Rubber Molding Materials: Elastomer & Durometer Guide

Guide to Vibration Isolation, Elastomers, and Durometer Selection 

Introduction

This guide has been designed by Enterprise Rubber Inc. to assist buyers in the selection of elastomer compounds and durometer (hardness) with a particular focus on vibration isolation mounts. It is intended to serve as an educational tool and to offer general information based on the fundamental properties of each basic compound. The general properties of the elastomers highlighted in this guide can be extended to other molded rubber products. It should be noted that there are thousands of unique formulations for each compound which can alter their properties to meet specific engineering requirements. The elastomers discussed in this guide are the compounds most often utilized by Enterprise Rubber Inc. This guide should not be used as a substitute for the recommendations of engineers, specialists, and industry experts. There are many environmental and application specific variables that must be considered when choosing elastomer compounds. Enterprise Rubber Inc. is not providing specific engineering guidance and recommends that customers should test and determine suitability for their applications.

Contents

Section 1 – Terms and Ratings

• Abrasion resistance
• Application
• Bonding
• Compression
• Compression set
• Deflection
• Elongation
• Oil Resistance
• Pull
• Resilience
• Shear
• Solvent Resistance
• Specific gravity
• Tear resistance
• Tensile
• Torque
• Vulcanization

Section 2 – Elastomers

• Natural Rubber
• Neoprene
• Nitrile
• Silicone
• EPDM

Section 3 – Durometer

Section 1 – Terms and Ratings

Abrasion resistance is the resistance of rubber to abrasion by scraping or rubbing.

Application refers to how a vibration isolation mount will be used.

Bonding is the process of cross-linking molecules between a rubber compound and individual component metals using an intermediate substrate to create a single, unified piece.

Compression describes a downward-force application where a vibration isolation mount is aligned on its vertical axis and the application load is located above the mount. Compression and shear applications are recommended for vibration isolation mounts.  

Compression set is the extent to which an elastomer fails to return to its original state upon releasing a compressive load. Repeated compression of rubber over time results in progressive stress relaxation. Compression set is the permanent deformation of a material after prolonged compressive stresses at a given temperature and deflection.

Deflection is the amount of change in either the height or symmetry of the rubber in a mount during use in its application. A maximum deflection of no more than 10% is typically recommended.

Elongation is defined as the percentage increase in the original length of a rubber sample with the application of a tensile force, or stress. Certain elastomers tend to stretch more compared to others.

Oil Resistance is the resistance of rubber in contact with oils that can cause degradation of the rubber.

Pull describes a downward-force application where a vibration isolation mount is aligned on its vertical axis and the application load is located below the mount. Mounts used in a pull application are being elongated and stretched apart. This is not recommended. Compression and shear applications are recommended for vibration isolation mounts.

Resilience, or rebound, is the ability of rubber to return to its original size and shape following a temporary deformation.

Shear describes an application where a vibration isolation mount is aligned on its horizontal axis and the application load is located to the side of the mount creating force along a parallel plane to the direction of the force. Compression and shear applications are recommended for vibration isolation mounts.

Solvent resistance is the resistance of rubber in contact with chemicals like bleach or other substances that can cause degradation of the rubber.

Specific gravity is the ratio of a material's weight to the weight of an equal volume of water. Elastomers with a low specific gravity provide more cubic inches per pound of raw material stock. Those with a higher specific gravity require more raw material per finished part.

Tear resistance is the resistance of rubber to the development of a tear when tension is applied.

Tensile strength is the amount of force needed to tear apart a rubber specimen until it breaks. Typically measured in pounds per square inch (PSI), tensile strength is important because it signifies the point of failure from stretching.

Torque is rotational force. Care must be taken to avoid transferring fastening torque from the metal into the rubber of an isolator. Allowing fastening torque to preload stress force into the rubber will alter the performance capabilities of the isolator and can cause premature failure.

Vibration Isolation Mounts are used to minimize the impact of unwanted vibrations. These go by many unofficial names, including bobbins, shock mounts, rubber buffers, dampeners, cylindrical mounts, rubber mounts, isolator mounts, anti-vibration mounts, motor mounts, rubber bumpers, snubbers, rubber feet, sandwich mounts, compression mounts, tapered mounts, cushion mounts, stud mounts, flex mounts, and more.

Vulcanization is a process that involves using heat and pressure to cross-link molecules and enhance the properties of rubber to form a homogeneous substance with desirable characteristics such as elasticity, strength, and stability.

Ratings

We have assigned a general rating for several key features for each elastomer. These are:

• Excellent
• Good
• Fair
• Poor

Proper elastomer selection will often rely on a combination of environmental and application specific factors. These ratings can assist in choosing the best option based on these variables.

Section 2 - Elastomers

Natural (Polyisoprene) Rubber – General Characteristics

Natural Rubber is a common selection for vibration isolation mounts. With an approximate specific gravity of 1.1, it is typically considered the most economical rubber choice and bonds well to metals. Natural rubber is a great choice for dry conditions and works well in temperatures from -40°F to 165°F. Prolonged contact with oils, solvents, and ozone are not recommended.

Neoprene (Polychloroprene) Rubber – General Characteristics

Neoprene is a common selection for vibration isolation mounts. With an approximate specific gravity of 1.4, it requires more raw material per part than natural rubber. It works well in temperatures from -50°F to 250°F. Neoprene bonds well to metals and performs better than natural rubber in some conditions, such as higher temperatures, as well as occasional contact with moderate chemicals, oils, and solvents.

Nitrile (Butadiene Acrylonitrile or Buna-N) Rubber – General Characteristics

Nitrile is sometimes necessary for vibration isolation mounts. With an approximate specific gravity of 1.2, its density is similar to natural rubber; however, it is a more expensive compound. Use of nitrile is recommended when conditions require regular contact with solvents, oils, water, and hydraulic fluids. It works well in temperatures from -30°F to 250°F. Use in highly polar solvents, ozone, or chlorinated hydrocarbons is not recommended.

Silicone (Polysiloxane) Rubber – General Characteristics

Silicone is sometimes necessary for vibration isolation mounts. With an approximate specific gravity of 1.35, its density is similar to neoprene; however, it is a more expensive compound. Due to its excellent temperature resistance silicone is a great choice in extreme temperature environments. It works well in temperatures from -100°F to 450°F. Silicone has moderate resistance to diluted solvents and is highly resistant to oxidation and ozone. Contact with most concentrated solvents, oils, concentrated acids, and sodium hydroxide is not recommended.

EPDM (Ethylene Propylene) Rubber – General Characteristics

EPDM is sometimes necessary for vibration isolation mounts. With an approximate specific gravity of 1.15, its density is similar to natural rubber. Because this compound requires extra steps to bond with metals effectively, it typically commands a higher price. EPDM has exceptional properties related to ozone and weather resistance, water and steam, chemical resistance, and does well with heat. It works in temperatures from -50°F to 250°F.  Use in oil, solvents, or aromatic hydrocarbons is not recommended.  

Section 3 – Durometer

Durometer refers to the hardness of a material and is measured on a “Shore A” scale from 0 to 100. Simply put, durometer measures the resistance of a material to deflection. Created in the 1920’s and named after its inventor, Albert Ferdinand Shore, Shore hardness offers different scales for measuring the solidity of different materials. The most common measurement scale for elastomers like the types used in vibration isolation mounts is called Shore A. Lower Shore numbers indicate softer material, higher Shore numbers indicate harder material. This system of measurement allows for a more precisely made rubber part and better performance in its application as a result.

An example of a product with a soft durometer is a rubber band, which will likely have a Shore A durometer of around 20 to 25. On the opposite end of the spectrum, an example of a product with a hard durometer is the wheel of a skateboard. These wheels will likely have a hardness of 80 or higher.

Why does hardness matter? Rubber materials that are harder are more resistant to compression; as the hardness of a vibration isolation mount is increased, its ability to carry a load increases as well. Two mounts with the same dimensions and different Shore A durometers will perform differently in their applications, as load capacity is rated according to the displacement of the mount under pressure. There are times when a softer mount is required to best isolate vibrations, and there are times when a harder mount is required to help carry load.

Many times, it is most effective to select a durometer somewhere between 40 and 60 Shore A. This allows the selector to take advantage of the central qualities gained from both soft and hard materials.