The durometer of rubber quantifies the hardness of materials like elastomers, polymers, and plastics. It is a dimensionless value that specifically measures a material’s resistance to permanent indentation. This property is fundamental in material science, providing a standardized way to ensure materials meet specifications and is a crucial step in quality control and material selection across countless industries.
Understanding Durometer and the Shore Scales
Durometer is measured using a specialized instrument known as a durometer gauge. The gauge applies a specific force to an indenter tip resting on the material’s surface, and the depth of penetration dictates the hardness reading. This reading is displayed on a scale ranging from 0 to 100, where a higher number signifies greater resistance to indentation and a harder material.
Different Shore hardness scales were developed to measure materials ranging from soft gels to hard plastics. The two most common scales, Shore A and Shore D, use distinct indenters and spring forces. The Shore A scale is used for softer, more flexible materials, such as rubbers, elastomers, and silicone.
In contrast, the Shore D scale is designed for harder materials, including semi-rigid plastics and rigid thermoplastics. The Shore A durometer uses a blunter, truncated conical indenter and a lower test force. The Shore D durometer employs a sharper, pointed indenter and a greater force to penetrate tougher surfaces. Materials that register below 10 or above 90 on a particular scale are generally measured using an adjacent scale for improved reliability.
How Durometer Readings Influence Material Performance
A material’s durometer value is directly linked to its performance in an application. Softer rubber compounds (lower Shore A durometer) are preferred for sealing because they exhibit better conformability. Their pliability allows them to compress and fill minute gaps on uneven mating surfaces, creating an effective seal even under low pressure.
Conversely, materials with a higher durometer offer superior resistance to abrasion and physical wear. This increased hardness indicates greater structural integrity, making the material better suited for environments involving rubbing, scraping, or tearing. For instance, a hard rubber roller withstands constant friction better than a softer compound.
Material hardness also affects compression set, which is the permanent deformation occurring under a sustained compressive load. Very soft elastomers are more susceptible to this permanent loss of resilience, meaning they may not return to their original thickness after the load is removed. This failure to recover can compromise the long-term effectiveness of components like gaskets and seals.
In contrast, softer materials are better at absorbing mechanical energy, making them ideal for vibration dampening and shock absorption. The lower durometer allows the material to deflect easily and dissipate vibrational energy across its structure. Applications requiring the isolation of movements or the cushioning of impact forces typically utilize compounds on the lower end of the Shore A or the softer Shore OO scale.
Practical Examples of Durometer Values
Common objects can be mapped to their approximate durometer values on the Shore A and D scales. A very soft rubber band typically registers around 20 Shore A. Moving into the mid-range, the rubber tread on a car tire commonly falls around 60 Shore A.
A moderately hard material, such as the sole of a running shoe, is often rated at about 70 Shore A, balancing cushioning and durability. As the material approaches the upper limit of the Shore A scale, it feels more like a rigid plastic, with a hard leather belt measuring near 80 Shore A. When an item becomes too hard for the Shore A scale, the Shore D scale is used, such as a hard plastic shopping cart wheel, which is likely rated around 60 Shore D.