Ejection Rubber Dilemma

Durometer or Compression/Deflection?

For as long as some of us can remember we have been using the standard of DUROMETER as the preferred method of comparing different types of ejection rubber. Although this type of measurement might help in comparing one type of rubber or elastomer to another; it does nothing in telling us how well it will perform on a die. The proper comparison method to use in determining which type of elastomer would work best on a particular die, is a test method called COMPRESSION/DEFLECTION. Let me explain…

Durometer is the measurement of firmness or surface tension of a compressible material. Within our industry we typically use two different scales: OO and A scales. The method used to determine the durometer value is done by using a tool that pushes a pin into the surface of the material and the amount of resistance the material surface exerts against the pin is the value used in defining the durometer of a material. The difference between the two scales is that OO uses a pin with a wider rounded tip whereas the A scale uses a narrower sharp pin that has a small blunt tip.

Here is the problem with using durometer measurements to qualify ejection materials. For our application, the softness or hardness isn’t what matters. For us, the spring force of the rubber is the key that determines what is or what is not the right rubber for a particular job. Consider what one instinctively does when comparing two different pieces of rubber. They squeeze it between their thumb and forefinger to determine if the rubber will work in their application. This test more closely represents Compression/Deflection rather than Durometer.

Compression/Deflection is the measurement of the spring force of a piece of rubber/elastomer. It measures the PSI of force the rubber generates at different percentages of compression. For example, Red Rhino generates 20 PSI at 20% compression and 31 PSI at 30% compression. A simple math equation determines what percentage of compression a piece of elastomer will be exposed to during a diecutting job.

Once this value is known it can be used in the Functional Range of Motion Chart below to determine the PSI of the various types of rubber under that percentage of compression. This chart will lead to an informed decision on which rubber will function best in your application. The PSI of each type of elastomer under the various compression percentages is listed within the colored boxes.

The Functional Range of Motion is the amount of compression a piece of rubber can withstand and function properly. In other words, it is the compression range where a piece of rubber can compress and snap back repeatedly without premature wear or failure. This is the GREEN zone.

The YELLOW zone is the densification point. This is where the rubber now becomes a solid and can no longer compress. If we were talking about springs this is where all the coils on the spring would be touching. Once the amount of compression gets close to this point, the rubber becomes densified and the life of that piece will be compromised.

Go beyond the yellow value and you are in the RED zone which is where the piece of rubber will fail and break apart.

Using Compression/Deflection as opposed to Durometer changes the paradigm that has been used for years in our industry in determining which rubber to use on a die. In this day and age where all companies are looking to get optimum performance out of their diecutter in the least amount of time, using Compression/Deflection to choose the right ejection material will go a long way in helping your jobs get on and off press quickly.

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