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Ejection Rubber Dilemma: Durometer or Compression/Deflection?

by David West, Essential Products, Inc.

November-December, 2012
Those of us in the business of supplying die ejection material have been misleading you, and it is high time we make it right. 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 will work best on a particular die is a test method called compression/deflection, rather than durometer.

How is the durometer value measured?
Durometer is the measurement of firmness or surface tension of a compressible material. There are nine primary durometer measurements ranging from Type A to Type M. Within our industry, we typically use two different scales: OO and A. The method used to determine the durometer value of a material is the same with either scale. It is done using a tool as pictured right. This tool 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.

Normally, the OO scale is used in measuring the firmness of softer materials like light foams and sponge rubber. For example, the OO scale would be used to measure the softness/hardness of a seat cushion. The wider, rounded tip of the OO pin allows for some resistance when it is pushed into the soft foam, and it is this resistance that is measured for the durometer value. The A scale would be used for measuring denser/harder materials such as rollerblade or skateboard wheels. The sharp pin of the A scale is able to depress into the surface so that a measurement can be recorded. If the wider, rounded tip of the OO scale was used on the rollerblade wheel, it would have so much resistance that it would not create any depression. On the other hand, if the sharp pin of the A scale was used to measure a soft foam, the pin would just penetrate into the material without any resistance and again, no measurement could be recorded.

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

How is compression/deflection measured?
Compression/deflection is the measurement of the spring force of a piece of rubber/elastomer. It measures the PSI (Pounds per Square Inch) of force the rubber generates at different percentages of compression. For example, Red Rhino generates 20 PSI at 20-percent compression and 31 PSI at 30-percent compression. A simple math equation determines what percentage of compression a piece of elastomer will be exposed to during a diecutting job. (See Illustration 2.)

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 as to which rubber will function best in each 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.

Click image to view a larger version.

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 jobs get on and off press quickly.

Oftentimes, when a job run has problems and the machine or operator is blamed, it is actually the result of either using the wrong rubber or placing the rubber in the wrong locations that is the cause of problem. The chart featured in this article holds valid information that a diemaker/operator can use to choose the best elastomer for each job simply by understanding what the percentage of compression will be and the PSI crush level of the material.

David West is the president for Essential Products, Inc. Essential Products manages sales and marketing for both Monroe Rubber and Rayner DieSupply. For more information, please visit www.monroerubber.com.