pressure is then applied to the durometer for the needle to press firmly perpendicularly into the material and the reading taken will indicate the hardness measured for the material. Sufficient force has to be applied to the durometer to ensure that readings taken are as precise as can be.

Shore A durometers would require less force at 0.822kg while a Shore D durometer would require a higher force of 4.55kg. This is because materials measured with Shore D durometers are of a harder nature and thus would require more force to be applied to take accurate readings. Regardless of the type of durometer scale, even pressure should also be applied for 15 seconds.

It is very important to ensure that the material tested has a flat surface because hardness readings with a durometer will not be accurate if it were done on surfaces that are round, rough or uneven. The sample of material tested should also be at least 19mm by 44mm in size with a thickness of 6mm.

Several thin layers of the material may be layered to achieve the 6mm thickness requirement but care must be taken to avoid air from being trapped between layers. An average of 5 readings should also be taken, each at least 13mm from the edges and 6mm apart from one another to ensure fair and uninterrupted readings.

Silicone extrusions are used in a wide range of applications, from aircraft seals to medical grade tubing. To make sure you have the right silicone for your application, the application requirements have to be closely matched to the silicone properties. There are many material properties that may affect our silicone’s performance in your application. One of the most important properties is hardness or “durometer”.

The hardness of a material refers to how easily the material resists deformation under compression. Put more simply, if something is pressed against the surface of a material, hardness tells us how easily the material is deformed or marked. Depending on the material, there are several standard tests that can be used to determine hardness. Metals are often tested using the Vickers or Rockwell procedures - these press a small tool into the surface of the metal under a standard load and measure the size of the resulting mark.

For elastomers such as silicone, hardness is measured using the Shore Durometer test. This test measures the depth that a small cone can be pressed into the surface of the silicone. The depth that the cone sinks into the silicone is converted to a value on the Shore Durometer scale. The cone sinks deeper into soft silicones and these have a low value on the scale. If the silicone is hard, the cone doesn’t sink very far and the value on the Shore scale is high.

There are several variations of the Shore Durometer test for different types of elastomers. The most common are Shore Durometer A for soft elastomers and Shore Durometer D for hard elastomers. Each test gives a hardness value from 0 to 100. The Shore A test is generally used to measure the hardness of silicones - most of our silicones fall between 25 and 80 Duro Shore A. Has equipment to test Shore hardness to the ASTM D2240 standard.

To get an idea of what the different hardness scales mean, 25 Duro silicone can be easily compressed with your fingers - think of the rubber that rubber bands are made of. 80 Duro silicone is much harder to compress, more like the rubber in a shoe sole.

To ensure that the silicone you choose for your application performs as required, it must have the right durometer. For applications such as vacuum sheeting, a low durometer might be just right, while gaskets may require a medium to high durometer.can tailor a silicone material to the durometer required for your application.

Durometer Hardness is used to determine the relative hardness of soft materials, usually plastics or rubber, is performed by the Shore Durometer Test according to the standard being referenced. Similar to the hardness tests for metal, this method determines a material’s resistance to indentation by penetration of an indenter into the test sample thus giving a hardness value.

Because the flexibility of polymers varies, LMATS is equipped with various indenters for use in testing different types of soft materials from elastomers and natural rubber products, neoprene, polyesters, soft P.V.C., leather, nitril rubbers, waxes and felt to rigid plastics and more.

The Shore durometer A scale indenter "soft samples" is a Hardened steel rod 1.1 mm - 1.4 mm diameter, with a truncated 35° cone (frustum), 0.79 mm diameter.
The Shore durometer D scale indenter "harder samples" is a Hardened steel rod 1.1 mm - 1.4 mm diameter, with a 30° conical point, 0.1 mm radius tip.

LMATS regularly undertake Durometer Hardness Testing to ASTM D2240 and ISO 868
Using the Shore hardness (using the Shore A and D scale) on the following items:

Hard and soft plastics (seals, gaskets, tyre treads, rubber wheels, o-rings, polyolefins, fluoropolymers, vinyl's, HDPE, LDPE)
Rubbers
Sealants
Elastomers

Testing For Durometer
An o-ring's hardness is often referred to as its durometer because hardness is measured by a durometer gauge. There are three main categories of hardness:

● Shore 00: Measures rubbers and gels that are extremely soft, including gummy candy and gel inserts for shoes.

● Shore A: The most commonly used category providing a wider range of hardness. This scale measures flexible mold rubbers that range from very soft to exceptionally hard rubbers with almost no flexibility. Some semi-rigid plastics can also fall onto the higher end of the scale.

● Shore D: Includes hard rubbers, from moderate hardness to extremely hard. The Shore D scale also encompasses semi-rigid plastics to hard plastics.

● Type M: Described as a micro-hardness gauge. Type M is appropriate for measuring the hardness of specimens that cannot be accurately measured using the Shore A device. Type M durometer is intended for use on specimens having a minimum thickness of 0.050 inches, although it can be used on thinner specimens if there is data supporting the accuracy of the measurement.

There are several standard testing methods that exist for measuring rubber durometer. ASTM D-2240 and ISO 868 are two of the most common. These specifications typically account for rubbers falling in the range of 20 to 90 Shore A.

●ASTM D-2240: This test method uses a tool that penetrates the rubber under specified conditions to measure the indentation. This procedure is used for determining the indentation of thermoplastic elastomers, vulcanized rubber, elastomeric materials, gel materials and some plastics.

●ISO 868: This test measures the indentation resistance of plastics by means of two types: type A and type D. Type A is used for softer materials while type D for harder plastics. This method permits the measurement of initial indentation or indentation after a specified period of time to understand how the material may function in certain environments.

The measurement of hardness across different surfaces is a critical aspect of material testing, often accomplished using a durometer. This device determines the resistance of indentation by pressing a needle onto the surface, and the resulting force is converted into a hardness value.

To ensure accuracy, the test is conducted at a minimum of three different locations, with each location having a material thickness of at least 6mm. The average of these measurements is then calculated to determine the overall hardness of the material under examination. The Shore A and the Shore D durometers (see Fig. 1 below) have useful applications for multifarious materials.

The Shore A durometer is suited for softer surfaces. Its applications can include: rubber, polyester, leather, wax, etc. The maximum hardness it can measure accurately is 100; anything higher is to be measured by the Shore D.

The Shore D durometer is suited for harder surfaces. Its applications can include: wood, hard rubber, plastics, flooring, bowling balls, etc.

In practical applications, durometers are incredibly versatile, making them invaluable tools across a wide range of industries. For example, Shore D durometers are frequently employed to measure the hardness of high-density polyethylene, highlighting their adaptability to various industrial needs.