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RubberTech Articles
Design Guide Article Series #1 - Selecting the Right Elastomer
Article: # 4
Author: Ernest B. Ferro, President, Corry Rubber CorporationElastomer Selection and Specification ConsiderationsKey Considerations
There are 4 primary considerations to bear in mind when specifying an elastomer type:
Is the application dynamic in nature?
To what fluids will the elastomer be exposed during service life?
To what temperatures will the elastomer be exposed during service life?
Will the elastomer be exposed to the weather for long periods of time during its service life?
To select the proper elastomer, the following information is needed during the design phase:
What is the frequency of vibration and the vibration amplitude that the elastomeric component will see?
Is it desired that the elastomeric component dampen vibration?
What is the desired service life in cycles?
What loads will the elastomeric component see during service?
Will the elastomeric component be required to provide sealing forces during its use?
Will the elastomeric component be outside in the weather or in a closed space?
What is the lowest temperature at which the elastomeric component must remain functional?
What is the highest temperature to which the elastomeric component will be exposed?
What is the highest temperature at which the elastomeric component must remain functional.
To what fluids will the elastomeric component be exposed during service and at what temperature will the component see these fluids?
Is fluid exposure continuous or intermittent?
Four Steps to Elastomer Selection
Elastomer selection and physical property definition is a 4 step process:
Listed below is a table that shows the different rubbers and their key properties. The table is arranged in order of increasing cost per pound from left to right. Natural rubber is considered 100 and all of the other elastomers are listed in relation to natural rubber. (From time to time, the price of natural rubber can fluctuate making some of the synthetics that are close to natural rubber in the table less expensive than natural. This table is provided as a general guide only.) Using the table below, start at the left and compare the properties of the elastomer to your answers to the 11 questions above. Keep moving to the right of the table until you find an elastomer that meets all or most of the requirements you outlined above. At this point, you will have found the most cost effective elastomer for your application. Keep in mind that applications that are at ambient temperature without exposure to fluids are generally best served by natural rubber. Natural rubber is still widely used and offers the greatest variety of compounds and properties at an attractive price.
The second step in elastomer selection is to select a hardness range. As we mentioned, hardness is measured using a durometer. Materials below approximately 30 durometer are extremely soft and similar to foam or sponge. Materials with hardnesses above 85 to 90 are extremely stiff and will be similar to rigid plastics. If you are unsure of the hardness that your application will need, please contact us for a set of harness test pieces. The table below lists the typical durometer ranges for most of the currently available elastomers.
The third step in elastomer selection is to select a tensile strength and elongation minimum. These requirements can be approximated through calculation or determined based on actual measurements made in the application environment itself. They can also be determined from testing a material that has proven to be satisfactory by trial or experiment. The table below lists some typical tensile strengths. Generally, costs go down moving from left to right in the table.
The fourth step in elastomer selection is to determine whether the component will be used in compression. If it will, we must define the maximum compression set allowed in the rubber when under load for a given period of time at a prescribed temperature. Compression set is usually expressed as a percentage of the compression which is not recovered within 30 minutes after removal of the load. Compression set requirements should only be specified when the product will be used in compression.
The fifth step in elastomer selection is to enumerate any other mechanical properties that might be relevant in the application. These properties can be tear strength, static or dynamic spring rates, dynamic damping, tan delta, resistance to specific fluids, etc.
Finally, a material specification that complies with ASTM D-2000 or SAE J-200 can be developed based on the properties outlined above. The ASTM or SAE specifications are identical and provide a framework for specifying elastomers. Generally all elastomer processors can understand and interpret the ASTM and SAE specifications. It should be mentioned, however, that many compounds can be made to meet a specification but depending on the application, not all materials may perform as well as others even though all materials meet the same specification.
Corry Rubber will be glad to answer your questions regarding design and engineering of rubber materials and products. Just Ask Dr. Stretch at www.CorryRubber.com.
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